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Monday, December 15, 2008

The Importance of Mixing Your NMR Samples

It is very important that your NMR samples are mixed well before NMR data are acquired. If the sample has a concentration gradient (i.e. more concentrated at one end of the sample column compared to the other) it will be very difficult to shim the magnet over the entire volume of the sample as the magnetic susceptibility is not constant over the sample volume. As a result the NMR lines may be skewed and will be much broader than necessary. This will lead to a much lower signal-to-noise ratio based on signal heights. The figure below shows partial 300 MHz 1H NMR spectra for 2-bromobutane in CDCL3. The spectrum on the left was acquired on a sample where 1 drop of 2-bromobutane was added to CDCL3 in an NMR tube. The tube was gently swirled but not shaken. The magnet was shimmed using a gradient shimming routine and the data collected. The spectrum on the right was acquired on the same sample except the tube was removed from the magnet, shaken and reinserted. The magnet was reshimmed with the same gradient shimming routine and the same number of scans were collected. The difference in the quality of the NMR data is obvious.

Monday, December 8, 2008

Optimizing Decoupler Pulses for CP/MAS NMR

The 13C line width of protonated carbons, the signal-to-noise ratio and often the resolution in a CPMAS spectrum depends on effective high power proton decoupling during the acquisition time. A convenient sample to optimize the decoupling power or pulses for 13C CPMAS NMR is glycine, as the line width of the methylene carbon is very sensitive to the quality of the decoupling. The figure below shows such an optimization of the pulse widths used in TPPM (Two Pulse Phase Modulation) decouipling. The data were collected at 11.7 T.

Friday, November 28, 2008

QCPMG

The quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) sequence can be used to measure the NMR spectra quadrupolar I = n/2 nuclei in the solid state. This technique is essentially a T2 sequence where a series of echos are collected. The entire echo train represents the time domain data and is Fourier transformed to produce a frequency domain spectrum. The QCPMG spectrum consists of spikelets separated in frequency by the reciprocal of the time separation between the echos in the echo train. The intensity envelope of the spikelets mimics the static line shape. This is analogous to the rotational echoes in the FID's of MAS data and the associated spinning sidebands in the frequency domain MAS spectra. The QCPMG technique represents an improvement in sensitivity compared to a single conventional Hahn echo as the intensity is concentrated in the spikelets rather than spread across the entire frequency span of the spectrum. The figure below shows the pulse sequence, an echo train and a QCPMG 23Na spectrum of solid sodium sulfate at 4.7 Tesla. The 23Na Hahn echo spectrum is also ahown as a comparison to the QCPMG spectrum. The spectrum represents the central transition only. The satallite transitions are not visible.

Monday, November 24, 2008

90 Degree Pulses for I = n/2 Quadrupolar Nuclei in the Solid State

The 90 degree pulse for an I = n/2 quadrupolar nucleus in the solid state depends on the strength of the rf pulse with respect to the quadrupolar frequency. If the strength of the pulse is much greater than the quadrupolar frequency, the pulse is non-selective and excites all transitions equally. If however it is much less than the quadrupolar frequency, then the pulse is selective to the central (m = 1/2 - m = -1/2) transition. The duration of the pulse producing a maximum signal is shorter for selective vs. non-selective pulses at a similar power level. In solution, where the quadrupolar interactions is averaged by random isotropic molecular motion or in the solid state, if the symmetry around the I = n/2 nucleus is cubic, the quadrupolar frequency is small with respect to the strength of the rf pulses and the pulses are non-selective. When the symmetry around the I = n/2 nucleus in the solid state is non-cubic, the quadrupolar frequency is significant and the pulses are very often selective to the central transition. This is illustrated in the figures below for the 23Na MAS spectrum of a mixture of NaCl (cubic) and Na2SO4 (non-cubic). The first figure shows the 23Na MAS spectrum labelling each component of the mixture. The second figure shows the effect of increasing the pulse duration. One can clearly see that the 90 degree pulse for NaCl is close to twice that of Na2SO4.

Friday, November 21, 2008

Before You Leave .....

This may seem to be a strange post .... a rant really...... but very important.

Over the years I have seen many students start a long acquisition (or series of acquisitions) on spectrometers and then immediately leave the lab. After a long lunch, an afternoon of playing billiards, a good night sleep or perhaps a weekend of skiing, they return to the lab and find no useful data waiting for them.

Why? ......

Well ....... perhaps the spectrometer was set up to run 4 rather than 20,000 scans, perhaps the receiver was saturated, perhaps the recycle delay was set to 1000 seconds rather than 2 seconds, perhaps the pulses were not set correctly, perhaps the spectral width was set too small, perhaps the probe was not tuned and matched, perhaps a delay was set to 10 seconds rather than 10 milliseconds. perhaps a typing error was made in the command to start the acquisition..... etc.

The NMR lab charges you for your time whether you get useful data or not, so it is important to be careful.

Before you leave the lab......

1. Double check the parameters in your experiment and for all queued experiments.

2. Query the spectrometer as to how long the experiment will take ("expt" (Bruker), "time" (Varian)) and ask yourself if the response makes sense.
3. Check the probe tuning and matching.
4. Make sure the receiver gain has been set correctly.
5. Look at the first few scans to make sure you have a signal.

Thursday, November 20, 2008

The Importance of Grinding Solid Samples

When the heteronuclear dipolar coupling interaction has been removed by high power decoupling, the NMR spectra of dilute spin I = 1/2 nuclei in a single crystal give rise to relatively sharp lines. The frequencies of the lines depend on the chemical shift tensor and the orientation of the single crystal with respect to the magnetic field. Finely powdered samples have many thousands of crystallites and all orientations of the crystallites with respect to the magnetic field are represented equally. As a result, for powders, one obtains a broad powder pattern. Samples that are not ground into a powder contain many fewer crystals than crystallites in a powder and will yield spectra with partially resolved lines. The envelope of lines for all of the crystals will approximate the true powder spectrum. An example of this is shown in the figure below.

Thank you to Victor Terskikh of the National Ultrahigh Field NMR Facility for Solids. for suggesting this post and kindly providing the data for the figure.

Monday, November 17, 2008

Complexed Solvents

I was once asked by an inorganic chemist: why do I have two THF signals in the spectrum of my compound dissolved in THF-d8? Many inorganic compounds crystallize with complexed solvent molecules as a fundamental component of their structure. This is particularly true of tetrahydrofuran (THF). The complexed solvent molecules are released when the solid compound is re-dissolved in solution and can easily be detected by high resolution NMR. The figure below shows the 500 MHz 1H NMR spectrum of an inorganic compound containing complexed THF which was re-dissolved in THF-d8. One can see the spectrum of the residual protons of the THF-d8 solvent and the spectrum of the complexed THF that was released when the solid was dissolved. The signals are separated due the isotope effect.

Thursday, October 30, 2008

The Effect of the Contact Time on CP/MAS NMR Spectra

One parameter for CP / MAS data collection that must be set by the user is the contact time during which magnetization is transferred from the abundant nucleus (usually 1H) to the dilute nucleus (e.g. 13C). In the case of the 13C nuclei in organic samples, the build up of magnetization for each type of carbon depends on the extent of the dipolar coupling to the proton network. The extent of 13C - 1H dipolar coupling depends on both the degree of protonation for each type of carbon and any molecular motion (such as methyl group rotation) which may average the dipolar coupling. At longer contact times, the magnetization decays as a function of the T1(rho) of the protons. It should be noted that cross polarization is also affected by MAS. The length of the contact time should be chosen such that all types of carbons have had sufficient time to polarize yet not so long as to loose significant magnetization due to the proton T1(rho). For 13C CP/MAS an appropriate choice is usually between 1 and 10 ms. The figure below shows the effect of the duration of the contact time for the two 13C resonances of glycine. The 50 MHz 13C CP/MAS spectra were run as a function of contact time and plotted side by side. The intensities of each resonance are marked with color coded points. One can see that the carbonyl carbon builds up more slowly than the protonated carbon. An appropriate choice of contact time for glycine is 2 -3 msec.

Friday, October 17, 2008

Kinetic Experiments on Bruker Spectrometers

Students often have to monitor the progress of a chemical reaction as a function of time using NMR spectroscopy. I have written three simple programs for XWINNMR (which should work with little or no modification for TOPSPIN). Each program uses a different method to control the time allowed between collecting spectra. All are very simple and easily implemented. They should be added to the Bruker/XWINNMR/exp/stan/au/src directory. The first two programs, kinetic_ds and kinetic_t, are suitible for slow reactions where precise timing is not critical as they do not take into account the time required to initialize each acquisition. The third program, kinetic_2d avoids the problem by using a pseudo 2d approach and is suitible for faster reactions.

1. kinetic_ds
This program uses dummy scans to control the time allowed between spectra. (A dummy scan is a scan taken without turning on the receiver.) The more dummy scans, the longer the time between experiments. The user should set up the appropriate parameters and then run the program (by typing xau kinetic_ds). You will be asked for the total number of spectra to be collected, the number of scans to be collected for each spectrum and the number of dummy scans to be used in all but the first spectrum. The first spectrum will be collected in the current experiment and the others in subsequent experiments.

/* kinetic_ds */
/* written by Glenn Facey, August 24, 2005 */
/* This program will set up a kinetic run based on the use of dummy scans */
/* The user is asked for the number of spectra, the number of scans for */
/* each spectrum and the number of dummy scans for all but the first spectrum */
/* the first spectrum uses no dummy scans. */
GETCURDATA
GETINT("Enter total number of spectra",i1)
GETINT("Enter the number of scans for each spectrum",i2)
GETINT("Enter the number of dummy scans for all but the first spectrum", i3)
STOREPAR("ns",i2)
STOREPAR("ds",0)
Proc_err(0,"Kinetic Run in Progress");
RGA
ZG
TIMES(i1-1)
IEXPNO
SETCURDATA
STOREPAR("ds",i3)
STOREPAR("ns",i2)
ZG
END
QUITMSG("Data Collection Complete!")


2. kinetic_t
In this program, the user should set up the appropriate parameters and then run the program (by typing xau kinetic_t). You will be asked for the total number of spectra to be collected, the number of scans to be collected for each spectrum and the time in seconds allowed between the end of one acquisition and the beginning of the next acquisition. The first spectrum will collected in the current experiment and the others in subsequent experiments.

/* kinetic_t */
/* written by Glenn Facey, August 24, 2005 */
/* This program sets up and runs a kinetic experiment */
/* The user is asked to input the number of spectra, */
/* the number of scans for each spectrum and the time in */
/* seconds between the end of an acquisition and the */
/* beginning of the next. The program will measure the */
/* receiver gain and start the acquisitions. */
GETCURDATA
GETINT("Enter total number of spectra",i1)
GETINT("Enter the number of scans for each spectrum",i2)
GETINT("Enter the time interval (in seconds)", i3)
STOREPAR("ns",i2)
Proc_err(0,"Kinetic Run in Progress");
RGA
ZG
TIMES(i1-1)
IEXPNO
SETCURDATA
STOREPAR("ns",i2)
ssleep(i3);
ZG
END
QUITMSG("Data Collection Finished")


3. kinetic_2d
This program avoids initialization delays by collecting the data in a pseudo 2D format where each slice of the experiment is a spectrum. The program uses a pulse program called zg30kin.gf (see below) which should be put in the directory Bruker/XWINNMR/exp/stan/lists/pp (This pulse program program should be modified to suit the needs of the user). A variable delay list called kinetic must also be set up. This list contains the same number of lines as the number of spectra to be collected. Each line in the variable delay list defines the time interval (in seconds) to be allowed before each acquistion. The user must set up the appropriate parameters (including the number of scans to be collected for each spectrum) and then run the program (by typing xau kinetic_2d). You will be asked only for the total number of spectra to be collected. The program will set up a pseudo 2d acquisition. Data collection is started with the zg command. The data are processed with the xf2 command.

/* kinetic_2d */
/* written by Glenn Facey, August 24, 2005 */
/* This program sets up a pseudo 2D kinetic run */
/* using the pulse program zg30kin.gf with a Variable */
/* delay list called "kinetic". */
GETCURDATA
GETINT("How many spectra do you want to acquire?", i1)
FETCHPAR("SFO1",&d1)
FETCHPAR("DW",&f2)
FETCHPAR("SW",&d2)
FETCHPAR("SF",&d3)
XCMD("parmode 2D")
XCMD("pulprog zg30kin.gf")
XCMD("vdlist kinetic")
STOREPAR("SFO1",d1)
STOREPAR("DW",f2)
STOREPAR("SW",d2)
STOREPAR("SF",d3)
STOREPAR1("TD",i1)
STOREPAR1("SI",i1)
QUITMSG("Setup Complete!\n1. Define 'VD' List called 'kinetic'.\n2. Run the experiment with 'zg'.\n3. Process data with the 'xf2' command.")


Pulse program zg30kin.gf

;zg30kin.gf
;zg30 modified to run kinetic experiment in pseudo 2D mode
;using VD list
;avance-version (00/02/07)
;1D sequence
;using 30 degree flip angle

#include
"d11=30m"
1 vd
ze
2 d1
p1*0.33 ph1
go=2 ph31
d11 wr #0 if #0 ivd
lo to 1 times td1
exit

ph1=0 2 2 0 1 3 3 1
ph31=0 2 2 0 1 3 3 1

;pl1 : f1 channel - power level for pulse (default)
;p1 : f1 channel - 90 degree high power pulse
;d1 : relaxation delay; 1-5 * T1
;d11 : short delay for I/O

Wednesday, October 8, 2008

Proton Spin Pairs

In the solid state, in the absence of very fast magic angle spinning or homonuclear multiple pulse decoupling schemes, the 1H NMR spectrum of a typical solid is a broad featureless line greater than 50 kHz in width. This is due to the homonuclear dipolar coupling interactions between the many protons present in the system. The situation is different for an isolated pair of protons. For an isolated pair of protons, there is only one dipolar interaction between the protons and the energy level diagram for the system has only three levels corresponding the combination of spin states among the two protons and the dipolar coupling between them. There are two transitions and therefore two resonances. The separation between the resonances depends on the magnitude of the dipolar coupling constant, R, and the orientation of the internuclear vector with respect to the applied magnetic field. For powdered samples where all orientations with respect to the applied magnetic field are represented, one observes a "Pake" doublet. This situation is very similar to the solid state NMR of 2H where, in that case, the three energy levels arise from the Zeeman states of a single 2H nucleus and their coupling to an electric field gradient.

Isolated proton pairs occur naturally in the waters of hydration of inorganic salts and the solid state 1H NMR spectrum is a Pake doublet. The separation between the inner peaks of the Pake doublet is 3/2 R and between the two shoulders is 3R, where R is the dipolar coupling constant. The dipolar coupling constant is directly proportional to the inverse cube of the distance between the protons. Therefore from a single spectrum, one can measure the internuclear separation, r. For the case of the waters of hydration, one can measure the H-O bond length with knowledge of the H-O-H bond angle. The figure below illustrates the Pake doublet spectrum obtained for CaSO4. 2 H2O. The asymmetry in the spectrum is the result of chemical shielding anisotropy and the broadening is the result of dipolar coupling to distant protons.

Tuesday, October 7, 2008

Positive NOE's and the Decision on Which Decoupling Mode to Use

Nuclei with positive gyromagnetic ratios, such as 13C, exhibit positive NOE's with nearby protons. When observing 13C directly with proton decoupling, the intensity of the resonances will be increased to an extent dependant on the magnitude of the NOE's and the amount of time over which they are allowed to build up. When quantitative results are not sought after, it is always best to collect the NMR data with decoupling during both the acquisition time and the relaxation delay so that the effect of the NOE's on the intensity of the lines is maximized. The figure below compares the 13C NMR spectra for formamide collected with inverse gated decoupling (left) and full decoupling (right).
The above spectra were collected at 11.7 tesla with 4 scans, using a recycle delay of 30 seconds and an acquisition time of 2.3 seconds. This effect of positive NOE's on the resonance intensity should be compared to that for negative NOE's.

Monday, October 6, 2008

Negative NOE's and the Decision on Which Decoupling Mode to Use

Nuclei with negative gyromagnetic ratios, such as 15N and 29Si, exhibit negative NOE's with nearby protons. When observing these nuclei directly with proton decoupling, the intensity of the resonances will be decreased to an extent dependant on the magnitude of the NOE's. Usually it is best to collect the NMR data for these nuclei with inverse gated decoupling so that the effect of the NOE's on the intensity of the lines is minimized. There are cases however where the magnitude of the NOE is so large that the intensity of the resonance collected with full decoupling becomes negative and even stronger than that observed with inverse gated decoupling. In such cases it is advantageous to leave the decoupler on 100 % of the time. As the figure below shows, this is indeed the situation with the 15N resonance of formamide where the magnitude of the signal is smaller when inverse gated decoupling decoupling is used.
The above spectra were collected at 11.7 tesla with 4 scans, using a recycle delay of 120 seconds and an acquisition time of 2.3 seconds.

Thursday, October 2, 2008

Dilute "D2O" in Benzene-d6

The 1H NMR spectrum of a mixture of H2O and D2O is a single line at about 4.8 ppm. The H2O and HDO resonances are unresolved in the spectrum due to fast chemical exchange between the isotopomers and possibly line broadening due to radiation damping. When traces of D2O are added to benzene-d6, which already contains traces of H2O, the situation is different. The resonance is shifted by more than 4 ppm to lower frequency compared to the bulk and since the water is now dilute and in small quantities, chemical exchange is slow on the NMR time scale and radiation damping is no longer a problem. The figure below shows the 500 MHz 1H NMR spectrum of dilute D2O in benzene-d6. The isotope shift between H2O and HDO and the HD coupling constant can easily be measured from the spectrum.

Monday, September 29, 2008

T1 Anisotropy

In the solid state, in the absence of magic angle spinning, the frequency of NMR lines depends on the orientation of the molecules with respect to the static magnetic field. For powdered samples, all orientations are represented in the sample and one obtains a broad envelope of peaks resulting from all possible orientations. Such broad resonance are called powder patterns and are said to be anisotropic. The frequency is not necessarily the only orientation dependant parameter. In some cases, the T1 relaxation time also depends on the orientation of the molecules with respect to the magnetic field. In such cases the T1 is said to be anisotropic. In contrast to the NMR resonances in solution which are characterized with a single T1, the powder pattern can be characterized with many different T1 relaxation times. Furthermore, the presence or absence of anisotropy in the T1 can help discriminate between certain types of molecular motion. An example of an anisotropic T1 is illustrated in the figure below for the wide line 2H inversion recovery spectra of acetone-d6 trapped in an organic inclusion compound. The line shape indicates that the acetone molecules undergo both fast methyl group rotation and fast two-fold flips about he carbonyl bond. One can see that the entire powder pattern does not have the same T1 as the line shapes are a function of the inversion recovery delay, tau. The T1 depends on the frequency within the powder pattern which in turn depends on the orientation of the molecules with respect to the magnetic field.

Wednesday, September 17, 2008

The Effect of Dissolved Oxygen on Relaxation Times

Proton T1 relaxation occurs, to a very large extent, due to inter-proton dipole-dipole interactions and their time dependence as a result of molecular motion. Potentially, there is also dipole-dipole interactions between protons and unpaired electrons which also contribute greatly to proton relaxation. In fact, for 13C NMR, paramagnetic materials are sometimes added to the sample to make relaxation more efficient. Dissolved molecular oxygen from air is a paramagnetic material that is often overlooked. Like other paramagnetic materials it contributes significantly to the relaxation rate of protons. It can be eliminated from NMR samples by simply bubbling nitrogen through the sample for a few minutes or freeze-pump-thawing the sample a few times. The following data show the effect of dissolved oxygen on the proton relaxation times of ethyl acetate in acetone-d6.

T1's for sample under air 
quartet = 8.7 seconds
singlet = 7.7 seconds
triplet = 6.9 seconds

T1's for sample under nitrogen
quartet = 17.7 seconds
singlet = 13.2 seconds
triplet = 12.5 seconds


One can see that the relaxation times are nearly 100% longer when oxygen is eliminated from the sample. The figure below shows the proton T1 inversion recovery measurement for the acetate methyl singlet of ethyl acetate for the same sample before and after nitrogen was bubbled through the solution.

Tuesday, September 16, 2008

Fast Molecular Motions and Solid State Wide Line 2H NMR

On occasion, I have been asked why people make such a big deal about solid state wide line 2H NMR. After all, 2H has a low natural abundance and isotopic labelling is necessary to collect the data. The answer is that the 2H line shapes depend on the interaction between the 2H quadrupole moment and the electric field gradient tensor surrounding the 2H nucleus. The electric field gradient tensor is dramatically affected by the averaging of different types of molecular motions and this effect is readily observed in the NMR line shapes. The line shapes are sensitive to the rates, order and axes defining the motion. For molecular motions occurring at rates fast with respect to the width of the static 2H NMR spectrum, one can determine the type of molecular motion by doing reasonably straight forward calculations or often simply by inspecting the motionally averaged spectrum. The figure below shows the effects that some common fast molecular motions have on 2H NMR line shapes. The same static quadrupolar coupling constant of 160 kHz was used in the calculation of all of the spectra in the figure.
The line shapes observed for molecules undergoing motions at rates comparable to the width of the static 2H NMR spectrum can also be calculated however, the calculations are a bit more involved.

Friday, September 12, 2008

Baseline Correction in 2D NMR Spectra

Sometimes a 1D NMR spectrum will have a baseline roll. One way of correcting this is to fit the baseline of the spectrum (between two limits) to a polynomial and then subtract the polynomial from the spectrum to produce a flat baseline. Baseline roll is not an exclusive problem to 1D data. It can also be present in each domain of a 2D data set. Baseline orrection can be applied to each of the dimensions. The figure below shows a proton NOESY spectrum which is uncorrected (top left), baseline corrected in the rows (top right), baseline corrected in the columns (bottom left) and baseline corrected in both domains (bottom right).

Thursday, September 11, 2008

Output Power Expressed in decibels (dB)

It is often very confusing for students to know exactly how much RF power (in Watts) they are using in their NMR experiments, especially since the two major instrument manufacturers express the output power differently. It is very expensive to use a power level too high as NMR probes can be damaged if proper safety measures are not taken. Both Varian and Bruker express the output power of their instruments in decibels (dB). The maximum power level (pl1, pl2, pl3 .... etc) on a Bruker spectrometer is -6 dB and the minimum power is 120 dB. On Varian spectrometers (at least our INOVA), the maximum power (tpwr, dpwr etc) is 63 dB and the minimum power is 0 dB. Because of amplifier non-linearity, one may get maximum power below 63 dB on a Varian instrument (perhaps 55 dB to 60 dB) with no further gain at higher settings. On Bruker instruments, the non-linearity can be taken into account by calibrating with known attenuators and creating a correction table (CORTAB). If one is using an amplifier with a maximum output power of 100 Watts and the effects of amplifier non-linearity are neglected, the output power will be approximately:

100 W @ 63 dB (Varian) and -6 dB (Bruker)
1 W @ 43 dB (Varian) and 14 dB (Bruker)
1 mW @ 13 dB (Varian) and 44 dB (Bruker)

The figure below shows a plot of the power level in dB vs. the fraction of total amplifier power. The expressions in the figure do not take into account amplifier saturation.

CAUTION : Exact output power on any particular instrument must always be measured and calibrated.

Thank you to Victor Terskikh of the National Ultra-high Field NMR Facility for Solids for suggesting this post.

Tuesday, September 9, 2008

Double Quantum Filtered COSY

COSY spectra are very useful in structure elucidation as they provide correlations between coupled spins. Often, NMR spectra have large singlet signals from uncoupled protons (such as t-butyl methyls, methoxy protons, excess water or a solvent signal) which provide no information in the COSY spectrum and perhaps even get in the way of looking for smaller coupled spins. In such cases one can use a double quantum filtered COSY sequence rather than a standard COSY 90 or COSY 45 sequence. Double quantum filtered COSY spectra filter out uncoupled singlets. A comparison of a standard COSY 90 and a double quantum filtered COSY sequence for ethyl acetate is shown below. One can see that the singlet is present in the COSY 90 spectrum but absent in the double quantum filtered COSY spectrum.

Friday, September 5, 2008

Backward Linear Prediction to Correct for Receiver Saturation

If the receiver gain is set too high the initial portion of the FID is clipped and the NMR spectrum is distorted. In such cases the spectrum should be run again with an appropriate receiver gain setting. Sometimes however, this may not be convenient as the sample may have decomposed. One way to improve the quality of the data is to take a close look at the FID, discard the initial clipped points, use backward linear prediction to calculate the discarded points and then do the Fourier transform. An example of the improvement you can expect is shown in the figure below.

Wednesday, September 3, 2008

Echoes and Fourier Transforms

NMR data are often collected using pulse sequences involving echoes. These sequences are usually of the form:

pulse1 - delay1 - pulse2 - delay 2 - acquire data

In theory the echo occurs when delay1 = delay2. Often, this is not quite correct in practice due to the pulses having finite duration and short hidden pre-acquisition delays before the receiver is turned on. It is a good idea to set delay2 < delay1 so that the entire echo is captured. In such cases it is important to discard the data before the top of the echo before Fourier transformation. Failure to do so will result in improper line shapes (in the case of broad lines) or phasing errors. The figure below shows the 2H quadrupolar echo data for perdeuterated PMMA. The effect on the line shape is shown when the data are Fourier transformed before the echo (top trace) and after the echo (bottom trace). The correct line shape is only obtained when the data are Fourier transformed precisely at the top of the echo (middle trace).

Friday, August 29, 2008

The NMR Time Scale

Many people use statements like, "It is fast on the NMR time scale" without really having an understanding of what the "NMR time scale" means. Assume we have a compound with two inter-convertible conformations, A and B, each of which gives a different NMR spectrum. If the interconversion between A and B is very slow then we expect to get a spectrum with both conformations A and B resolved. If the interconversion is very fast then we would get an unresolved spectrum which represents the average conformation of A and B. The interconversion between A and B is fast on the NMR time scale if it occurs at a rate much greater than the difference in frequency between A and B. Since the difference in frequency between A and B depends on the Larmor frequency of the nucleus being observed and the strength of the magnetic field, the "NMR time scale" depends on the particular experiment and the field strength. Therefore, when one uses the expression, "NMR time scale" one must qualify it with details of the measurement. An example of different NMR time scales is shown in the figure below. Here the 1H (300 MHz) and 13C (75 MHz) NMR spectra of the N-methyl groups of N,N-dimethylacetamide are shown at various temperatures. The spectra were plotted on the same scale in Hz. In this molecule there is a hindered rotation about the central C-N bond. One can see that the rotation is fast on the 1H NMR time scale before it is fast on the 13C NMR time scale because the difference in frequency between the methyl groups is smaller in the 1H spectrum compared to the 13C spectrum.

Wednesday, August 27, 2008

What is Quadrature Detection?

In order to distinguish between negative and positive frequencies relative to the carrier frequency (i.e. the frequency at the center of the spectrum), quadrature detection must be employed. Quadrature detection involves the collection of time domain NMR data on both the x and y axes of the rotating frame of reference. One of these FIDs is called "real" and the other one is called "imaginary". The assignment of the labels "real" and "imaginary" stems from the mathematics of a Fourier transform, which requires a complex input. The Fourier transform in turn produces a "real" spectrum and an "imaginary" spectrum". Having these two signals allows us to phase one of the NMR spectra (eg. the "real" one) entirely in absorption mode after the data have been collected. In an ideal world, it would be nice to have the two FID's collected via separate receiver coils. In this case, with a bit of manipulation, we could improve our signal-to-noise ratio by a factor of the square root of 2 by adding the real and imaginary spectra together. A very nice discussion of this has been presented by Carlos Cobas. In the real world we do not have two receiver coils in quadrature and we must rely on electronic "tricks" to achieve quadrature detection. These "tricks" do not allow us to gain a signal-to-noise advantage by combining the real and imaginary spectra. A simple block diagram of how a spectrometer collects data in quadrature is shown in the figure below. The yellow signal is the intermediate frequency (IF) of the spectrometer. The purple signal is the intermediate frequency of the spectrometer modulated with the NMR spectrum information (IF plus or minus delta nu). Two phase sensitive detectors (PSDs) are used. These devices produce a difference signal between two inputs. The inputs to the first PSD are the IF and the IF plus or minus delta nu. The output is therefore plus or minus delta nu (i.e. the information content of the NMR spectrum). The inputs of the second PSD are identical to the first except the phase of the IF is shifted by 90 degrees. The output is therefore similar to the first PSD except that it has a 90 degree phase difference. The two outputs are the "real" and "imaginary" FID's which provide the input for the Fourier transform.

Tuesday, August 26, 2008

Establishing the Lock with Varian's VNMR

Many students have problems establishing the deuterium lock on a spectrometer manually. There is an excellent description of how to do so on a Varian spectrometer running VNMR here. The procedure however is always the same.
1. Turn the lock off.
2. Adjust the lock power and lock gain until you see a signal.
3. Adjust the field (Z0) until the signal is as flat as possible.
4. Adjust the lock power and lock gain to get a stable signal on scale.
5. Adjust the lock phase to maximize the lock signal.
6. Turn the lock on.

Saturday, August 23, 2008

Happy Birthday !

The University of Ottawa NMR Facility BLOG is one year old today!

I am surprised at the interest shown for this BLOG over the last year. Since March of 2007 the number of readers has steadily increased (even over the summer months).

There are now more than 65 people who receive the posts via email. What started as a way for me to answer the questions and problems posed by the University of Ottawa NMR users has turned out to be useful for a larger audience. 92% of the readers are outside of the University of Ottawa. There are now 188 posts and they are searchable. I'm glad that you have found the contents of this BLOG to be useful.

Friday, August 22, 2008

The Fourier Transform of a Single Rotational Echo

The spinning sidebands in MAS NMR spectra are the result of rotational echos in the free induction decay. These echos show up as spikes in the free induction decay (and can be used conveniently to set the magic angle). The intensity envelope of the spinning sideband manifold in the NMR spectrum mimics the static wideline spectrum. One can obtain an NMR spectrum similar to the wideline spectrum from MAS data by Fourier transforming a single rotational echo rather than the entire FID. This is illustrated in the figure below for the 27Al MAS data for kaolinite at 21.1 Tesla.

Wednesday, August 20, 2008

Measuring Chemical Shift Parameters for Spin I=1/2 Nuclei in the Solid State

The observed frequency of a spin I = 1/2 nucleus in a molecule depends on the orientation of the molecule with respect to the magnetic field. For powdered solid samples, all possible orientations of the molecules are present in the magnetic field and one obtains an envelope of peaks called a powder pattern. If one can neglect all other interactions (such as dipolar coupling), the powder line shapes are dominated by the chemical shielding interaction. These line shapes provide valuable electronic, geometrical and dynamic information about the molecule. The chemical shift parameters can be read directly or calculated trivially from the spectrum. These parameters are indicated in the figure below.
The parameters shown in the figure are those recommended by IUPAC. There are other conventions used in the older literature and at times it can be confusing. The other conventions are described here by Klaus Eichele. The site also contains a very useful tool to convert the parameters from one convention to another. Thank you Klaus!

Monday, August 18, 2008

Measuring Parameters from Solid State 2H NMR Spectra

Solid State 2H NMR spectra of rigid molecules or molecules moving fast with respect to the overall line width of the static spectrum are characterized by two parameters: the quadrupolar coupling constant and the asymmetry parameter. These parameters are easily measured directly from the NMR spectrum as shown in the figure below.

Thursday, August 14, 2008

Pulse Sequences to Minimize Acoustic Ringing

Acoustic ringing can be a real problem at low frequencies. Its effects can be minimized in a simple one-pulse spectrum by either throwing away the initial bad data and applying a large phase correction or by using backward linear prediction to calculate the lost data. There are a number of pulse techniques one can use when collecting the data to cancel out the ringing. Two such examples are shown in the figure below. The bottom spectrum is the result of a simple one pulse experiment. The middle spectrum was collected using Bruker's "aring" pulse program and the top spectrum was acquired using Bruker's "aring2" pulse program. In all three cases a simple Fourier transform was applied with exponential line broadening. The spectra were collected with the same number of scans on a 300 MHz instrument.

Tuesday, August 12, 2008

MOOT NMR Minisymposium

For more than 20 years I have enjoyed attending the MOOT NMR Minisymposium. This is a regional (but growing) NMR meeting held over a weekend usually in late September or early October. It is a friendly forum for students and researchers to present NMR results and a great way to socialize with old friends or meet new contacts. I have attended all but 3 of the 20 meetings held thus far and even had the privilege of hosting MOOT VIII in 1995. MOOT XXI will be held on October 4 -5 this year, hosted by Robert Schurko at the University of Windsor. This year, for the first time, there are scheduled a number of tutorial talks. You can register on line and find all of the information here. I highly recommend this meeting for all those interested in NMR spectroscopy.

Friday, August 8, 2008

Setting the Magic Angle with Glycine

One of the most precise ways of setting the magic angle is to maximize the number of rotational echos in the FID of a suitable spin I = n/2 quadrupolar nucleus (n =3, 5, 7 ....). When setting up for 13C CPMAS, one usually uses the 79Br resonance of KBr as the resonance frequency of 79Br is very close to that of 13C. An alternative method of setting the magic angle is to use the 13C carbonyl resonance of glycine. This has the advantage in that the glycine can also be used to set the Hartman Hahn matching condition and to check the decoupling power. The width of the carbonyl resonance is very sensitive to the setting of the magic angle. The angle can easily be adjusted and set properly while maximizing the duration of the signal in the FID interactively. The figure below shows the 13C CPMAS FID and spectrum for glycine on- and off-angle with digital filtering such that the methylene resonance is outside of the spectral width. The spectra were collected at 11.7 Tesla using a spinning speed of 12 kHz. When the angle is mis-set, one can see that the line shape for the resonance is a miniature version of the powder pattern observed in the absence of magic angle spinning.

Thursday, August 7, 2008

Chemical and Magnetic Equivalence

Many students are unclear about the difference between chemical equivalence and magnetic equivalence. The clearest explanation I have seen on this is in Robin Harris' book, "Nuclear Magnetic Resonance Spectroscopy" (1983). Chemically equivalent nuclei behave the same as one another chemically but do not have the same NMR properties as one another, whereas magnetically equivalent nuclei are chemically equivalent and they have the same NMR properties. One can determine whether two nuclei are chemically or magnetically equivalent by considering the following:

If the nuclei under consideration are not isochronous (i.e. they do not have the same chemical shift) then they are neither chemically nor magnetically equivalent. All chemically or magnetically equivalent nuclei are isochronous however, isochronous nuclei need not be chemically or magnetically equivalent as their chemical shifts may be fortuitously identical.

If the two nuclei being considered are isochronous, one should consider how they couple to a third magnetic nucleus in the molecule which is not equivalent to them. If the coupling to the third magnetic nucleus is different for each of the nuclei being considered, then the nuclei are chemically but not magnetically equivalent to one another. If the coupling to the third magnetic nucleus is identical (and this is true for every magnetic nucleus other than those being considered) then the two nuclei are both magnetically and chemically equivalent.

This is illustrated in the figure below. (The examples from Harris' book were used here.) In the molecule on the left, the two protons are both chemically and magnetically equivalent as they both have the same coupling with the fluorine. In the molecule on the right, the two protons are chemically equivalent but not magnetically equivalent as they do not have the same coupling to each of the fluorines. Similarly, the two fluorines are chemically equivalent but not magnetically equivalent as they do not have the same coupling to each of the protons.

Friday, August 1, 2008

What Does an RF Pulse Actually Look Like?

NMR books and papers always show pulse sequences where the pulses are represented by perfect rectangles indicating perfect rectangular manifolds of monochromatic radiation. This is not the case in the real world. Despite the very impressive timing specifications given by instrument companies which greatly simplify the designing of pulse programs, the pulses at the output of the amplifiers are subject to the imperfect response of electronic components in the console. This manifests itself as pulses with imperfect edges (i.e. measurable rise and decay times). The figure below illustrates this point. The top two panels show 5 microsecond and 1 microsecond pulses measured with a 1 GHz digital oscilloscope at the output of the amplifier of a very modern NMR spectrometer. One can see that to a good approximation, the 5 microsecond pulse is rectangular whereas the 1 microsecond pulse shows an obvious rise and decay.The bottom panel shows an expansion of the beginning and end of the 5 microsecond pulse. The pulses seen by the sample will be even worse due to the response of the capacitors in the tank circuit of the probe.

Wednesday, July 30, 2008

Tuning Problems for Samples of High Ionic Strength

Tuning and matching an NMR probe are essential in getting optimum NMR results (also, see this link). The pulse widths and power levels called up by the spectrometer for use with parameter sets are based on the NMR probe being optimally tuned and matched. When the probe is not optimally tuned and matched, the pulse widths and power levels stored in the parameter files of the spectrometer are no longer calibrated correctly (eg. what the spectrometer calls a 90 degree or 180 degree pulse may actually be an 80 degree or 160 degree pulse). The tuning and matching of the NMR probe are affected by the sample inside the coil. It is therefore good practice to tune and match the NMR probe for each sample to insure that the pulses will be correctly calibrated for your NMR measurement. Problems may arise with samples of high ionic strength. These samples very strongly affect the tuning and matching of a probe especially at higher frequencies. The effect can be so large that the probe cannot be tuned and matched optimally. In fact, several times per year, NMR users will report to me that they are unable to tune the NMR probe on their sample. In such cases the pulses should be recalibrated for the specific sample or (if this is not possible) on a sample of similar ionic strength. This effect is illustrated at 300 MHz in the figure below. The top panel is a tuning curve for the probe containing a sample of water after the the tuning and matching were optimized. The middle panel shows the tuning curve for the probe after the water sample was replaced with a concentrated solution of NaCl. Note the gross mismatching. The bottom panel shows the tuning curve for the concentrated NaCl solution after the probe was retuned and rematched. It should be noted that the matching is not as good as that for the water sample and therefore the pulses stored in the spectrometer parameter files will not be correctly calibrated for this sample. Again note, that this problem is more pronounced at highr frequencies.

Tuesday, July 29, 2008

Resolution of Overlapping Signals Based on T1's

When students are asked what defines an NMR signal, they will most often say: the chemical shift, the coupling pattern, and the line width. One parameter which is often overlooked is the T1 relaxation time. The T1 is the time constant for the build up of magnetization along the magnetic field direction (z axis) when a sample is first placed in a strong magnetic field or after a pulse has been applied. It governs how long a spin system takes to come to equilibrium. T1's are measured with a 180-tau-90 inversion recovery sequence as a function of tau. For each resonance in an inversion recovery spectrum, there is a value of tau for which the signal will be nulled. If two signals with different relaxation properties overlap one another, tau values can be found which will null each of the signals individually and thus reveal the other signal. An example of this is shown in the figure below where the overlapping 1H signals of the methylene protons one either side of the carbonyl group of 3-heptanone are examined (i.e. those in the 2 and 4 positions). The black trace in the bottom panel is the 1H spectrum of the overlapping signals. The blue trace in the middle panel is an inversion recovery spectrum (phase corrected by 180 degrees) where a 4.1 second delay was employed. This delay nulls the protons on the 4 position revealing the quartet in the 2 position. The red trace in the top panel is an inversion recovery spectrum where a 5.5 second delay was employed. This delay nulls the protons on the 2 position revealing the triplet in the 4 position.

Monday, July 28, 2008

T2 vs T2*

The T2 relaxation time is the exponential decay constant for transverse magnetization (i.e. magnetization in the xy plane). In principle, one should be able to measure the T2 relaxation time by applying a 90 degree pulse to create transverse magnetization and measuring the decay constant of the FID. In reality however, the decay rate of the FID is also affected by such things as magnetic field homogeneity, unresolved coupling, temperature gradients.....etc. Because of these effects, the decay constant of the FID is called T2* rather than T2. T2* is an instrumentally dependant parameter and it determines the line width of an NMR resonance. T2, on the other hand, is a physically meaningful parameter independent of field inhomogeneity, J coupling and other factors. It is measured with a 90-tau-180-tau-FID pulse sequence as a function of tau. T2 is always greater than or equal to T2*. The figure below compares T2 to T2* for the proton resonance of CHCl3 for the lineshape sample in a reasonably well shimmed 300 MHz magnet. The line shape specifications were 0.3 Hz (at 50%), 2.9 Hz (at 0.55 %) and 6.2 Hz (at 0.11%). Even in a well shimmed magnet, the T2 for CHCl3 is nearly 19 times longer than the T2*.

Friday, July 25, 2008

Magnitude COSY-90 vs. Phase Sensitive COSY-90

Most commonly, chemists run simple COSY spectra in magnitude mode. A magnitude COSY provides positive peaks for both the diagonal and off-diagonal responses due to the magnitude calculation. The responses in a standard COSY-90 sequence have lines with a phase twist shape. The magnitude calculation is necessary to provide positive responses and the time domain data are usually treated with a sine bell weighting function (or something similar) to enhance the resolution lost in the phase twist lineshape. There are however, several versions of the COSY experiment. One such version is a phase sensitive COSY-90. This version provides off-diagonal responses that can be phased. Furthermore, the coupling giving rise to the cross peak (the active coupling) will be antiphase. The disadvantage to this method is that the diagonal responses are 90 degrees out of phase and can obscure off-diagonal responses close to the diagonal. The figure below compares a magnitude COSY-90 to a phase sensitive COSY-90 for ethyl acetate. In the figure, black is positive and red is negative.

Thursday, July 24, 2008

What are Those Positive Peaks in My NOESY Spectrum?

Students will sometimes ask me, "What are those positive off-diagonal peaks in my NOESY spectrum?". Since the NOESY pulse sequence is exactly the same as the EXSY sequence, a NOESY spectrum will show all possible correlations due to cross relaxation. These correlations include those from NOE's, chemical exchange and conformational (or rotational) exchange. For small molecules, if one phases the diagonal responses such that they are positive, the NOE's correlations will be negative and the exchange correlations will be positive. The answer to the question is therefore that the positive peaks are due to exchange. The figure below shows an example where three different types of cross peaks are visible. The molecule in the figure has two distinct rotational conformations in slow exchange with one another and gives a proton spectrum with every resonance doubled. Each resonance for one rotational conformation gives a positive (black) cross peak correlating it to the corresponding resonance of the other conformation. There is also a positive cross peak correlating the -NH- proton in the molecule to the residual water in the DMSO-d6 solvent. These protons exchange chemically with one another. Finally, the negative (red) correlations in the figure are due to NOE's.
Thank you to Jean-Gregoire Roveda of Dr. Beauchemin's group for giving me permission to use his spectrum as an example.

Wednesday, July 23, 2008

2D EXSY

The 2D EXchange SpectroscopY (EXSY) technique is exactly the same as the same as the 2D NOESY technique. The pulse sequences are identical. The method provides off-diagonal responses for spins which exchange slowly with one another (either conformationally or chemically) and also between spins with NOE's. The EXSY method is useful for showing exchange when the rate of the exchange is greater than or of the same order as the T1 relaxation rate (1/T1) but less than the frequency difference between the two spins (in the absence of exchange). Depending on the experimental conditions, the responses due to exchange are often much more intense than those due to NOE's. The figure below shows the 300 MHz 2D 1H EXSY spectrum of N,N-dimethylacetamide at room temperature. At this temperature, the molecule exhibits slow rotation about the (CH3)2N - C bond such that both methyl groups exchange with one another rotationally yet are distinct in the spectrum. This is evident by the cross peaks in the spectrum between the two methyl groups on the nitrogen. Note that the off diagonal peaks are of the same phase as the diagonal peaks.

Tuesday, July 22, 2008

1D Selective Gradient TOCSY as a Function of Mixing Time

Total Correlation SpectroscopY , TOCSY (see links here, here and here) is a technique that employs a spin lock during the mixing time of the sequence for which chemical shifts are invariant but J couplings evolve. The technique is used to correlate spins in the same J coupled spin system. The 1D selective TOCSY uses a shaped pulses to select a single spin and one gets a convenient 1D spectrum with all spins in the same spin system as the selected spin. The intensity of the coupled resonances depends on the duration of the mixing time as well as the magnitude of the coupling between spins. For short mixing times, one can often trace out the spin system by running a series of spectra as a function of the mixing time. The figure below shows a series of 1D selective gradient TOCSY spectra for 3-heptanone (selectively irradiated at the 6 position) collected as a function of mixing time. This molecule has two spin systems, separated by the carbonyl group, namely the protons in positions 6, 5, 4 and 3 and those in positions 1 and 2. With a short 10 msec mixing time, one observes the nearest protons at position 5. For 20 msec, one observes the protons at positions 5 and 4. For 30 msec and 50 msec mixing times, one observes all of the protons in the same spin system as the protons in the 6 position. Furthermore, one is able to observe the protons in the 3 position independent of the overlapping protons in position 2 which are not in the same spin system.

Friday, July 18, 2008

Second Order 1H NMR Spectra of Isopropyl Groups

One of the first things a chemistry student learns about NMR is how to interpret the coupling patterns in first order NMR spectra. With the high magnetic fields available for NMR today, this really goes a long way in interpreting spectra. Chemistry students also learn that when the chemical shift difference between two spins is comparable to their coupling constant that second order NMR spectra are observed and furthermore, that these second order spectra are "very complicated". Many do not bother to understand the line shapes. Sadly, many students carry around laptops with software packages capable of simulating these spectra and do not even know it. Several years ago, a student came to me with a proton NMR spectrum of an isopropylsilyl compound and asked why he could not see the typical septet - doublet isopropyl pattern in the spectrum. He was very concerned that he did not have the right compound. I told him he had a second order spectrum and that it was just as "beautiful" as any first order spectrum.

The first figure below shows simulations (carried out in TOPSPIN) for an isopropyl group as a function of the C-H chemical shift. One can see the typical septet -doublet pattern when the chemical shift difference between the methyl and CH protons is much greater than the coupling constant. When the shift difference is comparable to the coupling constant, complicated second order spectra are obtained. When the shift difference is zero one obtains a singlet.

The left panel of the second figure shows the isopropyl region of the experimental NMR spectrum of (triisopropylsilyl)acetylene. The complicated second order spectrum is simulated in the right hand panel.

Thank you to Mattieu Leclere for providing the sample used in the figure above.

Thursday, July 17, 2008

New Data Processing Option at the U of O

University of Ottawa students now have another option for processing their NMR data. The University of Ottawa has just purchased a university-wide site license for ACD's NMR processing software package. This software will run on WINDOWS 2000, XP and VISTA computers. It is currently installed on the workstation in D'Iorio room 430. Please try it out and visit the NMR lab to arrange to have it installed on your PC.

Tuesday, July 15, 2008

Apodization of 2D Data

In the interest of data collection time and disk storage space, 2D data sets are often collected with short t2 acquisition times and as few as possible t1 slices. In such cases, the FID's in the t2 domain do not decay into the noise and the interferograms in the t1 domain do not decay completely. Applying a 2D Fourier transform to such truncated data will cause ripples in the 2D frequency domain spectrum analogous to those observed in 1D spectra where the acquisition time is too short. The application of an appropriate apodization function (and/or forward linear prediction) to the t2 and t1 domains is important to produce high quality spectra. The figure below illustrates the effect of the more common apodization functions on the cross peak in the phase sensitive COSY spectrum of ethyl acetate. The panel on the upper left shows the appearance of the cross peak when no apodization is applied. One can easily see the ripples in both the F2 and F1 frequency domains. The panel on the upper right shows the effect of applying a 2 Hz exponential line broadening function to each domain. The data are improved but the ripples are still visible. The panel on the lower left shows the effect of applying a sine bell weighting function with the maximum at the midpoint of each of the t2 and t1 domains. This apodization function is suitable for magnitude mode data where the phase is irrelevant. In this example of a phase sensitive data set, one can see a major distortion in the cross peak. The panel on the lower right shows the effect of using a sine squared weighting function with the maximum at the beginning of the t2 and t1 signals. The spectrum is free of ripples and clearly shows the phase information.

Friday, June 27, 2008

Glenn is on Vacation!

The U of O NMR Facility BLOG will be quiet for a couple of weeks while I am on vacation. If you leave comments on any post, they will not appear until I return.

This photo was taken by my wife, Patty (I don't paint my nails or wear an ankle bracelet), however it reflects an activity I hope to enjoy in the next couple of weeks.

Artifacts Due To Setting the Receiver Gain Too High in 2D Homonuclear Experiments

Setting the receiver gain too high leads to very characteristic artifacts in 1D NMR spectra. If the receiver gain is set too high in 2D experiments, one can also expect artifacts in the 2D Fourier transformed data. In homonuclear experiments, setting the receiver gain too high will lead to parallel diagonal signals. This is illustrated in the COSY data in the figure below. In the left hand panel, the receiver was set correctly while in the right hand panel it was set too high. Dotted lines were drawn through the artifacts.

Tuesday, June 24, 2008

Spin-Spin Coupling Between Equivalent Nuclei

When many chemists are asked what is the 2JH-H coupling for compounds like methane, acetone, methylene chloride, dimethyl ether or DMSO, they will often return a look of confusion. "There is no coupling," they will say, "the proton spectrum is a singlet". Indeed the proton spectrum is a singlet for these compounds but 2JH-H is not equal to zero. The only reason that the coupling is not observed in the spectrum is because the chemical shifts of each proton are identical. The coupling can easily be measured by observing the spectrum of a partially deuterated isotopomer. The 2JH-H coupling constant is equal to 2JH-D multiplied by the ratio of the gyromagnetic ratios of 1H to 2H. This is illustrated in the figure below for methylene chloride.In fact, 2JH-H is -7.192 Hz not +7.192 Hz however, this cannot be determined simply by observing the spectrum. Both spectra were measured for dilute solutions with CDCl3 as solvent. The residual protons of CDCl3 were used as the chemical shift reference (7.26 ppm). The chemical shift difference between CH2Cl2 and CHDCl2 is due to an isotope effect.

Monday, June 23, 2008

COSY- 90 vs COSY- 45

Aside from the standard 1H and 13C NMR 1D experiments, 1H COSY experiments are among the most commonly used NMR techniques by organic chemists. There are many different modifications to the standard two pulse COSY experiment and often the organic chemist does not even know which one they are using. Two of the most common experiments for routine work are the gradient magnitude COSY- 90 and COSY- 45 experiments. The only difference between the two methods is the flip angle of the second pulse (90 degrees for the COSY- 90 and 45 degrees for the COSY- 45). For a concentrated sample, these experiments can be acquired in a matter of minutes. Although the signal to noise ratio is higher for a COSY- 90, the COSY- 45 is usually the preferred experiment because the diagonal signals are smaller and less intense allowing correlations between close resonances to be resolved more easily. The figure below shows magnitude gradient COSY- 90 and COSY- 45 spectra for 3-heptanone. Note the smaller diagonal responses in the COSY- 45.

Friday, June 20, 2008

APT vs DEPT-135

Both the APT (Attached Proton Test) and DEPT (Distortionless Enhancement by Polarization Transfer) sequences are very commonly used to help assign 13C NMR spectra. Both experiments yield 13C NMR spectra where the number of attached protons (the multiplicity) is encoded in the phase of the 13C NMR signals. APT spectra have quaternary carbons, and methylene carbons phased negative and methine and methyl carbons phased positive. DEPT-135 spectra show no quaternary carbons and have methylene carbons phased negative and methine and methyl carbons phased positive. A modification to the DEPT-135 method (the DEPTQ-135) will also show quaternary carbons phased negative. Although the APT and DEPT methods provide similar information, the mechanism for multiplicity selection is different for each method. The multiplicity selection in APT experiments is based on 13C magnetization dephasing during a delay equal to the reciprocal of the average one-bond 13C - 1H coupling constant. Although DEPT experiments also employ a delay related to the average one-bond 13C - 1H coupling constant, the multiplicity selection is accomplished by adjusting the final 1H pulse (a DEPT-135 uses a 135 degree 1H pulse). Another major difference between the two techniques is that the DEPT technique transfers proton magnetization to carbon giving it a sensitivity advantage over the APT method. This is illustrated in the figure below which compares the 13C APT and DEPT-135 spectra of menthol. The signal-to-noise ratio is higher in the DEPT 135 spectrum.

Thursday, June 19, 2008

11B Background Signals

Unfortunately NMR probes and NMR tubes cannot be "transparent" for all of the isotopes one may want to observe. Depending on the NMR probe, it is very common to have background signals for 19F, 23Na, 27Al, 29Si, 65Cu, 10B and 11B. These background signals must be taken into account when interpreting NMR spectra. The background signal for 11B on a Bruker AVANCE 300 with a 5 mm broadband probe is shown in the figure below with several different types of NMR tubes commonly used for routine work in our laboratory. The probe was tuned before running each spectrum and the spectra were collected with proton decoupling. The left hand panel shows the background signal for the NMR probe and the other spectra show the combined background of the probe and the indicated NMR tubes. It is obvious that the magnitude and shape of the 11B background depends on the type of NMR tube used. It should be noted that all of the major NMR tube manufactures offer quartz NMR tubes which have little (if any) 11B background signal.

Tuesday, June 17, 2008

The Available RF Field for MAS NMR Probes

In order to rotate an equilibrium magnetization vector from the z axis into the transverse plane, one must provide a pulse with an oscillating magnetic field transverse the static field, Bo, at the Larmor frequency of the nucleus being observed. This is usually provided with a vertically oriented Helmholtz coil for liquids and a horizontal solenoid coil for solids. In both cases the coils provide radio frequency fields transverse to the static magnetic field. MAS coils are solenoids oriented at 54.7 degrees from the static magnetic field. They produce an oscillating radio frequency field at the magic angle. It is only the horizontal component of this field which is capable of rotating magnetization vectors. The available radio frequency field is therefore only 82% of that compared to an identical horizontal solenoid coil.