University of Ottawa NMR Facility Web Site

Please feel free to make suggestions for future posts by emailing Glenn Facey.



Monday, December 21, 2015

NMR of the Christmas Tree

One of my fondest memories as a child is the colorful lights and especially the smell of a decorated Christmas tree.  The hot incandescent lights used years ago would heat up the tree evaporating the fragrant compounds in the needles producing the very memorable and wonderful smell of Christmas.  Although modern artificial Christmas trees and cool LED lights have made the holiday season safer with respect to fires, they have taken much of the magic out of Christmas.  Among many other compounds, it is pinene, bornyl acetate and citronellol that contribute to the Christmas smell of evergreen needles.
We can use NMR spectroscopy to look for these compounds and perhaps recover a bit of the Christmas magic.  The bottom panel of the figure below shows the 13C CPMAS spectrum of spruce needles.  One can easily identify the signals from cellulose in the CPMAS spectrum of the needles while some of the smaller peaks can be attributed to fragrant compounds.  Many of the fragrant compounds in the needles are likely to be in a liquid-like state and not cross polarize very well.  These will either be absent or under-represented in the CPMAS spectrum.  The top panel of the figure shows the 1H - 13C HSQC spectrum of a benzene-d6 extract prepared from crushed spruce needles.  The top and left-side projections are the high resolution 1H and 13C NMR spectra, respectively.  This sample is expected to contain all of the benzene soluble compounds.  The spectrum is free of cellulose resonances and shows a mixture of fragrant compounds.
These data don't recover the childhood magic of Christmas but they do bring a little bit of joy to this NMR spectroscopist.

Merry Christmas  

Wednesday, November 18, 2015

NMR Signals in Tuning Curves

The quality of the tuning and matching of an NMR probe on a Bruker NMR spectrometer can be monitored by using the wobble (or "wobb") routine in the TOPSPIN software.  This routine sweeps the frequency using low power and displays a plot corresponding roughly to the absorbance vs frequency for the probe electronics.  If the probe has very high sensitivity (eg: a cryoprobe) and contains a sample rich in protons (eg: water) then one is able to observe the proton spectrum in the frequency swept wobble curve.  This is demonstrated in the figure below which shows wobble curves for a 600 MHz cryoprobe containing a sample of water.
The curves show the tuning profile of the probe with an anomaly at the proton frequency.  The anomaly is the proton signal of the water in the NMR probe. The top panel is the wobble curve for the probe in a well-shimmed magnet and the bottom panel is the same curve after the magnet shims had been grossly maladjusted.  One can see that the anomaly is much more broad in the wobble curve collected on a poorly shimmed magnet as one would expect the signal from the water to be much broader in an inhomogeneous magnetic field.



Thank you to Stan Woodman of Bruker Canada for pointing this phenomenon out to me.

Wednesday, June 24, 2015

Radiation Damping and Pulse Calibration

Radiation damping causes broadening in the NMR resonances of very strong signals (such as the 1H signal of pure water) as a result of currents induced in the coil from the strong transverse magnetization.  Radiation damping can also produce asymmetry and phase irregularities in the affected resonances.  These problems make pulse calibration by the standard nutation curve problematic when very strong signals are used for the calibration.  The left-hand panel (black) of the figure below shows the standard 1H nutation curves for 0.1% H2O in D2O (bottom) and 80% H2O in D2O (top).  In both cases, single-scan spectra with a recycle delay of 30 sec were collected and plotted horizontally.  The pulse was varied from 1 µsec to 24 µsec in steps of 1 µsec.  In the case of 0.1% H2O in D2O, the nutation curve is well behaved and one is easily able to read off the 90°, 180°, 270° and 360° pulse durations.  In the case of 80% H2O in D2O, where radiation damping is a problem, the nutation curve is not well behaved.  There are asymmetry and phase distortion problems which make it impossible to determine the 90° pulse, based on maximum signal height, with any accuracy.  Nor is it possible to determine a reliable 180° based on the first minimum.  The spectra show very little distortion in the vicinity of the second minimum so the 360° pulse can be used reliably to determine the 90° pulse.  The right-hand panel of the figure (red) shows the integrals of the corresponding nutation spectra.  The integrals for both samples behave similarly.  It is clear that even in the case of severe radiation damping, one is able to determine a well behaved nutation curve from the integrals.

Friday, May 8, 2015

TROSY

The chemical shift resolution and sensitivity of NMR generally benefit from an increase in magnetic field strength.  As a result, large sums of money are spent on magnets with higher and higher fields.  The boost in sensitivity means that smaller and smaller quantities of sample are needed and measurements can be completed in shorter periods of time.  There are particular cases however, where an increase in magnetic field can lead to a loss of sensitivity and resolution.  This is the case for 15N decoupled 1H spectra of the 1H-15N spin pairs in very large 15N labelled proteins.  The very long correlation times of the protein combined with the high resonance frequencies associated with high field strength lead to very short T2 relaxation times and therefore broader lines.  The broad lines account for a significant loss in resolution and sensitivity.  One might then wonder why protein structural chemists spend so much money on very high field magnets.  What follows is one possible answer to this question.

The two main relaxation mechanisms for the 1H and 15N in proteins are dipolar coupling and chemical shielding anisotropy.  These two mechanisms are also cross correlated with one another.  The cross correlation term is of different sign for each of the two peaks in a 1H-15N doublet resulting in one of the peaks of the doublet having a shorter T2 (and broader line) than the other one.  If 15N decoupling is applied, one sees a single resonance with a line width determined by the average of the two components of the doublet.  This is illustrated in the figure below for a 1H-15N spin pair in a small and large molecule at high field .  The same is true in the 15N-1H doublets in the 15N spectra of 15N-1H spin pairs.
At very high fields, One of the lines in the 1H-15N doublet is very sharp and the other very broad.  If, in the 1H spectrum of a protein, we could eliminate all of the broad doublet components leaving only the sharp ones, we would have a high resolution 1H spectrum.  Further, if we could combine such a measurement with an HSQC, we would have a high resolution 1H-15N HSQC at high field.  The combination of these two measurements is called transverse relaxation optimized spectroscopy (TROSY).  TROSY data collection employs an HSQC measurement with neither 1H nor 15N decoupling elements (as described in a previous post) as well as other elements which suppress the broad lines of the doublets and retain the sharp lines.  The results of this are illustrated in the figure below for small and large proteins at high field.
Clearly, it is not advantageous to use the TROSY technique on small proteins rather than the conventional HSQC.  For large proteins at high field however, there is a significant sensitivity and resolution advantage compared to a conventional HSQC.  It should be noted that the TROSY cross peaks are shifted by ½ 1JHN in both the F2 and F1 domains.  The figure below shows a superposition of a conventional HSQC (black) and a TROSY (blue) for a protein at 500 MHz.
One can clearly see the ½ 1JHN shift in the F2 and F1 domains of the TROSY compared to the HSQC.  In this case, the conventional HSQC gives higher sensitivity than the TROSY.

Thank you to Adam Damry of Professor Roberto Chica’s research group at the University of Ottawa for providing the sample of 15N labelled protein.

Wednesday, May 6, 2015

Decoupling in 2D HSQC Spectra

HMQC and HSQC NMR data are commonly used to correlate the chemical shifts of protons and 13C (or 15N) across one chemical bond via the J coupling interaction.  The data are 1H detected, with the 1H chemical shift in the horizontal F2 domain and the 13C (or 15N) chemical shift in the vertical F1 domain.  In the case of 1H and 13C, the technique depends on protons bonded to 13C.  1H–12C spin pairs provide no coupling information and are suppressed by the method.  If one is to observe the 1H signal of a 1H-13C spin pair, one expects to observe a doublet with splitting 1JH-C (i.e. the 13C satellites).  Likewise, if one is to observe the 13C signal of a 1H-13C spin pair, one expects to observe a doublet with the same splitting.  2D HSQC spectra are normally presented with both 1H and 13C decoupling yielding a simplified 1H-13C chemical shift correlation map over one chemical bond.  The figure below shows one of the most commonly used gradient HSQC pulse sequences.  The 1H and 13C decoupling elements of the sequence are highlighted in yellow and pink, respectively.
During the evolution time, t1, the 13C chemical shift and 1H-13C coupling evolve.  The 1H 180° pulse (color coded in yellow) in the center of the evolution time refocuses the coupling and as a result decouples protons in the F1 (13C) domain of the spectrum.  13C is broadband decoupled from the F2 (1H) domain by applying a GARP pulse train (color coded in pink) at the 13C frequency during the collection of the FID.  One can turn each of these elements “on” or “off” for data collection.  The figure below shows the 1H-13C gradient HSQC spectrum of benzene with all possible combinations of 1H and/or 13C decoupling.
In the top left panel both 1H and 13C decoupling are turned “on” and one observes a singlet in both the F2 (1H) and F1 (13C) domains.  In the top right panel, the 1H decoupling element is “on” while the 13C decoupling element is “off”.  The result is a 1H-13C doublet in the F2 (1H) domain and a singlet in the F1 (13C) domain.  In the bottom left panel, the 1H decoupling element is “off” while the 13C decoupling element is “on”.  The result is a 13C-1H doublet in the F1 (13C) domain and a singlet in the F2 (1H) domain.  In the bottom right panel, both the 1H and 13C decoupling elements are “off”.  The result is a 1H-13C doublet in both the F2 (1H) and F1 (13C) domains.

Tuesday, April 28, 2015

Dead Time and Phase

The phase of an NMR peak depends on the sine/cosine character of the free induction decay (FID) when the receiver is turned on.  Positive and negative cosine FIDs will yield positive and negative in-phase peaks, respectively whereas positive or negative sine FIDs will yield peaks 90° out of phase.  FIDs that are neither a pure sine nor pure cosine with yield peaks which are out of phase to an extent dependent on the cosine/sine character of the FID.  In a perfect world, the receiver is gated on immediately after a perfect 90° pulse and all FID’s are cosines producing positive in-phase NMR peaks.  In the real world however, there are problems with acoustic ringing, pulse breakthrough imperfect pulses of finite duration and finite electronic switching times.  These problems produce a dead time between the end of the pulse and time at which the receiver is gated on during which data cannot be collected.  As a result the FID may not be a perfect cosine function and a phase correction will need to be applied after Fourier transform.  The first two figures below illustrate this point for a single off-resonance NMR signal as the dead time is increased.  The first figure shows the FID’s as a function of increasing dead time from bottom to top while the second figure shows the Fourier transformed spectra without phase correction as a function of dead time from left to right.
When there is more than one signal, the FID is an interferogram representing the sum of all time domain signals, each with a different frequency.  Since each component has a different frequency, its phase is affected to a different extent as a result of the dead time.  Higher frequency time domain components  (i.e. those representing peaks further off-resonance) are affected more than lower frequency components (i.e. those representing peaks closer to being on-resonance).  This is illustrated in the figure below for the 1H NMR data for p-xylene.  The left-hand portion of top panel of the figure shows the FID containing both methyl and aromatic components while the right-hand portion of the top panel shows an expansion of the initial portion of the same FID.  The bottom panel of the figure shows a stacked plot of the NMR spectra collected as a function of dead time.  One can see that the phase of the aromatic peak furthest off-resonance is affected to a greater extent by an increased dead time than the methyl peak closer to resonance.
The last figure also shows a stacked plot of the 1H NMR spectra of p-xylene as a function of dead time.
In this case, the methyl signal was set on-resonance.  One notices immediately that the phase of the on-resonance methyl peak is unaffected by an increase in the dead time whereas that of the off-resonance aromatic peak is severely affected.  The on-resonance methyl peak is not affected by an increase in dead time as its time domain signal is a simple exponential with no sine/cosine oscillations.  A loss of the beginning of a simple exponential FID due to the dead time still leaves a simple exponential and thus the phase is not affected.

Thank you to Dr. Michael Lumsden of the NMR Facility of Dalhousie University for suggesting the subject of this post.

Friday, April 17, 2015

The Information Content of an FID

The free induction decay (FID) is a function representing the decay of transverse magnetization as a function of time after the application of a pulse (or pulse sequence).  The NMR spectrum is obtained by Fourier transforming the FID.  All of the information in the NMR spectrum (line width, intensity, phase, line shape ....) is contained in the FID.  Knowledge of what part of the FID represents a particular property of an NMR spectrum allows a user to process the raw time domain data in a way that maximizes the quality and information content of the processed frequency domain spectrum.  The figure below shows the real component of the complex FID for a sample in which there is a single resonance.  The FID is labelled in terms of its information content.
The shape of the envelope of the overall decay defines the line shape of the NMR resonance.  Exponential decays yield Lorentzian line shapes.  The frequency in the FID represents the offset of the resonance from the carrier frequency or in other words, how far the resonance is from the center of the spectrum.  The higher the frequency, the further off-resonance the peak.  The FID for an on-resonance peak is a simple decaying exponential with no oscillation frequency present.  The first point of the FID contains the integrated intensity and phase information.  Since time and frequency are reciprocals of one another, the duration of the decay before it disappears into the noise determines the sharpness of the NMR resonances and ultimately the resolution in cases where there is more than one resonance in the spectrum.  The longer the decay - the narrower the lines.  The early portion of the FID (short time) defines the broad features (wide frequency distributions) whereas the the latter portion of the FID (long time) defines the sharp features (narrow frequency distributions).  Also, since the FID is a decaying function, the early portion has a higher signal-to-noise ratio than the latter portion.  With this knowledge its is very easy to understand how best to apply apodization functions to the raw data for the purpose line broadening and resolution enhancement.  Processing techniques such as zero filling, forward linear prediction, backward linear prediction and signal-to-noise optimization as well as spectral artifacts such as truncation errors, baseline roll and receiver saturation errors are easily understood with knowledge of the information content in the FID.   

Wednesday, April 1, 2015

NMR of Edible Oils

NMR spectroscopy is one of the most informative techniques for the study of structure, composition and dynamics of matter.  One of the many thousands of applications of NMR spectroscopy is in the study of edible oils.  Plant and animal oils are composed of complex mixtures of fatty acid tri-esters of glycerol.  The fatty acid moieties are generally straight chains of 16 - 24 carbons in length with various degrees of unsaturation.  In natural oils the double bonds are all cis-.  Fatty acids with trans- double bonds are usually the result of food processing.  The double bonds in polyunsaturated fatty acids are generally separated by single methylene groups.  The end methyl carbon of each fatty acid chain in the glycerol tri-esters is referred to as the omega position.  Omega-3 fatty acids are those with a double bond on the third carbon from the omega methyl position.  The most common omega-3 fatty acid in plant oils is α-linolenic acid (ALA), a C18 acid with three cis- double bonds in the 9-, 12- and 15- positions.  Two of the most common omega-3 fatty acids in marine oils are eicosapentaenoic acid (EPA), a C20 acid with five cis-double bonds in the 5-, 8-, 11-, 14-, and 17- positions and docosahexaenoic acid (DHA), a C22 acid with six cis- double bonds in the 4-, 7-, 10-, 13-, 16- and 19- positions.  The human body benefits from EPA and DHA which are only inefficiently synthesized from ALA in the human body.  Also, it is recommended that consumption of saturated oils should be limited and that polyunsaturated oils are a better alternative.  With these concerns, the study of edible oils has become important.  One might expect that the complexity of the mixtures constituting the natural edible oils would limit the usefulness of NMR as a method of study however; the spectra contain a great deal of information as can be seen in the figures below.  The 13C NMR spectra of 4 plant oils and 1 commercial “wild fish” oil are shown in the first figure.
The olefinic carbons are color-coded in yellow and give an indication of the degree of unsaturation in the oils.  Clearly, the coconut oil is saturated and the fish oil contains the highest degree of unsaturation.  The 1H NMR spectra of the same oils are shown in the second figure.
The resonances color-coded in yellow are those of the protons on olefinic carbons and are a direct indication of the degree of unsaturation.  The resonances color-coded in pink are methylene protons on carbons adjacent to two olefinic carbons and represent the degree of polyunsaturation.  The resonances color-coded in blue are methylene protons attached to carbons adjacent to both methylene carbons and olefinic carbons.  The methyl resonances are at the lowest chemical shift.  Those color-coded in green are from the omega-3 fatty acid moieties.  Qualitatively, from the data, it is obvious that the coconut oil is saturated and the fish oil contains the most omega-3 polyunsaturated fatty acid moieties.

Tuesday, March 31, 2015

Cross Polarization Based Mixture Resolution in Solids

One of the most common techniques used to collect solid-state NMR data for spin I = ½ nuclides is a combination of cross polarization (CP) and magic angle spinning (MAS).  CPMAS provides high sensitivity from the CP and high chemical shift resolution from the MAS.  Furthermore, the scan repetition rate depends on the shorter relaxation time of the protons rather than the longer relaxation time of the spin I = ½ nuclide therefore, more scans can be collected per unit time.  It must be remembered however, that the success of the CP technique depends on the dipolar coupling interaction between proximate protons and the nucleus being observed.  In the absence of dipolar coupled protons, CP signals are not observed.  For this reason, it is sometimes necessary to use a conventional one-pulse method (Bloch decay) which can be used to observe the spin I = ½ nuclide, albeit with lower sensitivity, whether protons are present or not.  When a sample consists of a mixture with some protonated components and some components without protons, then it may be advantageous to collect both a CPMAS and a Block decay spectrum.  When both methods are used, the data can be used to resolve the spectrum of the mixture into subspectra; the protonated components in one spectrum and those components without protons in another.  An example of this is shown with the 13C NMR data for a common antacid tablet in the figure below.

The two most abundant carbon containing components of an antacid tablet are calcium carbonate and sucrose.  Spectrum (a) is the CPMAS spectrum.  It consists only of the resonances of sucrose (color coded in yellow) since calcium carbonate (color coded in pink) contains no protons.  Spectrum (b) is the Bloch decay spectrum with high power proton decoupling.  It consists of both the resonances of sucrose and calcium carbonate.  Spectrum (c) is a linear combination of (a) and (b) and represents, primarily, the spectrum of calcium carbonate.  Another interesting example of CP based mixture analysis is given here using Christmas shortbread cookies as an example.

Thursday, March 26, 2015

Mixture Resolution in 13C CPMAS NMR

The recycle delay necessary to get the highest signal-to-noise ratio in a multi-scan 13C CPMAS NMR spectrum depends on the relaxation properties of the protons in the sample.  The protons in pure solid samples normally belong to a single homogeneous dipolar coupled network.  As a result, all of the protons in the coupled network have a common T1 relaxation time.  One would expect the same behavior for a mixture of compounds only if the components were mixed at the molecular level.  If the compounds are not mixed at the molecular level, the sample consists of domains of pure materials, each of which has a common proton T1.  If the proton T1's of the domains are significantly different, then one has a means of discriminating between the domains and hence the compounds of the mixture with 13C CPMAS NMR data.  The figure below illustrates this principle for a tablet of vitamin C ground into a powder.  The vitamin C tablet consists primarily of ascorbic acid for which the structure is shown in the figure.  The other major solid organic additives are hypromellose (hydroxypropyl methylcellulose), stearic acid (n-C17H35COOH), magnesium stearate and carnauba wax (a complex mixture of C26 to C30 acids, esters and alcohols).  When the tablet is ground up, the powder consists of ascorbic acid domains, stearic acid domains, magnesium stearate domains and carnauba wax domains.

13C CPMAS NMR spectra were acquired with a 30 second and a 2 second recycle delay and are shown in (a) and (b), respectively.  One can see that relative intensity of the components in the mixture depends on the recycle delay.  The proton T1 of the ascorbic acid is obviously longer than that of the other components of the mixture.  The spectra in (c) - (e) are linear combinations of (a) and (b).  The linear combination in spectrum (c) was created such that the ascorbic acid resonances were nulled.  The resulting spectrum is that of only the organic additives. The hypromellose resonances are in the 50 ppm to 110 ppm range.  The aliphatic resonances of the stearic acid, magnesium stearate and carnauba wax overlap in the 10 ppm to 50 ppm range and appear to have similar proton T1's.  The linear combination in spectrum (d) was created such that the aliphatic stearic and wax resonances were nulled.  The resulting spectrum is that of ascorbic acid and the inverted spectrum of the hypromellose.  The linear combination in spectrum (e) was created such that the hypromellose resonances were nulled. The resulting spectrum is that of the ascorbic acid with the stearic acid, magnesium stearate and carnauba wax additives.  This combination allows observation of the ascorbic acid with no overlapping resonances from the additives.

Wednesday, March 25, 2015

13C NMR of Vitamin C - Solids vs. Liquids NMR

In 13C[1H] NMR spectra of liquids, one observes a single resonance for each symmetrically nonequivalent carbon atom.  The same is true of the 13C CPMAS spectra of solids.  The difference is that the molecular symmetry is the determining factor in liquids NMR whereas the crystallographic symmetry is the determining factor in solids NMR.  As a result, solids NMR can give different spectra for different solid polymorphs and multiple resonances due to multiple nonequivalent molecules in the asymmetric unit of the crystal structure.  The figure below compares the solids 13C CPMAS and liquids 13C[1H] NMR spectra of a vitamin C (ascorbic acid) tablet.

The structure is provided in the figure and the assignment for all of the carbon atoms is color coded.  In the liquids spectrum, one observes resonances for each carbon in the molecule.  In the 13C CPMAS spectrum, one observes that three of the six 13C resonances are doubled.  This doubling is consistent with there being two nonequivalent ascorbic acid molecules in the asymmetric unit of the crystal.  Presumably, the other three resonance are doubled as well but there is insufficient resolution in the spectrum for this observation.  The broad features in the spectrum are due to additives in the vitamin C tablet which include, hypromellose, stearic acid and carnauba wax. 

Wednesday, March 4, 2015

BLOG Index



University of Ottawa NMR Facility BLOG INDEX
March 2015

               H20 vs D2O
1D selective HOESY
               1H-19F
               1H-31P
               for exchanging roramers
1D TOCSY measurements
               as a function of mixing time
               for mixture analysis
1H NMR with X nucleus decoupling
               1H[11B]
               1H[13C]
               1H[19F]
               1H[27Al]
               1H[31P]
2D correlation spectroscopy (COSY)
               11B COSY
               COSY 90 vs COSY 45
               double quantum filtered
               ECOSY for measurement of coupling constants
               magnitude vs phase sensitive
               vs TOCSY
               and NOE (1)
               and NOE (2)
               better data – more scans or more slices?
               data apodization
               phasing (video tutorial)
2D NOESY measurements
               19F NOESY
               and exchange (1)
               and exchange (2)
               choice of mixing time
               effect of viscosity
               phasing
               phasing (video tutorial)
               small vs large molecules
               HSQC - TOCSY
               TOCSY vs COSY
2H NMR of liquids
               chemical shift referencing in
               on Bruker AVANCE spectrometers
               backward linear prediction to correct for
               pulse sequence to minimize
ASCII file generation
               in TOPSPIN (1)
               in TOPSPIN (2)
Background signals
               11B
               23Na
               from dirty NMR probe
Background suppression
               11B in liquids
               11B in solids
Baseline correction
               improving
               in 2D NMR spectra
               in solids MAS spectra of quadrupolar nuclides
Benchtop NMR
               (1)
               (2)
Chemical exchange agents for spectral simplification
               D2O shake
               trifluorocaetic acid
Chemical shift referencing
               1H in aqueous solution
               external
               in 2H NMR
Chemical shift tensors
               and MAS sideband manifolds (1)
               and MAS sideband manifolds (2)
               from static CP spectra
               measurement from static solids spectra
Chemical shifts
               concentration dependent
               temperature dependent
Concentration gradients
               effect on spectral quality (1)
               effect on spectral quality (2)
Coupling
               13C – 14N
               13C-14N-31P
               13C – 19F
               13C – 2H (1)
               13C - 2H (2)
               13C – 2H (3)
               13C - 59Co
               signs of coupling constants vis ECOSY
               virtual
               CPMAS of household dust
               effect of contact time
               effect of MAS spinning speed
               importance of Hartman-Hahn match
               in relation to MAS and high power decoupling
               measurement of 13C 90 degree pulses with
               measurement of relaxation times with
               optimizing 1H decoupling in
               ramped contact pulses
               sensitivity improvement from
               to distinguish solid polymorphs
               vs Bloch decay
               with FSLG HETCOR
               as an assignment tool
               effect of spinning speed
               with long dephasing delays (1)
               with long dephasing delays (2)
Decoupler pulse calibration
               in liquids
               in solids
Decoupling
               1H and 31P decoupling
               1H decoupling and 13C signal-to-noise ratio
               1H decoupling and 31P signal-to-noise ratio
               11B decoupling
               13C decoupling
               19F decoupling
               2H decoupling
               27Al decoupling
               31P decoupling
               high power 1H decoupling in solids
               homonuclear decoupling
               modes of heteronuclear decoupling in liquids
DEPT
               and quaternary alkyne carbon sites
               APT vs DEPT 135
               effect of 1H tuning on
               missing signals in
               of “acetone-d6”
               of “perdeuterated “ solvents
               vs DEPTQ
               with 29Si
DEPTQ
               vs DEPT
Diffusion and DOSY
               diffusion in CPMG measurements
               DOSY
Dynamic processes studied by 1D NMR
               exchange studied by saturation transfer
               exchanging rotamers
               the NMR time scale
Echoes
               CPMG
               Fourier transform of
               gradient spin echoes and selective excitation
               simple spin echo
               to remove 11B background in solids
               tutorial video
               tutorial video
Floor vibrations
               effect on NMR spectra
Food and drink
               candy cane
               coffee
               fruit cake
               popcorn
               rum and eggnog
               shortbread cookies
Free induction decay
               effects of truncation
HMBC experiments
               HMQC responses in HMBC spectra
               measuring  19F – 13C coupling in 1H - 13C HMBC
               missing signals in              
               Nyquist fold-backs in
HMQC, HSQC and edited HSQC experiments
               1H – 11B
               19F – 13C
               31P -109Ag
               31P – 13C
               doubled signals / poor decoupling
               HMQC vs HSQC
               HSQC - TOCSY
               HSQC vs edited HSQC
               isotope effects in 19F – 13C HMQC
               removing t1 noise in
               Bloch-Siegert shifts
               broadband 1H decoupled 1H spectra
iPad / iPhone apps
               Bruker Almanac
               educational
Isotope effects
               acetone
               chloroform
               complexed solvents
               ethane
               in 19F – 13C HMQC
               methylene chloride
               “perdeuterated” solvents
               triphenyl phosphate
               water    
               erroneous
Linear prediction
               backward
               backward LP to correct for receiver overload
               forward
               forward LP for 2D data
Lock
               consequence of locking on the wrong solvent
               how high should the lock signal be?
               paramagnetic samples
               spectra acquired with a sweeping field
               spectral distortion from weak lock signal
               Varian
               1H MAS
               and available rf field
               effect on Hartman-Hahn match
               field dependence of chemical shift resolution
               how fast to spin
               how much sample
               increasing signal-to-noise ration in
               setting the magic angle (1)
               setting the magic angle (2)
               solution vs MAS spectra
               rotor crashes
Magnet
               accessing a used magnet
               measurement of field drift
               quench
Magnet cryogen fills and spectral quality
               helium fills
               helium one-way valve oscillation
               nitrogen fills
               nitrogen pressure in the magnet
Magnetic resonance imaging (MRI)
               gradient calibration / 1D MRI
               MRI photocopier
               Nyquist fold-backs in MRI images
               slice selection
               why MRI scanners are loud
NMR of more than one isotope
               23Na and 51V
NOE’s and decoupling
               negative NOE’s and decoupling
               positive NOE’s and decoupling
               and digital filtering
               and mode of data acquisition
               in HMBC data
               in MRI
Paramagnetism
               13C spectra of paramagnetic compounds
               determination of paramagnetic susceptibility
               paramagnetic shifts
               the effect of paramagnetic oxygen
               first order phase errors
               phasing a 1D spectrum (video tutorial)
               phasing a 2D spectrum (video tutorial)
Probes
               broadband vs inverse broadband
               coil geometry
               solids
               why so expensive
               effect of 1H tuning on 13C spectra
               effect of cable length
               effect of sample spinning
               effect on 90 degree pulse
               probe electronics
               problems with salty samples
               effect of probe tuning
               fast determination
               for shaped pulses              
               for spin I = n/2 quadrupolar nuclei in solids
               expressed in dB Bruker vs Varian
               expressed in Hz  
               pulses
Pulsed field gradients
               calibration
               dephasing ability
               gradient spin echoes
               recovery times
               distortions in QCPMG spectra
               for solid state 2H NMR
Quadrature spikes
               how to remove
               finding lost 13C signals
Receiver gain
               and signal-to-noise ratio
               distortion from mis-setting
Relaxation
               effect of paramagnetic oxygen
               faster relaxation time measurements in solids
               T1 anisotropy in solids
               T1 measurement
               T1r
               T2 CPMG filter to enhance sharp lines
               T2 measurements and diffusion
               T2 vs T2*
               by using benzene as a solvent
Sample limitation
               making the best of
Scans
               and signal-to-noise ratio
               broadband 180 degree pulses
               calibration
               excitation profiles (1)
               excitation profiles (2)
               for selective excitation
Shimming
               effect of concentration gradients
               effect of sample depth
               effect of sample mixing
               effect of sample volume
               line shapes resulting from bad shimming
               without a lock signal
               1H spin pairs
Solid state 2H NMR spectroscopy
               echoes and Fourier transforms
               increasing signal-to-noise ratio
               measuring spectral parameters
               QCPMG
               T1 anisotropy
               to determine molecular motions
               90 degree pulse calibration
               baseline correction
               field dependence
               Fourier transform of a single rotational echo
               satellite transitions
Solvent effects
               improving resolution
Solvent suppression
               absolute water suppression
               double presaturation
               presaturation
               watergate
               watergate vs presaturation
Spin simulations
               butane
               ethane
               isopropyl groups
               quadrupolar line shapes in solids (QUEST)
               virtual coupling
t1 noise removal in 2D spectra
               heteronuclear (tutorial video)        
               homonuclear (tutorial video)
Triple resonance experiments
               13C [2H][1H]
               13C [31P][1H]
               31P -13C HMQC with 1H decoupling
Variable temperature measurements
               temperature calibration (1)
               temperature calibration (2)
               temperature dependent chemical shifts
               temperature gradients
               variable temperature to improve resolution
Video tutorials
               1D spectrum phasing
               2D spectrum phasing
               exponential line broadening
               EZ NMR
               removing t1 noise from homonuclear 2D spectra
               and throwing away noise