University of Ottawa NMR Facility Web Site

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Tuesday, April 25, 2017


NMR spectroscopy is an indispensable tool for assigning the structure of organic compounds.  One very useful method in the NMR toolbox is the Heteronuclear Multiple Bond Correlation (HMBC) experiment.  HMBC data are 1H detected and provide a 2D correlation map between 1H and 13C similar to HMQC or HSQC except that the correlations are between protons and carbons separated by two, three and sometimes even four bonds.  This long range information is very helpful in elucidating chemical structures, especially those with non-protonated carbons.  The problem, however with HMBC data is that the correlations depend only on the magnitude of the long-range 1H-13C coupling constants.  Two- or three- bond coupling constants are very similar in magnitude to one another and therefore it is not possible to distinguish between two- and three- bond correlations.  Also, since many long range 1H-13C coupling constants (including two-bond coupling constants) are near zero, some correlations may be absent.  These problems may make structure elucidation frustrating or impossible.  The Heteronuclear 2 Bond Correlation (H2BC) experiment1 provides an HMBC-like correlation map with (almost) exclusively two-bond 1H-13C correlations.  Unlike the correlations in the HMBC measurement, which rely exclusively on long range 1H-13C coupling constants, the 1H-13C correlations in the H2BC experiment rely on three-bond 3JH-H coupling between the protons on adjacent carbons.  It is a combined HMQC-COSY experiment.  The size of the H2BC correlations depends on the magnitude of the 3JH-H coupling constant.  Three-bond 1H-13C correlations are possible only if four-bond 4JH-H coupling is significant.  One disadvantage to the H2BC experiment is that all correlations between protons and non-protonated carbons are necessarily absent because of the absence of H-H coupling.  In general, two-bond 1H-13C correlations that are weak or absent in HMBC spectra are strong in H2BC spectra and three-bond 1H-13C correlations which are strong in HMBC spectra are absent or very weak in H2BC spectra.  The techniques are very complimentary.  The figure below illustrates  the complimentary nature of the two methods for styrene.

The HMBC spectrum in the left panel was scaled up until some of the HMQC artifacts (color coded in blue) were visible.  The data show only one 2-bond 1H-13C correlation (color coded in pink). The three-bond 1H-13C correlations are color coded in yellow.  In comparison, the H2BC spectrum in the right panel shows exclusively two-bond 1H-13C correlations with the exception of those involving the C1 non-protonated carbon.

1. Nyberg, Duus, Sorensen.  J. Am. Chem. Soc. 127, 6154 (2005).

Tuesday, January 24, 2017

Improved 1H Resolution with 14N Decoupling

The J coupling between 13C and quadrupolar nuclides can be resolved, for example, in the cases of the 13C NMR spectra of deuterated compounds, some cobalt complexes and some tetraalkyl ammonium salts.  The ability to resolve the coupling depends on the relaxation rates among the Zeeman levels of the quadrupolar nuclide with respect to the reciprocal coupling constant.  When the relaxation is slow, the J coupling can be resolved and when it is very fast, the 13C is a sharp singlet and said to be "self decoupled".  When the relaxation rates among the Zeeman levels of the quadrupolar nuclide are on the same order of the coupling constant, the NMR resonance of the 13C will be broadened.  This is a very common observation for the 13C resonances of nitrogen bearing carbons.  It is also possible to see broadened 1H or 19F resonances due to coupling to 14N.  Such is the case for the resonances of the proton on C6 and the fluorine on C2 in 2,3-difluoropyridine as can be seen from the figure below which clearly shows these resonances broadened compared to the resonances of 1H or 19F further removed from the nitrogen.
The broadening of the resonance of the 1H on C6 can be reduced by applying 14N decoupling during the acquisition time, thus providing much improved resolution.  This is  demonstrated in the figure below.

Monday, January 23, 2017

PSYCHE to Evaluate 1H-19F Coupling Constants

Even small molecules can yield very complex 1H NMR spectra as the result of spin - spin coupling.  This is particularly true for small molecules that contain fluorine.  It can sometimes be challenging to determine which splittings are due to 1H-1H coupling and which are due to 1H-19F coupling.  One can collect a 1H spectrum with 19F decoupling to give a spectrum with only 1H-1H coupling present.  Even with this data, it may be difficult to evaluate the 1H-19F coupling constants by comparing the 1H[19F] spectrum to the 1H spectrum due to the complexity of the multiplets.  The 1H-19F coupling constants can however be read directly from a 1H PSYCHE spectrum.  The PSYCHE spectrum provides a 1H decoupled 1H spectrum, leaving only the 1H-19F coupling behind.  The bottom trace of the figure below shows the 300 MHz 1H NMR spectrum of 2,3-difluoro pyridine.  The spectrum is quite complex, making it difficult to assign 1H-1H and 1H-19F couplings.  The middle trace shows the 1H[19F] spectrum which allows the evaluation of all of the 1H-1H coupling constants (3JH5-H4 = 4.8 Hz, 4JH5-H3 = 1.6 Hz and 3JH4-H3 = 8.0 Hz.  The top trace shows the 1H PSYCHE spectrum which allows one to evaluate all of the 1H-19F coupling constants.  For this compound, 4JH3-F1 = 3JH3-F2 = 9.8 Hz, 4JH4-F2 = 3.2 Hz and 4JH5-F1 = 1.8 Hz.

Friday, January 20, 2017

Pure Shift 1H NMR - PSYCHE

Much effort has been directed to obtain broadband 1H decoupled 1H NMR spectra.  The subject has been reviewed recently.1  One technique used to obtain such spectra is the pseudo-2D Zangger - Sterk method2,3 based on a selective refocusing pulse applied simultaneously with a weak field gradient centered in the t1 evolution period allowing all chemical shifts to be measured at the same time but from different slices of the column of sample in the NMR tube.  For each resonance, the coupling from all of the coupling partners is refocused.  The data are collected in a conventional 2D matrix however, a single FID is constructed by concatenating a chunk from each of the individual 2D time domain signals.  The Fourier transform of the reconstructed FID is a pure shift, 1H decoupled 1H NMR spectrum.  The PSYCHE (Pure Shift Yielded by CHirp Excitation) modification4 of the Zangger - Sterk method uses a pair of small flip angle, frequency swept chirp pulses rather than a selective 180° pulse applied simultaneously with the weak spatially selective field gradient.  This modification offers improved sensitivity.  The details of implementing this technique are kindly provided on-line by the Manchester NMR Methodology Group.  As an example, the figure below shows the 600 MHz PSYCHE spectrum of sucrose in in DMSO-d6, collected in less than 3 minutes.  One can observe the collapse of all multiplets into singlets.
The PSYCHE technique can dramatically simplify complex 1H NMR spectra as shown in the figures below. The second figure is an expansion of the low frequency region of the first.

1.  CastaƱar and Parella. Mag. Res. Chem. 53, 399 (2015).
2.  Zangger and Sterk. J. Mag. Reson. 124, 486 (1997).
3.  Aguilar, Faulkner, Nilsson and Morris. Angew. Chem. Int. Ed. 49, 3901 (2010)
4.  Foroozandeh, Adams, Meharry, Jeannerat, Nilsson, Morris. Angew. Chem. Int. Ed. 53, 6990 (2014).

Thursday, January 12, 2017

Exchange Effects in HSQC Spectra

The effects of chemical or dynamic exchange on NMR spectra are very well known.  Exchange is often studied by observing line shape changes as a function of temperature, by 2d EXSY, inversion transfer or saturation transfer methods.  Effects due to exchange can also be observed in 1H - 13C HSQC spectra.  The HSQC method works by transferring 1H magnetization to 13C magnetization via an INEPT transfer through the one-bond J coupling across the 1H - 13C chemical bond.  The 13C magnetization evolves during the incremented delay, t1, of the 2D pulse sequence according to its chemical shift.  The 13C magnetization is then transferred back to 1H magnetization where is observed during t2.  HSQC spectra thus exhibit cross peaks between 1H resonances and the resonances of their attached carbons.  If there is exchange between nonequivalent carbon sites during t1, some 1H resonances may appear to be correlated to two carbon resonances.  An example of this is shown in the figure below.
The 13C spectrum of cannabidiol has equally intense broad, resolved aromatic resonances for non-protonated carbons 2 and 6 (not shown) as well as for the protonated carbons 3 and 5.  The 1H spectrum has broad resolved resonances for both aromatic protons.  This indicates that either the aromatic ring undergoes 180° flips about the 1 - 4 axis or it has two equally probable rotomers defined by a rotation about the 1 - 4 axis.  In either case, the dynamic exchange is slow enough on the NMR time scale to produce resolved resonances yet fast enough to cause significant line broadening.  For each of the two aromatic protons, the HSQC spectrum shows correlations to both C3 and C5; a strong correlation to the carbon to which it is chemically bonded and a weaker correlation to the carbon site in exchange with its attached carbon.