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Pure shift NMR
A significant improvement in resolution in a proton spectrum can be reached by broadband homonuclear 1H decoupling. This is demonstrated by a new class of pure shift experiment, named PSYCHE (Pure Shift Yielded by Chirp Excitation).
the upper trace below shows the regular 1H NMR of D-Maltose. The lower trace shows the pure shift 1H NMR acquired by TSE_PSYCHE which is very efficient in homonuclear decoupling. Both spectra were acquired on AVIII 600_QCI.
For more details: M. Foroozandeh, et al., Angew.Chem.Int.Ed., 2014, 53, 6990-6992
CSSF - TOCSY
The method of chemical shift selective filters (CSSF) has very high selectivity in exciting the NMR signal of interest. It acquires the data with variable chemical shift evolution. The addition of the data eliminates the off resonance signals and remains the on resonance signal. The CSSF can be combined with TOCSY, NOESY, ROESY to get those 1D selective experiments with very high selectivity.
The spectra below are the 1H NMR of sucrose in D2O, acquired on 600_QCI. The bottom two are the standard 1H and the pure shift 1H spectra, respectively. The pureshift NMR was acquired to get the true chemical shift of 5 and 5' which were used latter in the CSSF-TOCSY experiments.
CSSF-TOCSY@5' and CSSF-TOCSY@4' give same spectra. Similarly, CSSF-TOCSY@3 and CSSF-TOCSY@5 show same spectra, which suggests the very high selectivity of CSSF method in these experiments.
For more details: P.T. Robinson, T.N. Pham, D. Uhrin, J. Magn. Reson., 2004, 170, 97-103.
S.J. Duncan, R.Lewis, M.A. Bernstein, P. Sandor, Magn. Reson. Chem., 2007, 45, 283-288.
LR-HSQMBC can be used to explore correlations for herternuclear coupling over very long range, typically from 2 to 3, 4, 5 or even up to 6 chemical bonds. This can be very useful for protond deficient molecules where there are not many correlations for heteronuclear couplings over 2, 3 bonds.
The spectra below are the standard 1H-13C HMBC and the 1H-13C LR-HSQMBC for ethyl-trans-cinnamate. Both spectra were acquired on our AVIII 600_QCI with long range coupling constant set to 2 Hz. Compared with the conventional HMBC, LR-HSQMBC clearly shows more 5J correlations, e.g. H3,5 - C8 and H7 - C4 . In addition, the 13C decoupling during acquisition, which is used in the LR-HSQMBC but not in the standard HMBC, also helped improve the resolution, as well as the sensitivity of the experiment.
For more details: R.T. Williamson, A.V. Buevich, G.E. Martin, T. Parella, J. Org. Chem. , 2014, 79, 3887-3894.
Heternuclear long range coupling constants can be measured by the J-HMBC experiment. The splittings of the doublets in the F1 dimension in a J-HMBC spectrum will be scaled up to allow small coupling constants to be measured, depending on the scaling factor used in the experiment. The spectrum below is the J-HMBC of ethyl-trans-cinnamate, acquired on QCI-600 with a scaling factor of 29. Two-fold low-pass J-filter was used to suppress the one-bond correlations. The obtained coupling constants for 3J(H7C6) and for 3J(H8C1) were 5.2 Hz and 5.3 Hz respectively which are same as those reported in the literature.
For more details: A. Meissner, O. W. Sorensen, Magn. Reson. Chem., 2001, 39, 49-52
A real-time J-upscaled HSQMBC can be used to measure very weak heteronuclear coupling constants which can usually be difficult for methods that rely on the splittings in F1 dimension due to limited digital resolution in F1. Since the selective excitation of the signal of interest is important in J(up)-HSQMBC, this method works well for well resolved protons. The spectrum below is the J(up)-HSQMBC of ethyl-trans-cinnamate, acquired on QCI-600 with a scaling factor of 20. The proton H8 was selctively excited for coupling constant measurement. Coupling constans of 0.45 Hz and 1.1 Hz were obtained for 4J(H8C2,6) and 2J(H8C7), respectively. For 2J(H8C7), both the J(up)-HSQMBC and J-HMBC obtained almost same values.
For more details: S. Glanzer, O. Kunert, K. Zangger, J. Magn. Reson., 2016, 268, 88-94
band-selective constant-time HMBC
Band-Selective Constant-time HMBC (BSCT-HMBC) selects only the region in f1 dimension which contains the most crowded resonances. This significantly improves the f1 resolution. A further improvement in resolution is achieved by eliminating proton-proton coupling in f1 using the contant-time principle. The spectra below show the standard 1H-13C HMBC (the left) and the BSCT-HMBC (the bottom right) of the mixture of caffeine and maltotrios. In the BSCT-HMBC, the region of selective excitation in f1 is from 68 ppm to 78 ppm.
For more details: T. D. W. Claridge, I. Perez_Victoria, Org. BioMol. Chem., 2003, 1, 3632-3534
The frequency switched Lee Goldburg heteronuclear correlation (FSLG-HETCOR) experiemnt correlates 1H chemical shifts with X-nuclei(e.g. 13C, 15N, etc.) chemical shifts. This experiment provides very good 1H resolution in the indirect dimension which is achieved with FSLG homonuclear decoupling during 1H evolution period. The spectrum below is the 13C-1H FSLG-HETCOR spectrum of tyrosine HCl, acquired at MAS=12.25kHz.
For more details: B. -J. Van Rossum, H. Forster, H. J. M. DeGroot, J. Magn. Reson., 1997, A120, 516-519
In the MAS-J-HMQC (heteronuclear multiple quantunm correlation) experiment, the magnetization transfer used to obtain the correlations is based on scalar heteronuclear J couplings. The resulting 2D spectrum provides through-bond chemical shift correlations between directly bonded pairs of spins, e.g. 13C-1H. It is a complementary technique to the above FSLG-HETCOR which is powerful in probing spatial conectivities. The spectrum below is the 13C-1H MAS-J-HMQC spectrum of tyrosine HCl, acquired at MAS=12.25kHz. Similar to the solution state HMQC experiment, only carbons bonded directly to proton show correlations.
For more details: A. Lesage, D. Sakellariou, S. Steuernagel, L. Emsley, J. Am. Chem. Soc., 1998, 120, 13194-13201