ISSN:
0749-1581
Keywords:
Multi-dimensional spectroscopy
;
Nuclear magnetic resonance
;
NMR
;
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Multi-dimensional NMR spectroscopy must satisfy the Nyquist sampling condition in all frequency dimensions. This entails very long experiments and very large data arrays, and may mean that fine structure of cross-peaks cannot be adequately digitized. This sampling restriction can be sidestepped in an alternative approach which explores the new frequency dimensions by scanning selective radiofrequency pulses in small steps over narrow frequency ranges. A pulse envelope shaped according to the first half of a Gaussian curve is particularly well suited to this task. Thus a form of double resonance experiment can be used to generate two-dimensional correlation spectra that have all the features of the well known COSY experiment, with an additional ‘zoom’ capability that reveals the detailed fine structure information. For systems of three coupled spins the corresponding triple resonance experiment generates a three-dimensional correlation specrum, the frequency scans being restricted to the region of interest (the 3D cross-peak) based on information from the conventional NMR spectrum. For correlation spectroscopy of higher dimensionality no frequency search is employed, the selective pulses simply being set at predetermined chemical shift frequencies. A four-dimensional correlation experiment is described which uses population transfer to establish that four non-equivalent protons are coupled in a chain I—S—R—P. It employs an initial ‘ZZ-pulse’ applied at the I-spin frequency to excite longitudinal two-spin order (2IzSz) which is then propagated along the chain by the application of selective ‘ZZZ-pulses’ to the intermediate spins S and R, creating an antiphase intensity perturbation (2RzPz) on the P multiplet. The procedure is recursive and can in principle be extended to N spins coupled pairwise in a ‘linear’ chain. This is a powerful diagnostic tool for structure determination and can be adapted to recognize other topological features such as chain branching and ring closure.
Additional Material:
12 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/mrc.1260261016
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