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Multidimensional NMR

As we learned above, the observable in pulse-acquired Fourier transform NMR is the

decay of the transverse magnetisation, calledfree induction decay(FID). The detected

signal thus is a function of the detection timet 2. Within the pulse sequence, the timet 1

(evolution time) describes the time between the first pulse and signal detection. Ift 1 is

systematically varied, the detected signal becomes a function of botht 1 andt 2 , and its

Fourier transform comprises two frequency components. The two components form

the basis of a two-dimensional spectrum.

Correlated 2D-NMR spectra show chemical shifts on both axes. Utilising different

pulse sequences leads to different methods, such ascorrelatedspectroscopy(COSY),

nuclearoverhauser effect spectroscopy (NOESY), etc. Such methods yield the

homonuclear^1 H couplings. The 1D-NMR spectrum now appears along the diagonal

and long-range couplings between particular nuclei appear as off-diagonal signals

(Fig. 13.10).

Summary of NMR parameters

The parameters obtained from NMR spectra used to derive structural determinants of

a small molecule or protein are summarised in Table 13.1.

13.5.2 Instrumentation

Schematically, an analytical NMR instrument is very similar to an EPR instrument,

except that instead of a klystron generating microwaves two sets of coils are used to

generate and detect radio frequencies (Fig. 13.11). Samples in solution are contained

in sealed tubes which are rotated rapidly in the cavity to eliminate irregularities

and imperfections in sample distribution. In this way, an average and uniform signal

F 2

F 1

dH1

dH2

dH3

dH3dH2 dH 1

dH

dH

Fig. 13.10Schematics of a correlated 2D-^1 H NMR spectrum. H3 couples with H2 and H1. H1 and H2 show

no coupling.

541 13.5 Nuclear magnetic resonance