due to the methyls, methylenes and methines have been assigned. A set of DEPT spectra for ethyl
benzoate is shown in Figure 9.43.
Although many pulse sequences have been investigated for 2-D NMR, only a very limited number are
of practical interest. One deterrent to their more widespread use is the time required and hence the cost
of collecting the data. The time factor arises because the pulse sequences have to be repeated hundreds
of times whilst varying the time interval during the evolution period, t 1 , in order to provide sufficient
data for the final 2-D contour plot. Three of the more valuable 2-D techniques are described below.
Figure 9.43
DEPT results for ethyl benzoate. (a)–(c): CH 3 , CH 2 , and CH subspectra.^
(d) Subspectrum of all protonated carbons.
(e) Unedited spectrum. 16 FIDs were accumulated for each θ value.(i) Correlated spectroscopy (COSY) or homonuclear correlation spectroscopy (HOMCOR-2D)
provides proton-proton coupling information (^1 H–^1 H connectivities). This information can also be
obtained from a series of 1-D spin decoupled spectra but difficulties arise with strongly coupled
protons, overlapping multiplets and in optimizing conditions.
A theoretical COSY spectrum for two coupled methine protons (AX system) is shown in Figure 9.44,
the chemical shifts of the two protons being δA and δx ppm. The theoretical 1-D proton spectrum is
shown alongside each frequency axis, F 1 and F 2 , the two scales being equal. Two types of