Pulsed FT-NMR has facilitated the study of nuclei other than^1 H where the sensitivity obtainable from a
CW instrument is totally inadequate. In particular,^13 C NMR, the sensitivity of which is nearly 10-^4 less
than that of the proton (Table 9.9), is now a well-established technique that yields information on the
skeletal structure of complex molecules. The pulsed technique also enables proton spectra to be
obtained from samples as small as a few micrograms.
Multiple Pulse Techniques:
1 - D and 2-D NMR
A variety of computer-controlled pulse sequences consisting of two or more pulses of appropriate
length, frequency range, power and phase, and separated by variable time intervals, has been developed,
giving rise to families of 1-D (one-dimensional) and 2-D (two-dimensional) techniques. These
techniques provide additional or more easily interpreted data on coupled nuclei, facilitating the
identification of signals from chemically different groups of nuclei and correlations between spectra
from different elements in the same compound.
It is confusing that the term '1-D' is used to describe a conventional absorption vs frequency (expressed
in ppm) spectrum but which is in reality a two-dimensional representation. Likewise, the term '2-D'
describes a spectrum where two frequency axes (F 1 and F 2 ) are shown at right angles and peak
intensities are plotted perpendicular to them. Although this is actually three-dimensional, a 2-D contour
plot, which is analogous to a topographical map is the simplest and most convenient way of displaying
the spectral data. One frequency axis may represent chemical shift in Hz or ppm of one nucleus, usually
(^1) H or (^13) C, while the other is either chemical shift of the same or another nucleus or a coupling constant
scale in Hz. As in a map, the 'contour lines' depict the intensities or heights of the peaks (Figure 9.45).
A diagram of a typical pulse sequence is shown in Figure 9.42(a). A preparation period, if required, is
followed by an initial or magnetization RF pulse, then by one or more further pulses with controlled
time intervals (evolution and mixing period, t 1 ) between them. The FID is acquired at the end of the
sequence (detection and acquisition period, t 2 ) and this is followed by one or two Fourier
transformations to give the final 1-D or 2-D frequency domain spectrum. The pulse sequence may be
applied to protons and/or carbon-13 nuclei in the sample (or any other nuclei of interest) and spin-
decoupling may be used simultaneously. For 2-D techniques, the sequence is repeated many times, the
time interval during the evolution period, t 1 , being increased by equal increments with each repetition.
Fourier transformation of the FIDs produces a two-dimensional matrix of stored data. By subjecting
sets of data points in corresponding positions in the matrix to further Fourier transformations, a 2-D
frequency domain spectrum is generated. In effect, this is a Fourier transform of a Fourier transform.
The sequence is shown in Figure 9.42(b).
The most useful 1-D pulse sequence applied to^13 C nuclei is known as distortionless enhancement by
polarization transfer (DEPT). A decoupled