Physical Chemistry Third Edition

1028 24 Magnetic Resonance Spectroscopy

intervals. The experiment is repeated a number of times with different time intervals

between the pulses. The dependence of the spin echo on the time between the pulses

gives the value ofT 2 , the spin–spin relaxation time. Since the decay is exponential, the

natural logarithm of the magnitude of the spin echo is plotted against the time required

for the spin echo to occur. The negative slope of the line that fits the data is equal to

the reciprocal ofT 2.

Two-Dimensional NMR Experiments

These experiments use pulse sequences similar to that of the spin–echo experiment.

There are a number of experiments with different pulse sequences. The different

experiments are commonly named with acronyms. For example, COSY (COrrelation

SpectroscopY) was the original two-dimensional NMR experiment.^7 Some other exper-

iments are NOESY (Nuclear Overhauser Effect SpectroscopY) and HETCOR

(HETeronuclearCORrelation). The usual goal of a COSY experiment is to determine

which lines belong to which multiplet in a complicated spectrum with overlapping

multiplets. We will give a simplified description of the COSY experiment for proton

NMR (this is sometimes called HH-COSY).

In a COSY experiment two 90◦pulses are used, separated by a timet 1 (theevolution

time). The FID signal is acquired immediately after the second pulse. A number of runs

are carried out with different values oft 1 , so that there are two time variables,t 1 and

the time that passes as the FID signal is monitored, denoted byt 2 (theacquisition time).

The pulse sequence is shown schematically in Figure 24.10. With the externally imposed

magnetic fieldBzparallel to thezaxis, the magnetic moment of an unshielded pro-

ton with spin up would precess around thezaxis at the reference frequency (see

t 1 t 2

t 2

FID

Figure 24.10 The Pulse Sequence for the COSY Experiment.

(^7) The COSY experiment was first proposed in 1971 by Prof. Jean Jeener of the Universite Libre de
Bruxelles.