BioPHYSICAL chemistry

at different frequencies and start to separate. The magnetization vector

decreases with a time constant, T 2 , resulting in an exponentially decreas-

ing signal. The form of this signal is called the free-induction decay. Each

spin makes a different contribution to the decay depending upon the specific

frequency for the spins that can be separated out by performing a Fourier

transform on the signal.

A normal NMR spectrum is measured using a pulse as described. Modern

NMR makes use of multiple pulses to generate two-dimensional spectra.

Depending upon the sequence of pulses, the resulting spectra will be

sensitive to different types of interaction between protons. For example,

correlation spectroscopy (COSY) uses two 90° pulses and is sensitive to

standard through-bond interactions between protons. Use of an alternate

pulse sequence allows determination of through-space interactions through

the nuclear Overhauser effect, as described below.

350 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY

Detecting

coil

Z

M

π pulse

Precession

and decay

Z

M

BBZ

B 1 , X

Time

Signal

M

Equal

populations

B  0

M  0

(a)

 (^12)
 (^12)
(b) B  0
M  0
Net magnetization
Unequal
populations
 (^12)
 (^12)
Figure 16.5The presence of a magnetic field, B, shifts the relative
populations of the spins from (a) equal populations to (b) unequal
populations and a net magnetization, M.
Figure 16.6After a 90° pulse the net magnetization Mis turned from the z-direction to the
x-direction and it begins to precess, yielding an oscillating signal.