Physical Chemistry , 1st ed.

This perpendicular pulse causes the magnetic moments of the nuclei to pre-

cess in a circular path, an effect that can be measured by a detector coil around

the sample. As soon as the pulse stops, the precession decreases as nuclei re-

align with the static B 0 magnetic field. This process is called relaxation,and has

several mechanisms that we won’t discuss but that are related to the identity of

the sample. A plot of the signal measured by the detector versus time is called

free induction decay(FID).

A mathematical function called a Fourier transform (FT) is applied to the

FID signal, which converts it into an NMR spectrum. The advantages of such

pulse techniques include the ability to store data digitally in a computer, which

can record multiple spectra and average them together. In this way, noise is re-

duced and a clearer spectrum is possible. In fact, some NMR spectra are so

noisy that magnetic resonance spectra are illegible when performed in a scan-

ning mode, so they must be performed in a pulsed mode.^13 C spectra are one

example. An FT-NMR^13 C spectrum is shown in Figure 16.24.

NMR has aspects not discussed here that make it one of the most powerful

techniques for studying the structures of molecules that have NMR-active

16.5 Nuclear Magnetic Resonance 581

10

ppm

0

Si(CH 3 ) 4

CH 4

RCH 3

RRCH 2 '

R 3 CH

CH 3 CC

CH 3 CO

HC C R

CH 3 O

CH 2 CR 2

ArOH

ArH

RCHO

ArCHO

987654321

Figure 16.23 NMR correlation chart, show-

ing where different types of hydrogens attached

to different atoms absorb in an NMR spectrum.

200

ppm

0

TMS

180

199.7

Figure 16.24^13 C NMR spectrum of phenyl^16014012010080604020
propyl ketone, C 6 H 5 COC 3 H 7.