Physical Chemistry Third Edition

1034 24 Magnetic Resonance Spectroscopy

absence of the field to have different energies. The two principal types of magnetic

resonance spectroscopy are electron spin resonance (ESR) and nuclear magnetic res-

onance (NMR). In ESR spectroscopy, a substance with unpaired electrons absorbs

radiation of a fixed frequency when the externally applied magnetic field is such that

the magnetic field is

Bres

hν

gβe

wheregis known as thegfactor of the electron, andβeis called the Bohr magneton.

Since the magnetic field “felt” by the electrons includes a term due to nuclear spins in

the molecule, structural information can be obtained from the ESR spectrum.

NMR spectroscopy involves transitions of nuclear spins from one state to another

in a magnetic field with absorption of radiation. The applied magnetic field at a given

nucleus must be

Bres

hν

gNβN

wheregNis the nucleargfactor for the particular nucleus, andβNis the nuclear

magneton (same for every nucleus).

The field at a given nucleus contains two contributions in addition to the externally

applied field. One is due to shielding of the electrons around the nucleus and is expressed

by the chemical shift. The other is due to the presence of other spins in the molecule

and gives rise to spin–spin splitting of spectral lines. Interpretation of the chemical

shifts and spin–spin splittings gives structural information.

Fourier transform NMR techniques have become the standard for many laboratories.

In these techniques, the precession of magnetization vectors is sensed to produce a free-

induction decay (FID) signal. This signal is subjected to a Fourier transformation to

obtain a spectrum as a function of frequency. Two-dimensional NMR techniques can

provide additional information.

ADDITIONAL PROBLEMS

24.40 a.Would H 2 have an ESR spectrum? If so, describe the
spectrum.
b.Would H 2 have an NMR spectrum? If so, describe the
spectrum.

24.41Describe qualitatively the following spectra:

a.The ESR spectrum of an isolated^17 O atom (I 5 /2).

b.The ESR spectrum of^16 O 2 (I0).

c.The ESR spectrum of an isolated^35 Cl atom (I 3 /2).

d.The proton NMR spectrum of benzene.

24.42Tell how you would distinguish between each pair of
substances.

a.Dimethyl ether and ethanol

b.3-Pentanone and 2-pentanone

c.1-Chloropropane and 2-chloropropane (The spin–spin

coupling between protons and chlorines does not lead

to spin–spin splitting.)

24.43Label each statement as either true or false. If a statement

is true only under special circumstances, label it as false.

a.Every molecule exhibits an electron spin resonance

spectrum.

b.Every molecule exhibits a nuclear magnetic resonance

spectrum.