BioPHYSICAL chemistry

364 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY

Eqn 14.1:

λν=c

Since the waveguides operate at specific frequencies (with adjustments

possible only over a narrow region), the frequency for a typical EPR ex-

periment is fixed and the magnetic field is varied. Because the magnetic

fields are substantially smaller, conventional electromagnets can be used

and the magnetic field strength can be varied by changing the applied

current.

In an EPR experiment, the microwave field is monitored as the mag-

netic field is swept. When the magnetic field strength creates an energy

difference that matches the energy of the microwaves (eqn 16.16) reso-

nance absorption occurs. Experimentally the absolute change in the

microwaves is susceptible to error from factors such as baseline drifts.

For increased sensitivity, most experiments not only sweep the magnetic

field but also apply a small alternating magnetic field simultane-

ously. Thus, the absolute EPR signal is not measured, only the

difference in the signal as a result of the small alternating field;

that is, the slope of the signal is measured. This technique, termed

phasesensitive detection, is more sensitive as only those signals

that oscillate with the proper frequency are allowed.

As the magnetic field is swept, the EPR signal is zero and so is

the slope. When the magnetic field approaches the resonance con-

dition, the EPR signal increases and the slope initially increases

but then decreases until a slope of zero is reached (Figure 16.18).

Increasing the magnetic field further results in a decrease of the

EPR signal and negative slope until both return to zero. Thus,

EPR spectra are normally reported in terms of the derivative

spectrum.

As with NMR, the resonance frequency is proportional to the

magnetic field and is not normally reported. For EPR, the samples

are described by the g factor:

(16.18)

In terms of the derivative signal, the g factor corresponds to

the crossing point in the middle of the signal. For some radicals,

differences compared to the free electron value of 2.0023 are

interpreted in terms of the interaction of the electronic spin

with the local environment. However, many electron spins show

large g values that arise due to sensitivity of electrons for the local

environment, as discussed below.

g

h

BB

=

ν

μ

First derivative ofabsorption, d

nχ

/d

B

Absorption,

nχ

B

Slope

Figure 16.18EPR spectra can
be measured as absorption (χ′′)
but are usually measured
using phase-sensitive detection
that results in a signal
representing the derivative
of the microwave absorption,
dχ′′/dB.