BioPHYSICAL chemistry

Just as for the nuclear spins, the electronic spins will split the atomic energy
levels in the presence of an external magnetic field, B(Figure 16.1):

E=−μzB=geμBmsB (16.14)

Note that because the electrons and protons have opposite charges, the
lower-energy state for this case is the −1/2 spin.
Transitions will occur when the energy of the applied light is equal to
this energy difference, giving:

ΔE=geμBB=hν (16.15)

This resonance frequency is substantially different than that used for the
NMR, primarily due to the difference in the dipole moments (eqn 16.14).
Using typical values of νNMR=100 MHz, BNMR=10 T, and BEPR=0.3 T gives:

(16.16)

This frequency is in the microwave region of light. Older systems typic-
ally used klystrons as the microwave source whereas newer systems use
diodes (Figure 16.17). The microwaves are carried to the sample by use
of waveguides whose size is matched to the wavelength of the microwave.
For a 10-GHz frequency, the wavelength:

(16.17)

λ==

×

×

=

c −

9

310

10 10

3

10 1

9

cm s

/s

cm

ννEPR NMR N
e

EPR

NMR

m

m

B

B

=≈×× 100 2000

0

MHz

..3

10

10

T

T

≈ GHz

CHAPTER 16 MAGNETIC RESONANCE 363

Phase-sensitive

detector

Signal

converter

Circulator

Print

Microwave

source

Electromagnet

Electron paramagnetic spectrometer

Sample cavity

Attenuator

Figure 16.17Schematic diagram of an EPR spectrometer.