ELECTRON PARAMAGNETIC RESONANCE 125
the focus here will be on EPR behavior of transition metal centers that occur
in biological species. An excellent presentation of the subject, written by
Graham Palmer, is found in Chapter 3 of reference 29. The discussion here is
summarized mostly from that source.
When an electron is exposed to a magnetic fi eld, BL , the electron can be
either stabilized ( ms = − 1/2) or destabilized ( ms = +1/2), with the magnitude of
the effect varying linearly with the intensity ofBL. This interaction of magnetic
moments withBL is called the Zeeman effect or Zeeman interaction. Figure
1 on page 126 of reference 29 , along with Figures 3.19 and 3.20 , relates the
energy level diagram for the electron to the absorption and fi rst - derivative
modes of EPR spectral presentation. Standard EPR instrumentation utilizes
a fi xed frequency (usually the 9 GHz “ X band ” ) and a variable magnetic fi eld.
Other frequencies may also be used to enhance spectra, thereby increasing
resolution of unresolved hyperfi ne structure (3 GHz) or resolution associated
withg anisotropy (35 GHz).
If one calculates g from equation 3.41 , the measurable experimental quan-
tity would appear to be a single number of approximately 2. Observed g factors
for paramagnetic metal ions range from < 1 to 18 (measured for some lantha-
nide ions). Two phenomena, known as spin – orbit interactions (spin – orbit cou-
pling) and zero – fi eld splitting, are responsible for g factor deviations from the
free electron value. Spin – orbit coupling arises because the magnetic dipole
associated with the orbital momentum of the electrons ( L ) tends to align itself
with the magnetic dipole due to the electrons ’ intrinsic spin ( S ). Spin – orbit
coupling tends to be quenched if the metal ion exists in a ligand fi eld that lifts
the degeneracy of the d orbitals, and in practice the g factor value will lie
somewhere between the free ion value (favored by spin - orbit coupling) and
the free electron value (quenched spin – orbit coupling). The greater the lifting
of the degeneracy of the d orbitals, the more effective the quenching of
spin – orbit coupling, and the closer g will be to free electron value. A simple
spin – orbit interaction is illustrated in Figure 3.21.
Figure 3.21 The motion of an electron in orbit about a nucleus generates an orbital
momentum ( L ) adding a component to the magnetic fi eld experienced by the electron
spin ( S ). (Adapted with permission of John Wiley & Sons, Inc. from Figure 2.18 of
reference 3. Copyright 1997, Wiley - VCH.)
electronSZ
LZnucleus