126 INSTRUMENTAL METHODS
For the simple case of one unpaired electron ( S = 1/2), the associated mag-
netic moment is not a simple number but is directionally oriented — that is, is
anisotropic. Taking spin – orbit interactions into account for EPR spectra leads
to four limiting cases:
- gx = gy = gz. The magnetic moment is independent of orientation. This case
is called isotropic, and a single symmetric EPR absorption is obtained.
The paramagnet can be represented by a sphere. The EPR spectrum will
resemble Figures 3.20A, 3.20B, and 3.22A. - gx = gy < gz. The paramagnet, represented by a football shape, exhibits a
minor feature at low fi eld (from gz , often called g|| or g parallel) and a
major feature at high fi eld (from gx and gy , often called g⊥ or g perpen-
dicular). The spectrum is said to be axial. If Cu(II) d^9 ions in octahedral
ligand fi elds exhibit z - axis elongated d - orbital splitting because of the
Jahn – Teller effect (see Section 1.6 for a discussion of these topics), the
EPR envelope will appear as shown in Figure 3.22B. All g values will be
- gx = gy > gz. The paramagnet, represented by a discus shape, exhibits a
minor feature at high fi eld (from gz , g|| or g parallel) and a major feature
Figure 3.22 EPR Absorption curves. (A) Isotropic spectrum, gx = gy = gz. (B) Axial
spectrum, gx = gy < gz. (C) Axial spectrum, gx = gy > gz. (D) Rhombic spectrum, gx ≠ gy ≠ gz.
(Reprinted with permission from Figure 4 of reference 29. Copyright 2000, University
Science Books.)
(a) ISOTROPICgx= gy= gzg//g//gx= gy= gzgxgygzABSORPTIONABSORPTIONDERIVATIVEgx= gy< gz gx= gy> gz gx≠ gy≠ gz(b) AXIAL (c) AXIAL (d) RHOMBICgg