Applications of EPR Spectroscopy to Understanding Carotenoid Radicals - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 175

HYSCORE spectra of zeaxanthin radicals photo-generated on silica-alumina were taken at two
different magnetic i elds B 0 = 3422 G, respectively. In order to combine the data

from the two spectra, the i eld correction was applied (Dikanov and Bowman 1998). The correction
consists of a set of equations that allow transformation of spectra to a common nuclear Zeeman
frequency. The set of new frequencies was added to that of the former spectrum and plotted as
the squares of the frequencies

3450 G and B 0 =

ν

2

and ν

2

. Examples of these plots can be found in Focsan et al.

α β

2008.

9.13 g -ANISOTROPY: HIGH-FIELD g -TENSOR RESOLUTION
High-i eld EPR (HFEPR) spectroscopy greatly improves the resolution of the EPR signals for spec-
tral features such as the g -tensor. Deviations of the g -value from free electron g 2.0023 are due to

spin-orbital interactions, which are one of the most important structural characteristics (Kevan and
Bowman 1990). Using a higher frequency results in enhanced spectral resolution in accordance with
the resonance equation:

=

h

ω

H =

2 πβ

where
h is the Planck constant
β is the Bohr magneton

ω

9 GHz

is the frequency of electromagnetic radiation

95 GHz

If inhomogeneous broadening of the EPR linewidth is primarily due
to unresolved hyperi ne couplings (hfc), at higher frequencies the
g -anisotropy will dominate over the hyperi ne interactions, i.e., the
condition

32 7 GHz

gg H H must be fuli lled.
The advantage of high-frequency EPR in g -anisotropy resolution
is provided by the spectrum of canthaxanthin radical cation adsorbed
on silica-alumina (Figure 9.8). The X-band (9 GHz) EPR spectrum of
a carotenoid radical cation consists of an unresolved single line with
g iso =

( Δ ) > Δ

iso o hfc

3 74 GHz

4 4 0 GHz

-radicals
(Wertz and Bolton 1972). The line shape most closely resembles that
of a Gaussian line, which indicates that the line is inhomogeneously
broadened by unresolved proton hyperi ne structure.
The 327-670 GHz EPR spectra of canthaxanthin radical cat-
ion were resolved into two principal components of the g -tensor
(Konovalova et al. 1999). Spectral simulations indicated this to be the
result of g -anisotropy where g II =

2.0027 ± 0.0002, which is characteristic for organic π

6 70 GHz

5 mT

2.0023. This type of
g -tensor is consistent with the theory for polyacene

2.0032 and g ^ =

FIGURE 9.8 HF-EPR spec-
tra of canthaxanthin radical
cation adsorbed on silica-
alumina: (solid line)—experi-
mental spectra recorded at 5 K;
(dotted line)—simulated spec-
tra using g -tensor values g zz =
2.0032 and g xx = g yy = 2.0023
and linewidth of 13.6 G. (From
Konovalova, T.A., J. Phys.
Chem. B , 103, 5782, 1999.
With permission.)

π -radical cations

(Stone 1964), which states that the difference g xx − g yy decreases with

g yy approaches zero, the g -tensor
becomes cylindrically symmetrical with g xx =

increasing chain length. When g xx −

g II . The
cylindrical symmetry for the all- trans carotenoids is not surprising
because these molecules are long straight chain polyenes. This also
demonstrates that the symmetrical unresolved EPR line at 9 GHz is
due to a carotenoid

g yy = g ^ and g zz =

-radical cation with electron density distributed
throughout the whole chain of double bonds as predicted by RHF-
INDO/SP molecular orbital calculations. The lack of temperature

π