Chemistry Reference
In-Depth Information
TABLE 9.2
Comparison of
g
-Values for Various Radical Cations with Those Observed for
Canthaxanthin Radical Cation
Radical Cations
g
xx
g
yy
g
zz
g
iso
Structure
Reference
TP
•+
/AsF
5
−
2.0031
2.0028
2.0022
2.0027
Polymer
Robinson et al. (1985)
2.00312
2.00230
2.00206
2.00249
Stacked array
Kispert et al. (1987)
TP
•+
/PF
6
−
2.00217
2.00217
2.00310
2.00248
Stacked array
Kispert et al. (1987)
QP
•+
/PF
6
−
2.00337
2.00248
2.00208
2.00264
Dimer
Klette et al. (1993)
P
865
•+
2.00317
2.00260
2.00226
2.0027
Dimer
Bratt et al. (1997)
P
700
•+
Bchl
a
•+
2.0033
2.0026
2.0022
2.0027
Monomer
Burghaus et al. (1991)
2.0023
2.0023
2.0032
2.0026
Symmetrical
π-RC
•+
Konovalova et al.
(1999)
Car
•+
Source:
Konovalova, T.A.,
J. Phys. Chem. B
, 103, 5782, 1999.
dependence of the EPR linewidths over the range of 5-80 K at 327 GHz suggests rapid rotation of
methyl groups even at 5 K that averages out the proton couplings from three oriented
-protons.
Determination of
g
-tensor components from resolved 327-670 GHz EPR spectra allows dif-
ferentiation between carotenoid radical cations and other C-H
β
-radicals which possess different
symmetry. The principal components of the
g
-tensor for Car
•
+
differ from those of other photosyn-
thetic RC primary donor radical cations, which are practically identical within experimental error
(Table 9.2) (Robinson et al. 1985, Kispert et al. 1987, Burghaus et al. 1991, Klette et al. 1993, Bratt
et al. 1997) and exhibit large differences between
g
xx
and
g
yy
values.
π
9.14 HIGH-FIELD EPR MEASUREMENTS OF METAL CENTERS
The HF-EPR can also be used to good advantage to study high-spin systems, where the zero-i eld
splitting (ZFS) term is often dominant in the spin Hamiltonian. The examples of such systems
are transition metal ions like Mn(II), Ni(II), and Fe(III), which have been used for introducing
active sites in mobile crystalline material (MCM-41) mesoporous materials. MCM-41 containing
well-organized nanometer-sized channels has been found to be a good photoredox system where
long-lived photoinduced electron transfer from bulky biomolecules such as carotenoids can occur.
Although the MCM-41 framework can act as an electron acceptor, replacement of some tetrahedral
Si(IV) in the MCM-41 framework by transition metal ions produces a long-lived charge separation
between the carotenoid radicals and the metal electron acceptor sites.
9.14.1 C
AROTENOIDS
IN
N
I
-MCM-41
Photo-oxidation of b-carotene and canthaxanthin in mesoporous Ni(II)-containing MCM-41 molec-
ular sieves was studied by 9-220 GHz EPR spectroscopy (Konovalova et al. 2001b). The presence
of Ni(II) ions in Ni-MCM-41 was verii ed by 220 GHz EPR spectroscopy. Ni-containing MCM-41
samples measured at 9 GHz showed no EPR signals consistent with Ni(II) ions. The 220 GHz EPR
spectrum of activated Ni-MCM-41 exhibits a broad line with g-value of 2.26 (Figure 9.9) providing
direct evidence of Ni(II) incorporation into the MCM-41 framework. It has been reported (Abragam
and Bleaney 1970) that Ni(II) ions in an octahedral environment give rise to very broad EPR lines
with g-values of 2.10-2.33.
Irradiation of Ni-MCM-41 at 350 nm generates new paramagnetic species stable at 77 K
whose spectra are superimposed. From the 110 GHz spectrum of Ni-MCM-41 (5 K), two different
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