Chemistry Reference
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5G
300 K
280 K
260 K
240 K
230 K
225 K
215 K
210 K
200 K
Fast exchange
10
−
7
s
−
1
9.86
×
10
−
7
s
−
1
6.51
×
10
−
7
s
−
1
3.70
×
10
−
7
s
−
1
2.82
×
10
−
7
s
−
1
2.46
×
10
−
7
s
−
1
1.76
×
10
−
7
s
−
1
1.41
×
Slow exchange
Experimental
Simulated
Figure 2.20
Experimental (left) and simulated (right) EPR spectra of
15
−
•
at different temperatures in
dichloromethane. (Reprinted with permission from [78a]. Copyright 2001 Wiley-VCH Verlag GmbH & Co.
KGaA.)
is localized on the EPR timescale on only one-half of the molecule, that is on one
p
-phenylenevinylene-
PTM moiety. When the temperature is increased, a new central line in the EPR spectra gradually emerges
between the two initial ones (Figure 2.20). This evolution is consistent with the increase of the electron
transfer on going from 200 K (the slow-exchange limit) to 300 K (the fast-exchange limit) due to a
thermally activated IET between the two equivalent sites of this M-V species. Under these conditions, the
unpaired electron of
15
−
•
is coupled with two equivalent
1
H nuclei. The EPR spectra were simulated by
using the jumping rates,
k
th
, given in Figure 2.20 and the resulting
k
th
values plotted using a linear Eyring
plot [ln(
k
th
/
G
±
, of 0.117 eV
)
T
vs 1/
T
], from which the energy barrier of the thermal electron transfer,
was obtained.
The EPR spectrum of radical anion
16
−
•
is unchanged in the temperature range 150 - 300 K and consists
of two symmetrical lines arising from the hyperfine coupling of the unpaired electron with one hydrogen
atom of the ethylene moiety with a
1
H hyperfine coupling constant close to that of the corresponding
monoradical anion at low temperature. This is a clear indication that the extra electron of radical anion
16
−
•
is always localized on the EPR timescale on one-half of the molecule regardless the temperature.
This result can be ascribed to the localization of frontier orbital in the latter radical anion
16
−
•
, because
of the
meta
connectivity of this non-Kekule molecule.
Finally, EPR spectra of diradicals
15
and
16
also provide information about the degree of electronic
delocalization in such species. Thus, the absolute values of zero field splitting parameters for diradi-
cals
15
and
16
, obtained from the simulated spectra in frozen trichloromonofluoromethane (CFCl
3
)
,are
10
−
4
10
−
4
cm
−
1
, respectively, with null
|
values for both diradicals. These
parameters arise from the dipolar magnetic interactions between the two unpaired electrons and can be
used to calculate the average interspin separation. Therefore, from the
D
/
hc
|=
3
.
9
×
and 2
.
3
×
|
E
/
hc
|
parameter in cm
−
1
and
Equation 2.5, average interspin separations of 19 and 22 A were found for diradicals
15
and
16
, respec-
tively. The average interspin separation found for diradical
15
is smaller than the nominal separation
between the two alpha carbon atoms where most of the spin density of the two PTM units is localized.
This result is in agreement with the existence of a certain degree of electron delocalization for the
para
|
D
/
hc
|
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