Chemistry Reference
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The dominant effect of the rapidly relaxing metal spin on the
T
M
of the slowly relaxing spin is
analogous to the effects of a physical motion, such as rotation of the methyl group, which averages
nuclear spins to which the unpaired electron is coupled. The most dramatic effect on
T
M
occurs
when the metal relaxation rate is of the same order of magnitude as the dipolar couplings to the
slowly relaxing spin. This phenomenon is shown in Figure 9.15, which exhibits the logarithmic
temperature dependence of the canthaxanthin relaxation rate, 1/
T
M
, in siliceous and Ti-containing
materials. As temperature increases, the metal relaxation rate increases and becomes comparable to
the dipolar splittings in the vicinity of 125 K. As a result in Ti-MCM-41, both components, fast and
slow, show a signii cant increase in 1/
T
M
around this temperature. On the other hand, siliceous sam-
ples showed little dependence on 1/
T
M
. The temperature dependence of 1/
T
M
for BI
in MCM-41 and
in Ti-MCM-41 also showed an increase in 1/
T
M
for BI
•+
in Ti-MCM-41 compared to that in MCM-
41, especially near 40 K. A signii cant increase in 1/
T
M
for the (
1
) radical in Ti-MCM-41 compared
to the MCM-41 sample indicates interaction of the carotenoid with the Ti
3+
ion. In contrast to can-
thaxanthin and BI, 7
-carotene containing the terminal carboxy group
shows a monotonic increase in relaxation rate. No prominent peak in relaxation was observed.
The relaxation enhancement displayed for canthaxanthin, 7
′
-apo-7
′
-(4-carboxyphenyl)-
β
-carotene
and BI was analyzed to provide interspin distances. The dipolar interactions and the distances can
be determined according to procedures described elsewhere (Budker et al. 1995, Rakowsky et al.
1995, Eaton and Eaton 2000, Rao et al. 2000) and based on simulations of the paramagnetic metal
ion contribution,
W
dd
:
′
-apo-7
′
-(4-carboxyphenyl)-
β
1
1
(9.26)
T
=+
W
dd
M
0
where 1/
T
M
and 1/
T
M0
are the Car
•+
relaxation rates in the presence and absence of the metal ion,
respectively. The
W
dd
can be numerically simulated on the basis of relaxation enhancement in a
two-pulse echo,
W
(
), due to electron-electron interaction between the two spins. The simulation
includes the experimental 1/
T
M
-1/
T
M0
value and
T
1
of the Ti
3+
ion. The adjustable parameter is the
distance
r
(nm). At the proper distance, the simulations should match the experimental echo decay
curves. If the relaxation rate increases signii cantly at a certain temperature, this procedure allows
τ
7.4
7.2
7.0
6.8
6.6
6.4
6.2
T
= 125 K
6.0
5.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
log
T
(K)
FIGURE 9.15
Temperature dependence of log 1/
T
M
(
T
M
[Hz]) for canthaxanthin radical. The ESEEM curves
were best i tted as double exponentials: (−
■
−) slow component for Car/MCM-41, (−
❑
−) slow component for
Car/Ti-MCM-41, (−
●
−) fast component for Car/MCM-41, and (−
❍
−) fast component for Car/Ti-MCM-41.
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