Chemistry Reference
In-Depth Information
CO
2
H
CO
2
Bu-
t
S
S
S
S
S
S
S
S
S
S
CF
3
CO
2
H
S
S
HO
2
C
C
•
t
-BuO
2
C
C
OH
S
S
(1.26)
SS
SS
S
S
S
S
S
S
CO
2
H
CO
2
Bu-
t
86
85
CO
2
Me
O
O
O
O
Ar
C
•
O
O
O
O
CO
2
Me
87
The presence of ionizable carboxyl groups on the TAM radicals leading to pH sensitivity of the EPR
parameters has been examined, as has the presence of limited numbers of aryl ring protons leading to
doublet or triplet EPR signals.
29a
These serve as dual function pH and oxygen probes. Dendritic TAM
molecules were prepared in which encapsulation of the radical by the dendrimer enhances their stability
and solubility in water over a wide pH range, and the copper(II) complex of
88
is a highly effective pH
probe.
29f
Amino substituted TAMs
90
and related derivatives were prepared by selective reduction of one or more
carboxyl groups of
85
to hydroxylmethyl substituents, and these were converted to mesylates that were
displaced by amino groups forming
89
. Conversion to radicals
90
was accomplished using trifluoroacetic
acid (Equation 1.27), and provides derivatives useable in other pH ranges.
29b
Collisions of these probe molecules reduce relaxation times of the probes and provide a measure of
local oxygen concentration; to investigate the viscosity and frequency dependence on the EPR spectra,
measurements of TAM radicals in glycerol/water ranging from 10 to 90 % concentration were carried
out, including the use of deuterated solvents.
30a
The conformations of the TAM radicals in solution were
investigated using
1
H,
2
H, and
13
C measurements,
30b
with hyperfine coupling tensors determined using
X-band pulsed electron-nuclear double resonance (ENDOR) spectroscopy for two triarylmethyl (trityl)
radicals used in EPR imaging and oximetry.
30b
Calculated geometries were used to predict the experimental
hyperfine tensors, and these gave satisfactory agreement.
30b
Rapid-scan EPR spectra of a 0.2 mM aqueous solution of deuterated
86
(trityl-CD
3
) radical at 9.8 GHz
using a rectangular resonator were obtained.
31a
Larger effects of magnetic field inhomogeneities were
found for the extended structure of
86
than in smaller lithium phthalocyanine crystals.
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