Chemistry Reference
In-Depth Information
optimized geometries for the C8 intermediate, the C8-methyl intermediate, and the
C8-bromo intermediate with selected bond lengths (in A
) given next to the
relevant bonds (see Table 1S in Ref. 46 for more information). Figure 7.29
compares the ns-TR
3
spectra in Figures 7.22-7.24 (time-delays of 15, 50, and
50
s, respectively) with the BPW91/cc-PVDZ computed normal Raman spectra
(whose relative intensities were convoluted with a Lorentzian function) for the C8
intermediate, the C8-methyl intermediate, and the C8-bromo intermediate.
Examination of Figure 7.28 shows that the experimental TR
3
spectra exhibit
reasonable agreement with the computed normal Raman spectra for the C8 inter-
mediate, the C8-methyl intermediate, and the C8-bromo intermediate, respectively
with modest differences in the relative intensities that could be due to the exper-
imental spectra are resonantly enhanced while the computed spectra are for normal
(or nonresonant) Raman spectra.
Inspection of Figure 7.27 shows that there are modest differences in the structure
and properties of the C8-methyl intermediate and the C8-bromo intermediate that are
seen mostly in the ring 1 of the guanosine part of the intermediate, the carbon-
m
nitrogen bonds connecting the guanosine and 2-fluorenyl nitrenium ion parts and
ring 1 of the 2-fluorenyl nitrenium ion part of the respective intermediates. The
C37
N34 bonds connecting the guanosine and 2-fluorenyl nitrenium
ion parts increase 0.0093 A
and 0.0317 A
, respectively in C8-methyl intermediate
compared to the C8-intermediate, and decrease 0.0190 A
and 0.0640 A
in the
C8-bromo intermediate compared to the C8 intermediate, and these results indicate
that connection of the 2-fluorenyl nitrenium ion at the C8 position of C8-methyl
guanosine leads to weaker C37
N34 and C8
N34 bonds in the C8-methyl
intermediate compared to the C8 intermediate formed from the corresponding
reaction with the guanosine substrate. In comparison, the connection of the
2-fluorenyl nitrenium ion at the C8 position of C8-bromo guanosine leads to stronger
C37
N34 and C8
N34 bonds in the C8-bromo intermediate compared to the C8
intermediate formed from the corresponding reaction with the guanosine substrate.
The C8-methyl and C8-bromo substituents on the C37
N34 and C8
N34 bonds in
their respective C8 intermediates cause different bond length changes relative to the
unsubstituted C8 intermediate and this can be attributed to the methyl group being an
electron donor and the bromo group being an electron acceptor. These different
properties also affect the C8
N7 and C8
N7 bonds of ring 1 of the guanosine
moiety in the C8-methyl intermediate to increase by 0.0142 A
and 0.0066 A
compared to the unsubstituted C8 intermediate while they decrease by 0.0395 A
and 0.0332 A
, respectively in the C8-bromo intermediate relative to the unsubstituted
C8 intermediate. The differences in the C37
N9 and C8
N34, C8
N34, C8
N9, and
C8
N7 bonds in the substituted C8 intermediates compared to the C8 intermediate
are usually greater for the C8-bromo substitution than for the C8-methyl substitution,
and it is interesting that the N7
C5, N9
C4, and C4
C5 bonds of ring 1 of the
guanosine part are not influenced as much as the C37
N34, C8
N34, C8
N9,
and C8
N7 bonds in the substituted C8 intermediates compared to the C8
intermediate, and all three intermediates have close to the same carbon-nitrogen
double bond character for the N7
C5 and N9
C4 bonds.
