C O M M U N I C A T I O N S
3
resonance enhancement of the TR spectra may make the relative
agreement with the assignment proposed by McClelland and co-
workers. The Raman spectrum of C8 exhibits significant intensity
3
intensities of the Raman spectra different from those calculated for
normal Raman spectra. When the chromophore for the enhancement
in modes associated with both the FN and G groups, and
comparison with results from BPW91/cc-PVDZ computations
indicates the C8 species has two CdN conjugated bonds in ring 1
of the G moiety. We note our results do not rule out the possibility
that an N7 intermediate pathway may be in competition with the
C8 intermediate pathway, or that initial attack may be at N7 and
3
is somewhat delocalized, the TR and calculated normal Raman
spectra can have similar intensity patterns, as was previously
observed for the related 4-methoxyphenylnitrenium ion in ref 6d,
and we expect that this is also the case here. Figure 1B reveals the
Raman vibrational frequencies, and relative intensities calculated
3
3
for C8 agree well with the TR spectrum of the second species
very quickly convert to the C8 intermediate observed in the TR
while the N7 calculated Raman spectrum does not agree very well.
We tentatively assign the TR spectrum of the reaction product to
spectra. Further work using different guanine derivatives should
prove useful in better characterizing how 2FN reacts with guanine
derivatives. We also note the caveat that reactions with a free
nucleoside in solution may be noticeably different than reactions
that occur in guanine-containing oligomers or genomic DNA.
3
C8 produced from reaction of the 2FN and G. The calculated
absorption spectra for N7 and C8 (Figure 1C) are noticeably
different. The C8 absorption has a strong band at 353 nm and
substantial absorption at the 369 nm probe wavelength used in the
Acknowledgment. This work was supported by grants from
the Research Grants Council of Hong Kong (HKU/7112/00P) to
D.L.P. W.M.K. thanks The University of Hong Kong for the award
of a Research Assistant Professorship.
3
TR experiments, consistent with our assignment of C8 to the
3
second species in the TR spectra. However, the N7 absorption
has a strong band at 316 nm and little absorption at 369 nm, and
3
this suggests that our TR experiment would not be very sensitive
to N7.
Supporting Information Available: Description of the synthesis
3
In C8 and N7, the most intense Raman bands are associated
of the 2-fluorenyl azide precursor and TR experiments. Figures of the
-
1
3
3
with the aromatic CdC stretches of the FN group near 1606 cm
The second most intense Raman bands in this region are the 1640
.
TR spectra and comparison to previous TR spectra of 2FN. Descrip-
tion of the BPW91/cc-PVDZ computations. Cartesian coordinates, total
energies, and zero-point vibrational energies of the optimized geometry
for the lowest energy C8 and N7 species. Simple schematic diagrams
of C8, N7, 2FN, and G with selected bond length parameters shown
from BPW91/cc-PVDZ calculations. This material is available free of
charge via the Internet at http://pubs.acs.org.
-1
-1
cm band associated the G moiety for C8 and the 1614 cm band
associated with the FN moiety of N7. This leads to the Raman
spectra of C8 and N7 being distinctly different in this region, with
the predicted C8 Raman spectrum in good agreement with the
3
experimental TR spectrum while the predicted N7 spectrum is not.
The C8 Raman spectrum also has substantial intensity in the 1338
References
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1
-1
cm (C-H bend) and 1411 cm (C-C stretch + C-H bend +
O-H bend) Raman bands associated with the G moiety, and these
bands are also in good agreement with the experimental TR3
spectrum in Figure 1B. Similar features are very weak and not
clearly discernible in the predicted N7 Raman spectrum, and this
(
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does not agree with the experimental TR spectrum. The fairly
-1
intense Raman bands at 1640, 1411, and 1338 cm associated with
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of ring 1 of G, so that the C -N (1.4989 Å) and C -N (1.4558
26
27
Å) bonds have single bond character, the N -C (1.3112 Å) and
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2
9
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The first time-resolved vibrational spectroscopic observation of
an arylnitrenium ion reaction with a guanine derivative (2FN with
G) and the formation of a C8 intermediate species were reported.
Comparison of the TR3 spectra and previous TA spectra to
predicted Raman and absorption spectra for C8 and N7 confirms
that the reaction product observed from 2FN with G is C8, in
3b
19068-19075.
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