2646 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Zhu et al.
10 to 20 of these readouts were accumulated to get a resonance Raman
spectrum. Pump only, probe only, and pump-probe transient resonance
Raman spectra as well as a background scan were obtained and the
known Raman bands of the water/acetonitrile solvent were used to
calibrate the wavenumber shifts of the resonance Raman spectra. The
solvent and precursor 2-azidofluorene Raman bands were removed from
the pump-probe transient resonance Raman spectrum by subtracting
a probe only Raman spectrum. The pump only spectrum was also
subtracted from the pump-probe spectrum to get the transient resonance
Raman spectrum of the photochemically generated 2-fluorenylnitrenium
ion.
All of the density functional theory computations reported here made
use of the Gaussian program suite.38 Complete geometry optimization
and vibrational frequency computations were done analytically by using
the BPW91 method39,40 with the cc-PVDZ basis set41 for the ground
electronic states of the 2-fluorenylnitrenium ion.
such study confirmed that the diphenylnitrenium ion has an
iminocyclohexadienyl cation-like structure.28 A very recent
investigation used TRIR to characterize substituent effects on
the symmetrical aromatic CdC stretch mode(s) in the 1580-
1628 cm-1 region for four N-methyl-N-phenylnitrenium ions.31
The experimental vibrational frequencies were found to have
excellent agreement with values computed from BPW91/cc-
PVDZ density functional theory calculations. The computational
results exhibited noticeable bond length alternation in the phenyl
rings accompanied by smaller C-N bond lengths and significant
positive charge delocalization into the phenyl rings.31
In this paper we report a preliminary transient resonance
Raman investigation of the 2-fluorenylnitrenium ion produced
from photolysis of a 2-azidofluorene precursor in a 75%water/
25%acetonitrile by volume mixed solvent. To our knowledge,
this is the first transient vibrational spectrum reported for the
2-fluorenylnitrenium ion and the first transient resonance Raman
spectrum for an arylnitrenium ion formed via a photochemical
reaction. We also note that this is the first transient vibrational
spectrum obtained for a photochemically produced arylnitrenium
ion in a largely aqueous system. We observe a resonance Raman
spectrum that contains a number of fundamental bands in the
1100-1700 cm-1 region as well as several combination bands
in the 2400-3300 cm-1 region. These Raman vibrational
frequencies show very good agreement with those computed
from BPW91/cc-PVDZ density functional calculations for the
2-fluorenylnitrenium ion. Our results confirm the assignment
of the ∼460 nm transient absorption band to a singlet 2-
fluorenylnitrenium species. Our transient resonance Raman and
BPW91/cc-PVDZ computational results indicate the 2-fluor-
enylnitrenium ion has substantial iminocyclohexadienyl char-
acter with significant carbon-carbon bond alternation in both
phenyl rings. We compare our results for the 2-fluorenylnitre-
nium ion to results for several other arylnitrenium ions.
Results and Discussion
Flash photolysis of 4-azidobiphenyl or 2-azidofluorene in
water/acetonitrile or water solvent leads to formation of intense
broad characteristic transient absorption bands with a maximum
at ∼460 nm that have been assigned to the 4-biphenylylnitre-
nium ion21 and 2-fluorenylnitrenium ion,18,21 respectively. These
ions were observed to be fully present after the 20 ns laser pulse
used in the transient absorption experiments and this implies
they are formed with a rate constant of >5 × 107 s-1 21
. A recent
picosecond measurement showed that the lifetime of the
appearance of the 2-fluorenylnitrenium ion was about 100 ps.25
We used a 309 nm pump laser wavelength (close to the 308
nm excitation wavelength used in the previous transient absorp-
tion study21) and a 416.0 nm probe wavelength (on the blue
side of the broad ∼460 nm transient absorption band) for our
transient resonance Raman experiments of 2-azidofluorene in
a 75% water/25% acetonitrile solvent. Thus, our transient
resonance Raman spectra should directly probe the identity and
structure of the species responsible for the strong transient
absorption band observed following ultraviolet photolysis of
2-azdiofluorene in water/acetonitrile solution. Figure 1 presents
the 416 nm transient resonance Raman spectrum obtained
following 309 nm excitation of 2-azidofluorene in 75% water/
25% acetonitrile solution.
Experimental and Computational Details
The 2-azidofluorene sample was synthesized according to the
literature method (further details of the synthesis are given in the
Supporting Information).18,29,32 Samples of the 2-azidofluorene precursor
were prepared with concentrations of ∼5 mM in a 75% water/25%
acetonitrile (spectroscopic grade) by a volume mixed solvent system
with a 2 mM acetate buffer and a pH of 3.5. The transient resonance
Raman spectra were obtained by using the experimental apparatus and
methods previously described in refs 33-37 and only a brief description
will be provided here. Hydrogen Raman shifted laser lines generated
from the harmonics of a nanosecond pulsed Nd:YAG laser supplied
the pump (309 nm) and probe (416 nm) beams for the transient
resonance Raman experiments. Optical delays of 0-10 ns between the
pump and probe pulses were used in the experiments. A near collinear
geometry was employed to focus the pump and probe beams onto a
flowing liquid stream of sample and the Raman scattering was collected
with reflective optics and imaged through a polarization scrambler
mounted on the entrance slit of a 0.5 m spectrograph. The grating of
the spectrograph dispersed the Raman light onto a liquid nitrogen cooled
CCD and the Raman signal was acquired by the CCD for ∼300 to 600
s before being read out to an interfaced PC computer. Approximately
Table 1 lists the vibrational frequencies and tentative assign-
ments for the resonance Raman bands observed in the transient
Raman spectrum of Figure 1. Inspection of Figure 1 shows the
transient Raman spectrum has a number of fundamental bands
in the 1100-1700 cm-1 fingerprint region that also appear to
have some combination bands in the 2500-3300 cm-1 region.
Several reports have used time-resolved infrared absorption
(TRIR) spectra and comparison to density functional theory
computations to convincingly assign several arylnitrenium ions
(biphenylylnitrenium ion28 and four N-methyl-N-phenylnitre-
nium ions31) to their singlet ground electronic states. Therefore,
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Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
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