Time-resolved resonance raman and computational investigation of the influence of 4-acetamido and 4-N-methylacetamido substituents on the chemistry of phenylnitrene

Article abstract of DOI:10.1021/jp201821d

A time-resolved resonance Raman (TR³) and computational investigation of the photochemistry of 4-acetamidophenyl azide and 4-N-methylacetamidophenyl azide in acetonitrile is presented. Photolysis of 4-acetamidophenyl azide appears to initially produce singlet 4-acetamidophenylnitrene which undergoes fast intersystem crossing (ISC) to form triplet 4-acetamidophenylnitrene. The latter species formally produces 4,4′-bisacetamidoazobenzene. RI-CC2/TZVP and TD-B3LYP/TZVP calculations predict the formation of the singlet nitrene from the photogenerated S ₁ surface of the azide excited state. The triplet 4-acetamidophenylnitrene and 4,4′-bisacetamidoazobenzene species are both clearly observed on the nanosecond to microsecond time-scale in TR³ experiments. In contrast, only one species can be observed in analogous TR ³ experiments after photolysis of 4-N-methylacetamidophenyl azide in acetonitrile, and this species is tentatively assigned to the compound resulting from dimerization of a 1,2-didehydroazepine. The different photochemical reaction outcomes for the photolysis of 4-acetamidophenyl azide and 4-N-methylacetamidophenyl azide molecules indicate that the 4-acetamido group has a substantial influence on the ISC rate of the corresponding substituted singlet phenylnitrene, but the 4-N-methylacetamido group does not. CASSCF analyses predict that both singlet nitrenes have open-shell electronic configurations and concluded that the dissimilarity in the photochemistry is probably due to differential geometrical distortions between the states. We briefly discuss the probable implications of this intriguing substitution effect on the photochemistry of phenyl azides and the chemistry of the related nitrenes.

Full text of DOI:10.1021/jp201821d

ARTICLE
pubs.acs.org/JPCA
Time-Resolved Resonance Raman and Computational Investigation of
the Influence of 4-Acetamido and 4-N-Methylacetamido Substituents
on the Chemistry of Phenylnitrene
†
‡
†
‡
†
†
Jiadan Xue, Shubham Vyas, Yong Du, Hoi Ling Luk, Yung Ping Chuang, Tracy Yuen Sze But,
†
‡
‡
,†
,‡
Patrick H. Toy, Jin Wang, Arthur H. Winter, David Lee Phillips,* Christopher M. Hadad,* and
Matthew S. Platz*
,‡
†
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong S.A.R., People's Republic of China
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States
‡
S Supporting Information
b
3
ABSTRACT: A time-resolved resonance Raman (TR ) and
computational investigation of the photochemistry of 4-acet-
amidophenyl azide and 4-N-methylacetamidophenyl azide in
acetonitrile is presented. Photolysis of 4-acetamidophenyl
azide appears to initially produce singlet 4-acetamidophenylni-
trene which undergoes fast intersystem crossing (ISC) to form
triplet 4-acetamidophenylnitrene. The latter species formally
0
produces 4,4 -bisacetamidoazobenzene. RI-CC2/TZVP and
TD-B3LYP/TZVP calculations predict the formation of the
0
singlet nitrene from the photogenerated S surface of the azide excited state. The triplet 4-acetamidophenylnitrene and 4,4 -
1
3
bisacetamidoazobenzene species are both clearly observed on the nanosecond to microsecond time-scale in TR experiments. In
3
contrast, only one species can be observed in analogous TR experiments after photolysis of 4-N-methylacetamidophenyl azide in
acetonitrile, and this species is tentatively assigned to the compound resulting from dimerization of a 1,2-didehydroazepine. The
different photochemical reaction outcomes for the photolysis of 4-acetamidophenyl azide and 4-N-methylacetamidophenyl azide
molecules indicate that the 4-acetamido group has a substantial influence on the ISC rate of the corresponding substituted singlet
phenylnitrene, but the 4-N-methylacetamido group does not. CASSCF analyses predict that both singlet nitrenes have open-shell
electronic configurations and concluded that the dissimilarity in the photochemistry is probably due to differential geometrical
distortions between the states. We briefly discuss the probable implications of this intriguing substitution effect on the
photochemistry of phenyl azides and the chemistry of the related nitrenes.
’
INTRODUCTION
is much greater than the rate of its ring expansion in room
52,61
temperature solution.
The photochemistry of aryl azides has been extensively studied
Certain singlet nitrenes can even react with water to form
and laser flash photolysis experiments have enabled direct
investigation of the reaction intermediates so produced, as well
as the stable photoproducts. This has led to a much improved
arylnitrenium ions such as the short-lived arylnitrenes with para-
26,28,31ꢀ37,40,47,48,61
substituted electron-donating groups.
The
1
ꢀ60
4-N-methylacetamido and 4-acetamido substituents on the phe-
nylnitrenium ions have been found to significantly stabilize these
two related arylnitrenium ions. These ions have lifetimes of
understanding of reaction mechanisms.
A singlet arylnitrene
species and a nitrogen molecule are typically produced upon
photolysis of aryl azides in room temperature solution, and this
singlet arylnitrene may then undergo very rapid ring expansion
reactions to form 1,2-didehydroazepines (cyclic ketenimines) or
decay by intersystem crossing (ISC) to form triplet arylnitrene
species. The ring expansion rates of singlet arylnitrenes increase
with increasing temperature, but the ISC rates are relatively
independent of temperature. Some electron-donating groups in
the para position of singlet arylnitrenes have been reported to
greatly accelerate the ISC rate. For example, singlet phenylni-
trene decays to the lower energy triplet state with a time
62,63
several milliseconds in room temperature solutions.
Density
functional theory (DFT) calculations of these two arylnitrenium
ions revealed that the N-methyl substitution on the acetamido
group has little effect on the structure and properties of arylni-
trenium ion and both species had similar iminocyclohexadienyl
5
5
63
character. However, there are relatively few studies of the
influence of the acetamido and N-methylacetamido groups on
Received: February 24, 2011
6
ꢀ1
constant of about (3.2 ( 0.3) ꢁ 10 s , but the ISC rate of
Revised:
May 4, 2011
9
ꢀ1
p-dimethylaminophenylnitrene is (8.3 ( 0.2) ꢁ 10 s , which
Published: June 07, 2011
r 2011 American Chemical Society
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The Journal of Physical Chemistry A
ARTICLE
Scheme 1
phenylnitrenes. We have recently reported time-resolved reso-
scattered Raman light was collected employing reflective optics
and a back scattering geometry. The collected Raman light was
imaged through a depolarizer mounted on the entrance of a
monochromator that dispersed the light onto a liquid nitrogen
cooled CCD detector that collected the Raman signal for 30ꢀ60
s before data transfer to an interfaced PC computer. About
10ꢀ20 of these readouts were summed to obtain the resonance
3
nance Raman (TR ) spectra for several arylnitrenes and their
6
0a,b,64
subsequent reactions.
In this paper, we present a nanose-
3
cond TR study of 4-acetamido- and 4-N-methylacetamidophe-
nylnitrenes (2a, 2b) (Scheme 1) in an organic solvent. The TR
3
spectra show that acetamido and N-methylacetamido substitu-
tions lead to very different photochemical outcomes for the two
substituted phenylnitrenes generated from photolysis of the
azide precursors. The singlet 4-acetamidophenylnitrene 2a was
observed to give a triplet nitrene 3a which then formed the
3
Raman spectrum. The TR spectra were obtained by subtracting
the pumpꢀprobe spectrum at negative 100ns from pumpꢀprobe
spectra acquired at positive time delays in order to remove the
solvent and precursor Raman bands. The known Raman
0
corresponding azo compound, 4,4 -bisacetamidoazobenzene
(
0
77
4a). The triplet 4-acetamidophenylnitrene (3a) and 4,4 -bisa-
wavenumbers of acetonitrile were used to calibrate the frequen-
3
ꢀ1
cetamidoazobenzene species (4a) were both clearly observed
cies of the TR spectra to an estimated accuracy of (5 cm . A
3
during the TR experiments, and characterized using resonance
Lorentzian function was used to integrate the relevant Raman
bands in the TR spectra in order to determine their areas and
3
Raman spectroscopy. In contrast, after photolysis of 4-N-methyl-
acetamidophenyl azide (1b), only one species was observed on
the nanosecond to microsecond time scale, and it was assigned to
to extract the decay and growth of the species observed in
the experiments. During the experiments, no noticeable deg-
radation was observed for the sample by UV/vis absorption
spectroscopy.
7
b formed from dimerization of ring-expansion intermediates,
1
,2-didehydroazepines (6b). These results strongly suggest that
the 4-acetamido substituent has a substantial influence on the
ISC rate of the substituted singlet phenylnitrene, but the 4-N-
methylacetamido group does not.
All of the density functional theory calculations presented
here, in particular, for Raman frequencies, were performed using
67
the Gaussian 98 program suite operated on the High Perfor-
mance Computing clusters installed at the University of Hong
Kong or the Ohio Supercomputer Center. Complete geometry
optimization and vibrational frequency calculations were analy-
tically performed using the (U)B3LYP method with the 6-31G*
basis set for the ground states with the lowest energy configura-
’
EXPERIMENTAL AND COMPUTATIONAL METHODS
Samples of 4-acetamidophenyl azide, 4-N-methylacetamido-
0
phenyl azide, and 4,4 -bisacetamidoazobenzene were synthesized
62
0
according to literature methods. Solutions of these two phenyl
azides were prepared with concentrations of 2 mM or/and 1 mM
azide in acetonitrile for the nanosecond TR (ns-TR ) experi-
ments. Spectroscopic grade acetonitrile was used in preparing the
sample solutions.
tions obtained for the triplet 4-acetamidophenylnitrene, 4,4 -
bisacetamidoazobenzene, and 4-N-methylacetamido substituted
3
3
dimer formed from the 1,2-didehydroazepines. A Lorentzian
ꢀ
1
function with a 20 cm bandwidth was employed with the
calculated Raman vibrational frequencies and relative intensities
to obtain the (U)B3LYP/6-31G* computed Raman spectra
reported in this paper.
3
The experimental apparatus and methods used for the ns-TR
experiments have been detailed elsewhere,
60,63ꢀ66
so only a brief
account will be given here. The fourth harmonic of a Nd:YAG
nanosecond pulsed laser system provided the 266 nm laser as
pump and 319.9 nm as probe (the third anti-Stokes hydrogen
Raman shifted laser line of the 532 nm second harmonic)
To investigate the photolysis of the azides and to characterize
the singlet excited states, we performed ground and excited state
calculations with second-order coupled cluster calculations with
6
8
the resolution-of-the-identity approximation (RI-CC2) and
time-dependent density functional theory, using Becke’s three
parameter exchange functional along with the LeeꢀYangꢀParr
3
3
wavelengths used in the ns-TR experiments. The ns-TR
experiments used two Nd:YAG lasers electronically synchro-
nized to each other by a pulse delay generator employed to
control the relative timing of the two lasers. A fast photodiode
whose output was displayed on a 500 MHz oscilloscope was used
to measure the relative timing of the pump and probe pulses with
the jitter between the pump and probe pulses determined to be
69
correlation functional (TD-B3LYP) method with the triple-ζ
70
valence polarized (TZVP) basis sets using the Turbomole-5.91
71
program. Ground-state geometries of both 4-acetamidophenyl
and 4-N-methylacetamidophenyl azide were optimized at the RI-
CC2/TZVP and TD-B3LYP/TZVP levels of theory. Calcula-
tions of vertical excitations followed by evaluation of the gradi-
ents for the FranckꢀCondon excited states were performed to
<
5 ns. The laser beams were loosely focused onto a flowing liquid
stream of sample using a near collinear geometry and the
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The Journal of Physical Chemistry A
ARTICLE
Table 1. Calculated Vertical Excitations for 4-Acetamidophenyl and 4-N-Methylacetamidophenyl Azide
RI-CC2/TZVP
TD-B3LYP/TZVP
excited state
wavelength (nm)
oscillator strength
orbital contributions
wavelength (nm)
oscillator strength
orbital contributions
4-Acetamidophenyl Azide (1a)
ꢀ
3
2
ꢀ4
1
2
292
266
1.1 ꢁ 10
4.8 ꢁ 10
46 f 48
46 f 49
334
280
2.0 ꢁ 10
46 f 48
46 f 47
46 f 49
ꢀ
ꢀ1
2.7 ꢁ 10
46 f 47
4
-N-Methylacetamidophenyl Azide (1b)
ꢀ
4
ꢀ4
1
2
289
262
9.3 ꢁ 10
50 f 52
50 f 53
330
276
4.0 ꢁ 10
50 f 51
50 f 52
ꢀ2
ꢀ1
1.8 ꢁ 10
2.6 ꢁ 10
50 f 53
Figure 1. Difference density plots for 4-acetamidophenyl azide (1a) at
the TD-B3LYP/TZVP, (a) S and (b) S , and at the RI-CC2/TZVP, (c)
S and (d) S , levels of theory. (The green contours depict the
1 2
accumulation of electron density in the excited state, and the red
1
2
Figure 2. Difference density plots for 4-N-methylacetamidophenyl
azide (1b) at the TD-B3LYP/TZVP, (a) S and (b) S , and at the RI-
CC2/TZVP, (c) S and (d) S , levels of theory. (The green contours
1
2
contours illustrate the loss of electron density from the S ground state.
1
2
0
depict the accumulation of electron density in the excited state, and the
The isocontour values are adjusted differently for appropriate visualiza-
red contours illustrate the loss of electron density from the S ground
tion: TD-B3LYP, S
1
(0.005, S
2
(0.002; RI-CC2, S
1
ꢀ0.02, þ0.005;
0
state. The isocontour values are adjusted differently for appropriate
S
2
ꢀ0.01, þ0.0005 au.)
visualization: TD-B3LYP, S
1
(0.005, S
2
(0.002; RI-CC2, S
1
ꢀ0.02,
þ0.005, S ꢀ0.01, þ0.0005 au.)
2
obtain the electron density changes for the different excited states.
Furthermore, to gain insight into the character of the nitrenes, we
performed CASSCF calculations for both open-shell and closed-
shell descriptions of the singlet phenylnitrenes with the Gaussian
we speculate that the azides are initially excited to the S surface
2
when photoexcited by 266 nm light. The comparison between
the calculated and experimental spectra is represented in Figure
1S in Supporting Information.
7
2
0
7
3 program and CASPT2 energy corrections with MOLCAS
73
.4 program. All of these excited-state calculations and CASSCF
To gain insight into the character of these singlet excited states,
we computed the electronic difference densities between the ground
and excited state wave functions. These difference density plots are
useful in the cases of multiconfigurational vertical excitations, for
which the inspection of individual orbitals may not be as useful. We
have recently utilized this strategy in many different photochemical
analyses were performed at the Ohio Supercomputer Center.
’
RESULTS AND DISCUSSION
A. Excited State Calculations on Azide Singlet Surfaces. All
74
of these calculations were performed with C symmetry. The
problems. Subtraction of the ground-state electron density from
the FranckꢀCondon, excited-state electron density yields a differ-
ence density plot which reveals the movement of electron density
upon excitation. A green color shows the accumulation of electron
density in the FranckꢀCondon excited state, while a red color
indicates the loss of electron density from the ground state.
Difference density plots for both the S and S excited states are
1
4
-N-methylacetamidophenyl azide has a 50° torsion angle be-
tween the substituent and the phenyl ring, whereas the 4-acet-
amidophenyl azide is almost planar in the ground state (See
Supporting Information.) Calculated ground-state absorption
features for both of the azides are tabulated in Table 1. For both
azides, the transition to the lowest singlet excited state is
calculated to be very weak as the oscillator strength is less than
1
2
shown in Figure 1 for 4-acetamidophenyl azide (1a) and in Figure 2,
for 4-N-methylacetamidophenyl azide (1b) at both RI-CC2/TZVP
and TD-B3LYP/TZVP level of theories.
ꢀ
3
1
0
. On the other hand, the second excited state is calculated to
have about 50ꢀ1000 times higher oscillator strength; therefore,
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3
Figure 4. 319.9 nm probe TR spectra obtained after 266 nm photolysis
of 2 mM 4-N-methylacetamidophenyl azide in acetonitrile. The time
delays between the pump (266 nm) and probe (319.9 nm) laser beams
are shown to the right of each spectrum, and the Raman shifts of selected
bands are presented at the top of the 500 μs spectrum. See text for
more details.
Raman shifts of selected bands are presented at the top of the
10 μs spectrum. Inspection of Figure 3 shows there are three
species observed on the 0 ns to 10 μs time scale. The first species
has characteristic Raman bands at 1224, 1419, 1528, and
3
Figure 3. Overview of 319.9 nm probe TR spectra obtained after
66 nm photolysis of 2 mM 4-acetamidophenyl azide in acetonitrile
under air. The time delays between the pump (266 nm) and probe
319.9 nm) laser beams are shown to the right of each spectrum, and the
2
ꢀ
1
(
1
693 cm and appears to decay completely within 50 ns to
Raman shifts of selected bands are presented at the top of the 10 μs
form a second species. The second species has characteristic
spectrum. See text for more details.
ꢀ1
Raman bands at 1258 and 1557 cm and decays to produce a
third species with characteristic Raman bands at 1401 and
ꢀ
1
3
The difference density plots are quite similar for azides 1a and
b, and both of the theoretical methods are in good agreement
1444 cm . Similar TR experiments were also performed under
oxygen purging conditions, and the resulting transient spectra are
shown in Figure 2S (Supporting Information). Close examina-
1
with each other. The plots corresponding to the S surface
1
3
suggest that the lowest singlet excited state is localized mainly
on the azide unit. The electron density is depleted from the π-
orbital of the azide and is transferred to the in-plane π* orbital of
the azide. The difference density changes indicate that the
ArNꢀN bond will lengthen on the S surface, followed by
tion of the TR spectra shows that the presence of oxygen
reduces the yield of the third species, as under identical experi-
mental conditions, the yield of the third species obtained with
oxygen purging is only 80% of that obtained under air. We
propose that the third species in the TR spectra in Figure 3 be
assigned to an azo compound formed from the triplet nitrene and
that the second species is the triplet nitrene.
3
2
1
formation of the singlet nitrene and molecular nitrogen. Similar
difference density plots were reported earlier by us for different
7
4
3
aryl azides and are characteristic of the azide dissociative states.
Figure 4 displays TR spectra acquired at various time delays
On the other hand, for the S state, the electron density move-
ment is localized on the phenyl ring, which indicates that this
excitation is a (π,π*) excited state on the aromatic unit. This is
after 266 nm photolysis of 2 mM 4-N-methylacetamidophenyl
azide in acetonitrile. The time delays between the pump
(266 nm) and probe (319.9 nm) laser pulses are shown to the
right of each spectrum, and the Raman shifts of selected bands are
presented at the top of the spectrum recorded 500 μs after the
laser pulse. Inspection of Figure 4 shows that only one species
can be observed between 0 ns and 500 μs. This species forms on
the time scale of about several tens of microseconds, which is
much slower than the formation of the third species observed in
2
also supported by the observation that the S state has a much
2
higher oscillator strength compared to the S state (Table 1).
1
Overall, these difference density plots suggest that irradiation of
4
2
-acetamidophenyl and 4-N-methylacetamidophenyl azides with
66 nm light should excite each system to the S state which is
2
phenyl (π,π*) in character, with subsequent relaxation to the S1
surface which is the azide dissociative state, thereby forming the
substituted phenylnitrene.
3
the TR spectra of Figure 3. Moreover, the species observed in
ꢀ
1
Figure 4 has the strongest Raman band at 1585 cm and the
3
ꢀ1
B. Time-Resolved Resonance Raman (TR ) Spectra Ac-
quired after 266 nm Photolysis of 4-Acetamidophenyl Azide
second most intense Raman band at 1636 cm . This is
significantly different from the Raman spectra in Figure 3 that
ꢀ
1
and 4-N-Methylacetamidophenyl Azide in Acetonitrile. Fig-
have their most intense Raman bands in the 1400ꢀ1500 cm
3
ure 3 displays TR spectra acquired at various time delays after
region. These significant differences in the Raman spectra and
the formation rates between the species observed in Figures 3
and 4 indicate that the species in Figure 4 obtained from
photolysis of 4-N-methylacetamidophenyl azide in acetonitrile
photolysis of 2 mM 4-acetamidophenyl azide in acetonitrile. The
time delays between the pump (266 nm) and probe (319.9 nm)
laser pulses are shown to the right of each spectrum, and the
7
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3
Figure 5. Comparison of (A) the experimental 50 ns and (C) 10 μs TR spectra from Figure 3 to the UB3LYP/6-31G* calculated normal Raman
0
spectrum for (B) triplet 4-acetamidophenylnitrene and (D) 4,4 -bisacetamidoazobenzene and (E) 319.9 nm pumped resonance Raman spectrum of
0
synthesized authentic 4,4 -bisacetamidoazobenzene. The calculated relative Raman intensities were convoluted with a Lorentzian function. See text and
Tables 1S and 2S (Supporting Information) for more details.
has a very different structure from the species observed in
Figure 3 obtained from photolysis of 4-acetamidophenyl azide
and therefore cannot be an azo compound. Consequently, singlet
convoluted with a Lorentzian function. Inspection of Figure 5
shows that the experimental 50 ns and 10 μs TR spectra
(spectra A and C) agree well with the calculated normal Raman
3
4
-N-methylacetamidophenylnitrene does not relax to its lower
spectra for the triplet 4-acetamidophenylnitrene species and the
0
energy triplet state but mostly undergoes the ring expansion
rearrangement pathway instead.
C. Assignments of the Species Observed in the TR Spectra.
4,4 -bisacetamidoazobene product (spectra B and D), respec-
tively, with some moderate differences in the relative intensities
that can be easily accounted for by the fact that the experimental
spectra are resonantly enhanced while the calculated spectra are
for normal (nonresonance) Raman spectra. Tables 1S and 2S of
the Supporting Information compare the experimental and the
calculated Raman band vibrational frequencies for the species
shown in Figure 5 and the differences between the experimental
3
Comparison of experimental vibrational frequencies to those pre-
dicted from density functional theory (DFT) or ab initio calcula-
tions for probable intermediates has been successfully employed
to identify and assign time-resolved infrared (TR-IR) and time-
3
resolved resonance Raman (TR ) spectra to arylnitrenium
48,56,57b,60,63ꢀ65
ꢀ1
ions, arylnitrenes, and arylnitrene photoproducts.
and calculated frequencies are, on average, only 11.8 cm for
ꢀ1
0
We shall use similar methodology to help assign the four species
the triplet 4-acetamidophenylnitrene and 9.1 cm for the 4,4 -
bisacetamidoazobene. All of these results indicate that the
second and the third species observed in Figure 3 are the triplet
3
observed in the TR spectra of Figures 3 and 4. DFT calculations
were performed to test these possibilities. (U)B3LYP/6-31G*
calculations were used to predict the total energy, the optimized
geometry, and vibrational frequencies for the triplet 4-acetamido-
0
4-acetamidophenylnitrene and the azo compound, 4,4 -bisace-
tamidoazobene, respectively. In order to unequivocally confirm
0
0
phenylnitrene and the azo compound, 4,4 -bisacetamidoazoben-
the assignment of the third species, the authentic 4,4 -bisaceta-
3
zene. Figure 5 compares the experimental 50 ns and 10 μs TR
midoazobenzene compound was synthesized and we compared
its 319.9 nm pumped resonance Raman spectrum (spectrum E)
spectra, post laser pulse, from Figure 3 to the (U)B3LYP/6-31G*
calculated normal Raman spectra whose relative intensities were
3
to the experimental 10 μs postpulse TR spectra as shown in
7
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Resonance Raman spectra have been obtained for a number of
azo compounds, and these resonance Raman spectra typically
have most of their intensity in Raman bands associated with the
NdN chromophore and corresponding vibrational modes in the
ꢀ
1
ν = 1400ꢀ1500 cm region with moderate intensity in bands
ꢀ
1
64a,65e,75
above 1500 cm associated with aromatic CdC bonds,
which are in agreement with the 10 μs TR spectrum in Figure 3.
3
But the species observed in Figure 4 has most of its Raman
ꢀ
1
intensity at about 1600 cm associated with the CdC stretch
vibrational modes, and only moderate intensity in the
ꢀ
1
1
400ꢀ1500 cm region. Hypothetically, the Raman spectrum
of the 4-N-methylacetamido substituted azo compound should
be similar to that of the 4-acetamido substituted azo compound
because of the great similarity in their structure, but this is not the
experimental result. Thus, it is unlikely that the species observed
in Figure 4 is that of an azo compound with an NdN chromo-
phore. Instead, the species may be eventually produced from the
ring expansion rearrangement reaction. Another class of species
with dimeric structures was found upon photolysis of phenyl
azide, 3-hydroxyphenyl azide, and 3-methoxyphenyl azide in
room temperature solutions. These dimers were believed to
form by dimerization of 1,2-didehydroazepine and were char-
acterized using Raman spectroscopy. The Raman spectra in
Figure 4 are very similar to those of dimers formed from 1,2-
didehydroazepines in terms of both their Raman band patterns
and formation rate. Consequently, this kind of dimer with 4-N-
methylacetamido substitution was considered with DFT calcula-
tions. The comparison of the experimental Raman spectrum of
Figure 4 to the calculated normal Raman spectrum of a dimer
formed from 1,2-didehydroazepines is displayed in Figure 7 and
comparisons of some specific vibrational frequencies are listed in
Table 3S (Supporting Information). Examination of Figure 7 and
Table 3S (Supporting Information) shows that there is reason-
able agreement between the computational and experiment
results considering the differences associated with normal and
resonance Raman in regard to their relative intensities. These
results indicate that the species observed in Figure 4 obtained
after photolysis of 4-N-methylacetamidophenyl azide in acetoni-
trile is the dimer formed from dimerization of 4-N-methylaceta-
Figure 6. Raman spectrum of singlet 4-acetamidophenylnitrene (red)
obtained by subtraction of a scaled Raman spectrum of triplet nitrene
3
(
blue) from the TR spectrum (black) obtained at 0 ns, in Figure 3. Star
symbols mark solvent-subtraction artifacts and stray-light and ambient-
light artifacts.
3
mido substituted 1,2-didehydroazepines. The didehydroazepine
Figure 7. Comparison of (A) the 100 μs experimental TR spectrum in
3
6
b was not observed during TR experiments even though it is
Figure 4 to (B) the B3LYP/6-31G* calculated normal Raman spectrum
for the dimer formed from 1,2-didehydroazepines. The calculated
relative Raman intensities were convoluted with a Lorentzian function.
See text for more details.
well-known that a didehydroazepine has a strong characteristic
ꢀ
1
IR band at ∼1880 cm . However the DFT calculation predicts
that 6b has an IR intensity of 164 Km/mol for the CdCdN
4
vibration mode while the Raman activity is only 12 Å /amu. This
is due to different selection rules for Raman scattering and
Figure 5. These two spectra (spectra C and E) are essentially
identical within experimental uncertainty, and this unambigu-
ously confirms the assignment of the third species to the azo
type compound.
infrared absorbance. It also accounts for the failure to observe
3
the didehydroazepine in TR experiments.
D. Discussion of the 4-Acetamido and 4-N-Methylaceta-
mido Substitution Influence on ISC Rate of Singlet Phenyl-
nitrenes. Parent phenylnitrene has a relatively slow ISC rate with
The first species in Figure 3 was observed at 0 and 10 ns time delays
and decayed completely within 50 ns to form the triplet 4-acetami-
dophenylnitrene. The singlet 4-acetamidophenylnitrene was found to
have a sufficiently long lifetime to be trapped by water to produce the
6
ꢀ1
a time constant of (3.2 ( 0.3) ꢁ 10 s because singlet
phenylnitrene is an open-shell singlet species for which the
effective intersystem crossing mechanism, spinꢀorbit coupling,
63
69
corresponding arylnitrenium ion Furthermore, a singlet aryl azide
excited state typically has a lifetime of only 150ꢀ300 fs; thus this
experimentally observed species is not a singlet azide excited state, but
the singlet nitrene. The Raman spectrum of the singlet 4-acetamido-
phenylnitrene, alone, is presented in Figure 6. It was obtained by
subtraction of an appropriately scaled triplet 4-acetamidophenylni-
is forbidden and thus ineffective in promoting ISC. Electron-
donating groups have been observed to greatly influence ISC
rates. p-Methoxy and dimethylamino substituents were observed
to accelerate the ISC rates with observed time constants of
8
ꢀ1
54
9 ꢀ1
about >5 ꢁ 10 s for p-methoxy and (8.3 ( 0.2) ꢁ 10 s for
69
p-dimethylamino phenylnitrene. Triplet 4-biphenylylnitrene
3
3
trene spectrum (50 ns TR spectrum) from the TR spectrum
obtained at 0 ns in Figure 3.
and its corresponding azo compound dimer were also observed
3
64a
by TR spectroscopy. All of these singlet phenylnitrenes have
7
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The Journal of Physical Chemistry A
ARTICLE
preferred to be open shell in their lowest singlet state and the
corresponding closed-shell analogues are 19.3 and 20.4 kcal/mol
above the open-shell description, respectively.
The 4-acetamido and 4-N-methylacetamido groups are both
electron-donating groups; however, the two substituted singlet
phenylnitrenes have quite different geometries. Figure 8 shows
optimized geometries of singlet 4-acetamido and 4-N-methyla-
cetamido phenylnitrenes in their open-shell electronic config-
urations at the CASSCF(12,11) level of theory. (See Supporting
Information for optimized geometries at other active spaces.)
The CꢀN bond lengths at the nitrene end of the molecules are
ca. 1.276 Å, which suggest similar communication between the
nitrene orbitals and the aromatic π-system for both of the
molecules. Conversely, the geometry at the substituent end is
quite different in these two nitrene species. The CꢀN-
(
X)COCH bond is lengthened in the case of 4-N-methylaceta-
3
mido phenylnitrene. Furthermore, the substituent, 4-N-
methylacetamido group is also significantly out-of-plane (ca.
4
5ꢀ55°) with respect to the aromatic system while the 4-acet-
amido group is almost planar (ca. 0ꢀ1°). These factors might
play an important role in understanding the experimentally
observed different photochemical behavior of these singlet
phenylnitrenes.
Figure 8. Optimized geometries of 4-acetamido (top) and 4-N-methy-
lacetamido phenylnitrene (bottom) in open-shell electronic configura-
tion at the CASSCF(12,11)/6-31G* level of theory.
3
ionic character, which can increase the spin-obit coupling and the
rate constant of intersystem crossing. The ring-expansion path-
ways that form 1,2-didehydroazepines are sensitive to substitu-
tion, in the case of the strongest para donor, MeNHꢀ, this
In our TR experiments, the triplet 4-acetamidophenylnitrene
and its corresponding azo compound were observed, consistent
with the expectation that strongly electron-donating groups
accelerate the ISC rate. In contrast, it is neither the triplet nitrene
nor an azo compound, but the ring expansion product instead,
the dimer formed from 1,2-didehydroazepines that was observed
ꢀ
1 76
barrier increased to about 13 kcal mol
,
as compared to ∼6
3
ꢀ1
kcal mol for singlet phenylnitrene.
3
3
In order to characterize the electronic structure of the nitrenes
generated, we performed CASSCF calculations to assess the
open-shell or closed-shell nature of these singlet intermediates.
Ideally, all of the π-orbitals and π-electrons must be used to
perform accurate CASSCF calculations, i.e., 14 electrons in 12
orbitals in these cases. However, attempts to optimize the closed-
shell configurations with the ideal active space for 4-acetamido-
phenylnitrene and 4-N-methylacetamidophenylnitrene yielded
an open-shell electronic configuration. Furthermore, we tried to
optimize both open-shell and closed-shell configurations with
reduced active spaces. In the case of 4-acetamidophenylnitrene,
we were able to optimize both configurations at the CASSCF-
in the TR spectra after photolysis of 4-N-methylacetamidophe-
nyl azide in acetonitrile. Evidently, the 4-acetamido substitution
has a substantial influence on the ISC transition rate of singlet
phenylnitrene but the 4-N-methylacetamido group does not. In
order to provide an explanation to this photochemical disparity,
we evaluated the singletꢀtriplet energy gap for these substituted
nitrenes. Thus, we optimized the triplet nitrenes at the CASSCF-
(12,11)/6-31G* level of theory, and the optimized triplet states
were found to have their unpaired electrons localized on the
nitrogen of the substituted phenylnitrene. The difference be-
tween the open-shell singlet and the triplet nitrene is estimated to
ꢀ
1
be about 16.9 and 16.8 kcal mol for 4-acetamidophenylni-
3
(
8,7), CASSCF(6,5), and CASSCF(4,3) levels with the 6-31G*
trene and 4-N-methylacetamidophenylnitrene, respectively, with
the triplet as the energetically favored state in each case. This
suggests that the energetic separation between the singlet and
triplet states is not a significant factor in the divergence in the
photochemistry. However, the optimized geometries of these
singlet and triplet nitrenes provided an insight as to why 4-N-
methylacetamidophenylnitrene does not follow the expected
mechanism. The 4-acetamidophenylnitrene remained to be
planar in singlet as well as triplet manifolds allowing interaction
of the lone pair of electrons on nitrogen with the aromatic ring
and the nitrene nitrogen. It is well-known that p-methoxy and
dimethylamino substituents accelerate ISC relative to parent
basis set. These calculations revealed the energy difference
between the two configurations of 13.4, 24.2, and 5.3 kcal mol
ꢀ
1
3
making the open-shell electronic configuration the preferred
description of the singlet state of both nitrenes. The inconsis-
tency in the energy gap is probably due to the lack of certain π-
orbitals in the active space. However, in all of these cases, the
open-shell configuration is consistently the preferred description.
Furthermore, attempts to optimize the closed-shell configuration
for the 4-N-methylacetamidophenylnitrene with reduced active
spaces also converged on the open-shell solution. This CASSCF
analysis predicts that both nitrenes are open shell in their lowest
energy singlet state; however, we were not able to obtain the
exact energy difference between the two electronic structures of
the singlet nitrenes. To calculate the energy gaps between the
closed-shell and open-shell configurations of the two singlet
nitrenes, we performed CASSCF(12,11)/6-31G* calculations. A
second-order perturbation level correction (CASPT2) was made
to the CASSCF(12,11)/6-31G* wave function using the same
active space and basis set. These calculations revealed that both
singlet 4-acetamido and 4-N-methylacetamidophenylnitrenes
54,69
phenylnitrene.
ionic resonance structures that are essential to spinꢀorbit
This has been attributed to the stabilization of
78
coupling.
However, both the singlet and triplet 4-N-methylacetamido
phenylnitrene have the N-methylacetamido substituent about
50° out-of-plane relative to the plane of the aromatic unit, thereby
disrupting overlap between orbitals of the electron-donating
acetamide nitrogen and the aromatic π system. Consequently,
there is a diminished contribution of the electron-donating
7
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dx.doi.org/10.1021/jp201821d |J. Phys. Chem. A 2011, 115, 7521–7530

The Journal of Physical Chemistry A
ARTICLE
qeffect by the N-methylacetamido substituent, less spinꢀorbit
coupling and reduced acceleration of ISC. Thus singlet nitrene
isomerization to form 1,2-didehydroazepines is now faster than
ISC to the triplet.
’ AUTHOR INFORMATION
Corresponding Author
*
E-mail: phillips@hkucc.hku.hk; hadad.1@osu.edu; platz.1@
osu.edu.
’
CONCLUSIONS
Time-resolved resonance Raman investigations of the photo-
chemistry of 4-acetamidophenyl azide and 4-N-methylacetami-
dophenyl azide are reported. Computational study of the excited
’
ACKNOWLEDGMENT
This work was supported by grants from the Research Grants
Council (RGC) of Hong Kong (HKU-7039/07P) to D.L.P.,
while C.M.H. and M.S.P. acknowledge financial support of this
work by the US National Science Foundation (CHE-0743258).
Generous computational resourced from the Ohio Supercom-
puter Center is also gratefully acknowledged. S.V. acknowledges
the OSU University Presidential Fellowship.
states suggests excitation to the S surface, followed by fast
2
internal conversion and formation of arylnitrene on the S1
surface. Photolysis of 4-acetamidophenyl azide in acetonitrile at
3
2
66 nm showed the signatures of three species in TR spectra
within 10 μs after excitation. These three species were assigned to
0
be the singlet and triplet 4-acetamidophenylnitrenes and 4,4 -
bisacetamidoazobenzene, respectively. In contrast, only one
3
species was observed in the TR spectra after photolysis of
’
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