Spectroscopic studies on methyl torsional behavior in 1-methyl-2(1H)pyridone, 1-methyl-2(1H)-pyridinimine, and 3-methyl-2(1H)-pyridone. I.
Excited state
Rajeev K. Sinha, B. Pradhan, S. Wategaonkar, Bhanu P. Singh, and T. Kundu
Citation: J. Chem. Phys. 126, 114312 (2007); doi: 10.1063/1.2566574
View online: http://dx.doi.org/10.1063/1.2566574
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THE JOURNAL OF CHEMICAL PHYSICS 126, 114312 共2007兲
Spectroscopic studies on methyl torsional behavior
in 1-methyl-2„1H…-pyridone, 1-methyl-2„1H…-pyridinimine,
and 3-methyl-2„1H…-pyridone. I. Excited state
Rajeev K. Sinha, B. Pradhan, S. Wategaonkar,a兲 Bhanu P. Singh, and T. Kundub兲
Department of Physics, Indian Institute of Technology Bombay, Mumbai-400076, India
共Received 16 October 2006; accepted 17 January 2007; published online 21 March 2007兲
The laser induced fluorescence excitation and dispersed fluorescence spectra of three nitrogen
heterocyclic molecules 1-methyl-2共1H兲pyridone 共1MPY兲, 1-methyl-2共1H兲pyridinimine 共1MPI兲,
and 3-methyl-2共1H兲pyridone 共3MPY兲 have been studied under supersonic jet cooled condition. The
methyl torsional and some low frequency vibrational transitions in the fluorescence excitation
spectrum were assigned for 1MPY. These new assignments modify the potential parameters to the
methyl torsion reported earlier. Some striking similarities exist between the torsional and vibrational
transitions in the fluorescence excitation spectra of 1MPY and 1MPI. Apart from pure torsional
transitions, a progression of vibration-torsion combination bands was observed for both these
molecules. The excitation spectrum of 3MPY resembles the spectrum of its parent molecule,
2-pyridone. The barrier height of the methyl torsion in the excited state of 3MPY is highest amongst
all these molecules, whereas the barrier in 1MPI is higher than that of 1MPY. To get an insight into
the methyl torsional barrier for these molecules, results of the ab initio calculations were compared
with the experimental results. It was found that the conformation of the methyl group undergoes a
60° rotation in the excited state in all these molecules with respect to their ground state
conformation. This phase shift of the excited state potential is attributed to the ␲*-␴*
hyperconjugation between the out-of-plane hydrogen of the methyl group and the molecular frame.
It has been inferred that the change in lowest unoccupied molecular orbital energy plays the
dominant role in the excited state barrier formation. © 2007 American Institute of Physics.
关DOI: 10.1063/1.2566574兴
I. INTRODUCTION
Methyl group in a methylated molecule has the ability to
undergo large amplitude quantized motion under the influence of the periodic potential arising from the local electronic structure. This hindered motion of the methyl group
plays an important role in the intramolecular and the photochemical dynamics of organic molecules.1 The minimum energy conformation and the barrier to the internal rotation of
the methyl group strongly depend on various electronic interactions present in the molecule.2 In many cases, it was
observed that the nature of the torsional potential changed
drastically upon electronic excitation.3
The azines or azabenzenes constitute a valuable and important series of compounds for both practical and theoretical interests. Since the process of internal rotation has its
importance in many chemical and biological processes, studies of the structural and electronic characteristics of azaheterocycles assume great significance. The methyl substitution
has a dramatic effect on the photophysics of the molecules,
and thus, spectroscopic studies provide an excellent opportunity to explore the effects that the position of the methyl
substitution and the electronic structure have on the characa兲
Department of Chemical Sciences, Tata Institute of Fundamental Research,
Colaba, Mumbai-400005, India.
b兲
Electronic mail: tkundu@phy.iitb.ac.in
0021-9606/2007/126共11兲/114312/10/$23.00
teristics of the methyl torsion. A few methyl substituted azabenzenes, both monocyclic as well as polycyclic azines, have
been investigated in this regard. The low frequency vibration
of the methyl group in 2-methyl pyridine and 4-methyl pyridine was studied by Rudolph and co-workers4 using microwave spectroscopy. The methyl torsional barrier in 4-methyl
pyridine is in correspondence to that of toluene with a nearly
free rotor character 共V6⬙ = 4.75 cm−1兲 in the ground electronic
state, whereas in 2-methyl pyridine the threefold term 共V3⬙
= 90 cm−1, V6⬙ = −4.13 cm−1兲 appears due to the symmetry reduction of this molecule. Mass resolved excitation spectroscopic studies of 2-methyl pyridine under jet cooled condition by Seeman et al.5 did not show any Franck-Condon 共FC兲
activity of the large amplitude motion in the excited state.
The ab initio calculated potential barrier 共V3⬙ = 90 cm−1兲 by
Kawamura et al.6 was in agreement for the ground state, but
the estimated excited state potential using configuration interaction singles 共CIS兲 calculation showed a large increment
in the barrier energy 共V3⬘ = 768 cm−1兲 for this molecule.
Caminati et al.7 had studied monocyclic highly symmetric
diazines, namely, 5-methyl pyrimidine and 2-methyl pyrimidine using microwave spectroscopy. Both the molecules
show a toluenelike character with V6⬙ = 1.56 cm−1 for
2-methyl pyrimidine and V6⬙ = 4.1 cm−1 for 5-methyl pyrimidine in the ground states. Later, Bandy et al.8,9 investigated
these two molecules using laser induced fluorescence 共LIF兲
excitation and single vibrational level fluorescence spectros-
126, 114312-1
© 2007 American Institute of Physics
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114312-2
Sinha et al.
FIG. 1. Molecular structures of 共a兲 1-methyl-2共1H兲-pyridinimine, 共b兲
1-methyl-2共1H兲-pyridone, and 共c兲 3-methyl-2共1H兲-pyridone.
copy, but failed to observe any low frequency torsional band
in the excited state. However, on the basis of the high resolution spectrum of the origin band, a very little change in the
excited state potential barrier was predicted. On the other
hand, in 4-methyl pyrimidine9 it was observed that the methyl torsional barrier in the ground state 共V3⬙ = 95 cm−1兲 increased drastically in the excited state 共V3⬘ = 740 cm−1兲, but
no conformational change was observed. The ab initio calculated barrier heights V3⬙ = 87 cm−1 for the ground state and
V3⬘ = 746 cm−1 for the excited state are in good agreement
with the experiment.6 Another well studied monocyclic diazine, 2-methylpyrazine, was investigated by Tomer et al.10
using LIF spectroscopy under jet cooled conditions. In this
case also the torsional potential V3⬙ = 105 cm−1 increases to
V3⬘ = 465 cm−1 upon electronic excitation. The methyl group
substituted at different positions in indole, a bicyclic
monoazine, was investigated by Bickel et al.11 using resonantly enhanced multiphoton ionization spectroscopy. From
the positional dependence of the torsional potential studies, it
was concluded that the changes in the torsional potential
upon electronic excitation could be interpreted as arising
from the changes in the electron density adjacent to the methyl rotor. From a systematic theoretical study on several
methyl azabenzenes, Kawamura et al.6 found a quasilinear
correlation between the rotational barrier of the methyl group
in the excited state and the splitting between the lowest and
the next lowest unoccupied molecular orbitals. Experimental
difficulty for these azabenzenes lies in the detection of fluorescence due to its low quantum yield, a consequence of fast
nonradiative 共internal conversion and intersystem crossing兲
processes arising from the existence of close lying ␲-␲* and
n-␲* states.
In this article, we present the spectroscopic studies of
three azines, namely, 1-methyl-2共1H兲-pyridinimine 共1MPI兲,
1-methyl-2共1H兲-pyridone
共1MPY兲,
and
3-methyl2共1H兲-pyridone 共3MPY兲. Molecular structures of these molecules are shown in Fig. 1. 1-methyl-2共1H兲-pyridinimine
共1MPI兲 is a stabilized imino form of pyridinimine obtained
by substituting a methyl group on a ring nitrogen atom. In
both 1MPI and 1MPY, the methyl group is connected to two
vicinal single bonds, whereas in 3MPY the methyl group is
attached to the ring carbon atom and has one single and one
double bond vicinal to it. 1MPI has been studied by UVvisible and infrared spectroscopies. Traore et al.12 carried out
ultraviolet absorption and fluorescence studies of 1MPI in a
solution phase. Inuzuka et al.13 studied the IR spectrum of
J. Chem. Phys. 126, 114312 共2007兲
1-MPI in solution and carried out the normal coordinate
analysis for the assignments of the vibrational structures in
the ground state. More recently, the laser induced fluorescence spectra of supersonic jet cooled14 1-MPI and 1-MPI-d
were studied, and the observed low frequency modes were
assigned qualitatively to the torsional motion involving methyl group. Further, Raman spectroscopy of 1MPI 共Ref. 15兲
facilitated ease of assignments of the ground state normal
modes. However, no effort has been made towards the study
of methyl internal potential barrier in 1MPI. Features of the
IR spectrum of 1-MPI were found to exhibit a similarity with
that of 1MPY.16 Recently, Pradhan et al.17 studied 1-methyl2共1H兲-pyridone 共1MPY兲 in supersonic jet cooled molecular
beam by using fluorescence excitation 共FE兲 and dispersed
fluorescence 共DF兲 spectroscopies along with quantum chemical calculations and successfully assigned the observed
bands in the ground electronic state and a few bands in the
excitation spectrum based on observed DF spectra upon excitation of these bands.
Here, we have investigated the DF spectra of few more
bands in the excitation spectrum of 1MPY. On the basis of
this experimental result, we have carried out the torsional
and vibrational assignments for the transitions observed in
the FE spectrum. We also present the FE spectra of supersonic jet cooled 1MPI. In view of the similarities in the spectral features of 1MPY and 1MPI, we have used the excitation
spectrum of 1MPY to explain the observed bands in the excitation spectrum of 1MPI. The methyl torsional potential
shape and the barrier for 1MPY and 1MPI in both the ground
and the first excited states have been extracted and compared
with the aid of quantum chemical calculation. Excitation
spectrum of 3MPY is also presented here to understand the
differences in the excitation spectrum with respect to 1MPY.
It has also been examined in the light of excitation spectrum
of 2-pyridone.
II. EXPERIMENTAL SECTION
2-pyridinimine, 1MPY, and 3MPY were purchased from
Aldrich Chemical Company. 1MPI was prepared from
2-pyridinimine by using the method described by Young
et al.,14 whereas 1MPY and 3MPY were used without further
purification. The fluorescence excitation experiment was performed using a free jet expansion system under vacuum
⬃3 ⫻ 10−5 to 1 ⫻ 10−4 torr. The sample vapor seeded in He
at 1 – 2 atm was expanded into the vacuum chamber through
a pulsed nozzle valve 共0.4 mm diameter兲. A nanosecond
Nd:YAG 共yttrium aluminum garnet兲 laser pumped dye laser
was used for the excitation of the molecules. Excitation
wavelengths in the region of interest were obtained by an
appropriate frequency conversion of the dye laser radiation
共second harmonic and its mixing with the fundamental of the
Nd:YAG laser兲. Integrated fluorescence of the given molecule was detected by photomultiplier tube in combination
with a filter. The signal was digitized and averaged using a
digital storage oscilloscope. The dispersed fluorescence was
recorded using a monochromator and a cooled charge
coupled device detector. The typical resolutions of the dispersed fluorescence spectra were 6 – 7 cm−1.
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114312-3
Methyl torsional behavior
FIG. 2. Fluorescence excitation spectra of 1MPY. 共a兲 Experimental and 共b兲
simulated 共see text兲. The O00 band is at 29 822.3 cm−1.
III. RESULTS AND DISCUSSION
A. FE spectra and assignments
1. 1-methyl-2„1H…pyridone „1MPY…
The molecule 1MPY has 39 normal modes accompanying 26 in-plane 共a1兲 and 13 out-of-plane 共a2兲 normal modes
under the G6 molecular symmetry group.18,19 The fluorescence excitation spectrum of 1MPY is shown in Fig. 2共a兲. In
our earlier work,17 the observed band at 29 822.3 cm−1 was
assigned as S0 → S1 共O00兲 of the ␲-␲* transition. The other
bands at 86 and at 120 cm−1 relative to this O00 band were
assigned as the methyl torsional transitions to 2e and 3a1
levels, respectively, from the ground zero point energy state,
and the band at 172 cm−1 was assigned to the 3410 ring deformation vibrational mode in the excited state. These assignments were based on the observed transitions from these
bands allowed to the ground state torsional energy levels 2e
at 99 cm−1, 3a1 at 162 cm−1, 4e at 175 cm−1, 5e at 223 cm−1,
and 6a1 at 271 cm−1 in the dispersed fluorescence spectra.
The potential of the methyl torsion in the excited state was
obtained with V3⬘ = 164 cm−1, V6⬘ = 40 cm−1, and F = 5.2 cm−1
by fitting these 2e 共86 cm−1兲 and 3a1 共120 cm−1兲 levels.
However, the predicted transitions to 4e 共142.7 cm−1兲 and 5e
共185 cm−1兲 levels were not observed in the FE spectrum.
Although the calculated Franck-Condon 共FC兲 intensity distributions for the transitions from 0a1, 1e, and 2e levels of
the excited state to the ground state torsional levels were in
good agreement with the observed ones, they failed to predict that from the 3a1 band. These discrepancies prompted us
to investigate other observed bands in the FE spectrum further. To understand the nature of these bands, we examined
the dispersed fluorescence spectra upon excitation of bands
located at 203.0, 285.0, 313.0, 351.0, and 359.0 cm−1 with
respect to O00, as shown in Fig. 3. Table I presents the assignments of spectral features observed in these DF spectra on
the basis of frequency calculations as well as the FC intensity calculations for the torsional levels of this molecule. The
DF spectrum obtained by exciting the O00 + 203 cm−1 band
J. Chem. Phys. 126, 114312 共2007兲
FIG. 3. Dispersed fluorescence spectra of 1MPY upon excitation of 0
+ 203, 0 + 285, 0 + 313, 0 + 351, and 0 + 359 cm−1 bands 共see Fig. 2兲. The
intensities at the excitation energies include scattering also.
shows a progression of the torsional bands of the e symmetry
along with vibrational bands and their overtones. The intensity of transition to the 4e torsional level is largest, and its
intensity distribution bears similarity to that from the 2e
band 共86 cm−1兲 of the excited state in the DF spectrum. The
DF spectra from O00 + 313 cm−1 and O00 + 359 cm−1 also show
a similar progression of e symmetry torsional bands and
other vibrational bands. This kind of transitions indicates that
these bands in the excitation spectrum are either bands of the
e symmetry of torsional mode or a combination of the e
symmetry torsional band with other vibrational modes. The
DF spectrum of the band at O00 + 285 cm−1 shows the progression of the vibrational bands as well as the torsional
bands of both a and e symmetries. Since a to e transition is
symmetry forbidden and the linewidth of this band is higher
than the other bands, we speculate that there is an overlap of
two bands of a and e symmetries. The next band at O00
+ 351 cm−1 does not have FC activity with any of the torsional levels of the ground state. The appearance of this band
could be due to the vibrational mode in the excited state that
is not coupled with the torsional motion.
From the above observations, bands located at
172 cm−1共3a1兲,
261 cm−1共5e兲,
and
86 cm−1共2e兲,
−1
307 cm 共6a1兲 could be assigned as a progression of methyl
torsional transitions building upon O00 electronic origin.
These bands were fitted to obtain the excited state torsional
parameters V3⬘ = 313 cm−1, V6⬘ = −60 cm−1, and F = 5.29 cm−1.
A comparison of the calculated and experimentally observed
torsional frequencies is presented in Table II 共progression 1兲.
Assignment for most of the bands could fit into this scheme
except the 4e band that was not observed. The absorption
spectrum of 1MPY in solution20 shows a vibrational band
associated with the first ␲-␲* state located at 334 nm, that is,
118 cm−1 away from the observed O00 band at 29 822 cm−1.
In 2-methyl pyridine, although the methyl group activity was
not seen in the mass resolved excitation spectrum, the low
frequency band observed at 120.6 cm−1 was assigned as a
vibronic band.4 The band at 120 cm−1 in the FE spectrum is
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114312-4
J. Chem. Phys. 126, 114312 共2007兲
Sinha et al.
TABLE I. Assignments of observed bands in dispersed fluorescence spectra of 1 MPY.
Excitation at
O00 + 203 cm−1
共30 025 cm−1兲
Excitation at
O00 + 313 cm−1
共30 135 cm−1兲
Excitation at O00 + 285 cm−1
共30 107 cm−1兲
Relative
separation
共cm−1兲
Ground state
levels
0.0
98
150
173
203
219
231
298
378
395
413
429
522
550
0
2e
␯38
4e
¯
␯37
5e
2␯38
¯
␯35
¯
2␯37
2␯372e
␯32
Relative
separation
共cm−1兲
Ground state
levels
0.0
96
137
162
232
266
279
307
346
369
399
432
460
503
535
553
0
2e
␯38
3a1
␯382e
6a1
2␯38
␯372e
␯37␯38
2␯382e
␯35
2␯37
¯
¯
␯35␯38
␯32
Excitation at
O00 + 351 cm−1
共30 174 cm−1兲
Excitation at
O00 + 359 cm−1
共30 181 cm−1兲
Relative
separation
共cm−1兲
Ground
state
levels
Relative
separation
共cm−1兲
Ground
state
levels
Relative
separation
共cm−1兲
Ground
state
levels
0.0
98
235
305
446
553
0
2e
␯382e
␯372e
␯34
␯32
0.0
350
399
465
519
539
0
␯37␯38
␯35
¯
¯
␯35␯38
0.0
144
239
284
304
452
525
557
0
␯38
5e
2␯38
␯372e
␯34
2␯372e
␯32
thus assigned as the second low frequency vibration 共3810兲 of
the excited state. The atomic displacements corresponding to
this mode shown in Fig. 4共a兲 reveal that this vibrational
mode includes out-of-plane ring deformation coupled with
methyl torsional motion. The optimized geometry with
CIS/ 6-31G共d , p兲 in the excited state showed that the molecule has a ring deformation and is thus nonplanar. Consequently, most of the out-of-plane vibrations get coupled with
the torsional motion. The bands at 203, 285, and 359 cm−1
which are 83, 165, and 239 cm−1 away, respectively, from
this vibrational band are assigned as combination bands with
torsional progressions 2e, 3a1, and 5e. Both 203 and
359 cm−1 bands show e progression, whereas the 285 cm−1
band shows a1 progression in the DF spectra, as listed in
Table I. We have assigned the 203 cm−1 band as a vibration2e combination band because its FC activity with the ground
state torsional bands seen in the DF spectra is in close agreement with the calculated FC intensity pattern for the 2e band.
This torsional progression yields the potential parameters
V3⬘ = 288 cm−1, V6⬘ = −47 cm−1, and F = 5.04 cm−1. A comparison of the observed and the fitted energy values is given in
Table II as progression 2. The band at 233 cm−1 is assigned
to the first overtone 共3820兲 of the vibrational band 共3810兲 at
120 cm−1, whereas the 313 cm−1 band is assigned as a com2e
bination band 共3820␶1e
兲 because it exhibits e progression in its
DF spectrum 共Table I兲. Since the DF spectrum upon excitation of the 285 cm−1 band shows both a1 and e torsional
progressions, this could originate from the existence of the e
torsional combination band with another vibration. This
prompted us to assign the 196 cm−1 band as the third vibrational mode 共3710兲 in the excited state and the transition at
2e
combination band. The atomic dis285 cm−1 as a 3710␶1e
placements of this vibrational mode 关Fig. 4共b兲兴 also show
ring out-of-plane vibration and methyl torsional motion. The
DF spectrum of 351 cm−1 did not show any torsional progression, and hence, we assign this band as the fourth vibrational mode 共3610兲 in the excited state. Assignments of these
observed bands in the FE spectrum are listed in Table III.
TABLE II. Comparison between calculated and observed torsional frequencies of methyl group in 1MPY.
Progression 1
Progression 2
Torsional energy
levels
Calc.
共cm−1兲
Obs.
共cm−1兲
Torsional energy
levels
Calc.
共cm−1兲
Obs.
共cm−1兲
2e
3a2
3a1
4e
5e
6a2
6a1
86
87
175
181
253
283
312
86
2e
3a2
3a1
4e
5e
6a2
6a1
84
85
165
172
237
83
V⬘3 = 313 cm−1, V⬘6 = −60 cm−1, F = 5.29 cm−1
172
261
307
165
239
V⬘3 = 288 cm−1, V⬘6 = −47 cm−1, F = 5.04 cm−1
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114312-5
J. Chem. Phys. 126, 114312 共2007兲
Methyl torsional behavior
FIG. 4. Atomic displacements of 共a兲 the ␯38 vibrational mode and 共b兲 the ␯37
vibrational mode of 1MPY in the excited state.
To ascertain the above assignments we have calculated
the FC factor of only torsional transitions and torsionvibration 共3810兲 combination band intensities normalized with
the observed intensity of the 3810 transition. We have used the
wave functions obtained from energy level calculations with
V3⬘ = 313 cm−1, V6⬘ = −60 cm−1, and F = 5.29 cm−1 for progression 1; V3⬘ = 288 cm−1, V6⬘ = −47 cm−1, and F = 5.04 cm−1 for
progression 2; and V3⬙ = 244 cm−1, V6⬙ = 15 cm−1, and F
= 5.2 cm−1 for the ground state17 to calculate the FC factor.
The shift of the potential between the excited state and the
ground state for both the progressions has been taken to be
60° as observed previously. Using a Lorentzian line shape
for the transitions, the simulated excitation spectrum presented in Fig. 2共b兲 shows very good agreement with the observed one.
2. 1-methyl-2„1H…pyridinimine „1MPI…
The molecule 1MPI has 42 fundamental vibrational
modes with 28 in-plane 共a1兲 and 14 out of plane 共a2兲 under
the G6 molecular symmetry group.18,19 The power normalized FE spectrum of jet cooled 1MPI in the spectral region of
the first excited state is shown in Fig. 5共a兲. Observed transitions and their frequency displacements from the first band in
the spectrum are listed in Table III. The first peak at
FIG. 5. FE spectra of 1MPI. 共a兲 Experimental and 共b兲 simulated 共see text兲.
The O00 band is at 24 899.9 cm−1.
24 899.9 cm−1 is assigned to the S0 → S1 共O00兲 electronic origin transition, which is in good agreement with the earlier
reported14 value 共24 899.2 cm−1兲. The weak origin transition
is apparently due to the shift in the excited state potential
energy surface with respect to that of the ground state. As
compared to 1MPY, the O00 band is redshifted by 4922 cm−1
in this case due to the replacement of the oxygen atom by the
N-H group. The solution phase study12 of 1MPI shows that
the S1 state is of ␲␲* character. Molecular orbital
calculation21 also confirms the lower electronic excited state
to be of ␲␲* character. We, therefore, assign this O00 band to
the ␲ → ␲* transition.
The spectrum exhibits many features common with the
excitation spectrum of 1MPY. However, it is less congested.
There are 14 clearly observed bands below 400 cm−1,
whereas in the case of 1MPY it is more than 30. Both these
spectra display a weak origin transition followed by strong
vibronic bands. The vibrational structure abruptly disappears
beyond 400 cm−1 from the O00 transition in both cases. There
exists a striking similarity between the vibrational bands in
TABLE III. Assignments of the observed bands in fluorescence excitation spectra of 1MPY and 1MPI.
1MPY
Peak
positions
共cm−1兲
Relative
separation
共cm−1兲
29 822.3
29 844.6
29 908.2
29 942.6
29 994.5
30 018.8
30 025.7
30 030.9
30 055.3
30 083.1
30 107.5
30 129.3
30 135.3
30 174.0
30 181.0
30 195.0
0.0
22
86
120
172
196
203
209
233
261
285
307
313
351
359
373
1MPI
Assignment
Peak
positions
共cm−1兲
Relative
separation
共cm−1兲
Origin
24 899.9
0.0
2e
␶1e
3810
␶3a1
0a1
3710
2e
3810␶1e
24 996.0
25 025.0
25 092.9
25 117.3
25 148.1
96
125
193
217
248
2e
␶1e
4110
␶3a1
0a1
2e
4110␶1e
2
410 / 4010
3820
5e
␶1e
2e
1 3a1
380␶0a1 / 3710␶1e
␶6a1
0a1
2e
3820␶1e
1
360
5e
3810␶1e
25 178.3
25 196.3
25 210.2
25 234.7
25 254.4
25 261.8
25 286.5
25 298.8
278
296
310
335
354
362
386
398
5e
␶1e
3910
4110␶3a1
0a1
2 2e
1 2e
␶6a1
/
41
0␶1e / 400␶1e
0a1
2910
Assignment
5e 7e
4110␶1e
/ ␶1e
2e
3910␶1e
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114312-6
J. Chem. Phys. 126, 114312 共2007兲
Sinha et al.
TABLE IV. Comparison between calculated and observed torsional frequencies of methyl group in 1MPI.
Progression 1
Progression 2
Torsional energy
levels
Calc.
共cm−1兲
Obs.
共cm−1兲
Torsional energy
levels
Calc.
共cm−1兲
Obs.
共cm−1兲
2e
3a2
3a1
4e
5e
6a2
6a1
7e
96
97
193
197
278
303
339
385
96
2e
3a2
3a1
4e
5e
6a2
6a1
7e
93
94
184
188
262
288
318
363
92
193
278
V⬘3 = 358.3 cm−1, V⬘6 = −57.4 cm−1, F = 5.31 cm−1
both the spectra, except for a few. Therefore, taking the spectrum of 1MPY as a basis, we have assigned the bands in the
excitation spectrum of 1MPI. The 22 cm−1 band seen in
1MPY is absent in the 1MPI spectrum. Like 1MPY, several
transitions in the FE spectrum belong to the methyl torsional
mode. Remaining transitions are FC allowed in-plane or outof-plane vibrations. Torsion-vibration combination bands are
also found active in this spectrum. Transition frequencies and
their corresponding assignments are presented in Table III. A
progression for bands at 96, 193, and 278 cm−1 was identified with the methyl torsional transitions. The potential parameters for the excited state, V3⬘ = 358 cm−1, V6⬘ = −57 cm−1,
and F = 5.31 cm−1, were obtained from the best fit to the
methyl torsional transitions. A comparison between calculated and the observed frequencies is given in Table IV 共progression 1兲. The threefold potential term here is higher than
that of 1MPY, while the sixfold term remains almost similar
in magnitude. The 4e transition is found to be absent in this
case as well.
Similar to the 120 cm−1 band in 1MPY, a band is observed at 125 cm−1 in the 1 MPI spectrum. This band can
also be assigned to another low frequency vibration. Ab initio frequency calculation for excited state at the
CIS/ 6-31G共d , p兲 level predicts four vibrations besides methyl torsion to lie below 400 cm−1. Amongst these, three are
out-of-plane vibrations 共a2 symmetry under G6兲 and one is
an in-plane vibration 共a1兲. The vector displacements of the
first four normal modes with calculated frequencies are
shown in Fig. 6. The band at 125 cm−1 could be attributed to
the transition to the fundamental of the ␯41 共4110兲 mode with
its calculated value of 110 cm−1. Though the transitions to a2
fundamentals have a rigorously vanishing FC factor, such
transitions can gain intensity if it is either coupled with the
torsion or if the molecular frame undergoes distortion to relax symmetry restrictions. As it is evident in Fig. 6, the vector displacements of atoms for this mode encompass the torsional motion with the out-of-plane ring vibration. Excited
state geometry also indicates a heavy distortion in the planarity of the molecular frame21 and thus corroborates our
observation. The assignment of band at 125 cm−1 to the 4110
transition allows us to infer that the bands at 217, 310, and
386 cm−1 are the torsion-vibration combination bands. The
torsional frequencies of the bands which appear as combina-
185
261
V⬘3 = 333.2 cm−1, V⬘6 = −45.2 cm−1, F = 5.04 cm−1
tions with 4110 are 92, 185, and 261 cm−1. These frequencies
were fitted to obtain the torsional parameters V3⬘
= 333.2 cm−1, V6⬘ = −45.2 cm−1, and F = 5.04 cm−1. Calculated
frequencies and the corresponding experimental ones are
grouped in Table IV as progression 2. The 248 cm−1 band
could be assigned as the first overtone of ␯41 共4120兲 or the
transition to the fundamental of ␯40 共4010兲, and the band at
335 cm−1 could be a 2e combination band with this vibration. The band at 296 cm−1 has been identified as a 3910 transition.
To ascertain the above assignment, dispersed fluorescence spectra for the first few bands near origin were recorded. However, we were unable to capture any distinct
bands up to 1000 cm−1 beyond laser excitation within our
experimental sensitivity limit. A close inspection reveals few
weak features in these spectra. With the help of ab initio
calculation we derived the ground state torsional potential
parameters. Using potential parameters obtained from different levels of theory, torsional energy levels were calculated
and matched with the observed transitions. An observed
weak feature at 264 cm−1 in the emission spectrum upon
excitation of band at 217 cm−1 closely matches with the 3a1
and 4e transitions calculated using the potential obtained
with the Hartree-Fock 共HF兲 / 6-31G共d , p兲 level 共Table V兲. As
the emission originates from the 217 cm−1 band excitation
FIG. 6. Atomic displacements of four low frequency normal modes of 1MPI
in the excited state at the CIS/ 6-31G共d , p兲 level of calculation.
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114312-7
J. Chem. Phys. 126, 114312 共2007兲
Methyl torsional behavior
TABLE V. Torsional energy levels 共cm−1兲 calculated using the potential parameters obtained from different levels of theory for 1MPI.
Energy
levels
HF/ 6-31G共d , p兲
V⬙3 = 454 cm−1, V⬙6 = 31 cm−1,
and F = 5.3 cm−1
HF/ 6-311+ + G共d , p兲
V⬙3 = 482 cm−1, V⬙6 = −32 cm−1,
and F = 5.3 cm−1
MP2 / 6-31G共d , p兲
V⬙3 = 530 cm−1, V⬙6 = −10 cm−1,
and F = 5.3 cm−1
B3LYP/ 6-311+ + G共d , p兲
V⬙3 = 433 cm−1, V⬙6 = −15 cm−1,
and
F = 5.3 cm−1
2e
3a2
3a1
4e
5e
6a2
6a1
145
145
264
267
351
376
405
127
127
245
247
348
363
418
142
142
271
272
379
392
451
124
124
234
237
325
345
386
which is assigned as a 2e torsion transition in combination
with 4110, it is unlikely that the 264 cm−1 band in the DF
spectrum is 3a1. The transition from 2e → 4e is symmetry
allowed and the transition to the 4e level should be intense,
as observed in the case of 1MPY. Therefore, the band at
264 cm−1 can be assigned as a 4e torsional energy level in
the S0 state. However, only this observed band will not be
sufficient to extract the torsional potential. We consider the
potential to be V3⬙ = 454 cm−1 and V6⬙ = 31 cm−1 for the ground
state as predicted by the HF/ 6-31G共d , p兲 level calculation
for further discussion. With these potential parameters for the
ground state along with those obtained experimentally for
the excited state, simulated excitation spectrum presented in
Fig. 5共b兲 is obtained with a 60° shift between the minima of
the excited and the ground states. Although it shows close
agreement with the observed one, a few discrepancies in intensity could be due to the use of pure torsional wave functions for the calculation of FC factors.
It is quite surprising that despite large total fluorescence
intensity, dispersed fluorescence spectra lack any prominent
features in the low frequency region. Our investigation into
this quandary reveals that 70% of the fluorescence was emitted above 450 nm by the excitation of all these bands.
Higher quantum efficiency of fluorescence at longer wavelength could be due to the existence of another electronic
level below the ␲␲* level. Ab initio calculation21 predicts
that there exist three n␲* singlet states between the first and
second ␲␲* singlet states, and these lie close to the ␲␲*
state. Generally, in the case of N-heterocyclic compounds,
the singlet excited states ␲␲* and n␲* are close to each other
in energy, and methyl substitution often changes their relative separation. Like 1MPY, where a complicated vibronic
structure and anomalous vibronic bands in the excitation
spectrum predict the existence of a low lying n␲* state,
1MPI may also be containing one of the n␲* states below the
␲␲*. Lack of emission much above the origin due to the
activation of radiationless processes gives some evidence to
this assignment.
3. 3-methyl-2„1H…pyridone „3MPY…
The fluorescence excitation spectrum of 3MPY in the
400 cm−1 region of the S0 → S1 transition is shown in Fig. 7.
The peak positions and relative frequencies of the observed
transitions are given in Table VI. The observed bands in the
spectrum appear to be broad because of inadequate rotational
cooling in the jet. Change in the backing pressure permissible in our experimental setup did not improve the cooling
condition. The strong feature at 30 355 cm−1 is assigned as
the S0 → S1 共O00兲 electronic origin transition. This transition is
525 cm−1 blueshifted as compared to that in the case of
2-pyridone. While the substitution of the methyl group at the
nitrogen atom in the case of 1MPY does not bring about any
change in excitation energy, substitution at ring carbon
causes destabilization by raising the electronic energy of the
excited state. There are many weak features seen close to the
O00 band, which may be due to the hot bands resulting from
improper cooling. The spectral features resemble more the
2-pyridone22 than the 1MPY excitation spectrum. Unlike
TABLE VI. Assignments of the observed bands in fluorescence excitation
spectrum of 3MPY.
Peak Peak positions 共cm−1兲 Relative separation 共cm−1兲
FIG. 7. Fluorescence excitation spectrum of 3MPY in supersonic jet. The
origin O00 is at 30 355 cm−1.
1
2
3
4
5
6
7
30 355
30 441
30 480
30 500
30 546
30 566
30 588
0.0
86
125
145
191
211
233
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Assignment
Origin 共1兲
Origin 共2兲
2e
␶1e
3710
2e
Origin 共2兲 + ␶1e
¯
3a1
114312-8
J. Chem. Phys. 126, 114312 共2007兲
Sinha et al.
TABLE VII. Comparison between calculated and observed torsional frequencies of methyl group in 3MPY.
Torsional level
Obs. 共cm−1兲
Calc. 共cm−1兲
2e
3a2
3a1
4e
5e
6a2
6a1
125
¯
233
¯
¯
¯
¯
125
126
233
237
317
344
375
V⬘3 = 408 cm−1, V⬘6 = −5 cm−1, F = 5.4 cm−1
FIG. 9. Optimized geometries of 1MPI 共a兲 ground state at HF/ 6-31G共d , p兲
and 共b兲 excited state CIS/ 6-31G共d , p兲.
1MPY,17 here the O00 transition is found to be very intense
and the spectrum is less congested. An equally strong band is
seen at 86 cm−1 from the S0 → S1 共O00兲 transition. Two similar
intense transitions with 100 cm−1 separation were also seen
in the excitation spectrum of 2-pyridone.22 These were interpreted as two different electronic origins corresponding to
two structural isomers in the excited state.23 Following this,
the band at 86 cm−1 is assigned as another electronic origin.
These two peaks are sensitive to backing pressure just as in
the case of 2-pyridone electronic origin bands. No vibronic
band was found to be active beyond 400 cm−1 just as in the
case of 1MPY and 1MPI excitation spectra. This can be attributed to radiationless processes induced by interelectronic
vibronic interaction between close lying n␲* and ␲␲* states.
On the other hand, many low frequency bands are seen near
these origin transitions. We could fit several of the observed
bands to pure torsional transitions using one-dimensional torsional Hamiltonian. The bands at 125 and 233 cm−1 were
fitted as 2e and 3a1 torsional transitions to obtain the potential parameters V3⬘ = 408 cm−1, V6⬘ = −5 cm−1, and F
= 5.4 cm−1 in the excited state. A comparison of the observed
and the calculated torsional frequencies is given in Table VII.
The threefold barrier of 3MPY is 95 cm−1 higher than that of
1MPY in the excited state. Besides pure methyl torsional
transition, other vibrational transitions are also found to be
active in the spectrum. We assign these bands with the help
of ab initio calculation carried out at the CIS/ 6-31G共d , p兲
level. The frequencies and the corresponding vector displacements of three lowest frequency modes in the excited state
are shown in Fig. 8. The lowest frequency mode 共3910兲 of
3MPY is ring out-of-plane vibration calculated to be
93 cm−1. The ␯38 mode corresponds to the methyl torsion
with the predicted frequency of 109 cm−1 in close proximity
to the 2e band observed at 125 cm−1. The band observed at
145 cm−1 matches with the calculated 129 cm−1 frequency
corresponding to the ␯37 mode. Hence, this band can be assigned as a 3710 transition. Since there is no normal mode
vibration lying near 191 cm−1, this band can be tentatively
assigned to the 2e torsional band building upon the second
electronic origin. We could not record dispersed fluorescence
spectra for 3MPY because of its low fluorescence quantum
yield. Modest scale ab initio calculation often predicts correctly the ground state methyl torsional potential energy barrier as established for several other similar molecules. Hence,
further analysis is based on the HF/ 6-31G共d , p兲 ab initio
calculated barrier.
FIG. 8. Atomic displacements of three low frequency normal modes of
3MPY in the excited state at the CIS/ 6-31G共d , p兲 level of calculation.
B. Methyl torsional potential
In 1MPY, the excited state torsional potential is 60° out
of phase with respect to that of the ground state. This phase
shift in the torsional potential was explained successfully on
the basis of nonconventional ␲*-␴* hyperconjugation, introduced originally by Nakai et al.24 The FE spectrum of 1MPI
also indicates the change in conformation of the methyl
group upon excitation. In order to investigate the phase shift
as well as the barrier height theoretically, the geometry of
this molecule was optimized in both S0 and S1 states using
HF and CIS methods, respectively, with the 6-31G共d , p兲 basis set. The optimized geometries are shown in Fig. 9. The
minimum energy conformation of the methyl group in the S0
state is staggered with the in-plane C–H bond placed away
from the N-H group. On the other hand, in the S1 state, the
methyl group is 60° out of phase with respect to the S0 state
and acquires eclipsed conformation with the in-plane C–H
bond towards the N-H group. The potential energy curve for
the excited state was determined from the excitation energies
calculated using CIS/ 6-31G共d , p兲 at discrete torsional
angles. The variation of these energies relative to the minimum energy was plotted against the torsional angle. The calculated barrier height 250 cm−1 is ⬃100 cm−1 lower than the
experimentally determined value 共358 cm−1兲. The potential
energy curve for the excited state is shown in Fig. 10. The
conformational preferences of the methyl group in both the
electronic states can be understood on the basis of the molecular orbital picture. Figure 11 shows the orbital contour
plot of the highest occupied molecular orbital 共HOMO兲 and
lowest unoccupied molecular orbital 共LUMO兲 of 1MPI at ␶
= 0° and ␶ = 60° displaying the relative stability of the corresponding orbital energies. In 1MPI, the ␲*-␴* hyperconjuga-
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114312-9
Methyl torsional behavior
FIG. 10. Potential energy curves for methyl internal rotation of 1MPI in the
ground 共S0兲 and excited 共S1兲 states. Upper curve shows the excitation energy
calculated 关CIS/ 6-31G共d , p兲兴 at different torsional angles using ground state
geometry 关HF/ 6-31G共d , p兲兴.
tion between the methyl C–H antibond and the ring ␲ antibond is found to be present in both the occupied and
unoccupied molecular orbitals. The ␲*-␴* hyperconjugation
in HOMO does not appear at ␶ = 60° but rather at ␶ = 0°,
giving rise to the energy stabilization for the staggered conformer in the ground state. LUMO shows ␲*-␴* hyperconjugation at ␶ = 60°, thereby stabilizing the eclipsed conformer
in the excited state. Figure 12 shows the HOMO energy,
LUMO energy, and their difference against torsional angle. It
is apparent from the figure that the change in the LUMO
energy is a dominant contributor to the change in the excitation energy, while the HOMO energy change of 1MPI has
little contribution to ⌬ES0→S1. This trend was also confirmed
for 1MPY and other substituted toluene and azabenzenes.23
Figure 13 shows the variation of the excitation energy,
the torsional potential in the S1 state calculated using
FIG. 11. Contour diagrams of HOMO and LUMO of 1MPI at staggered and
eclipsed conformations. In staggered conformer HOMO shows ␲*-␴* hyperconjugation between methyl CH␴* and ring ␲* stabilizing this configuration
in the ground state. The eclipsed conformer is stabilized in the excited state
due to hyperconjugation exhibited by LUMO. The contour diagrams are
plotted with the same sensitivity.
J. Chem. Phys. 126, 114312 共2007兲
FIG. 12. Variation of HOMO, LUMO, and LUMO-HOMO energies as a
function of torsional angle ␶.
CIS/ 6-31G共d , p兲, and the potential curve for S0 calculated by
HF/ 6-31G共d , p兲 with the potential constants V3⬙ = 555 cm−1
and V6⬙ = 6 cm−1 for 3MPY. From the potential curve of S1,
the barrier height in the excited state was estimated to be
458 cm−1, which is in reasonable agreement with the experimental value of 408 cm−1. Hence, in 3MPY, the barrier to
methyl internal rotation undergoes reduction in going from
the ground state to the excited state. It is evident in Fig. 13
that the potential minimum in the excited state is 60° shifted
with respect to that of the ground state, as in the case of
1MPY, 1MPI, and many other aromatic molecules. In 3MPY
also the ␲*-␴* hyperconjugation between the methyl C–H
antibond and the ring ␲ antibond was found to be present in
LUMO at ␶ = 60°, which is responsible for the stabilization of
the eclipsed conformer in the excited state. However, the
intense O00 peaks for 3MPY observed in the FE spectrum do
not comply with this conformational change of the methyl
group. Probably, it arises due to the oscillator strength borrowed from other nontorsional modes of vibration.
The maximum changes in the HOMO energy, the
LUMO energy, and their differences upon methyl rotation for
1MPY, 1MPI, and 3MPY are listed in Table VIII for comparison. The variations of HOMO and LUMO energies are
found to be 243 and 818 cm−1, respectively, for 1MPY and
116 and 908 cm−1, respectively, for 1MPI. The potential bar-
FIG. 13. Potential energy curves for methyl internal rotation for 3MPY in
the ground state 共S0兲 and excited state 共S1兲 calculated using HF/ 6-31G共d , p兲
and CIS/ 6-31G共d , p兲, respectively.
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114312-10
J. Chem. Phys. 126, 114312 共2007兲
Sinha et al.
TABLE VIII. Comparison of HOMO, LUMO, and barrier energy in 1MPY, 1MPI, and 3MPY.
1MPY
1MPI
3MPY
⌬EHOMO 共cm−1兲
⌬ELUMO 共cm−1兲
⌬ELUMO-HOMO 共cm−1兲
⌬Ebarrier 共cm−1兲
243
116
236
818
908
959
575
792
723
313
358
408
rier in the excited state for 1MPI 共358 cm−1兲 is greater than
that for 1MPY 共313 cm−1兲. The possible reason could be due
to the smaller change in HOMO energy and the larger
change in LUMO energy during methyl rotation for 1MPI
compared to those for 1MPY. The LUMO energy change has
a dominant contribution in the torsional potential barrier for
3MPY similar to the case of 1MPI and 1MPY.
IV. CONCLUSION
From the fluorescence excitation and dispersed fluorescence spectra of 1MPY, 1MPI, and 3MPY, the internal rotational levels of the methyl group and some of the low frequency vibrations in the excited state have been assigned.
The spectral analysis reveals considerable mixing of the methyl torsion with the low frequency vibrational modes such
as ring out-of-plane in the excited state. The potential barrier
of the methyl group in the excited state is higher in 1MPI
than in 1MPY, probably due to the hyperconjugation involving lone pair electrons. Such an interaction is expected to be
more in 1MPI than in 1MPY as the lone pair electrons of
nitrogen atom is more available for the participation by virtue of smaller electronegativity of nitrogen than oxygen
atom. The barrier increases upon electronic excitation in
1MPY, whereas it decreases in the case of 1MPI and 3MPY
and the methyl conformer changes by 60° in the excited
state. The ␲*-␴* hyperconjugation interaction model explains this switching of methyl group conformation reasonably well. In this kind of molecules, the change in LUMO
energy during the methyl group rotation seems to play a
dominant role in the formation of the excited state barrier.
ACKNOWLEDGMENT
This work was supported by the Department of Science
and Technology, India.
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