Structure and vibrational dynamics of aniline and aniline-Ar from high resolution electronic spectroscopy in the gas phase

Article abstract of DOI:10.1063/1.472710

Rotationally resolved S₁← S₀ electronic spectra of aniline and its single atom van der Waals complex with argon (An-Ar) have been observed. Analysis of these spectra leads to a determination of the vibrationally averaged structures of the bare molecule and the complex in the two electronic states. Aniline itself is pyramidally distorted at the NH₂ group in the S₀ state. Attachment of the Ar atom on the side of the ring opposite the two N-H bonds converts the symmetric double well along the inversion coordinate into an asymmetric one, in the ground state. The excited state is quasiplanar along this coordinate. Analyses of the spectra of An-Ar at higher energies in the S₁ state provide a probe of the vibrational predissociation (VP) behavior of the complex. We observe in these spectra line broadenings and spectral perturbations from which the important role of intra-intermolecular mode mixing (i.e., IVR) in promoting the VP process is elucidated.

Full text of DOI:10.1063/1.472710

Structure and vibrational dynamics of aniline and aniline–Ar from high resolution
electronic spectroscopy in the gas phase
Wayne E. Sinclair and David W. Pratt
Citation: The Journal of Chemical Physics 105, 7942 (1996); doi: 10.1063/1.472710
View online: http://dx.doi.org/10.1063/1.472710
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Structure and vibrational dynamics of aniline and aniline–Ar from high
resolution electronic spectroscopy in the gas phase
Wayne E. Sinclair and David W. Pratt
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
͑Received 15 April 1996; accepted 9 August 1996͒
Rotationally resolved S1←S0 electronic spectra of aniline and its single atom van der Waals
complex with argon ͑An–Ar͒ have been observed. Analysis of these spectra leads to a determination
of the vibrationally averaged structures of the bare molecule and the complex in the two electronic
states. Aniline itself is pyramidally distorted at the NH2 group in the S0 state. Attachment of the Ar
atom on the side of the ring opposite the two N–H bonds converts the symmetric double well along
the inversion coordinate into an asymmetric one, in the ground state. The excited state is quasiplanar
along this coordinate. Analyses of the spectra of An–Ar at higher energies in the S1 state provide
a probe of the vibrational predissociation ͑VP͒ behavior of the complex. We observe in these spectra
line broadenings and spectral perturbations from which the important role of intra–intermolecular
mode mixing ͑i.e., IVR͒ in promoting the VP process is elucidated. © 1996 American Institute of
Physics. ͓S0021-9606͑96͒04442-X͔
INTRODUCTION
Analyses of the observed bands yield accurate values of the
rotational constants of all species in their S0 and S1 states.
We report first on the derived inertial parameters of the bare
aniline molecule, confirming that it is a nonplanar molecule
in S0 and quasiplanar in S1. Then, we probe the motion of
the two states along various vibrational coordinates by com-
paring the observed inertial parameters of several vibrational
levels with calculated ones.
Structural and dynamical studies of van der Waals
͑vdW͒ molecules provide the basic information needed to
elucidate the nature of the weak chemical forces that domi-
nate their behavior. Extensive investigations have focused on
rare gas complexes of aromatic molecules. These complexes
are of particular interest since the rare gas atom acts as a
structureless probe of the shape of the intermolecular poten-
tial energy surface. Such surfaces often can be described by
a superposition of a number of dominant interaction energy
terms. Furthermore, such complexes typically exhibit bind-
ing energies much less than some chromophore vibrational
frequencies, thus making possible studies of the dynamics of
vibrational predissociation ͑VP͒.
A system that has attracted much attention is the vdW
complex of the aniline molecule with a single argon atom
͑An–Ar͒. Aniline itself has been extensively studied and its
spectroscopy is well known.1–14 Thus, An–Ar has been the
subject of numerous experimental and theoretical
studies.15–26 The S1←S0 fluorescence excitation spectrum of
An–Ar was first observed by Amirav et al.15 A spectral shift
of ϳϪ54 cmϪ1 from the 000 band of the bare aniline molecule
was reported. Yamanouchi et al.17 performed a rotational
contour analysis of the 00 band of the complex, yielding an
approximate position of the argon atom above the surface of
the aromatic ring in the two states. The vibrational dynamics
of An–Ar have been probed by picosecond time-resolved
studies performed by Nimlos et al.19 and by ZEKE studies
performed by Zhang et al.,26 providing much insight into the
VP process. The observed VP dynamics was explained in the
framework of a ‘‘serial’’ model.27 Here, intramolecular vi-
brational redistribution ͑IVR͒ from the initially excited chro-
mophore mode to the vdW modes is the rate limiting step,
followed by fast VP from the IVR populated states.
Next, we turn our attention to the structure of the An–Ar
vdW complex in the two electronic states. We find that the
˚
argon atom is at a distance of ϳ3.5 A above the aromatic
plane in the S0 state, and that this distance decreases slightly
on excitation to the S1 state. Additionally, the argon atom
exhibits significant large-amplitude motion in both states.
Nonetheless, comparisons of the rotational constants of the
different isotopomers show that the argon atom is localized
on one side of the ring in the S0 state, opposite to the two
N–H bonds. Thus, in An–Ar, the symmetric double well
along the inversion coordinate of the bare molecule is con-
verted into an asymmetric double well, owing to the pres-
ence of the argon atom. We find that an anisotropic attractive
interaction between argon and the lone pair electrons of the
NH2 group satisfactorily accounts for this effect.
Finally, we probe the VP dynamics in An–Ar. A pro-
gressive increase of the single rovibronic linewidths is ob-
served as a function of vibrational energy in the S1 state.
Additionally, perturbations are observed in the highest-
0
¯
1
¯
energy band recorded ͑10͒, resulting from coupling between
1
¯
the 1 vibrational level of the S1 state and near-degenerate
intermolecular vibrational levels. We find that the transitions
affected by these perturbations have linewidths that are mea-
surably larger than those of the unperturbed transitions. The
results support the necessity for IVR in promoting the VP
process in this system.
EXPERIMENT
In what follows, we present a detailed study of the rota-
tionally resolved S1←S0 fluorescence excitation spectra of
aniline, An–Ar, and their amino group deuterated analogs.
In the low resolution experiments, aniline was seeded in
a 1% mixture of Ar in He at a backing pressure of 60 psi and
7942
J. Chem. Phys. 105 (18), 8 November 1996
0021-9606/96/105(18)/7942/15/$10.00
© 1996 American Institute of Physics
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W. E. Sinclair and D. W. Pratt: Structure and dynamics of aniline–Ar
7943
expanded through a 10 Hz pulsed nozzle ͑General Valve
Series 9͒ into a vacuum of 10Ϫ5 Torr. The resulting super-
sonic free jet was crossed about 2.5 cm downstream of the
nozzle with a 10 Hz pulsed UV laser beam. The laser is a
Quanta-Ray Nd3ϩ/YAG pumped dye laser, frequency
doubled with an autotracking KDP crystal to give a spectral
width of about 0.3 cmϪ1 and a typical power of 100 ␮J per
pulse. Fluorescence was collected by a single lens system
and focused onto a standard PMT whose output was fed
through a Stanford Research System boxcar integrator, inter-
faced to a MASSCOMP MCS561 computer.
High resolution data were obtained using a CW molecu-
lar beam laser spectrometer, described elsewhere.28 The
sample was heated to ϳ345 K, seeded in pure argon at vari-
ous backing pressures, expanded through a 240 ␮m quartz
nozzle, skimmed once, and probed 15 cm downstream of the
nozzle by a frequency doubled, single-frequency, tunable
ring dye laser. A few experiments were performed in the
high resolution port, 100 cm downstream of the nozzle.
Typically, 300–600 ␮W of UV power was employed. Fluo-
rescence was collected using spatially selective optics, de-
tected by a PMT and photon counting system, and processed
by the MASSCOMP data acquisition system. For the bare
molecule experiments, each spectrum was taken with a 2000
s scan at an acquisition rate of 80 Hz. For the vdW complex
experiments, 8000 s scans at an acquisition rate of 25 Hz
were used. Each fluorescence excitation spectrum was nor-
malized by simultaneously recording the laser power curve.
Relative frequency calibration was performed using a near-
confocal interferometer having a mode-matched FSR of
299.7520Ϯ0.005 MHz at the fundamental of the dye. Abso-
lute frequency calibration was performed by simultaneously
recording the I2 absorption spectrum.
The rotationally resolved spectra were analyzed using
ASYROT,29 which employs Watson’s asymmetric rotor
Hamiltonian.30 Fits of the spectra were made using a stan-
dard nonlinear least squares analysis.28 Individual rovibronic
transitions exhibit Voigt profiles owing to the independent
contributions of residual Doppler broadening ͑Gaussian͒ and
radiative or nonradiative decay ͑Lorentzian͒. A computer
program was employed to determine each of these contribu-
tions.
FIG. 1. S1←S0 fluorescence excitation spectrum of An and An–Ar in a
supersonic jet. The 000 band of An is at 34 029 cmϪ1
.
clude the S1←S0 origin at 34 029 cmϪ1 and the 6a01, I02, and
110 vibronic bands at 492, 760, and 798 cmϪ1 above the ori-
gin, respectively. The 6a vibration is the totally symmetric
component of the vibronically active 6͑e2g͒ mode in ben-
zene, the I mode is the NH2 inversion vibration, and the 1
mode is a symmetric ring breathing vibration. Two promi-
nent bands are observed to the red of the 000 band of An, at
Ϫ53 and Ϫ106 cmϪ1. The band at Ϫ53 cmϪ1 has been as-
0
¯
signed as the S1←S0 electronic origin ͑00͒ of An–Ar,
whereas the second band at Ϫ106 cmϪ1 has been assigned as
the corresponding origin of the An–Ar2 complex.15 No other
single atom Ar atom complex bands were observed in the
vicinity of the bare molecule origin. Bands with shifts of
Ϫ53 cmϪ1 from the prominent vibronic features of the bare
molecule are attributed to the analogous vibronic excitations
of the An–Ar complex.
Several of the bands of An and An–Ar were examined at
high resolution. The upper traces of Fig. 2 show the high
resolution spectra of the 000, 6a01, and 101 bands of the bare
molecule. The overall appearance of each spectrum is simi-
lar. Each exhibits strong P and R branches and little or no Q
branch. These are the b-type bands of a near prolate sym-
metric rotor with the electronic transition dipole moment
͑TM͒ oriented along the short in-plane axis. Similar results
were obtained for the 000 bands of aniline–NHD͑AnHD͒ and
aniline–ND2͑AnDD͒ and for the I20 bands of An and AnDD.
Each band exhibits more than 100 assignable rovibronic
transitions, with Voigt line-shape profiles and linewidths
͑FWHM͒ of 30 MHz.
Rotational analyses of the spectra yielded the parameters
listed in Table I. Note, first, that the rotational constants of
the S0 vibrational levels involved in the four transitions are
all the same, within experimental error. This shows that all
transitions originate in the same level, the S0 zero-point vi-
brational level ͑ZPL͒. All rotational constants change signifi-
cantly on S1 excitation, differently for different vibronic
bands. The largest change is in the A constant, ⌬AϭϪ331.2
MHz in the 000 band. Additionally, the magnitude of the in-
ertial defect is relatively large in the S0 state, ⌬IЉϭϪ0.406
amu Å2, and decreases significantly on S1 excitation, to
Various expansion pressures were used. A pressure be-
tween 0.5–0.6 bar was normally employed for the bare mol-
ecule experiments. In the vdW complex experiments, the op-
timum conditions for the formation of the complex were
achieved by varying the sample vapor and Ar backing pres-
sures. Typically, 0.01 bar of sample vapor and between 0.6–
0.7 bar of Ar were employed. Aniline ͑99%͒ was purchased
from Aldrich and used without further purification. The
N-deuterated anilines were prepared from normal aniline by
direct exchange with pure D2O.
RESULTS
Figure 1 shows the low resolution S1←S0 fluorescence
excitation spectrum of aniline ͑An͒ and its vdW complex
with argon ͑An–Ar͒. The bare molecule bands observed in-
J. Chem. Phys., Vol. 105, No. 18, 8 November 1996
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7944
W. E. Sinclair and D. W. Pratt: Structure and dynamics of aniline–Ar
FIG. 2. Rotationally resolved fluorescence excitation spectra of the 000 , 6a01 , and 101 bands of the S1←S0 transition of An in a molecular beam. Also shown
are portions of the observed spectra and the corresponding calculated spectra.
⌬IЈϭϪ0.232 amu Å2. Substitution of the hydrogens of the
NH2 group by one or two deuterium atoms has a substantial
effect on all measured parameters.
Comparisons of a typical rovibronic lineshape in each of
the four bands in An to a Lorentzian lineshape function are
shown in Fig. 3. ͑Each line was actually fit to a Voigt line-
shape profile; Fig. 3 shows only the Lorentzian components
of these fits.͒ Table II lists all measured linewidths
␥m͑FWHM͒, together with the deconvoluted Lorentzian line-
widths ␥ and the corresponding fluorescence lifetimes,
␶ϭ͑2␲␥͒Ϫ1. The value of ␥ for the 000 band is 22Ϯ2 MHz,
corresponding to a fluorescence lifetime of the ZPL of the S1
state of 7.2Ϯ0.6 ns. This value was independently checked
by experiments in the high resolution port, yielding ␥ϭ20Ϯ2
MHz͑␶ϭ7.9Ϯ0.5 ns͒. Within experimental accuracy, the
fluorescence lifetime of the bare molecule was found to be
TABLE I. Rotational constants ͑MHz͒, asymmetry parameters, and inertial defects ͑amu Å2͒ of aniline–NH2 in its S0 ͑00͒ state and the S1 states ͑00, 6a1, I2,
and 11͒, and of aniline–NHD and aniline–ND2 in selected states, derived from analyses of their fully resolved S1←S0 excitation spectra. T is the rotational
temperature ͑K͒, ␯ is the band origin ͑cmϪ1͒, and ⌬␯ is the vibrational frequency shift ͑cmϪ1͒ from the 000 band of the protonated molecule ͑also in cmϪ1͒.
0
N refers to the number of assigned lines used in the fit; OMC is the observed minus calculated standard deviation of the fit ͑in MHz͒.
An
AnHD
AnDD
An
An
AnDD
An
000 band
000 band
000 band
6a10 band
I20 band
I20 band
110 band
00 state
A
B
C
5618.1͑1͒
2594.2͑1͒
1777.2͑1͒
Ϫ0.575
5572.3͑1͒
2493.8͑1͒
1726.2͑1͒
Ϫ0.601
5520.4͑1͒
2403.7͑1͒
1679.0͑1͒
Ϫ0.623
5617.5͑2͒
2593.8͑1͒
1777.1͑1͒
Ϫ0.575
5618.1͑1͒
2594.3͑1͒
1777.2͑1͒
Ϫ0.575
5520.9͑1͒
2403.9͑1͒
1679.3͑1͒
Ϫ0.623
5617.6͑1͒
2594.0͑1͒
1777.1͑1͒
Ϫ0.575
Љ
Љ
Љ
␬
Љ
⌬I
Ϫ0.406
Ϫ0.576
Ϫ0.795
Ϫ0.414
Ϫ0.396
Ϫ0.809
Ϫ0.411
Љ
00 state
6a1 state
I2 state
I2 state
11 state
⌬A
⌬B
⌬C
Ϫ331.2͑1͒
39.6͑1͒
Ϫ17.8͑1͒
Ϫ0.504
Ϫ324.6͑1͒
37.1͑1͒
Ϫ17.4͑1͒
Ϫ0.535
Ϫ317.7͑1͒
34.8͑1͒
Ϫ17.2͑1͒
Ϫ0.561
Ϫ330.7͑2͒
36.9͑1͒
Ϫ22.7͑1͒
Ϫ0.504
0.365
Ϫ351.9͑1͒
33.2͑1͒
Ϫ19.0͑1͒
Ϫ0.504
Ϫ330.1͑1͒
33.3͑1͒
Ϫ15.6͑1͒
Ϫ0.561
Ϫ332.9͑1͒
37.7͑1͒
Ϫ19.0͑1͒
Ϫ0.505
␬
Ј
⌬I
Ϫ0.232
Ϫ0.243
Ϫ0.270
Ϫ0.868
Ϫ0.926
Ϫ0.206
Ј
T
3.0͑3͒
34029.19͑1͒
0.0
7.0͑5͒
34 030.56͑1͒
1.4
3.0͑3͒
34 037.58͑1͒
8.4
3.0͑3͒
34 521.63͑1͒
492.4
5.0͑5͒
34 789.98͑1͒
760.7
5.0͑5͒
34 584.89͑1͒
547.2
5.0͑5͒
34 827.11͑1͒
797.9
␯
0
⌬␯
N
173
169
330
115
172
186
170
OMC
1.3
2.0
1.2
2.4
2.9
3.2
1.5
J. Chem. Phys., Vol. 105, No. 18, 8 November 1996
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W. E. Sinclair and D. W. Pratt: Structure and dynamics of aniline–Ar
7945
and R branches. These are clearly the c-type bands of a near
oblate asymmetric rotor, evidencing a reordering of the mo-
ments of inertia or a reorientation of the S1←S0 TM on Ar
complexation of the bare molecule. Similar results were ob-
0
¯
tained for the 00 band of aniline–ND2–Ar͑AnDD–Ar͒. All
bands exhibit more than 100 assignable rovibronic transi-
tions, which again could be fit with rigid rotor Hamiltonians
for both states. The derived values of the parameters are
listed in Table III.
Examining the data in Table III, we note first that the S0
rotational constants of all examined bands of An–Ar are,
again, identical within experimental error, showing that they
all originate in the same ground state level, presumably the
ZPL of the vdW complex. These constants also are much
smaller than those of An itself, reflecting the complex’s
larger mass. Notably, AЉ ͑An–Ar͒ϷCЉ͑An͒. The changes
⌬A, ⌬B, and ⌬C that occur on S1 excitation are comparable.
Additionally, the complex exhibits a large negative inertial
defect in both electronic states, ⌬IЉϭϪ176 and ⌬IЈϭϪ189
amu Å2.
The frequencies of all observed lines in the spectra of the
0
¯
1
¯
00 and 6a0 bands could be fit to standard deviations that are
significantly less than the observed linewidths. The analysis
1
¯
of the 10 band, despite its similarity in overall appearance,
proved more difficult. While the vast majority of lines were
found to have frequencies and intensities that were consistent
with model predictions, many lines having significant
strength were observed to have frequencies and intensities
that deviated considerably from the predicted ones. In some
cases, no correspondence could be found between theory and
experiment. By excluding such lines from the fit, the stan-
dard deviations improved significantly, with little change in
the derived values of the inertial parameters. Two such fits,
one excluding just seven such lines, are shown in Table III.
Comparisons of a typical rovibronic line shape in each
of the observed bands of An–Ar to a Lorentzian line-shape
function are shown in Fig. 5. The derived values of the
line-widths ␥m , Lorentzian widths ␥, and lifetimes ␶ also are
FIG. 3. Measured line shapes of the JKa,Kcϭ101 rovibronic transition in the
000 , 6a10 , and 110 bands of An. The solid lines represent the Lorentzian com-
ponents of the Voigt lineshapes that were fit to the experimental points; the
residual width is Gaussian in character. The widths shown are the total
͑LorentzianϩGaussian͒ widths.
0
¯
listed in Table II. The value of ␥ for the 00 band of An–Ar is
identical to that found in the bare molecule, 23Ϯ2 MHz ͑␶
ϭ6.9Ϯ0.6 ns͒. Thus, attaching a weakly bound Ar to An
independent of the particular S1 rotational level excited, of
excess vibrational energy, and of deuterium substitution.
0
1
1
¯
¯
¯
High resolution spectra of the 00, 6a0, and 10 bands of
does not affect its decay behavior in the ZPL of the S1 state.
2
¯
An–Ar are shown in Fig. 4. The I0 band was too weak to
record a fully resolved spectrum. Again, all measured bands
are very similar to each other. However, they are very dif-
ferent from those of the bare molecule. Each An–Ar band
exhibits a strong, blue-shaded Q branch and well-defined P
1
However, with ⌬Eϭ492 cmϪ1 ͑6a0 band͒, we observed an
¯
1
¯
increase of ␥ to 30Ϯ3 MHz, and with ⌬Eϭ798 cmϪ1 ͑10
band͒, we observed a further increase of ␥ to 50Ϯ5 MHz.
The corresponding lifetimes are 5.3Ϯ0.4 and 3.2Ϯ0.4 ns.
Similar results were obtained for all rovibronic lines, except
those that are strongly perturbed. These results indicate that
an excess energy-dependent decay channel opens up on ex-
citation of the complex to higher vibrational levels of its S1
state.
TABLE II. Measured linewidths ␥m , deconvoluted Lorentzian linewidths ␥,
and fluorescence lifetimes ␶ of single rovibronic lines in the 000 , 6a01 , and 101
bands of aniline, and of single rovibronic lines in the corresponding bands of
aniline–Ar.
DISCUSSION
An
An–Ar
Structure of aniline in its S0 and S1 electronic states
0
1
1
000
6a10
101
00
6a0
10
¯
¯
¯
First, we discuss the properties of the bare molecule in
its S0 and S1 electronic states. A complete substitution struc-
ture of the ground state has been determined from extensive
studies of the microwave spectra of An and 13 of its
␥
͑MHz͒
30͑2͒
22͑2͒
7.2͑6͒
30͑2͒
23͑2͒
6.9͑6͒
30͑2͒
23͑2͒
6.9͑6͒
30͑2͒
23͑2͒
6.9͑6͒
35͑3͒
30͑3͒
5.3͑4͒
53͑5͒
50͑5͒
3.2͑4͒
m
␥ ͑MHz͒
␶ ͑ns͒
J. Chem. Phys., Vol. 105, No. 18, 8 November 1996
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7946
W. E. Sinclair and D. W. Pratt: Structure and dynamics of aniline–Ar
0
¯
1
1
¯
¯
FIG. 4. Rotationally resolved fluorescence excitation spectra of the 00 , 6a0, and 10 bands of the S1←S0 transition of An–Ar in a molecular beam. Also shown
are portions of the observed spectra and the corresponding calculated spectra.
isotopomers.6 ͓Our independently determined values of the
S0 rotational constants ͑Table I͒ of An, AnHD, and AnDD,
though less accurate, agree with the microwave values,
within experimental error. Our S1 rotational constants for the
00, 6a1, 11, and I2 vibronic levels also agree with the recently
determined values of Kerstel et al.14͔ Key geometrical prop-
erties of the ground state are listed in Table IV. The phenyl
group is flat and is attached through the C–N bond to a
pyramidally distorted NH2 group, so that the planes defined
by the two groups make an angle of ␾Ϸ38° with respect to
each other. Currently,8 it is believed that inversion of the
NH2 group is controlled by a symmetric double well poten-
tial with a barrier to inversion of ϳ550 cmϪ1. Further, it is
believed that S1←S0 excitation of An flattens the molecule
2
0
1
1
¯
¯
¯
¯
TABLE III. Rotational constants ͑MHz͒, asymmetry parameters, and inertial defects ͑amu Å ͒ of aniline–NH2–Ar in its S0 ͑00͒ and S1 states ͑0 , 6a , and 1 ͒,
0
¯
¯
and of aniline–ND2–Ar in its S0 ͑00͒ and S1 ͑0 ͒ states, derived from analyses of their fully resolved S1←S0 excitation spectra. Two sets of constants are
1
¯
shown for the 10 band of aniline–NH2–Ar; see the text.
An–Ar
AnDD–Ar
0
1
1
0
¯
¯
¯
¯
00 band
6a0 band
10 band
00 band
¯
00 state
¯
00 state
A
Љ
B
Љ
C
Љ
␬
Љ
1795.7͑1͒
1143.7͑1͒
923.9͑1͒
Ϫ0.496
1795.7͑1͒
1143.8͑1͒
923.9͑2͒
Ϫ0.495
1795.7͑3͒
1144.4͑2͒
924.8͑4͒
Ϫ0.496
1795.6͑3͒
1144.3͑2͒
924.7͑4͒
Ϫ0.496
1702.8͑1͒
1130.1͑1͒
895.5͑2͒
Ϫ0.419
⌬I
Ϫ176.4
Ϫ176.2
Ϫ176.6
Ϫ176.6
Ϫ179.6
Љ
0
1
1
0
¯
¯
¯
¯
0 state
6a state
1 state
0 state
⌬A
⌬B
⌬C
Ϫ23.5͑1͒
22.5͑1͒
29.0͑1͒
Ϫ28.2͑1͒
22.3͑1͒
28.6͑1͒
Ϫ24.1͑4͒
21.6͑2͒
27.9͑4͒
Ϫ23.6͑4͒
21.5͑2͒
27.9͑4͒
Ϫ24.6͑1͒
23.9͑1͒
27.8͑2͒
␬
⌬I
Ϫ0.479
Ϫ188.3
Ϫ0.475
Ϫ188.6
Ϫ0.479
Ϫ188.2
Ϫ0.480
Ϫ188.2
Ϫ0.389
Ϫ191.7
Ј
Ј
T
2.2͑2͒
33 976.04͑1͒
0.0
297
3.9
2.2͑2͒
34 468.93͑1͒
492.9
197
3.4
2.5͑2͒
2.5͑2͒
2.5͑2͒
33 984.13͑1͒
8.1
229
4.6
␯
34 774.88͑1͒
798.9
0
⌬␯
N
OMC
113
11.4
106
5.2
J. Chem. Phys., Vol. 105, No. 18, 8 November 1996
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W. E. Sinclair and D. W. Pratt: Structure and dynamics of aniline–Ar
7947
TABLE IV. Experimental and theoretical structures and rotational constants
of aniline in its S0 and S1 states.
S0
S1
Expt.a
Calc.b
Expt.c
Calc.b
Distance/Å
0.997
1.397
1.393
1.383
1.386
1.077
1.076
1.075
Angle/deg
110.6
118.7
118.7
120.4
120.8
119.9
119.2
43
N–H1
C1–N
1.001
0.995
1.402
1.397
1.394
1.396
1.082
1.083
1.080
1.347
1.421
1.411
1.410
1.074
1.073
1.075
C1–C2
C2–C3
C3–C4
C2–H2
C3–H3
C4–H4
H1NH7
C6C1C2
C3C4C5
C1C2C3
C2C3C4
H2C2C3
H3C3C2
113.1
116.3
119.4
118.9
120.1
120.7
120.1
119.4
37.5
121.9
122.1
118.6
119.3
121.7
120.5
20
d
␾
Rotational constants/MHz
A
B
C
5617.4
2593.8
1777.0
5710.9
2620.6
1799.5
5276.9
2633.8
1759.4
5390.1
2662.8
1782.9
aReference 6.
b
*
6-31G values.
cThis work.
d
␾ is the dihedral angle between the NH2 and C6H5N planes.
More quantitatively, we have performed several tests of
the properties of An in its S0 and S1 states. First, we com-
puted the S0 and S1 structures of An using the Hartree–Fock
method for the ground state and the CI singles method for
FIG. 5. Measured line shapes of the JKa,Kcϭ101 rovibronic transition in the
0
¯
1
1
¯
¯
00 , 6a0 , and 10 bands of An–Ar. The solid lines represent the Lorentzian
components of the Voigt line shapes that were fit to the experimental points;
the residual width is Gaussian in character. The widths shown are the total
͑LorentzianϩGaussian͒ widths.
*
the excited state. A 6-31G basis set was used in both
calculations.31 All bond lengths and angles were varied to
obtain fully optimized geometries. The results, also listed in
Table IV, are in reasonably good agreement with experi-
ment. The computed dihedral angle of the S0 state is ␾ϭ43°;
it decreases to ␾ϭ20° in the S1 state. All C–C bond lengths
increase on S1 excitation, by as much as 0.025 Å. The C–N
bond length decreases by 0.05 Å and the C6C1C2 bond angle
increases by 3.2°, both changes evidencing increased inter-
action between the NH2 and phenyl groups. The calculated
rotational constants of the S0 state, and the changes in these
constants on S1 excitation, are at least in qualitative accord
with experiment.
along this coordinate, expands the aromatic ring, and short-
ens the C–N bond.4 The inversion potential function that
best fits the S1 spectroscopic data is essentially quartic with a
zero barrier.11
Our results are qualitatively consistent with these views.
The inertial defect of the ZPL of the S0 state of An is large
and negative, ⌬IЉϭϪ0.41 amu Å2. It also is sensitive to deu-
terium substitution; ⌬IЉ͑AnHD͒ϭϪ0.58 and ⌬IЉ͑AnDD͒
ϭϪ0.80 amu Å2. Both observations are consistent with a
significant out-of-plane distortion of the NH2 group. S1 ex-
citation produces a significant decrease in the magnitude of
the inertial defect. Further, it is less sensitive to deuterium
substitution; ⌬IЈ͑An͒ϭϪ0.23, ⌬IЈ͑AnHD͒ϭϪ0.24, and
⌬I͑AnDD͒ϭϪ0.27 amu Å2. Both of these observations are
consistent with a flattening of the NH2 group on S1 excita-
tion. Simultaneously, there is a large decrease in the A rota-
tional constant, reflecting the expansion of the ring, and an
increase in the B rotational constant, suggesting contraction
of the C–N bond. Clearly, there is enhanced conjugation of
the two groups in the electronically excited state.
Our experiments probe the vibrating molecule in the two
electronic states, whereas ab initio theory probes only the
stationary structure. To examine further the issue of vibra-
tional averaging, we determined the center-of-mass ͑COM͒
coordinates of the two amino hydrogen atoms using Kraitch-
man’s equations.32 Comparison of the observed rotational
constants of An, AnHD, and AnDD makes possible two such
determinations for the ZPLs of each state, one from the An/
AnHD pair and one from the An/AnDD pair. The results are
listed in Table V. Notably, the two determinations give the
same results, within experimental error. Thus, there is little
change in the vibrational amplitudes when the amino group
J. Chem. Phys., Vol. 105, No. 18, 8 November 1996
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7948
W. E. Sinclair and D. W. Pratt: Structure and dynamics of aniline–Ar
TABLE V. Center-of-mass coordinates of the amino hydrogen atoms for the
isotopic pairs An–NH2/An–NHD and An–NH2/An–ND2 in their S0 and S1
electronic states.
We now treat the inversion motion of S1 An in greater
detail. We use the ground state geometry to model this mo-
tion because vibrations along the inversion coordinate
sample, increasingly, noncoplanar configurations of the NH2
group. Shown in Fig. 6 are plots of the calculated A, B, and
C rotational constants of An as a function of the inversion
angle, ␾. The calculations were performed in two ways. In
the ‘‘invert’’ calculations, the rotational constants of the S0
state were calculated at selected values of the inversion
angle, without further modification of the initial, fully opti-
mized geometry. In the ‘‘IOPS’’ calculations, the S0 geom-
etry was fully optimized at each chosen inversion angle. The
This work
Lister et al.a
*
NH2/NHD
NH2/ND2
NH2/NHD NH2/ND2 6-31G
S0
͉
͉
͉
x
y
z
͉
͉
͉
͑Å͒
͑Å͒
͑Å͒
2.78͑1͒
Ϯ0.83͑1͒
0.29͑2͒
2.77͑1͒
Ϯ0.83͑1͒
0.33͑2͒
2.78
Ϯ0.83
0.30
2.77
Ϯ0.83
0.33
2.75
Ϯ0.82
0.32
S1
͉
͉
͉
x
y
z
͉
͉
͉
͑Å͒
͑Å͒
͑Å͒
2.79͑1͒
Ϯ0.87͑1͒
0.0͑1͒
2.77͑1͒
Ϯ0.87͑1͒
0.1͑1͒
2.77
Ϯ0.84
0.10
*
6-31G basis set was used in both calculations. Comparing
aReference 6.
these results to experiment, we find that only the IOPS cal-
culations correctly describe all the observed trends in the
rotational constants of the S1 state; i.e., decreasing A, B, and
C with increasing displacements along the inversion coordi-
nate. The magnitudes of the calculated changes also agree
with experiment. In contrast, the invert calculations predict
an increasing B and C with increasing ␾. The difference
between the results of these two calculations shows clearly
the importance of structural relaxation effects in the inver-
sion motion.
hydrogen atom͑s͒ is͑are͒ replaced by the heavier deuterium
atom͑s͒. However, it is important to note that the value of the
out-of-plane z coordinate is not well defined by this ap-
proach. There also is relatively good agreement between
these coordinates and those derived from both the structure
of Lister et al.6 and the ab initio calculations, as also shown
in Table V.
Next, we examine the inertial parameters of S1 An along
three vibrational coordinates; 6a, 1, and I. In the ground
state, the first two modes are in-plane vibrations; the 6a
mode is the totally symmetric component of the 6͑e2g͒ vi-
bration in benzene and the 1 mode is a symmetrical ring
breathing mode. The I mode is the out-of-plane NH2 inver-
sion mode. All three modes are shown below.
Shown below is the approximate location of the a, b,
and c inertial axes of S0An.
We see from the model that the effect of an increase in ␾
͑from ␾ϭ0°͒ is to increase Ia and decrease A, monotonically
until ␾ϭ90°, owing to the increasing displacement of the
amino hydrogen atoms from the a axis. This trend is shown
in Fig. 6 from both types of calculations; the change in A
from ␾ϭ0° to ␾ϭ90° is on the order of 40 MHz. As ␾
increases from ␾ϭ0°, the inertial contributions of the amino
hydrogen atoms to Ib and Ic are expected to decrease mono-
tonically due to the decreasing displacements of the amino
hydrogen atoms from the b and c axes, respectively. These
trends ͑i.e., increasing B and C͒ are reproduced only in the
invert calculations.
The differences between the invert and IOPS calcula-
tions must be associated with inversion-induced changes in
the C1–N bond length. When ␾ϭ0°, this bond is partially
double in character in the S1 state. The effect of increasing ␾
is to decrease the ␲ contribution, lengthening the bond. This
serves to change the magnitude of the differences in the ro-
tational constants in the IOPS calculations with respect to the
invert calculations. This effect is clearly significant enough
to alter the sign of the change in B and C with ␾, as shown
in Fig. 6.
Now, the A, B, and C rotational constants of the correspond-
ing S1 vibrational levels are all smaller than those of the S1
ZPL ͑Table I͒, reflecting increases in the expectation values
2
of ͑displacement͒ along all three coordinates. Exciting the
6a mode results in relatively large changes in B͑Ϫ3.1 MHz͒
and C͑Ϫ5.0 MHz͒, and a positive inertial defect ͑⌬IЈϭ0.37
amu Å2͒, as expected for an in-plane mode. Exciting the 1
mode results in relatively large changes in A͑Ϫ2.2 MHz͒ and
B͑Ϫ2.1 MHz͒, and an inertial defect ͑⌬IЈϭϪ0.21 amu Å2͒
that is not very different from that of the ZPL͑⌬IЈϭϪ0.23
amu Å2͒. And exciting the inversion mode results in rela-
tively large changes in A͑Ϫ20.7 MHz͒ and B͑Ϫ6.3 MHz͒,
and a significantly more negative inertial defect ͑⌬IЈϭϪ0.87
amu Å2͒, as expected for an out-of-plane mode. The ͉⌬IЈ͉
value of the I2 level increases only slightly upon isotopic
substitution, as expected for a planar molecule. We therefore
conclude from these results that the 6a, 1, and I vibrations of
An in its S1 state are very similar to the corresponding modes
in the ground state; i.e., there is little or no Duschinsky
rotation33 along these coordinates.
In view of these results, it is important to ascertain how
well the one-dimensional model used to fit the inversion
J. Chem. Phys., Vol. 105, No. 18, 8 November 1996
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W. E. Sinclair and D. W. Pratt: Structure and dynamics of aniline–Ar
7949
mode spectroscopic data correctly accounts for the inversion
level dependence of the observed inertial parameters. We
treat the motion as an internal rotation about an axis in the
ring plane that passes through the nitrogen atom and is per-
pendicular to the C1–N bond. The inversion potential can be
expanded in the form
͑An–Ar͒ϷCЈ ͑An͒, and since no axis reorientation is ob-
served on S1←S0 excitation. Thus, we conclude that the a,
b, and c axes of the complex are approximately parallel to
the c, a, and b axes, respectively, of the bare molecule, in
both electronic states.
V ␾͒ϭ 1V 1Ϫ cos ␾͒ϩ 1V 1Ϫ cos 2␾͒.
͑1͒
͑
͑
͑
2
2
1
2
First, we fit the inversion eigenlevels to the experimentally
determined S0 and S1 inversion mode spectroscopic data.11
Then, we computed the values of ͗␾͘ from the inversion
eigenfunctions of the two surfaces, ␺͑␯iϭ0͒ of the S0 state
and ␺͑␯iϭ0,2͒ of the S1 state. From the values of ͗␾͘, we
determined the corresponding ‘‘theoretical’’ values of the in-
ertial defects derived from the IOPS plot in Fig. 6. These
results are compared with those determined experimentally
in Table VI.
The experimental value of ⌬I for the ␯iϭ0 level of the
ground state is in best agreement with the IOPS calculation,
demonstrating that the potential energy surface for S0 de-
rived from the spectroscopic data accurately represents the
inversion motion, at least in the region of the symmetrically
displaced minima. Considerable disagreement is found in the
excited state, especially for the ␯iϭ2 level. The experimental
values of ⌬I are significantly larger in magnitude than the
theoretical ones. This demonstrates that using a one-
dimensional potential energy surface to represent the inver-
sion motion in the S1 state is inadequate, owing to the afore-
mentioned inversion-induced changes in the C1–N bond
length.
Vibrationally averaged structures of aniline–Ar
Next, we turn our attention to a determination of the
equilibrium geometries of An–Ar in its S0 and S1 electronic
states. Approximate structures may be deduced by compar-
ing two observables, the S1←S0 TM orientations and the S0
and S1 rotational constants. The two 000 bands are b type in
the bare molecule and c type in the complex. Attaching a
weakly bound Ar atom to An should have little or no effect
on the orientation of the TM in the molecular frame. There-
fore, the c axis of the complex must be approximately par-
allel to the b axis of the bare molecule. Next, we recall that
the inertial defect of the complex is large and negative, and
that AЉ ͑An–Ar͒ϷCЉ ͑Ar͒. Thus, the Ar atom in the ground
state complex must lie approximately along the axis that
is nominally perpendicular to the ring ͑the c axis in An͒,
above ͑or below͒ the aromatic plane. This must be the ap-
proximate structure of the complex in the S1 state, as well,
since its inertial defect also is large and negative, since AЈ
TABLE VI. Comparison of the experimental and theoretical inertial defects
of the S0 and S1 electronic states of aniline.
␯
͗␾͘/deg ⌬I from expt./amu Å2 ⌬I from theory/amu Å2
i
S0
S1
0
0
2
40
22
38
Ϫ0.41
Ϫ0.23
Ϫ0.86
Ϫ0.42
Ϫ0.11
Ϫ0.38
FIG. 6. Variation of the A, B, and C rotational constants of S0 aniline with
inversion angle, ␾; from ab initio calculations ͑IOPS͒, and from an inver-
sion of the amino group, without geometry optimizations ͑invert͒.
J. Chem. Phys., Vol. 105, No. 18, 8 November 1996
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7950
W. E. Sinclair and D. W. Pratt: Structure and dynamics of aniline–Ar
TABLE VII. Center-of-mass coordinates of the Ar atom in the ZPLs of the
S0 and S1 electronic states of aniline–Ar and aniline–ND2–Ar.
term gave similar results. The binding energy is increased to
450 cmϪ1, (x,y,z)ϭ͑Ϫ0.23, 0.00, 3.44 Å͒, and (⌬x,⌬y,⌬z)
ϭ͑0.30, 0.32, 0.12 Å͒. The S1 surface differs most from the
S0 surface along the x coordinate.
Our experimental results are in general accord with the
predictions of these model potentials, whose features have
been confirmed in independent calculations by us. The mea-
An–Ara
AnDD–Arb
S0
S1
͉
͉
͉
͉
͉
͉
x
y
z
x
y
z
͉
͉
͉
͉
͉
͉
, Å
, Å
, Å
, Å
, Å
, Å
0.201͑4͒
0.427͑7͒
3.508͑1͒
0.140͑5͒
0.376͑6͒
3.448͑1͒
0.267͑4͒
0.369͑10͒
3.512͑1͒
0.207͑4͒
0.318͑10͒
3.450͑1͒
sured values of
calculated values of z. Hence,
͉
z
͉
in the two states are nearly identical to the
is to a reasonable approxi-
͉z͉
mation an equilibrium COM coordinate, relatively unaf-
fected by vibrational averaging. It decreases on S1←S0 ex-
citation because the vdW bond is stronger in the more
aBased on a comparison of the rotational constants of aniline–NH2–Ar and
aniline–NH2 .
bBased on a comparison of the rotational constants of aniline–ND2–Ar and
aniline–ND2 .
polarizable S1 state. The measured values of ͉x͉ and ͉y͉, and
the changes in these values on S1 excitation, are comparable
in magnitude to the calculated values of ⌬x and ⌬y in the
two states. Thus, ͉x͉ and ͉y͉ are primarily measures of the
A more accurate measure of the vibrationally averaged
position of the attached Ar can be made using Kraitchman’s
equations.32 Thus, if we assume that there are no changes in
the geometry of the bare molecule on complex formation,
and that the bare molecule is a hypothetical complex with an
Ar atom having zero mass, we can determine the COM co-
ordinates of the attached atom from the changes in the rota-
tional constants that occur when it is replaced by the real Ar
atom in An–Ar. The results of these determinations for the
ZPLs of S0 and S1 An–Ar and AnDD–Ar are listed in Table
VII.
rms amplitudes of vibration along the x and y axes.
However, the model potentials do not provide an expla-
nation for some of the experimental results. For example, the
fact that ͉x͉ is found to be substantially less than ͉y͉ in the
ground state is inconsistent with a model potential that gives
⌬xϷ⌬y. NH2 substitution of benzene moves the COMϳ0.5
Å toward the N atom. If the potential function were centered
above the phenyl ring and cylindrically symmetric about the
perpendicular axis, one would expect ⌬x to be larger than
⌬y. Experimentally, the reverse is found. Hence, the poten-
tial must be significantly more attractive in the direction to-
ward the N atom than away from it.
Examining the data in this table, we see that the approxi-
mate structures deduced above are correct. The Ar atom in
An–Ar is located above ͑or below͒ the ring plane, close to
the a axis, in both electronic states. The determined values of
Deuterium substitution of the NH2 group moves the
COM still farther toward the N atom, by ϳ0.05 Å. Experi-
͉
x
͉
and
͉
y
͉
in the S0 state are 0.20 and 0.42 Å, respectively.
mentally, we find in AnDD–Ar that ͉x͉ increases and ͉y͉
Thus, either the equilibrium positions of the Ar atom along x
and y are different, or its vibrational displacements along the
two axes are different. The determined value of
decreases, compared to An–Ar ͑Table VII͒. The potential
itself should be invariant to such substitution, in first ap-
proximation. Thus, we again conclude that the vibrationally
averaged position of the Ar atom is shifted away from the
center of the aromatic ring, toward the N atom. Similar re-
sults are found for the S0 structures of other rare gas com-
plexes of aromatic molecules incorporating a single N atom
in the aromatic system. In the complexes of pyridine and
pyrrole with argon,34–36 the preferred binding site of the ar-
gon atom is shifted away from the center of the ring and
toward the N atom.
͉
z
͉
in the S0
found
state is 3.51 Å. This is comparable to the values of
͉z͉
in similar Ar atom-aromatic molecule complexes.34–36 The
magnitudes of all three COM coordinates decrease on S1
excitation, evidencing significant differences in the intermo-
lecular potential energy surfaces of the two electronic states.
Several authors have performed theoretical studies of
these surfaces. The most recent work is that of Parneix
et al.24 These authors constructed the intermolecular poten-
tial between the Ar atom and the An molecule as a sum of
pairwise additive atom–atom terms, with an induction term
added to account for the dipole–induced dipole interaction.
They found that the binding energy in S0 An–Ar is about
400 cmϪ1. The calculated mean values of the Ar atom COM
coordinates in the ZPL of the S0 state are (x,y,z)ϭ͑0.04,
0.00, 3.51 Å͒; the rms amplitudes of vibration along each
axis in this state were found to be (⌬x,⌬y,⌬z)ϭ͑0.36, 0.36,
0.12 Å͒. Thus, motion of the Ar atom in a plane parallel to
the phenyl ring is easier than in the direction perpendicular
to the ring. To model the S1 state, an additional positive and
anisotropic term centered on the N atom was added to the
potential, to describe the interaction between the Ar atom
and the N atom lone pair electrons. Additionally, the atom–
atom potentials were adjusted to reproduce the experimen-
tally observed redshift. Two different forms of the repulsive
Similar arguments apply to the S1 state of An–Ar. Here,
the vibrational amplitudes are smaller, a consequence of the
steeper nature of the potential along x and y, the Ar atom
being more tightly bound in the S1 state. But again, as for the
ground state, ͉x͉ is significantly less than ͉y͉, the difference
being larger in An–Ar than in AnDD–Ar. The S1 potential,
therefore, also must be significantly more attractive toward
the N atom than away from it.
Symmetry favors locating the Ar atom in the plane per-
pendicular to the aromatic ring and containing the An C2
axis; i.e., at yϭ0. Following Klots et al.,34 we show this to
be the case by comparing the planar moments of An–Ar and
AnDD–Ar with the corresponding planar moments of An
and AnDD ͑Table VIII͒. If the Ar atom is in the xz plane,
then only the nuclei of An will make a contribution to Py ,
and the planar moments of the NH2 and ND2 complexes
J. Chem. Phys., Vol. 105, No. 18, 8 November 1996
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W. E. Sinclair and D. W. Pratt: Structure and dynamics of aniline–Ar
7951
TABLE VIII. Planar momentsa ͑in amu Å2͒ of aniline (Py) and aniline–Ar
TABLE IX. COM coordinates of the amino hydrogen atoms in the S0 and
S1 states of aniline–Ar.
(PЈ) in the ZPLs of their S0 and S1 states.
y
Complex framea
Unsubstituted frameb
An–NH2–Ar
S0
An–ND2–Ar
S0
S0
S1
x ͑Å͒
y ͑Å͒
z ͑Å͒
x ͑Å͒
y ͑Å͒
z ͑Å͒
2.66
Ϯ0.96
Ϫ1.33
2.70
Ϯ0.98
Ϫ1.15
2.66
Ϯ0.96
Ϫ0.27
2.70
Ϯ0.98
Ϫ0.10
S1
S1
Py
89.75
87.99
1.8
95.5
93.9
1.6
91.15
89.81
1.3
97.0
95.8
1.2
PЈ
y
PyϪPЈ
y
aMoments of inertia were calculated from the rotational constants in Table
III. A conversion factor of 50.537 907ϫ104 amu Å2 was used for IϫB.
aBased on a comparison of the rotational constants of aniline–NH2–Ar and
aniline–ND2–Ar.
bAssuming z͑Ar͒ϭ3.51 Å.
should be equal to those of the corresponding bare mol-
ecules. The data in Table VIII show this to be approximately
the case. The two sets of moments differ by only 1–2
amu Å2. We attribute these differences to vibrational averag-
ing; motion of the Ar atom in one direction causes motion of
the ring in the opposite direction, reducing Py . Thus, we
conclude that the equilibrium position of the Ar atom in the
complex is at yϭ0. ͑In Table VIII, the S1 moments are typi-
cally larger than the S0 moments, owing to the overall ex-
pansion of the molecule on S1 excitation. Additionally, ND2
substitution increases Py because of the displacement of the
two amino H atoms from the xy plane.͒
the direction opposite to the displacement of the Ar atom
from the ring plane. To confirm this, we transformed the
COM coordinates of the two hydrogen atoms in the inertial
frame of the complex into the inertial frame of the bare mol-
ecule. The results of this transformation also are listed in
Table IX. The derived value of the z in the S0(S1) state is
Ϫ0.27 Å ͑Ϫ0.10 Å͒, if the z coordinate of the Ar atom in the
complex is assumed to be positive. Thus, we conclude that of
the two possible An–Ar structures shown below, the anti
structure is preferred. The vibrationally averaged position of
the Ar atom in An–Ar is on the side of the ring opposite to
the two amino H atoms.
Next, we turn to the coordinate z. As is the case with x
and y, a sign ambiguity exists in the determined values of
͉z͉, owing to the dependence of the observed rotational con-
stants ͑and their changes͒ on ͑displacement͒.2 But there are
two possible binding sites of the attached Ar, above or below
the ring. These sites are inequivalent in the S0 state because
of the nonplanarity of the amino group. To distinguish them,
we compare the measured values of ͉z͉ in the two different
isotopomers. If the Ar atom is on the side of the ring oppo-
site to the two H atoms, then ND2 substitution should in-
crease
substitution should decrease
increase in of 0.004 Å on ND2 substitution, suggesting
that the Ar atom and the two amino H atoms lie on opposite
sides of the ring. ͑The two values are the same in the S1
state, within experimental error, because of the planarity of
the NH2 group in this state͒. However, owing to the possible
influence of vibrational averaging along this coordinate in
the S0 state, we believe that this difference is too small to be
conclusive.
We therefore take a different approach. Using Kraitch-
man’s equations, we compare the rotational constants of the
NH2 and ND2 complexes in the ZPLs of their S0 states. We
thus determine the amino H atom coordinates in the COM
system of the complex. The results are listed in Table IX.
The vibrationally averaged value of the out-of-plane coordi-
͉
z
͉
. If the Ar atom is on the same side, then ND2
͉
z
͉
. We observe ͑Table VII͒ an
The model pair potentials discussed above uniformly
predict that the syn conformer of An–Ar is more stable than
the anti conformer, by 20 cmϪ1 or so.25 This prediction is not
in accord with the experimental result that the anti conformer
is preferred. Thus, some modifications to the theoretical po-
tential energy surfaces of An–Ar are required.
͉z͉
An estimate of the difference in the binding energies of
the Ar atom to the two sides of the ring can be obtained in
the following way. Inversion in the bare molecule is de-
scribed by a symmetric double minimum potential, with a
␯iϭ0–1 tunneling splitting ͑in An–NH2͒ of ␦ϭ40 cmϪ1 and
a barrier of Ebϳ550 cmϪ1.8 Attaching an Ar atom to one
side of the ring should convert the symmetric double well
into an asymmetric double well, with some energy difference
⌬ between the two minima. As a result, there should be a
change in both the energy eigenvalues and eigenfunctions
along the inversion coordinate. Modifications of the spectro-
scopic selection rules also are required. In particular,
͑ϩ͒↔͑Ϫ͒ transitions, forbidden in the symmetric case,
should become allowed. But there is no evidence for transi-
tions to ͑or from͒ odd levels in either the fluorescence exci-
tation or dispersed fluorescence spectra of An–Ar.19 Both
spectra are very similar to those of An itself. In An–Ar͑An͒,
nate in the S0(S1) state is
ing an Ar atom at the position
ordinate system of the complex in the S0 state displaces the
͉
z
͉
ϭ1.33 Å ͑1.15 Å͒. Now, attach-
ϭ3.51 Å in the COM co-
͉z͉
origin of the COM coordinate system of the bare molecule
by ϳ1.06 Å along the z axis. The measured values of the ͉z͉
coordinates of the amino hydrogen atoms in the two elec-
tronic states are significantly larger than this. Thus, the two
amino hydrogen atoms are displaced from the ring plane in
J. Chem. Phys., Vol. 105, No. 18, 8 November 1996
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7952
W. E. Sinclair and D. W. Pratt: Structure and dynamics of aniline–Ar
TABLE X. Intermolecular parameters of aniline–Ar and aniline–ND2–Ar
in their S0 and S1 states.
R/Å
␪/deg
͗␣͘/deg
S0
S1
An–Ar
AnDD–Ar
An–Ar
3.541͑3͒
3.542͑3͒
3.472͑3͒
3.472͑3͒
0.0͑5͒
0.9͑5͒
0
8.1͑2͒
9.1͑2͒
6.8͑5͒
7.8͑5͒
AnDD–Ar
0.9͑5͒
S0 inversion level. As shown in Fig. 7, the potential predicts
that this level is a level in which the amino H atoms are
primarily in the syn configuration. Alternatively, one could
determine the inversion level dependence of the electric di-
pole moment of An–Ar.
FIG. 7. Comparison of the NH2 inversion potentials of An ͑dashed͒ and
An–Ar ͑solid͒. Schematic wave functions squared for the ␷ϭ0 and ␷ϭ1
levels of S0 An–Ar and the ␷ϭ0 level of S1 An–Ar are shown.
Given that the two amino H atoms are largely localized
on one side of the ring, the Ar atom in the complex must be
largely localized on the other. What is responsible for this
localization? Some insights into the answer to this question
are provided by a comparison of the vibrationally averaged
structures of An–Ar and NH3–Ar. To make this comparison,
we first least-squares fit the observed rotational constants of
S0 An–Ar to the inertial model of Klots et al.,34 as modified
by Spycher et al.36 If modeled in this way, the determined
structure of the complex, described by the COM polar coor-
dinates R and ␪, also incorporates the large-amplitude mo-
tion of the Ar atom as an average angular displacement ␣
from its equilibrium position. The results are listed in Table
X. Then, we compare this structure with three proposed
structures of NH3–Ar, as shown in Fig. 8.
Ab initio calculations of the potential surface of NH3–Ar
have been performed by Chalasinski et al.38 and Bulski
et al.39 Additionally, Schmuttenmaer et al.40 have deter-
mined an ‘‘experimental’’ surface from a global fit to exten-
sive spectroscopic data. In general, three possible binding
sites have been identified, A, B, and C, all significantly in-
fluenced by vibrational motion. Of the three, B appears to be
the I20 band is at ⌬Eϭ760 cmϪ1 ͑758 cmϪ1͒ and the I02 band
is at ⌬EϭϪ421 cmϪ1 ͑Ϫ423 cmϪ1͒. Therefore, the asym-
metry in the inversion potential of An–Ar must be very
small.
Following Baughman et al.,37 we write the vibrational
wave function along this coordinate as a linear combination
of the two localized structures,
␺vϭs␺synϩa␺
.
͑2͒
anti
Solution of the resulting secular equation yields
Eϭ 12 ⌬Ϯ ⌬2ϩ␦2͒
,
͑3͒
͑4͒
1/2
͓
͑
͔
2
1/2
a
s
⌬
⌬
ϭϪ
Ϫ
ϩ1
.
ͩ ͪ
ͫ
ͩ ͪ
ͬ
␦
␦
Now, the value of ␦ in An–Ar is very similar to that in An
itself. However, there is an experimental uncertainty on the
order of several cmϪ1. The ␯iϭ0–1 interval in An–Ar could
be as large as 45 cmϪ1. In that event, ⌬ϭ20 cmϪ1 and
␺0ϭ0.52␺synϩ0.85␺
␺0ϭ0.85␺synϩ0.52␺
,
.
͑5a͒
͑5b͒
anti
anti
Qualitative plots of the resulting potential and the wave func-
tions squared are shown in Fig. 7. The energy levels of the
complex are the same as those of the bare molecule to within
1 cmϪ1. But, with a difference in energy of the two conform-
ers on the order of 20 cmϪ1 in the S0 state, there is a local-
ization of probability amplitude in the anti well of the com-
plex. The molecule spends ϳ72% of its time in the anti
configuration in the ZPL of the S0 state. Still, tunneling is not
completely quenched; the molecule spends ϳ28% of its time
in the syn configuration. Presumably, this motion is respon-
sible for the small decrease in ͉z͉ in An–Ar compared to the
bare molecule ͑Tables V and IX͒. Thus, we conclude that the
difference in the binding energy on the two sides of the ring
is of order 0.05Eb . Qualitatively similar behavior is pre-
dicted to occur in the S1 state ͑Fig. 7͒. Here, however, dis-
tortion of the quartic well leads to an increased value of ͉z͉.
A further test of this model would be to record the high
resolution spectrum of a band originating in the first excited
FIG. 8. ͑a͒ Calculated positions ͑Ref. 38͒ of the argon atom in Ar–NH3
corresponding to (A) the absolute minimum for dispersive interactions, (B)
the absolute minimum for repulsive interactions, and (C) the local mini-
mum in a superposition of dispersive and repulsive interactions. Experimen-
tally determined parameters ͑Ref. 40͒ are indicated. ͑b͒ Positions of the
argon atom in the syn and anti configurations of An–Ar. Experimentally
determined parameters are indicated ͑this work͒.
J. Chem. Phys., Vol. 105, No. 18, 8 November 1996
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W. E. Sinclair and D. W. Pratt: Structure and dynamics of aniline–Ar
7953
preferred. In this position, the Ar approaches between two
N–H bonds, giving minimal repulsion between the poten-
tially overlapping charge distributions. At the same time,
there is some long-range attraction from the N atom lone
pair. C is a secondary minimum whereas A is strongly dis-
favored. The geometrical properties of structure B, according
to the results of Schmuttenmaer et al.,40 are Rϭ3.57 Å and
band is consistent with the shorter calculated lifetime of the
I state, 1.7 ns.
2
¯
Currently, it is believed that vdW molecule dissociation
in complexes of this type is governed by a ‘‘serial’’
process.27 Pulsed laser excitation creates a nonstationary
state of the complex, a coherent superposition of the avail-
able molecular eigenstates ͑MEs͒ at the excitation energy.
These states are thought to be linear combinations of three
types of zeroth-order states; bound intramolecular modes of
the complex ͑which strongly resemble the corresponding
modes of the bare molecule͒, bound intermolecular modes
͑unique to the complex͒, and the dissociation continuum,
whose onset lies ϳ450 cmϪ1 above the minimum in the PES
of the An–Ar complex.
In a time-dependent description, the intramolecular vi-
brational energy must first flow into the intermolecular vdW
modes ͑IVR͒. Then, at sufficient energies, the intermolecular
vibrational energy flows into the dissociation continuum, and
the complex dissociates ͑VP͒. Thus, there are two time scales
in the serial model, the IVR time scale, and the VP time
scale. Whether or not these time scales can be distinguished
depends, of course, on the molecule, on its energy, and on
the method by which it is excited. Model calculations of the
observed kinetics of An–Ar and An–CH4 ͑Ref. 19͒ suggest
that IVR is slow and rate limiting in the former, whereas VP
is slow and rate limiting in the latter, at energies up to 700
cmϪ1 above the S1 origin under nanosecond excitation con-
ditions.
␪ϭ96.6°; its binding energy is 149.6 cmϪ1
.
Possible reasons for localization of the Ar atom in
An–Ar are revealed by a comparison of its structure with the
proposed structure of NH3–Ar ͑Fig. 8͒. The anti position in
An–Ar roughly corresponds to the C geometry of NH3–Ar.
Here, Rϭ4.2 Å and ␪ϭ53°. Replacing a NH3 hydrogen atom
with a phenyl group clearly stabilizes A and C relative to B.
But experiment shows that C is more favorable than A.
Therefore, the preference for the anti structure of An–Ar
must be a consequence of a combination of two factors; the
attractive nature of the N lone pair and the repulsive nature
of the two NH bonds. In the case of NH3–Ar,40 long-range
͑RϾ3.8 Å͒ attractive interactions are greatest when either a
N–H bond or the NH3 lone pair is directed toward the argon.
Vibrational dynamics and predissociation in
aniline–Ar
Finally, we discuss the excess energy-dependent dynam-
ics that appear on excitation of An–Ar to higher vibrational
levels of the S1 state. Recall that the homogeneous widths of
single rovibronic lines terminating in the ZPL of the S1 state
of the complex are the same as those of the corresponding
level of the bare molecule. Also recall that these widths are
the same for all measured vibronic bands of the bare mol-
ecule, whereas the corresponding bands of the complex ex-
hibit progressively larger widths. Finally, recall that the
highest-energy band of the complex examined in this work,
Our experiments were performed in the frequency do-
main, at eigenstate resolution. Nonetheless, as the following
discussion will illustrate, they provide unique information
about the dynamics of the IVR/VP process in the An–Ar
system.
0
¯
1
1
¯
¯
Figure 9 shows selected portions of the 00, 6a0, and 10
bands of An–Ar at full experimental resolution. On the left
are portions of the P and R branches that terminate in the
same upper state rovibronic levels, JЈKaKcϭ533 and 532, A
and B, respectively. Also shown are the corresponding cal-
culated spectra, based on rigid rotor fits of these bands and
the determined lineshape functions. Focusing on Fig. 9͑a͒, it
is immediately apparent that while the computed spectra of
1
¯
10, is perturbed. Several single rovibronic lines in this band
have frequencies and intensities that are significantly differ-
ent from the predicted ones. We believe that the appearance
of these larger homogeneous widths and spectral perturba-
tions signal the onset of an energy-dependent decay channel
in An–Ar. We also believe that this decay channel is vibra-
tional predissociation ͑VP͒.
0
¯
1
¯
those portions of the 00 and 6a0 bands shown are in excellent
agreement with experiment, there is significant disagreement
1
¯
Previous evidence for VP in An–Ar was obtained by
Nimlos et al.,19 employing both static and time-resolved
spectroscopic techniques and by Zhang et al.,26 employing
ZEKE photoelectron spectroscopy. These authors reported
between theory and experiment in the 10 band. The transition
to JЈKaKcϭ532 appears, but is shifted to the blue by ϳ50
MHz and broadened by ϳ100 MHz. The transition to
JЈKaKcϭ533 is not observed. In the corresponding R branch
1
1
¯
¯
dissociation of all levels above 6a0. Most importantly, it was
found that the decay of an initially excited vibronic level of
the complex was matched by the appearance of a dissociated
bare molecule, typically in the ZPL of the S1 state. The mea-
region ͓Fig. 9͑b͔͒, the transition to JЈKaKcϭ532 in the 10 band
is again shifted to the blue by ϳ50 MHz and exhibits a width
significantly larger than 50 MHz ͑the ‘‘unperturbed’’ SRVL
width͒. The transition to JЈKaKcϭ533 is observed, but appears
blue-shifted and significantly broadened. The irregular fea-
tures observed are common to both the P and R branches for
transitions terminating in the same S1 rovibronic level, evi-
dencing rotational perturbations in that state.
1
1
¯
¯
sured decay ͑and rise͒ times for excitation of the 6a0 and 10
bands were found to be 4.7 and 2.7 ns, respectively. Signifi-
1
1
¯
¯
cantly, these measured lifetimes of the 6a and 1 levels are
identical, within experimental error, to the lifetimes extracted
from the measured homogeneous linewidths of the corre-
sponding bands in the high resolution spectra, 5.3 and 3.2 ns,
Turning our attention to Figs. 9͑c͒ and 9͑d͒, we see that
similar effects are observed in other regions of the spectrum.
The portions of the spectra shown terminate in the
2
¯
respectively. Additionally, our inability to resolve the I0
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7954
W. E. Sinclair and D. W. Pratt: Structure and dynamics of aniline–Ar
0
¯
1
1
¯
¯
FIG. 9. Selected portions of the 00 , 6a0 , and 10 bands of An–Ar. A and B show portions that terminate in the upper state rovibronic levels JKЈa,Kc ϭ 532 and
533 in the P and R branch regions, respectively. C and D show portions that terminate in the upper state rovibronic levels JЈKa,Kc ϭ 432 and 433 in the P and
R regions, respectively.
0
¯
JЈKaKcϭ431 and 432 single rovibronic levels. As before, there
MHz, which exceeds the widths of SRVLs in the 00 band by
0
¯
1
¯
are no perturbations in the 00 and 6a0 vibronic bands. But
␥ϳ8 MHz. We attribute this excess width to VP;
kVPϭ2␲␥ϳ50 MHz. This decay width is independent of ro-
tational quantum number, as expected for dissociation in the
statistical limit of a nonradiative process.
1
¯
the 10 band shows both line shifts and selective line broad-
enings. In the P branch region, transitions to these two levels
are broadened and red-shifted, by ϳ40 MHz. And in the R
branch, where they are well resolved, both transitions are
red-shifted by ϳ40 MHz and broadened to ϳ70 MHz.
Voigt line-shape fits to several of these irregular regions
The situation is different at an excess energy of 798
cmϪ1. Here, VP also makes a major contribution to the de-
1
¯
cay; most SRVLs in the 10 band are broadened to a homo-
geneous width of 50 MHz. Hence, ␥ϳ28 MHz and
VPϭ2␲␥ϳ150 MHz. The decay width due to VP thus in-
1
¯
in the 10 band were performed. Figure 10 shows two ex-
amples. In Fig. 10͑a͒, we estimate a Lorentzian linewidth of
70Ϯ10 MHz for the R branch transition terminating in
JЈKaKcϭ431. Figure 10͑b͒ shows simulated spectra in the re-
gion of the R branch transition terminating in JЈKaKcϭ533,
performed with Lorentzian linewidths of 50 and 100 MHz.
By comparison with the observed overlapping transitions of
Fig. 9͑b͒, we estimate a homogeneous linewidth of 100Ϯ10
MHz for the transition. Notably, the integrated intensities of
the two lines are the same as those calculated from the pre-
k
creases with increasing energy content. Additionally, most
SRVLs are unperturbed. However, some lines are perturbed
and further broadened. We believe that these perturbations
arise from couplings between intramolecular modes and in-
termolecular modes. In the zeroth-order picture, specific
SRVLs of the complex, excited along the intramolecular
modes, interact with specific SRVLs of the complex excited
along intermolecular modes, of which there are a large num-
ber of possible candidates ͑␳ϳ30͒. The occurrence of such
‘‘accidental’’ perturbations thus signals the onset of mode
mixing, the frequency domain prerequisite to IVR. There
appears to be little or no rotational dependence of the widths
of the SVRLs, indicating that the coupling is essentially an-
harmonic. Significantly, no ‘‘extra’’ lines are observed, sug-
gesting that the IVR coupling matrix element are relatively
small. ͑Recall, also, that the integrated intensities of the per-
¨
dicted Honl–London factors.
These results show that, at an excess energy of 492
cmϪ 1, the major contributor to the observed decay of S1
1
¯
An–Ar is VP. The 6a0 band fits well to a rigid rotor model
for both states, with no evidence of spectral perturbations at
our resolution ͑ϳ20 MHz͒. The standard deviation of the fits
0
¯
is comparable to that of the 00 band. However, each SVRL in
1
¯
the 6a0 band is broadened to a homogeneous linewidth of 30
J. Chem. Phys., Vol. 105, No. 18, 8 November 1996
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W. E. Sinclair and D. W. Pratt: Structure and dynamics of aniline–Ar
7955
electronic state. Most of the intramolecular vibrational
modes are unaffected by this attachment, in either electronic
state. However, the symmetric double well along the inver-
sion coordinate in the S0 state of the bare molecule is con-
verted into an asymmetric double well in the complex, re-
sulting in significant vibrational localization. The Ar atom
exhibits large amplitude vibrational motion with respect to
the ring, in both transverse directions, in both electronic
states. Couplings between intra- and intermolecular modes
are revealed by studies of the spectra at higher energies in
the S1 state of the complex. Analyses of the line broadenings
and perturbations that appear in these spectra provide inde-
pendent estimates of the time scales for intramolecular vibra-
tional redistribution ͑IVR͒ and vibrational predissociation
͑VP͒, from which it is concluded that the dynamics occurs by
a two-step, serial process, IVR followed by VP.
ACKNOWLEDGMENTS
M. Becucci introduced us to the aniline problem. We
thank E. Kerstel for sending us preprints of Ref. 14 and for
valuable discussions. D.W.P. also thanks J. P. Simons and
the spectroscopy and dynamics groups at Oxford University,
where portions of this paper were written, for their kind hos-
pitality and stimulating conversations. This work has been
supported by National Science Foundation ͑CHE-9224398͒.
FIG. 10. ͑A͒ Measured line shapes of the JKa,Kcϭ431 rovibronic transitions
1
¯
in the 10 band of An–Ar. The solid lines represent the Lorentzian compo-
nents of the Voigt line shapes that were fit to the experimental points; the
residual width is Gaussian in character. ͑B͒ Simulated spectra in the region
of the R branch transition terminating in JЈKaKcϭ533 , with Lorentzian lin-
ewidths of 50 and 100 MHz, respectively.
1 J. C. Evans, Spectrochim. Acta 16, 428 ͑1960͒.
2 J. C. D. Brand, D. R. Williams, and T. J. Cook, J. Mol. Spectrosc. 20, 359
͑1966͒.
3 J. Christoffersen, J. M. Hollas, and G. H. Kirby, Mol. Phys. 16, 441
͑1969͒.
4 T. Cvitas, J. M. Hollas, and G. H. Kirby, J. Mol. Struct. 19, 305 ͑1970͒.
5 M. Quack and M. Stockburger, J. Mol. Spectrosc. 43, 87 ͑1972͒.
6 D. G. Lister, J. K. Tyler, J. H. Hog, and N. W. Larson, J. Mol. Struct. 23,
253 ͑1974͒.
turbed lines are conserved.͒ An–Ar is in the small molecule
limit with respect to IVR. Most notably, however, we also
find that the perturbed lines are broader that the unperturbed
lines. Excess widths of up to ␥ϳ80 MHz are observed, yield-
ing kVPϭ2␲␥ϳ500 MHz. Therefore, in agreement with pre-
vious work,27 we conclude that the mixing of intra- and in-
termolecular vibrational modes greatly enhances the rate of
VP in the vdW complexes of rare gas atoms and aromatic
molecules. Presumably, the perturbations that evidence such
mixing also exists at lower energies, but are too small to
observe at our resolution. On the other hand, the correspond-
7 N. W. Larsen, E. L. Hansen, and F. M. Nicolaisen, Chem. Phys. Lett. 43,
583 ͑1976͒.
8 R. A. Kydd and P. J. Krueger, Chem. Phys. Lett. 49, 539 ͑1977͒.
9 N. Mikami, A. Hiraya, I. Fujiwara, and M. Ito, Chem. Phys. Lett. 74, 531
͑1980͒.
10 J. M. Hollas and T. Ridley, J. Mol. Spectrosc. 89, 232 ͑1981͒.
11 J. M. Hollas, M. R. Howson, T. Ridley, and L. Halonen, Chem. Phys. Lett.
98, 253 ͑1983͒.
12 G. Roussy and A. Nonat, J. Mol. Spectrosc. 118, 180 ͑1986͒.
13 B. Kleibomer and D. H. Sutter, Z. Naturforschung 43a, 561 ͑1988͒.
14 E. R. Th. Kerstel, M. Becucci, G. Pietrapeizia, and F. Castellucci, Chem.
Phys. 199, 263 ͑1995͒; E. R. Th. Kerstel, M. Becucci, G. Pietraperzia, D.
Consalvo, and E. Castellucci, J. Mol. Spectrosc. 177, 74 ͑1996͒.
15 A. Amirav, U. Even, J. Jortner, and B. Dick, Mol. Phys. 49, 899 ͑1983͒.
16 J. L. Knee and P. M. Johnson, J. Chem. Phys. 80, 13 ͑1984͒.
17 K. Yamanouchi, S. Isogal, S. Tsuchiya, and K. Kuchitsu, Chem. Phys.
116, 123 ͑1987͒.
2
¯
ing perturbations in the I0 band must be very large, owing to
the out-of-plane character of the I2 state. As we have shown,
there is a weak but significant interaction of the argon atom
with the An molecule along the inversion coordinate.
SUMMARY
18 A. R. Bacon and J. M. Hollas, Faraday Discuss. Chem. Soc. 86, 129
͑1988͒.
We have studied the eigenstate resolved S1←S0 fluores-
cence excitation spectra of aniline and its single atom van
der Waals complex with argon in the collision-free environ-
ment of a molecular beam. Analyses of these spectra provide
new information about the structural and dynamical proper-
ties of the isolated molecule and how these properties are
modified by complex formation. We find that the Ar atom is
attached above the aromatic ring, opposite to the two N–H
bonds, in the pyramidally distorted molecule in its ground
19 M. R. Nimlos, A. Young, E. R. Bernstein, and D. F. Kelley, J. Chem.
Phys. 91, 5268 ͑1989͒.
20 E. J. Bieske, M. W. Rainbird, I. M. Atkinson, and A. E. W. Knight, J.
Chem. Phys. 91, 752 ͑1989͒.
21 E. J. Bieske, M. W. Rainbird, and A. E. W. Knight, J. Chem. Phys. 94,
7019 ͑1991͒.
22
´
S. Douin, P. Parneix, and Ph. Brechignac, Z. Phys. D 21, 342 ͑1991͒.
23 E. J. Bieske, A. S. Uichanco, M. W. Rainbird, and A. E. W. Knight, J.
Chem. Phys. 94, 7029 ͑1991͒.
P. Parneix, N. Halberstadt, Ph. Brechignac, F. G. Amar, A. van der Av-
24
´
J. Chem. Phys., Vol. 105, No. 18, 8 November 1996
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
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7956
W. E. Sinclair and D. W. Pratt: Structure and dynamics of aniline–Ar
oird, and I. van Bladel, J. Chem. Phys. 98, 2709 ͑1991͒.
P. Hermine, P. Parneix, B. Coutant, F. G. Amar, and Ph. Brechignac, Z.
Pople, GAUSSIAN 94, Revision B.2, Gaussian, Inc., Pittsburgh, PA, 1995.
25
32 J. Kraitchman, Am. J. Phys. 21, 17 ͑1953͒.
33 F. Duschinskii, Acta Physicochim. URSS 1, 551 ͑1937͒.
34 T. D. Klots, T. Emilsson, R. S. Ruoff, and H. S. Gutowsky, J. Phys. Chem.
93, 1255 ͑1989͒.
´
Phys. D 22, 529 ͑1992͒.
26 X. Zhang, J. M. Smith, and J. L. Knee, J. Chem. Phys. 97, 2843 ͑1992͒.
27 D. F. Kelley and E. R. Bernstein, J. Phys. Chem. 90, 5164 ͑1986͒.
28 W. A. Majewski, J. F. Pfanstiel, D. F. Plusquellic, and D. W. Pratt, in
Laser Techniques in Chemistry, edited by A. B. Myers and T. Rizzo
͑Wiley, New York, 1995͒, p. 101.
35 R. K. Bohn, K. W. Hillig, and R. L. Kuczkowski, J. Phys. Chem. 93, 3456
͑1989͒.
36 R. M. Spycher, D. Petitprez, F. L. Bettens, and A. Bauder, J. Phys. Chem.
98, 11863 ͑1994͒.
29 F. W. Birss and D. A. Ramsay, Comput. Phys. Commun. 38, 83 ͑1984͒.
30 J. K. G. Watson, J. Chem. Phys. 46, 1935 ͑1967͒.
37 S. L. Baughcum, R. W. Duerst, W. F. Rowe, Z. Smith, and E. B. Wilson,
J. Am. Chem. Soc. 103, 6296 ͑1981͒.
31 M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson,
M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgom-
ery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J.
B. Foresman, J. Cioslowski, B. Stefanov, A. Nanayakkara, M. Challa-
combe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E.
S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J.
Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez, and J. A.
38 G. Chalasinski, S. M. Cybulski, M. M. Szcesniak, and S. Scheiner, J.
Chem. Phys. 91, 7809 ͑1989͒.
39 M. Bulski, P. E. S. Wormer, and A. van der Avoird, J. Chem. Phys. 94,
491 ͑1991͒.
40 C. A. Schmuttenmaer, R. C. Cohen, and R. J. Saykally, J. Chem. Phys.
101, 146 ͑1994͒.
J. Chem. Phys., Vol. 105, No. 18, 8 November 1996
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