A re-examination of the S0 -> S1 excitation spectrum of dimethylaniline

Article abstract of DOI:10.1063/1.469967

A new assignment for the S₀->S₁, transition of N,N-dimethylaniline (DMA) and related derivatives is presented.The 1ow frequency bands and long Franck-Condon envelope observed in DMA-h₆ and DMA-d₆ are assigned to the coupled methyl torsion mode of the amino group, not to torsion of the amino substituent about the C-N bond.This new assignment is consistent with the change in frequency of the excitation bands upon deuteration of the methyl groups and the strong origin transitions observed in the excitation spectra of other alkyl anilines.The assignment was confirmed by simulations of the excitation spectra of DMA-h₆ and DMA-d₆, with parameters of the calculated potential energy surface determined to be V₃=148.0+/-0.5 cm^-1, V₊=-31.6+/-0.5 cm^-1, V_-=8.5+/-0.5 cm^-1, and V₆=-15+/-0.5 cm^-1.By Franck-Condon analysis, it was determined that the weak origin transition is due to the shifting of the S₁ torsion minimum by 40 deg along the gearing coordinate relative to the corresponding minimum in the ground state.

Full text of DOI:10.1063/1.469967

A reexamination of the S0→S1 excitation spectrum of dimethylaniline
Robert A. Weersink, Stephen C. Wallace, and Robert D. Gordon
Citation: The Journal of Chemical Physics 103, 9530 (1995); doi: 10.1063/1.469967
View online: http://dx.doi.org/10.1063/1.469967
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A re-examination of the S ˜S excitation spectrum of dimethylaniline
0
1
Robert A. Weersink and Stephen C. Wallace
Department of Chemistry, University of Toronto M5S 1A1, Canada
Robert D. Gordon
Department of Chemistry, Queen’s University, Kingston K7L 3N6, Canada
͑
Received 11 May 1995, accepted 6 September 1995͒
A new assignment for the S →S transition of N,N-dimethylaniline ͑DMA͒ and related derivatives
0
1
is presented. The low frequency bands and long Franck–Condon envelope observed in DMA-h and
6
DMA-d are assigned to the coupled methyl torsion mode of the amino group, not to torsion of the
6
amino substituent about the C–N bond. This new assignment is consistent with the change in
frequency of the excitation bands upon deuteration of the methyl groups and the strong origin
transitions observed in the excitation spectra of other alkyl anilines. The assignment was confirmed
by simulations of the excitation spectra of DMA-h and DMA-d , with parameters of the calculated
6
6
Ϫ1
Ϫ1
potential energy surface determined to be V ϭ148.0Ϯ0.5 cm , V ϭϪ31.6Ϯ0.5 cm
,
3
ϩ
Ϫ1
Ϫ1
V ϭ8.5Ϯ0.5 cm , and V ϭϪ15Ϯ0.5 cm . By Franck–Condon analysis, it was determined
Ϫ
6
that the weak origin transition is due to the shifting of the S torsion minimum by 40° along the
1
gearing coordinate relative to the corresponding minimum in the ground state. © 1995 American
Institute of Physics.
3
I. INTRODUCTION
A subsequent analysis of the same data again assigned
the bands to dimethylamino torsion, deriving a S potential
1
The twisted intramolecular charge transfer process
TICT͒ model requires a twisting of the donor group by
energy surface that exhibited the proper symmetry and accu-
rately fit several of the observed energies and intensities of
the observed DMA bands. The calculated PES has minima at
Ϯ25° relative to the planar configuration of the ground elec-
tronic state. The model, however, ignores several bands and
has difficulties in predicting the observed excitation spec-
trum of DMABN with the methyl groups deuterated,
DMABN-d6 .2
͑
9
0° relative to the acceptor group to stabilize the charge
1
transfer state. In derivatives of dimethylaniline ͑DMA͒,
such as dimethylamino benzonitrile ͑DMABN͒, the donor
group is the dimethylamino group and the acceptor group is
a benzene ring substituted in the para position. Because of
the twisting motion, there has been considerable interest in
determining the correct potential energy surface along this
A moderately high resolution band contour analysis of
1
1
4
coordinate for the S , L , and L electronic states. There
DMABN prepared in a jet expansion concluded that the
0
b
a
is much evidence in the literature supporting a TICT process
in solution, where solvation must play a role in stabilizing
the twisted state.
amino group is twisted from the planar position by about
30° in the S1 state, with the potential having a double-
minimum character. The estimate of the twist angle had a
high uncertainty since the calculated spectra were rather in-
sensitive to the dimethylamino torsion coordinate, indicating
to the authors that the barrier to amino torsion about the
planar position was either very small or non-existent.
This study aims to examine the situation in the gas
phase. Several studies of DMA and DMABN cooled in a
supersonic jet expansion have been done to elucidate infor-
mation regarding the excited state potential energy surfaces,
since such a preparation removes much of the spectral con-
gestion observed in the condensed phase. The observed spec-
tra exhibit a weak origin and long Franck–Condon progres-
sion, with a large number of low frequency vibrational
bands. A jet-cooled REMPI study of DMABN and DMA
assigned the observed low frequency modes in the S → S
The conformation of the amino substituent in aniline de-
rivatives depends on the electronic properties of the nitrogen.
Its hybridization and conjugation with the benzene ring de-
termine the potential energy surfaces for the amino torsion
and the inversion motions. In the ground electronic state, the
3
5–7
nitrogen is basically of sp hybridization, as evidenced by
0
1
spectrum to inversion and torsion of the whole dimethyl
the pyramidal structure of the amino group. The lone pair of
2
3
amino substituent about the CarylϪN bond. Based on a
electrons on the nitrogen in the sp hybrid form can undergo
Franck–Condon analysis, the authors concluded that the
minimum energy configuration of the amino group in DMA
conjugation with the ␲ electrons of the benzene ring. There-
fore, the Caryl –N bond is not exclusively a ␴ bond, but it has
some ␲ character as well. Because the nitrogen is essentially
and DMABN is twisted by about 30° in the S state relative
1
3
to the minimum energy configuration of the S state. The
sp hybridized, the substituents attached to the end of the
0
assumed symmetry of the torsion potential, however, is in-
correct since it neglects the plane of symmetry along the
long axis of the molecule. By assuming the amino group is
twisted out of the plane by 30°, the potential is antisymmet-
ric with respect to the vertical plane of symmetry along the
long axis of the molecule.
nitrogen are out-of-plane with respect to the benzene plane.
Torsion about the Caryl –N bond, however, is not the
only possible large amplitude vibration in the dimethylamino
substituent. Inversion of the CNC plane of the substituent
through the plane of the benzene ring is also expected to
occur. In aniline, this mode is prominent in the excitation and
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9
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Weersink, Wallace, and Gordon: Excitation spectrum of dimethylaniline
9531
emission spectra.5–8 As in aniline, an inversion barrier exists
and collimated to form a spot size of approximately 5 mm in
diameter on the jet axis. To avoid saturation of the excitation
transition and to maximize the ratio of two-colour to the
one-colour signal of ␭ alone, the intensity of ␭ was attenu-
at the planar position in the S state. Microwave studies of
0
DMA ͑Ref. 9͒ indicate a pyramidal conformation in S with
0
the Caryl –N bond making an angle of about 27° with the
1
1
Me-N-Me plane, and a barrier to inversion on the order of
ated either by neutral density filters or with a variable attenu-
ator.
Ϫ1
2
50 cm . For the S state, it has been determined that along
1
the inversion coordinate, the minimum occurs at the planar
position. As well, torsion of the methyl rotors can occur.
The ions produced in the expansion were deflected into a
quadrupole mass spectrometer ͑VG QXK300͒ where they
were detected and analyzed. When no mass resolution was
required, the mass spectrometer was left off. Details of the
ion extraction configuration and mass spectrometer are de-
scribed in a previous publication.14
Dispersed fluorescence emission data were collected us-
ing the time-correlated-single-photon counting method ͑TC-
SPC͒. The second harmonic of a cw mode-locked Nd:YAG
laser ͑Spectra Physics 3460 laser, Spectra Physics 342A
mode-locker͒ synchronously-pumped a tunable, extended
cavity dye laser ͑Spectra Physics 375B͒, with an acousto-
optic cavity dumper ͑Spectra Physics 344͒ operating at 4.1
MHz. Rh6G was used in the dye laser with an average power
of 300 mW at 603 nm. This output was doubled in a 1 cm3
4
Since the two rotors are sterically close together, it is ex-
pected that the motion of the rotors are coupled.1
0–13
Sepa-
ration of all of these large amplitude vibrations into distinct
modes may, in fact, be impossible, since the potential energy
surface for one vibration is influenced by the position that
the molecule is situated along the potential energy surface of
one of the other vibrations.
This paper presents a new analysis of the S →S spectra
0
1
of DMA and DMABN, assigning the observed bands to the
coupled methyl torsion mode of the dimethylamino substitu-
ent. This analysis is consistent with both the observed shift
of the S →S spectrum of DMA-d , and with the excitation
0
1
6
spectra of other alkyl anilines. It is found that there is a
moderate barrier to complete internal rotation of the rotors,
with a small degree of coupling between the rotors. Also, the
minimum energy configurations of the rotors shift dramati-
cally in the excited state relative to similar configurations in
the ground state, likely the result of coupling of the methyl
LiIO crystal to produce 15-30 mW of UV light, for a pulse
3
energy of 3-5 nJ. This excitation beam, focused to a 100
␮m waist crossed the jet axis at right angles, 20-30 nozzle
diameters downstream from the pinhole. Light from the fluo-
rescing region of the jet was collected on the axis mutually
perpendicular to that of the laser beam and the jet axis and
focused into the entrance slit of a monochromator ͑GCA
McPherson 0.35 m, f/6.5͒. The dispersed fluorescence which
exits the monochromator is detected by a photomultiplier
tube ͑Hamamtsu R1527͒. Details of the TCSPC electronics
are given in a previous publication.15
torsion modes with the inversion mode in the S state.
0
II. EXPERIMENT
Details of the experimental setup have been described
1
4,15
previously.
Therefore, only a brief discussion of the ex-
pansion conditions and the excitation sources for the multi-
photon ionization and fluorescence emission experiments
will be given.
III. RESULTS
The molecule was prepared in a continuous free-jet ex-
pansion with He as the carrier gas. The nozzle was fitted with
a circular pinhole of diameter 25, 50, or 100 ␮m and heated
to about 50-75 °C to increase the concentration of DMA in
the expansion. The backing pressure was typically between
A. Excitation Spectra of DMA
The excitation spectra of DMA-h and DMA-d are de-
6
6
picted in Figure 1. The spectrum of DMA-h is very similar
6
2
to that previously reported. The lowest energy transition is
Ϫ1
0.5 and 5 atm. DMA was used ͑Aldrich 99ϩ%͒ without fur-
the weak band observed at 32 895.8 cm . Examination of
ther purification. DMA-d was prepared by reacting aniline
with ethylchloroformate to make the required amide which
the region below this band found no evidence for any other
transitions and the relative intensities of the observed bands
did not change under a variety of expansion conditions.
6
was subsequently reduced with LiAlD to make NMA-d .
4
3
16
Ϫ1
The procedure was repeated once to make DMA-d6 .
Therefore, the weak band at 32 895.8 cm is assigned to the
Excitation spectra of the lowest excited state of the iso-
lated DMA were acquired using 2-color, 1ϩ1 resonance en-
hanced multiphoton ionization ͑REMPI͒. The doubled output
of a Nd:YAG laser ͑Spectra-Physics GCR-3͒, Q-switched at
a repetition rate of 10 Hz, was split to pump two dye lasers
origin transition. This assignment is consistent with previous
2
assignments of DMA and DMABN. Prominent bands occur
Ϫ1
at 70, 100, 130, and 165 cm above the origin transition,
Ϫ1
with a weaker band occuring at 118 cm . Large changes in
the intensities and frequencies are observed in the excitation
͑
Quanta-Ray PDL͒. The output of these lasers was then
spectrum of DMA-d ͑Figure 1b͒. The origin of DMA-d is
at 32 976.7 cm , higher in energy than the origin of DMA-
h6. Such a blue shift indicates that the modes involving the
6
6
Ϫ1
doubled with a 2 cm long KDP crystal to give pulsed ultra-
violet light of 5-7 ns duration, 0.5 cm linewidth, and a
Ϫ1
pulse energy of 5 mJ. Either a Quanta-Ray WEX ͑Wave-
length Extender͒, or a home-built computer-controlled track-
ing system was used to maintain the phase matching angle of
the doubling crystals while the dye lasers were being
scanned. Temporal overlap between the two pulses was
deuterated atoms have a higher zero-point energy in S than
0
in S . The band positions are very similar to those in the
1
excitation spectrum of DMABN-d ͑Ref. 2͒ but differences
6
occur in the intensities of some of the bands. This may be the
result of saturation in the excitation spectrum of DMA-d ,
6
achieved though a delay line in the path of ␭ . Both ultra-
which will increase the relative intensities of the weaker
2
violet beams were directed 3-5 mm in front of the nozzle,
bands. Since only a small amount of DMA-d sample was
6
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9532
Weersink, Wallace, and Gordon: Excitation spectrum of dimethylaniline
FIG. 1. 2-Color resonance enhanced multiphoton ionization spectrum of the
S1 state of ͑a͒ DMA-h6 and ͑b͒ DMA-d . The weak origins occur at 32
6
Ϫ1
Ϫ1
8
95.8 cm and 32 976.7 cm for DMA-h6 and DMA-d , respectively.
6
FIG. 2. Single vibronic fluorescence emission spectra following excitation
of various bands in the DMA excitation spectrum.
available, the regular tests for saturation were sacrificed in
favour of obtaining the correct band positions. It has been
postulated that the origin of the DMA-d6 spectrum is not
3
observed. To tests this hypothesis, the region to lower en-
abscence of torsion splittings in the microwave spectrum of
Ϫ1
ergy of the band at 32 976.7 cm was examined carefully
DMA ͑Ref. 9͒ and by the fact that dimethylamine has a bar-
Ϫ1 18
for any other weak transitions. None were observed and
rier to internal roation of about 1280 cm
.
The higher
Ϫ1
therefore the assignment of the band at 32 976.7 cm to the
barrier in S0 is also consistent with the observed blue shift in
the origin of the excitation spectrum of DMA-d6 relative to
the origin of DMA-h6 . The small shifts in the frequencies
of the lowest emission bands signify that the mode giving
origin transition is made with confidence. The large changes
in frequency of the S bands indicates that the large ampli-
1
tude motion in the excited state that gives rise to these low
frequency bands must be strongly affected by the change in
mass of the substituent. The mode most consistent with this
large change in frequency is methyl torsion.
B. Emission spectra of DMA
Single vibronic level fluorescence emission spectra of
DMA-h excited at a number of S bands are depicted in
6
1
Figure 2. The emission spectra resulting from excitation of
most of the noted bands in Figure 2 all have similar low
Ϫ1
frequency bands while the 118 cm band has a different
emission spectrum. The similarities in the emission spectra
Ϫ1
indicate that the 70, 100, 130, and 165 cm bands of the
excitation spectrum belong to the same progression. Any
model used to predict the the excitation spectrum of DMA
must therefore include these bands.
The single vibronic level fluorescence emission spectra
of DMA-h and DMA- d excited at their respective stron-
6
6
gest transitions are compared in Figure 3. Emission bands
Ϫ1
below 300 cm do not show a large deuteration shift and
are therefore unlikely to involve methyl torsion. Thus the
expected methyl torsion bands are likely to fall in the
crowded higher-energy region. Consequently, torsional fre-
quencies, and hence the barrier to internal rotation are sig-
nificantly higher in S than in S . This is supported by the
FIG. 3. Single vibronic fluorescence emission spectra following excitation
of the strongest bands in the excitation spectra of DMA-h6 and DMA-d6 .
0
1
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Weersink, Wallace, and Gordon: Excitation spectrum of dimethylaniline
9533
sult of methyl torsion. The potential energy surface of the
amino torsion in all five alkyl anilines should be approxi-
mately equivalent. The steric interactions of the alkyl groups
with the benzene ring should be the same for DMA, DEA
and PYRBEE. Therefore, one should expect the same
Franck–Condon envelope for the progression of this mode in
the S → S spectra of these alkyl anilines, although differ-
0
1
ences in the masses of the substituent would affect the vibra-
tional frequencies. The inversion potential may vary from
molecule to molecule, but the S state should be essentially
1
planar with a flat well as in aniline. Hence, the observed
progression should have similar Franck–Condon envelopes
for the progression of the inversion mode in the vibronic
spectra of these anilines. Only the alkyl torsion mode will be
different in all molecules and can be expected to result in
differences in the observed excitation spectra. From these
comparisons, it can be inferred that the weak origin transi-
tion and low frequency transitions are due to the methyl
torsion mode.
The only assignment of the the DMA excitation spec-
trum that is consistent with the deuteration data and the com-
parisons made between the excitation spectra of DMA and
other alkyl anilines is that of the coupled methyl rotor modes
of the dimethylamino group. To confirm this assignment,
simulations of the excitation spectra of DMA-h and DMA-
6
d6 were carried out.
FIG. 4. 2-Color resonance enhanced multiphoton ionization spectrum of the
S states of ͑a͒ NMA, ͑b͒ DMA-h , ͑c͒ NEA, ͑d͒ DEA. The origins occur at
1
6
Ϫ1
Ϫ1
3
3 302.4, 32 895.8 cm , 33 281.7, and 32 294.5 cm , respectively.
IV. HAMILTONIAN FOR COUPLED METHYL ROTORS
Swalen and Costain were the first to study the problem
of two internal methyl rotors as a means of assigning the
rise to these bands is minimally affected by the deuteration
of the methyl groups, and is therefore likely either the amino
torsion mode and/or the inversion mode of the amino group.
vibrational spectra of acetone.1
0,11,19,20
The theory has been
expanded and applied to a large number of molecules with
two internal rotors.1
2,13,21,22
Ϫ1
Congestion in the spectra above 300 cm makes it difficult
The correct form of the coupled methyl rotor potential is
determined by the symmetry of the molecule. For DMA, the
symmetries of the ground and excited states are different
because of differences in the out-of-plane bending mode of
the amino group in each electronic state. For the ground
state, the minimum energy configuration is with the amino
group bent out of the plane of the benzene ring. Using the
notation of Durig,13 the molecule can be described as
to assign those bands in this region that are the result of
methyl torsion.
C. Excitation spectra of other alkyl anilines
The excitation spectra of DMA and other alkyl anilines,
N-methylaniline ͑NMA͒, N-ethylaniline ͑NEA͒ and diethy-
laniline ͑DEA͒ are shown in Figure 4. In contrast to DMA,
the other alkyl anilines have strong origin transitions. The
C TϪC FϪC T, indicating that the two equivalent ro-
3
v
S
3v
origins of the S states of NMA, NEA, and DEA occur at 33
tors are of C3v symmetry attached to frame of C symmetry.
The correct symmetry of the potential energy surface for the
1
S
Ϫ1
302.4, 33 281.7, and 32 294.5 cm respectively. These alkyl
anilines have only a few low frequency bands that are weak
in intensity. In contrast, DMA has many strong low fre-
quency bands. Consequently, the low frequency modes and
Franck–Condon envelope of DMA must be due to a mode
that is not found in NMA, NEA, or DEA. Another pair of
compounds that are useful comparative references are N,N-
pyrrolidinobenzonitrile ͑PYRBN͒ and ethyl, N,N-
pyrrolidinobenzoate ͑PYRBEE͒. The excitation spectra of
the S →S transition of PYRBN and PYRBEE also exhibit a
coupled methyl rotors in the ground state is therefore G18 .
This means that the motion of the two rotors must be sym-
metric with respect to the plane of symmetry perpendicular
to the plane of the benzene ring. Therefore, the gearing mode
is symmetric and the anti-gearing mode is antisymmetric.
The only restriction placed on the minimum energy configu-
ration of the rotors is that they retain the mirror symmetry
required by the C point group.
S
In the excited state, the amino group is planar with re-
spect to the plane of the benzene ring, and therefore the
frame has C2v symmetry. Therefore the double rotor poten-
tial energy surface has G36 symmetry, placing further restric-
tions on the minimum energy configurations of the potential
energy surface. With a C2v frame the rotors must be sym-
metric with respect to the vertical and horizontal planes of
0
1
strong origin transition with a few very weak low frequency
1
7
transitions.
Consideration of the possible large amplitude, low fre-
quency modes of the alkylamino group of these alkyl anlines
suggests that the long Franck–Condon progression and low
frequency bands observed in the DMA spectrum are the re-
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9534
Weersink, Wallace, and Gordon: Excitation spectrum of dimethylaniline
The z axis is defined as the long axis of the molecule and the
x axis is defined as the short axis in the plane of the benzene
ring.
Standard bond lengths and angles were used to calculate
Ј
the values of F and F . Using equations 2 and 3, the values
of F and FЈ for dimethylaniline are calculated to be
Ϫ1
Ϫ1
Fϭ5.47 cm and FЈϭϪ0.06 cm
.
B. Potential energy
The potential function for internal rotation of two inter-
acting methyl rotors is an expansion of a Fourier
series.1
0,12,13
The terms present in the potential energy func-
tion are dependent on the symmetry of the molecule. For a
molecule of G36 symmetry such as DMA in the S state, the
1
potential is
V͑␾ ,␾ ͒ϭV /2͑2Ϫcos3␾ Ϫcos3␾ ͒
1
2
3
1
2
FIG. 5. Newman projections, looking down the CH -N bonds, of the pos-
3
ϩV /2͑1Ϫcos͑3␾ ϩ3␾ ͒͒
ϩ 1 2
sible conformations of DMA. The CH -N-CH plane is horizontal, with
3
3
torsion angles measured from the horizontal plane. In the planar S confor-
mation, the methyl groups must stagger ͑a͒ or eclipse ͑b͒ the Caryl –N bond.
ϩV /2͑1Ϫcos͑3␾ Ϫ3␾ ͒͒
Ϫ 1 2
1
In S , the C –N bond is approximately 27° out of the plane of the
ϩV /2͑2Ϫcos6␾ Ϫcos6␾ ͒,
͑4͒
0
aryl
6
1
2
horizontal plane, and the methyl torsional angles are displaced 40° in the
same ͑top row of S0 conformations͒ or the opposite ͑bottom row of S0
conformations͒ direction from the Caryl –N bond.
where ␾ and ␾ are the two torsional angles defined so that
1
2
both rotors are moving in the same direction. The first and
last terms are potential functions used for two separate but
equal rotors while the second and third terms result in cou-
pling between the two rotors and are representative of the
anti-gearing and gearing modes respectively.
symmetry. Therefore, there can only be two possible mini-
mum energy configurations, as depicted in Figure 5; with
each methyl rotor staggered ͑Figure 5a͒ or eclipsed ͑Figure
C. Basis set and intensity
5b͒ with respect to the benzene ring.
Since the calculations are for the S state of DMA, the
proposed Hamiltonian is appropriate to G36 symmetry.
1
The assumed basis set for the double rotor problem is a
product of two free rotor function,
im ␾
im ␾
2 2
1
1•e
͉
⌿͑␾ ,␾ ͒
͘
ϭ
ϭ
am•e
1
2
͚ ͚
m
m
2
1
A. Kinetic energy
The kinetic energy terms for the coupled rotor case are10
am
͉
m ,m2
1
͘
,
͑5͒
͚
͚
m
m
2
1
2
1
2
2
Ј
TϭF͑p ϩp ͒ϩF ͑p •p ϩp •p ͒,
͑1͒
1
2
2
1
where the single m labelling the coefficient denotes a specific
combination of m and m values. There are 4 A, 1 G, and 4
where the F is the rotation constant for the equivalent rotors,
1
2
Ј
F is the kinetic energy coupling term and p is the momen-
E symmetry species in the G36 molecular symmetry group.
In the high barrier approximation, a given vibrational level is
near 9-fold degenerate, with 1 A, 2 E and 1 G symmetry
species. The 2 E symmetry species are doubly degenerate
and the G species is quadruply degenerate. To calculate the
energy levels, the matrix is divided into four blocks using the
quantum numbers v and ␴ such that m ϭ3v ϩ␴ , where
i
tum operator for rotor i. The rotational constants are calcu-
lated using
ប2
Ϫ1
Ϫ1
Fϭ
͑rz ϩrx ͒,
͑2͒
͑3͒
4
I␾
ប2
i
i
i
i
i
FЈϭ
Ϫ1
Ϫ1
v is an integer, and ␴ ϭ0,Ϯ1, iϭ1,2. With ␴ ϭ␴ ϭ0, the
͑rz Ϫrx ͒,
i i 1 2
4
I␾
A levels are calculated. When ␴ ϭ0,␴ ϭϮ1, the G levels
i
j
are calculated, but only one set of ␴’s are required since all
four combinations of i and j will result in the same calcu-
lated energies. The E levels are separated into two blocks,
one in which ␴1ϭ␴2ϭϮ1, the other in which
where
2
r ϭ͓1Ϫ͑2␭ I /I ͔͒,
z
z
␾
z
2
z
r ϭ͓1Ϫ͑2␭ I /I ͔͒,
x
␾
x
␴1
ϭϮ1,␴ ϭϯ1.
2
where I is the moment of inertia of one internal rotor about
The intensities of the formally allowed progression are
␾
its symmetry axis, I and I are the moments of inertia about
determined by calculating the Franck–Condon overlap of the
two torsion-inversion levels of a transition. Symmetry re-
z
x
the z and x axes of the molecule, ␭ is the direction cosine
i
th
between the i axis and the axis of internal methyl rotation.
quires the torsional minima in the planar S to be symmetric
1
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Weersink, Wallace, and Gordon: Excitation spectrum of dimethylaniline
9535
TABLE I. Correlation table for C2v and G36 molecular symmetry groups
with statistical weights for hydrogen nuclei in parentheses for each level.
TABLE III. Calculated torsional levels of DMA-h . Zero-point energy is
6
Ϫ1
Ϫ1
59.80 cm
Energies are in relative cm . Hamiltonian parameters are
Fϭ5.47, FЈϭϪ0.06, V ϭ148.0, V ϭϪ31.6, V ϭ8.5, V ϭϪ15.0
3
ϩ
Ϫ
6
C2
v
G36
(
␷ϩ ␷Ϫ)
A
G
E
E
A (28)
A (6) ꢀ E (4) ꢀ E (2) ꢀ G(16)
1
1
1
3
A (28)
A (6) ꢀ E (4) ꢀ E (2) ꢀ G(16)
A (10) ꢀ E (4) ꢀ E (6) ꢀ G(16)
2 1 4
0
1
2
3
4
5
6
7
8
(0 0)
(1 0)
(0 1)
(2 0)
(1 1)
(0 2)
(3 0)
(2 1)
(1 2)
0.00
58.90
74.40
0.67
54.51
71.06
100.22
119.75
132.41
165.15
170.86
194.77
1.34
52.48
65.38
115.00
123.30
130.26
170.01
171.02
183.91
1.34
52.41
65.47
115.85
121.76
130.92
169.19
172.30
182.60
2
3
2
3
B (36)
1
B (36)
A (10) ꢀ E (4) ꢀ E (6) ꢀ G(16)
2
4
2
4
98.95
100.80
136.88
168.40
175.59
213.24
to the benzene plane, but there is no such restriction in py-
ramidal S . Thus an arbitrary change in the phase of the
0
barrier is possible and must be considered. This is accom-
plished by introducing a phase term into the overlap calcu-
lation. If the minimum of the torsion potential energy surface
undergoes a change in geometry of ␶ and ␶ from its ground
state equilibrium position, then the overlap of two wavefunc-
tions is then
D. Calculated excitation spectra
1
2
The calculations employed 121 basis functions in each
symmetry block, consisting of eleven free rotor functions for
each rotor. No change was found in the energies of the eigen-
functions when a larger basis set was used. The kinetic en-
ergy terms used in the simulation were calculated from the
geometry of DMA using equations 2 and 3. The potential
energy terms were varied using the Hellmann-Feynman
method26 to calculate changes in energies, until the calcu-
lated energies of the eigenfunctions matched the observed
͗
⌿Ј͑␾ ,␾ ͒
͉
⌿Љ͑␾ ϩ␶ ,␾ ϩ␶ ͒
͘
1
2
1
1
2
2
Ј
Љ
Љ
Љ
ϭ
ϭ
a a exp͑Ϫim ␶ Ϫim ␶ ͒
m m 1 2
1 2
͚
͚
m
m
Ј
Љ
Ј Љ
͓a a ͑cosm ␶ ϩcosm ␶ ͒ϩa a
m m
͚
͚
m
m
1
1
2
2
m
m
energies of the DMA-h spectrum. A least squares routine
6
was used to determine the best fit to the observed values. The
ϫ͑isinm ␶ ϩisinm ␶ ͒ϭGϩiH,
͑6͒
͑7͒
1
1
2 2
calculated DMA-h potential was then used in the simulation
6
where
of DMA-d , where the only change in the Hamiltonian was
6
the required change in the values of F and FЈ.
Ј
Љ
Gϭ
a a ͑cosm ␶ ϩcosm ␶ ͒,
The fits were confirmed by calculating the intensities of
the transitions, using equations 6, 7, and 8 and varying the
values of ␶ and ␶ such that ␶ ϭϪ␶ . A ground state po-
͚
͚
m
m
1
1
2 2
m
m
1
2
1
2
Ј
Љ
Ϫ1
Hϭ
a a ͑sinm ␶ ϩsinm ␶ ͒].
tential energy surface of V ϭ950 cm was used in the in-
͚
͚
m
m
1
1
2
2
3
m
m
tensity calculations to simulate the observed high frequency
torsion modes in the emission. It was found that there was
little variation in the calculated intensities with changes in
the ground state potential. Confidence in the calculated in-
tensities would increase, however, with an accurate ground
state potential that included any coupling between the inver-
sion and the torsion modes.
The intensity is then the square of the overlap,
2
2
IϭG ϩH .
͑8͒
The values of ␶ and ␶ are restricted by symmetry con-
1
2
siderations. Since the ground state is of lower symmetry,
G18 , the only restriction placed on the values of the phase
terms is that ␶ ϭϪ␶ , consistent with motion along the
The kinetic energy terms were calculated from the ge-
ometry of the molecule using standard bond lengths and
1
2
symmetric gearing mode of the ground state.
Ϫ1
angles and were set at Fϭ5.47 cm , FЈϭϪ0.06 for DMA-
In determining the calculated intensities, each level is
also multiplied by the statistical weight for its respective
symmetry. The correlations and statistical weights for DMA-
h , and DMA-d for each symmetry level are listed in
Ϫ1
h6 , Fϭ2.7 cm , FЈϭϪ0.07 for DMA-d . The potential
6
energy parameters found to fit the observed energies best
Ϫ1
Ϫ1
were V ϭ148.0Ϯ0.5 cm
,
V ϭϪ31.6Ϯ0.5 cm
,
3
ϩ
6
6
Ϫ1
Ϫ1
2
3–25
V ϭ8.5Ϯ0.5 cm , and V ϭϪ15.0Ϯ0.5 cm . The best
Tables I and II.
Ϫ
6
fit to the intensities was found with ␶ ϭϪ␶ ϭ40°Ϯ2°, a
1
2
change in the minimum of the excited state potential of
0° along the gearing coordinate with respect to the ground
state potential. The calculated transitions and calculated in-
4
TABLE II. Correlation table for C2 and G36 molecular symmetry groups
with statistical weights for deuterium nuclei in parentheses for each level.
v
tensities for DMA-h and DMA-d are listed in Tables III–
6
6
VI, while the calculated spectra are depicted in Figures 6 and
C2v
G36
7
.
A (370)
A (66) ꢀ E (64) ꢀ E (64) ꢀ G(176)
1
1
1
3
The calculated potential energy surface is depicted in
A (359)
A (55) ꢀ E (72) ꢀ E (56) ꢀ G(176)
3 2 3
A (66) ꢀ E (64) ꢀ E (64) ꢀ G(176)
2 1 4
A (55) ꢀ E (72) ꢀ E (56) ꢀ G(176)
4 2 4
2
Figure 8. Two coordinates are defined along the diagonals of
the plot. The anti-gearing motion is defined as
␾ϩϭ(␾ ϩ␾ )/2 and occurs along the diagonal for which
B (370)
1
B (359)
2
1
2
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9536
Weersink, Wallace, and Gordon: Excitation spectrum of dimethylaniline
TABLE IV. Observed and calculated torsional levels and intensities of major
TABLE VI. Observed and calculated torsional levels and intensities of major
bands of DMA-h . Calculated intensities with rotors twisted from S posi-
bands of DMA-d . Calculated intensities with rotors twisted from S posi-
6
0
6
0
tion by Ϯ40° in a gearing motion.
tion by Ϯ40° in a gearing motion.
Assignment
G36 (␷ϩ ␷Ϫ)
Observed
Calculated
Calculated
Intensity
Assignment
G36 (␷ϩ ␷Ϫ)
Observed
Calculated
Calculated
Intensity
Ϫ1
Ϫ1
Ϫ1
Ϫ1
͑cm
͒
͑cm
͒
Error
͑cm
͒
͑cm
͒
Error
0
2
3
G
G
G
(0 0)
(0 1)
(2 0)
(0 2)
(0 2)
(3 0)
(2 1)
(1 2)
0
0.0
70.4
99.5
129.6
131.7
164.5
170.2
193.2
0.0
Ϫ0.4
0.5
0.3
Ϫ1.7
0.5
0.05
0.11
0.14
0.19
1.00
0.23
0.20
0.50
0G
2G
3G
5G
5E3
8G
9G
(0 0)
(0 1)
(2 0)
(0 2)
(0 2)
(2 1)
(0 3)
0
55
90
95
95
0.0
51.0
84.2
97.8
98.5
0.0
4.0
5.8
Ϫ2.8
Ϫ3.5
Ϫ1.3
Ϫ2.0
0.03
0.11
0.03
1.00
0.19
0.19
0.84
70
100
130
130
165
165
5
E1
5
6
7
8
G
G
G
G
135
147
136.3
145.0
Ϫ5.2
intensity at ␷Ϫ 0 ͑the 130 cmϪ1 band͒, consistent with the
2
␾1
ϭ␾ , or from the bottom left corner of the potential to
2
4
0° change in conformation described above. Since the anti-
the top right corner of the potential in Figure 8. The gearing
motion is defined as ␾ ϭ(␾ Ϫ␾ )/2 the diagonal for
which ␾ ϭϪ␾ , or from the bottom right corner of the
potential to the top left corner of the potential in Figure 8. In
the calculated potential, a barrier exists along the anti-
gearing and gearing coordinates of 296 cm . These poten-
tials are depicted in Figure 9. Notice that the well of the
anti-gearing potential is flatter than the well of the gearing
potential, consistent with the lower frequency of the anti-
gearing fundamental band, calculated to occur at 58 cm
gearing mode is non-totally symmetric, the transition to the
Ϫ
1
2
1
2
␷ϩ 0 is forbidden, and is not observed. However, the ␷
ϩ 0
1
2
Ϫ1
transition ͑the 100 cm band͒ would be allowed and is ob-
served. At these lower energies, the agreement between cal-
culated and observed spectra is excellent.
Ϫ1
At higher torsional energies the ␷ /␷ classification,
ϩ
Ϫ
and the rigid-molecule C symmetry on which it is based,
S
are expected to break down. Selection rules based on G36
symmetry apply and for levels of G and E symmetry, tran-
sitions corresponding to all values of ⌬␷ϩ and ⌬␷Ϫ are per-
mitted. Several transitions corresponding to odd values of
Ϫ1
.
The flatter well of the anti-gearing potential relative to the
gearing potential is the result of lower steric interactions if
the rotors move in the anti-gearing direction. In Figure 10,
the rotors are rotated from the initial equilibrium conforma-
⌬
␷ϩ are predicted to have significant intensity and are in-
deed observed. The agreement between observed and calcu-
lated spectra is less good at higher energies, especially with
regard to intensities. This may be due to significant interac-
tions between coupled methyl torsion and other modes.
tions of the S state, as depicted in Figure 5, by 30° along the
1
anti-gearing ͑Figure 10a͒ and gearing ͑Figure 10b͒ coordi-
nates. The steric interactions between the methyl hydrogens
is lower for the rotors that have moved along the anti-gearing
coordinate, i.e., the conformation depicted in Figure 10a than
for the rotors that have moved along the gearing coordinate,
i.e., the conformation depicted in Figure 10b.
The lower torsional levels can be labelled by torsional
vibration quantum numbers ␷ϩ and ␷Ϫ for the antigearing
and gearing modes, respectively, as shown in Tables III–VI.
Each (␷ ,␷ ) level has nine degenerate components with
ϩ
Ϫ
symmetries Aϩ2EϩG. A progression is observed in the
totally symmetric ͑under C ) gearing mode, with maximum
S
TABLE V. Calculated torsional levels of DMA-d . Zero-point energy is
6
Ϫ1
5
3.69 cm
Hamiltonian parameters are Fϭ2.77, FЈϭϪ0.06, V3
ϭ148.0, V ϭϪ31.6, V ϭ8.5, V ϭϪ15.0.
ϩ
Ϫ
6
(
␷ϩ ␷Ϫ)
A
G
E
E
0
1
2
3
4
5
6
7
8
9
(0 0)
(1 0)
(0 1)
(2 0)
(1 1)
(0 2)
(3 0)
(2 1)
(1 2)
(0 3)
(4 0)
0.00
38.08
51.62
77.37
80.78
96.89
123.03
130.39
140.33
144.93
145.59
0.05
37.71
51.07
78.07
84.20
0.11
37.36
50.49
80.00
86.67
0.11
37.36
50.49
80.05
86.58
97.87
98.57
98.64
115.72
124.52
136.38
145.06
145.34
115.65
115.84
129.00
143.00
163.22
115.09
116.35
129.02
143.03
162.88
FIG. 6. Calculated ͑above͒ and observed ͑below͒ excitation spectra of
Ϫ1
DMA-h6 . The calculated Hamiltonian terms are Fϭ5.47 cm
,
,
Ϫ1
Ϫ1
Ϫ1
Ϫ1
FЈϭϪ0.06 cm , V ϭ148.0 cm , V ϭϪ31.6 cm , V ϭ8.5 cm
3
ϩ
Ϫ
Ϫ1
and V ϭϪ15.0 cm The minimum of the potential surface is shifted by
6
10
40° along the gearing coordinate relative to the ground state potential to
obtain the correct intensities.
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Weersink, Wallace, and Gordon: Excitation spectrum of dimethylaniline
9537
FIG. 9. Potential energy surfaces for the gearing and anti-gearing modes.
Ϫ1
The barrier to both of these modes is 296 cm . The anti-gearing mode has
a flatter potential well than the gearing mode, resulting in the fundamental of
the anti-gearing mode having a lower energy than the fundamental of the
gearing mode.
FIG. 7. Calculated ͑above͒ and observed ͑below͒ excitation spectra of
DMA-d . The potential energy terms are identical to those used for DMA-
6
h . The kinetic energy terms have been adjusted to account for the deutera-
6
Ϫ1
Ϫ1
tion, with Fϭ2.77 cm , and FЈϭϪ0.05 cm . As with the calculated
DMA-h6 spectrum, the minimum of the potential surface is shifted by 40°
along the gearing coordinate to obtain the correct intensities.
relative intensities of these emission bands for excitation of
different S torsion bands, however, indicates some coupling
1
of the methyl torsion and inversion modes. This coupling is
most likely to occur in the ground state with its double mini-
mum potential along the inversion coordinate. With move-
ment along the inversion coordinate, the minimum of the
torsion potential changes along the gearing coordinate.
The long Franck–Condon progression has been attrib-
uted to a change in the equilibrium configurations of 40°
along the gearing coordinate. This change in equilibrium
configuration can be attributed primarily to activity in the
The low frequency bands in the emission spectra of
DMA are reasonably assigned to inversion of the dimeth-
ylamino group and torsion of the whole amino group. In
previous I.R studies,27 the fundamental of the torsion of the
Ϫ1
amino group was assigned to a band at 66 cm , approxi-
Ϫ1
mately half the frequency of the 124 cm band observed in
the emission spectra. In the absence of torsion-inversion cou-
pling, excitation of the S torsion bands should result in simi-
1
S0 state, particularly with respect to coupling between the
lar intensity in the observed emission bands. Changes in the
torsion modes and the inversion mode. Consideration of the
possible DMA conformations in S and S results in four
0
1
possible paths consistent with such a change. The preferred
conformation in each electronic state is determined by steric
repulsion between the methyl hydrogens of the different me-
thyl groups, steric repulsions between the methyl C–H bonds
and the benzene hydrogens and electronic effects between
FIG. 10. Newman projections, looking down the CH –N bonds, of confor-
3
mations of DMA after the rotors have moved from the initial equilibrium
conformations by 30° along the anti-gearing coordinate ͑a͒, and the gearing
coordinate ͑b͒. Steric interactions are expected to be much smaller in ͑a͒
than in ͑b͒. This is consistent with the predicted flatter well of the potential
energy surface of the anti-gearing mode with respect to the potential of the
gearing mode.
FIG. 8. Contour map of the calculated potential energy surface of DMA
coupled methyl torsion mode in the S state. Contour lines occur every 20
1
Ϫ1
cm . The minimum occurs at 0° along each coordinate.
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9538
Weersink, Wallace, and Gordon: Excitation spectrum of dimethylaniline
the C–H bonds of the methyl groups and the Caryl –N bond
and between the methyl C–H bonds and the neighbouring
N–Cmethyl bond.
gered in S . Competition between these effects also make it
1
difficult to predict the barrier heights to complete internal
rotation. An 3 analogy may be considered with
0
Figure 5 depicts the staggered ͑a͒ and eclipsed ͑b͒ ge-
1-methylindole, in which the methyl torsion barrier is
ometries in the S state. Based on only steric effects between
smaller in the excited state than in the ground state.
As can be seen in Figure 5, transitions between either of
1
the rotors, DMA is likely to have a similar minimum energy
configuration as the rotors in dimethylamine,28 which has a
minimum energy configuration identical to that depicted in
Figure 5b, i.e. the eclipsed form. However, significant inter-
actions occur between the methyl rotor hydrogens and ben-
zene ring hydrogens. Calculations of the interatomic dis-
tances suggests that steric repulsions between the methyl and
the benzene hydrogens are greater than steric repulstions be-
tween the two methyl groups, thereby favouring the stag-
gered conformation ͑Figure 5a, top͒. The steric repulsion be-
tween the methyl groups and the benzene ring should be
the possible S equilibrium configurations to either the stag-
0
gered or eclipsed S configurations would result in a shift in
1
the potential minima along the gearing coordinate. This shift
is due to the change in orientation of the phenyl group with
respect to the plane of the dimethylamino group in the S and
0
S1 states and probable changes in the electronic interactions
upon excitation. Determining the actual configurations of the
excited and ground states and hence the path of the confor-
mation change upon excitation is undeterminable from the
present analysis.
larger in S than S due to the shortening of the C –N
1
0
aryl
4
bond from about 1.45 Å in S to 1.3 Å in S . In contrast,
0
1
V. DIMETHYLAMINO TORSIONAL MODE
electronic effects favour the eclipsed form ͑Figure 5 b, top͒,
since there is strong evidence indicating that methyl hydro-
gen bonds are attracted to the ␲ structure of a neighbouring
Previous analysis of the low frequency bands of DMA
and its derivatives, such as DMABN, has assigned these
bands to torsion of the dimethylamino substituent.2 Using
,3
2
9
bond. In DMA, conjugation along the Caryl –N bond pro-
this analysis, an S potential energy surface can be derived
vides the source of attraction. In S , this conjugation in-
1
1
2
that accurately reproduces several of the observed band en-
ergies and intensities of the observed DMA-h6 spectrum.
The model, however, ignores several bands and has difficul-
ties in predicting the observed excitation spectrum of
creases as the N atom changes to an almost sp hybridiza-
tion, thereby increasing this electronic attraction in the S1
state. Analogies with NMA indicate that electronic effects
should favour the same equilibrium configuration in both
DMABN-d . To approximately fit the observed DMABN-
S 0 and S . Electronic effects should be the principal reason
6
1
d6 spectrum, it was assumed that the origin transition was
for changes in the minimum energy configurations of the
rotor in the different electronic states in NMA since steric
interactions between the methyl group and the neighbouring
amino hydrogen are small and steric interactions betwen the
methyl hydrogen bonds and the benzene ring should be simi-
lar in S and S . The strong origin transition observed in the
not observed, and that the lowest observed band was a higher
frequency torsional band. As noted previously, however, the
region lower in energy than the 32 976.6 cm
Ϫ1
band of
DMA-d was examined carefully for other weak transitions,
6
with none observed, and therefore this band was assigned to
the origin transition.
0
1
NMA excitation spectrum ͑Figure 4a͒ indicates little change
Using the proper assignment of the origin transition of
in the configuration of the rotor in S and S states. There-
0
1
the DMA-d spectrum, the torsional analysis of the dimeth-
6
fore, the rotors in DMA would have the same configuration
in S and S if only electronic effects were considered.
ylamino mode should be reconsidered. Analysis of the dim-
ethylamino torsional mode can be performed using conven-
tional techniques.31 The torsional potential energy surface
used was of the form
0
1
These interactions are stronger than those between the
hydrogens on differing methyl groups, based on their sepa-
ration distance, thereby favouring the staggered conforma-
tion ͑Figure 5a͒. A further interaction, an electronic interac-
tion between the C–Hmethyl bond and ␲ electron density of
the ring and the CN bond, also occurs. Recent evidence in-
dicates that this electronic interaction favors an eclipsed form
1
2
1
2
V͑␻͒ϭ V ͑1Ϫcos2␪͒ϩ V ͑1Ϫcos4␪͒
͑9͒
2
4
with the complete torsional Hamiltonian
ץ2⌿
ץ␪2
ϪF
ϩV͑␪͒⌿ϭE⌿.
͑10͒
͑Figure 5b͒, with one of the C–Hmethyl bonds attracted to the
␲
electron density along the CN bond. Because of these
Fϭប/(4␲I c) is the reduced rotational constant of the tor-
m
competing effects, it is difficult to know the actual preferred
conformation of the methyl rotors without further experi-
ments involving rotational resolution.
sion mode and Im is the reduced moment of inertia of the
th
m
top as calculated by
3
The competing strengths of the steric and electronic ef-
fects results in two possible equilibrium conformations in the
S state, and four in the S state, as depicted in Figure 5 with
2
ImϭAm
1Ϫ
Am␭ /Ii
,
͑11͒
ͩ
͚
mi
ͪ
iϭ1
1
0
th
four different possible transitions to account for the shift in
the potential surface minima by 40° along the gearing coor-
dinate. The differences in magnitude of the steric and elec-
tronic effects in each state is such that even if one configu-
where Am is the moment of intertia of the m top about the
top’s axis of rotation, i represents the three principle axis of
the molecule, and ␭ is the direction cosine between the
mi
th
th
axis of the m top and the i principal axis. Using standard
ration is favoured in the S state, the opposing configuration
bond lengths and angles, for the dimethylamino torsion
0
Ϫ1
Ϫ1
may be favoured in the S state, i.e. if eclipsed in S , stag-
mode, Fϭ0.50 cm for DMA-h and Fϭ0.42 cm for
1
0
6
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J. Chem. Phys., Vol. 103, No. 22, 8 December 1995
129.24.51.181 On: Thu, 27 Nov 2014 11:36:19

Weersink, Wallace, and Gordon: Excitation spectrum of dimethylaniline
9539
torsion of the methyl rotors of the dimethylamino substitu-
ent, not to torsion of the amino substituent about the C–N
bond. Unlike the previous assignment, this new assignment
is consistent with the change in frequency of the excitation
bands upon deuteration of the methyl groups and the strong
origin transitions observed in the excitation spectra of other
alkyl anilines. The assignment was confirmed by a simula-
tion of the excitation spectra of DMA-h and DMA-d .
6
6
A potential energy surface for the coupled methyl torsion
modes in S was determined by fitting the potential to the
1
observed frequencies and intensities. The methyl rotors were
found to be coupled with the calculated torsion parameters
Ϫ1
determined to be V ϭ148.0Ϯ0.5 cm , V ϭϪ31.6Ϯ0.5
3
ϩ
Ϫ1
Ϫ1
Ϫ1
cm , V ϭ8.5Ϯ0.5 cm , and V ϭϪ15Ϯ0.5 cm . As
Ϫ
6
well, the weak origin of the excitation spectrum indicates
that the minium energy configuration of the methyl rotors is
substantially different in the excited and ground electronic
states. By Franck–Condon analysis, it was determined that
the minimum of the S torsion potential is shifted by 40°
1
along the gearing coordinate relative to the minimum in the
ground state. It is the strong interaction of the methyl rotors
with the inversion mode in the ground state which result in
the broad Franck–Condon envelope in the excitation spectra
of DMA-h and DMA-d .
6
6
FIG. 11. Calculated and observed excitation spectra of a͒ DMA-h6 and b͒
DMA-d6 using the dimethylamino torsional model. The calculated potential
Ϫ1
Ϫ1
ACKNOWLEDGMENTS
energy surface is V ϭ3706 cm
and V ϭϪ1443 cm
with Fϭ0.50
2
4
Ϫ1
Ϫ1
cm for DMA-h6 and Fϭ0.42 cm for DMA-d . The calculated excita-
6
The authors would like to acknowledge the support of
the Natural Sciences and Engineering Council of Canada and
CEMAID towards the completion of this work. The authors
would also like to thank Professor Mark Lautens for provid-
ing the DMA-d6 .
tion spectrum is good for DMA-h6 ͑a͒. However, the calculated excitation
spectrum is a poor fit to the observed spectrum of DMA-d6 ͑b͒.
3
DMA-d . The best fit to the DMA-h excitation spectrum is
6
6
Ϫ1
Ϫ1 3
with V ϭ3706 cm and V ϭϪ1443 cm . This produces
2
4
a double well potential centered about the ␪ϭ0° position
and with minima at ␪ϭϮ25°. The calculated excitation
1
For a review, see W. Rettig, Angew. Chem. Int. Ed. Engl. 25, 971 ͑1986͒.
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2
spectrum of DMA-h is compared with the observed in Fig-
6
ure 11a. Transition intensities were calculated by using a
3
simple V torsion potential in the S state and calculating the
4
2
0
O. Kajimoto, H. Yokoyama, Y. Oshima, and Y. Endo, Chem. Phys. Lett.
Franck–Condon overlap between the ground and excited
state wavefunctions. All the calculated excitation transitions
1
79, 455 ͑1991͒.
5
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͑
originated in the lowest vibrationless level of S . The size of
0
6
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The calculated excitation spectrum of DMA-d using the
6
9
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same potential energy parameters as above but with
͑
1982͒.
Ϫ1
Fϭ0.42 cm is depicted and compared with the observed
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DMA-d excitation spectrum in Figure 11b. In this calcula-
6
1
1
1
2
3
4
tion, the lowest observed transition is assigned to the origin
band. The fit to the observed spectrum is poor, indicating that
assignment of the low frequency bands to the dimethylamino
torsional mode is not consistent with the observed isotope
effect on the excitation spectra.
15
1
1
6
7
VI. CONCLUSION
1
1
2
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9
0
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1
0
niline is re-examined, with a new assignment for the ob-
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͑
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Weersink, Wallace, and Gordon: Excitation spectrum of dimethylaniline
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J. Chem. Phys., Vol. 103, No. 22, 8 December 1995
129.24.51.181 On: Thu, 27 Nov 2014 11:36:19