12202
J. Phys. Chem. A 2010, 114, 12202–12207
High Resolution Millimeter-Wave Spectroscopy of Vinyltellurol
Roman A. Motiyenko,*,† Laurent Margulès,† Manuel Goubet,† Harald Møllendal,‡ and
Jean-Claude Guillemin§,|
Laboratoire de Physique des Lasers, Atomes et Molécules, UMR CNRS 8523, UniVersité de Lille 1,
F-59655 VilleneuVe d’Ascq, France; Centre for Theoretical and Computational Chemistry (CTCC), Department
of Chemistry, UniVersity of Oslo, P.O. Box 1033 Blindern, NO-0315 Oslo, Norway; École Nationale Supérieure
de Chimie de Rennes, CNRS, UMR 6226, AVenue du Général Leclerc, CS 50837, 35708 Rennes Cedex 7, France
and UniVersité européenne de Bretagne
ReceiVed: September 1, 2010; ReVised Manuscript ReceiVed: October 7, 2010
The millimeter-wave rotational spectrum of vinyltellurol has been recorded and assigned for the first time.
To support the spectrum assignment, high level ab initio calculations have been carried out. Geometries, total
electronic energies, and harmonic vibrational frequencies have been determined at the MP2 level. A smallcore relativistic pseudopotential basis set (cc-pVTZ-PP) was employed to describe the tellurium atom. Two
stable conformers, synperiplanar (sp) and anticlinal (ac), have been identified. The sp conformer is planar
with a small negative inertia defect of -0.025 u Å2. The ac conformer was found to be nonplanar with a
C-C-Te-H dihedral angle of about 140° from sp. This conformer exhibits a large amplitude motion associated
with the torsion about the C-Te bond. The barrier to internal rotation is about 1 kJ/mol, according to the
theoretical calculations. For the ac conformation, a torsional potential function consisting of quartic and quadratic
terms of the torsional angle has been partially determined from the observed rotational constants.
Introduction
Vinyltellurol belongs to a class of compounds with the general
formula H2CdCHXH, where X is a main-group 16 element (X
) O, S, Se, and Te). Rotation about the C-X bonds may
produce rotational isomerism in these compounds.
In H2CdCHOH, two rotameric forms, where the C-C-O-H
chain of atom is synperiplanar (0°) in one1-3 and antiperiplanar
(180°) in the other conformer,4 have been identified by
microwave spectroscopy. The synperiplanar form was found to
be preferred by 4.5(6) kJ/mol over the antiperiplanar rotamer
in this case.4
The conformational properties of H2CdCHSH resemble those
of its alcohol analogue by having a synperiplanar5-7 as well as
an antiperiplanar8 conformer. However, the antiperiplanar form
of the thiol is different from the corresponding conformer of
its alcohol congener. Antiperiplanar H2CdCHSH has a small
hump at the exact planar position of 0.23 kJ/mol. The energy
difference between the two rotamers is now only 0.60(30) kJ/
mol with the synperiplanar form as the more stable,8 which is
quite different from the alcohol case (4.5(6) kJ/mol).4
Both a microwave9 and an infrared study10 are available for
the selenol analogue, H2CdCHSeH, demonstrating the existence
of two rotameric forms. One form has an exact synperiplanar
arrangement for the CdC-Se-H link. Ab initio calculations
at the MP2/SDB-cc-pVTZ level of theory indicate that this angle
is 151.7° from synperiplanar (0°) in the second form. This
rotamer was found to have an experimental inertial defect of
-0.394 u Å2, compared to -0.198 u Å2 in the corresponding
thiol, and 0.0099 u Å2 in the completely planar alcohol. The
* To whom correspondence should be addressed. Telephone: +33-320434943. Fax: +33-3-20337020. E-mail: motienko@phlam.univ-lille1.fr.
†
Université de Lille 1.
‡
University of Oslo.
§
ENSCR.
|
Université européenne de Bretagne.
large absolute value of the inertial defect of the selenol seems
to indicate that the second form is nonplanar.9 Intensity
comparison of rotational transitions revealed no significant
energy difference between these two forms.9
Thus, the main group 16 H2CdCHXH (X ) O, S, Se)
compound each exists as a mixture of two rotamers, an exact
synperiplanar form common for all three compounds, and a
second form, which tends to deviate from planarity as the
X-atomic number increases. Will this tendency be even more
pronounced for vinyltellurol, so it will exist as a mixture of
synperiplanar (sp) and the anticlinal (ac, roughly 120° from sp)
forms? These two conformers of vinyltellurol are depicted in
Figure 1.
The synthesis of the vinyltellurol has been reported quite
recently,11 allowing the experimental determination of its gasphase acidity12 and the recording of its photoelectron13 and
infrared spectra.10 Theoretical calculations predicted the existence of two conformers;13 a nonplanar anticlinal and a planar
synperiplanar conformer were calculated (MP2/(C, H, cc-pVTZ;
Te, SDBaug- cc-pVQZ) and B3LYP level) to be local minima
with an energy difference of less than 1 kJ/mol, whereas the
rotation barrier is only 4.35 kJ/mol. It was therefore concluded
that the TeH group rotates almost freely.13 The photoelectron
spectrum was not able to differentiate between the two conformers.13 However, the two forms could be identified by
infrared spectroscopy, but the ratio between sp and ac could
not be determined.10 Millimeter-wave studies of tellurols are
rare; in fact, only the rotational spectrum of one tellurol, namely
ethanetellurol (CH3CH2TeH), has been published very recently.14
In the present work, we report the rotational spectrum of a
second tellurol, vinyltellurol (H2CdCHTeH), assisted by quantum chemical calculations.
The studies of the vinyl alcohol, thiol, and selenol referred
to above have revealed that these compounds display interesting
dynamics associated with the comparatively low C-X barriers
10.1021/jp108312w  2010 American Chemical Society
Published on Web 10/29/2010
Rotational Spectrum of Vinyltellurol
J. Phys. Chem. A, Vol. 114, No. 46, 2010 12203
the cell, which had to be refilled every few minutes with fresh
sample. This made it impossible to perform continuous scanning
of larger spectral intervals. Instead, we had to concentrate on
measuring specific lines, whose frequencies could generally be
very accurately predicted using the spectroscopic constants
already available from the work in Lille.
Source Modulation Spectroscopy Experiment. Using the
Lille spectrometer the rotational spectrum has been recorded
in the frequency range 75-200 GHz. The accuracy of the
frequency measurement for an isolated line is estimated to be
better than 0.03 MHz. The experimental setup and conditions
used in the present study are almost identical to those ones used
in the previous study of the rotational spectrum of ethanetellurol.14 Therefore, we present only a brief description. Because
the absorption cell was kept at room temperature, the experiment
has been carried out in a flow mode, that is, the sample was
evaporated at -80 °C and continuously injected at one end of
the cell and pumped out at the other end. We have found that
the major impurity of the sample is ethanetellurol. The intensities
of observed rotational lines of ethanetellurol are comparable
with the intensities of those of the title compound.
Computational Methods
Figure 1. The structures of the ac (a) and sp (b) conformers of
vinyltellurol.
to internal rotation. It would be interesting to know if this is
also the case for the title compound. The structural, conformational and dynamical problems associated with vinyltellurol
motivated this first study of its rotational spectrum. A successful
investigation of a delicate conformational equilibrium such as
the one presented by H2CdCHTeH requires experimental
methods possessing high resolution. Rotational spectroscopy
meets this requirement because of its superior accuracy and
resolution, making this method especially well suited for
conformational studies of gaseous species. The spectroscopic
work has been augmented by high-level quantum chemical
calculations, which were conducted with the purpose of obtaining information for use in assigning the rotational spectrum and
investigating properties of the potential-energy hypersurface.
Experimental Methods
Caution: Vinyltellurol is malodorous and potentially toxic.
All reactions and handling should be carried out in a wellVentilated hood.
Synthesis. The synthesis of vinyltellurol was performed as
previously reported.11 The product, which contains about 5%
of the more stable ethanetellurol, is kinetically too unstable to
be kept in dry ice under nitrogen for more than a few hours
and should be kept at the liquid nitrogen temperature.
Stark Modulation Spectroscopy Experiment. The spectrum
of vinyltellurol was studied in the 20-80 GHz frequency
interval by Stark-modulation spectroscopy using the microwave
spectrometer of the University of Oslo, which has a 2 m
Hewlett-Packard absorption cell. Details of the construction and
operation of this device have been given elsewhere.15,16 This
spectrometer has a resolution of about 0.5 MHz and measures
the frequency of isolated transitions with an estimated accuracy
of ∼0.15 MHz. The experiments were performed at a pressure
of roughly 10 Pa with the cell cooled to about -30 °C by
portions of dry ice. Vinyltellurol decomposed very readily in
To our knowledge, only two theoretical investigations of
vinyltellurol have been previously reported.10,13 Although both
these investigation have been performed at a high level of theory
and provided accurate values of spectroscopic constants and
important data on the C-Te torsion potential function, some
additional information on dipole moment components and
centrifugal distortion constants, which is useful in microwave
spectroscopy, was missing from these studies. Therefore, we
decided to perform a new series of ab initio calculations at
slightly higher level of theory.
In this study, all the calculations were performed using the
Gaussian 03 software package.19 The geometries were fully
optimized, and the frequencies were calculated at the MP2 level,
including all electrons in the correlation calculations (MP2(full)).
The small-core relativistic pseudopotential basis set (cc-pVTZPP) was employed to describe the tellurium atom;20 the
cc-pCVTZ basis set with extra core/valence functions was used
for the carbon atoms,21,22 and the standard cc-pVTZ basis set21,22
was employed to describe the hydrogen atoms. Basis sets were
obtained from the EMSL basis set library.23,24 This calculation
level was chosen because the results previously obtained for
ethanetellurol were in very good agreement with experimental
values.14 Finally, the transition states existing between the stable
conformations were characterized using the QST2 procedure
as implemented in the Gaussian 03 software package.
Geometries optimizations indicated the existence of two stable
conformations, denoted sp (a planar skeleton with a C-C-Te-H
dihedral angle of 0°) and ac (with a C-C-Te-H dihedral angle
of about 140° from sp). The calculated geometrical parameters
are given in Table S1 of the Supporting Information. Two saddle
points along the C-Te bond torsion coordinate have been found,
one with an antiperiplanar conformation (C-C-Te-H dihedral
angle of 180°, denoted hereafter ap-TS), and another one with
a synclinal conformation (C-C-Te-H dihedral angle of about
64°, denoted hereafter sc-TS). It is worth noting that the ac and
sc-TS forms have two equivalent conformations with opposite
sign of the C-C-Te-H dihedral angle. The relative energies
of each calculated conformation with respect to the ac conformer
are listed in Table 1. For all forms, the total electronic energy
has been ZPE corrected with subtraction of the frequency, in
the stable conformers, corresponding to the imaginary one in
12204
J. Phys. Chem. A, Vol. 114, No. 46, 2010
TABLE 1: Relative Energies with Respect to the ac
Conformer ∆Eaca
conformation
∆Eac/ (kJ · mol-1)
∆Eac (kJ · mol-1) Ref 10
ac
sp
ap-TS
sc-TS
0.00
0.33 (0.13)
1.00 (0.89)
4.95 (5.10)
0.00
0.7
a
5.5
The ZPE corrected energies are reported in parentheses.
the TS (i.e., the C-Te torsional mode). Calculated harmonic
frequencies for the ac and sp conformers are listed in Table S2
of the Supporting Information.
The two stable conformer geometries and relative energy
agree well with the results of the previous studies.10,13 In the
present case, the energy difference between ac and sp is even
smaller, and the present calculations are not able to point out
which conformer is the more stable. However, the exploration
of the potential energy surface has given useful information for
the spectroscopic study.
The energy of the sc-TS form is about 4 kJ/mol higher than
the energy of ac and sp, which indicates that these two rotamers
should be well isolated. No major perturbations in the millimeter-wave spectrum of sp would be expected. However, the
barrier between the two ac forms is low because the energy of
ap-TS is found to lie about 1 kJ/mol above the energy of ac
which is very close to the energy of a half quantum of the
harmonic frequency of the C-Te torsional mode ν1 (≈0.8 kJ/
mol). In other words, the C-Te torsion mode going from one
ac form to the other passing by ap-TS represents a large
amplitude motion whose energy is very close to the barrier.
This might lead to pronounced perturbations in the ac millimeterwave spectrum at low J values, which was indeed observed.
Assignment and Analysis of the Spectra
Anticlinal Conformer. The assignment of the rotational
spectrum of vinyltellurol was complicated by its high density
due to impurities in the sample and relatively low intensity. At
first we were able to identify several origins of aR0,1-type bands.
The separation between bands origins, ∆f, provides direct
assignment of J quantum number for a band, since as it is wellknown for this type of band ∆f ) (B + C)(J + 1).25 The
assignment of the Ka quantum number for each transition within
a band was a much more difficult task. Several attempts were
made to assign Ka quantum number for a series of transitions
close to the band origin. However, each time the results of leastsquares fit were not satisfactory. We then focused on the low-
Motiyenko et al.
Ka transitions. For this purpose, we used approximate values of
the B and C rotational constants obtained in the previous fit
and ab initio values of the A rotational constant and centrifugal
distortion constants. The frequency predictions obtained using
this combination of parameters allowed us to assign the first
series of Ka ) 0 and Ka ) 1 aR0,1-type transitions. Since these
lines were the strongest ones observed in the spectrum they were
attributed to the most abundant 130Te (34%) isotopic species of
the tellurium atom. The rotational lines belonging to two others
less abundant isotopologues (128Te (30%) and 126Te (16%)) were
found shifted to higher frequencies. Their assignment was based
on relative intensities considerations.
By analyzing the spectra in detail, we found that each lowKa line has at least 3 satellites. One of the series of satellite
lines has nearly the same intensity as the corresponding groundstate transitions, and the two others have ∼60% of the intensity
of this state. The vibrational assignments of the two strongest
states were based on the inertial defect. The lowest vibrational
mode of vinyltellurol is the C-Te torsion, which is an out-ofplane vibration. Vibrational excitation would therefore increase
the absolute value of the inertial defect. In the present study
for the two strongest series of lines the inertial defects were
-1.13 u Å2 and -1.87 u Å2 (see Table 2) respectively,
indicating that the first one belongs to the V ) 0 state and the
second to V ) 1 state. In the same manner, the vibrational
numbers were attributed to the second pair of states, which has
an intensity of roughly 60% of the previous pair. Such quite
unusual behavior of the relative intensities was also observed
in rotational spectrum of ap conformation of vinylmercaptan.
In ref 8, similar series of lines were assigned to the ground and
three lowest excited states of the C-S torsional mode, and the
anomalous relative intensities were explained by large variations
of µa dipole moment component with vibrational excitation. We
suggest that vinyltellurol behaves similarly and consequently,
the assigned lines belong to the ac conformation. The alternating
variation of the rotational constants as a function of the
vibrational quantum number (see Figure 2) supports this
assignment and is characteristic for a double minimum potential
function. Indeed, the present ab initio calculations for the ac
conformer have indicated that the potential function of the C-Te
torsion has two equivalent minima with a very low barrier to
internal rotation of 0.89 kJ/mol.
While the ab initio calculations are inconclusive whether the
ground state is below or above the top of the barrier, some
additional information can be obtained from the torsional
potential function. A quartic-quadratic potential function may
TABLE 2: Spectroscopic Constants for the Ground and 3 Excited Torsional States of the
Conformation of Vinyltellurola
A (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJK (kHz)
d1 (kHz)
d2 (kHz)
HJK (Hz)
HKJ (Hz)
∆ (u Å2)
N
σ (MHz)
σwb
a
130
Te Isotopologue of ac
V)0
theory
V)1
V)2
V)3
41555.25 (79)
3181.8769 (18)
2975.3389 (15)
1.16026 (48)
-18.12 (13)
-0.09409 (48)
-0.00535 (22)
-1.379 (95)
[0.0]
-1.13614 (26)
92
0.113
1.16
41733.437
3208.638
3013.706
1.111
-20.18
-0.0897
-0.0101
41251.0 (32)
3178.8634 (27)
2984.1198 (18)
1.1395 (42)
-16.90 (45)
-0.1082 (11)
-0.0160 (18)
12.6 (13)
[0.0]
-1.87621 (96)
74
0.164
1.28
41324.083 (72)
3173.15912 (46)
2979.88523 (45)
1.117603 (98)
-18.0544 (72)
-0.11757 (15)
0.002783 (64)
-0.0859 (35)
-4.380 (74)
-1.899680 (41)
154
0.044
0.67
41030.61 (49)
3165.6534 (10)
2987.7921 (10)
1.12690 (17)
-16.0633 (56)
-0.10717 (32)
0.00130 (14)
[0.0]
[0.0]
-2.81319 (14)
119
0.061
1.27
Parameter DK is fixed to its ab initio value of 678.99 kHz. b Unitless rms deviation of the fit.
Rotational Spectrum of Vinyltellurol
Figure 2. Variation of rotational constants of
J. Phys. Chem. A, Vol. 114, No. 46, 2010 12205
130
Te isotopic species of vinyltellurol with the C-C-Te-H torsion quantum number.
TABLE 3: Spectroscopic Constants for the Ground and 3 Excited Torsional States of the
Conformation of Vinyltellurola
A (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJK (kHz)
d1 (kHz)
d2 (kHz)
HJK (Hz)
HKJ (Hz)
∆ (u Å2)
N
σ (MHz)
σwb
a
128
Te Isotopologue of ac
V)0
V)1
V)2
V)3
41563.09 (79)
3189.6931 (20)
2982.2057 (15)
1.16534 (54)
-18.17 (11)
-0.09497 (52)
-0.00540 (22)
-1.414 (88)
[0.0]
-1.13575 (27)
82
0.061
1.24
41256.7 (15)
3186.6659 (23)
2991.0249 (15)
1.13987 (93)
-17.38 (38)
-0.10824 (55)
-0.01805 (50)
11.33 (32)
[0.0]
-1.87624 (47)
49
0.059
1.18
41330.80 (24)
3180.94913 (58)
2986.78771 (51)
1.12270 (11)
-18.0523 (82)
-0.11838 (16)
0.002772 (76)
-0.0715 (46)
-4.190 (87)
-1.899594 (82)
136
0.022
0.70
41036.98 (45)
3173.42033 (98)
2994.73895 (93)
1.13208 (16)
-16.0730 (43)
-0.10796 (30)
0.00130 (13)
[0.0]
[0.0]
-2.81332 (15)
110
0.038
1.12
Parameter DK is fixed to its ab initio value of 678.99 kHz. b Unitless rms deviation of the fit.
be used to model the C-Te torsion. The Hamiltonian for a onedimensional quartic-quadratic oscillator is expressed as:
H)
P2x
+ Ax4 + Bx2
2µ
(1)
where x is a generalized vibrational coordinate describing the
torsional motion, µ is the reduced mass for this vibration, and
Px is the vibrational momentum conjugate to x. In reduced form
the Hamiltonian is re-expressed using a dimensionless coordinate
z as:
H ) χA(Pz2 + z4 + χBz2)
(2)
In the present study we have used the transformation described
in eq 2 of the paper by Laane.26 The parameter χB can be derived
from torsional dependence of the rotational constants:
⟨V|Bξ |V⟩ ) B0ξ + β2⟨V|z2 |V⟩ + β4⟨V|z4 |V⟩
(3)
where ξ ) a, b, c; and the Bξ0 constants refer to the planar
molecule. The fitting procedure is divided in two parts. First,
for a given value of χB (the parameter χA being fixed, for
example, to 1) the Hamiltonian matrix is set up and diagonalized
in the basis set using at least 20 harmonic oscillator eigenfunctions. In the present study, we used 100 harmonic oscillator
functions. The eigenvectors were used to calculate the expectation values of ⟨V|zn|V⟩. The observed rotational constants were
then fitted to eq 3. This procedure was repeated for a wide range
of values of χB, and the criterion of the best fit was the smallest
standard deviation. In case of ac, the best fit was found for χB
) -3.42 for all three most abundant isotopic species. By
considering the torsional potential function one can see that for
χB ) -3.42, and for any given value of the parameter χA the
states V ) 0 and V ) 1 lie below the barrier. These two states
are often denoted the 0- and 0+ states. As expected, both these
states are perturbed by an interaction of Coriolis-type, and we
were only able to fit within experimental accuracy a rather
limited set of lines with Ka e 2 for V ) 0 and Ka e 4 for V )
1 using Watson’s S-reduction Hamiltonian27 in Ir representation.
Although the lines with higher values of Ka can be assigned in
the spectra, they cause large errors in fit, unrealistic values of
the centrifugal distortion constants, and severe convergence
problems. A simultaneous fit of these two states using Coriolis
coupling terms, as it was previously done for the synclinal
conformation of ethanetellurol,14 was unsuccessful. The states
V ) 2 and V ) 3 lie above the barrier and we encountered no
difficulties in fitting their spectra to within the experimental
uncertainty, at least for all the transitions with Ka < 9. Thus,
for these states the sets of rotational parameters are better
determined compared to V ) 0 and V ) 1. The rotational
parameters obtained for the ground and three lowest excited
vibrational states for three most abundant isotopologues of
vinyltellurol are listed in the Tables 2-4. The final fits have
been undertaken using ASFIT/ASROT programs by Z. Kisiel
(PROSPE - programs for rotational spectroscopy, http://info.
ifpan.edu.pl/∼kisiel/prospe.htm). Because all the transitions
assigned are of a-type, the DK parameter could not be determined
12206
J. Phys. Chem. A, Vol. 114, No. 46, 2010
Motiyenko et al.
TABLE 4: Spectroscopic Constants for the Ground and 3 Excited Torsional States of the
Conformation of Vinyltellurola
A (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJK (kHz)
d1 (kHz)
d2 (kHz)
HJK (Hz)
HKJ (Hz)
∆ (u Å2)
N
σ (MHz)
σwb
a
126
Te Isotopologue of ac
V)0
V)1
V)2
V)3
41570.19 (71)
3197.7483 (17)
2989.2837 (13)
1.17052 (49)
-18.35 (13)
-0.09540 (44)
-0.00557 (23)
-1.543 (97)
[0.0]
-1.13581 (24)
73
0.059
1.18
41263.5 (15)
3194.7088 (27)
2998.1438 (17)
1.14474 (91)
-17.30 (40)
-0.10874 (65)
-0.01828 (52)
11.39 (32)
[0.0]
-1.87618 (47)
49
0.061
1.21
41337.36 (23)
3188.97824 (58)
2993.90108 (49)
1.12775 (12)
-18.070 (10)
-0.11912 (15)
0.002742 (77)
-0.0739 (60)
-3.82 (15)
-1.899661 (79)
110
0.024
0.62
41043.93 (47)
3181.4253 (10)
3001.8984 (10)
1.13753 (20)
-16.0996 (98)
-0.10849 (33)
0.00164 (13)
[0.0]
[0.0]
-2.81301 (16)
100
0.044
1.14
Parameter DK is fixed to its ab initio value of 678.99 kHz. b Unitless rms deviation of the fit.
TABLE 5: Spectroscopic Constants for Various Isotopologues of sp Conformation of Vinyltellurola
130
A (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJK (kHz)
d1 (kHz)
d2 (kHz)
HKJ (Hz)
∆ (u Å2)
N
σ (MHz)
σwb
a
Te
42880.33 (19)
3138.30985 (52)
2924.73791 (54)
0.945475 (64)
-17.6741 (35)
-0.09283 (16)
-0.004058 (56)
-1.743 (36)
-0.026578 (67)
196
0.059
0.60
128
theory
43709.894
3161.990
2948.683
0.9105
-19.34
-0.0917
-0.00339
Te
42880.71 (23)
3146.25592 (47)
2931.64033 (48)
0.950069 (60)
-17.7057 (37)
-0.09362 (15)
-0.004077 (66)
-1.646 (38)
-0.026605 (73)
170
0.052
0.51
126
Te
42881.29 (23)
3154.44802 (63)
2938.75362 (63)
0.954768 (81)
-17.7609 (45)
-0.09446 (19)
-0.003995 (74)
-1.861 (48)
-0.026560 (80)
160
0.076
0.69
Parameter DK is fixed to its ab initio value of 650.92 kHz. b Unitless rms deviation of the fit.
and is fixed to its ab initio value for the ground state of 130Te
isotopic species.
Synperiplanar Conformer. The ab initio values of the B
and C rotational constants of the sp conformer are predicted to
be smaller than those of ac. We therefore expected the aR0,1type bands of sp to be shifted to lower frequencies compared
to ac. To provide better accuracy of the initial frequency
predictions we used the differences of the rotational constants
between sp and ac obtained in the quantum chemical calculations
added to the experimental rotational constants of ac. By using
these data the low-Ka rotational transitions of the most abundant
isotopic species (130Te) were easily found. The following
analysis has led to the assignment of the 128Te and 126Te
isotopologues. The inertial defect can be used as additional
evidence that the assigned spectrum (Table 5) belongs to sp.
The value obtained in the present study for all three most
abundant isotopologues is -0.025 u Å2, indicating that the
molecule has a planar configuration. Quite similar values of
inertia defect were obtained for synperiplanar forms of the others
main group 16 H2CdCHXH (X ) O, S, Se) compounds, 0.046
u Å2 for vinylalcohol, 0.0312 u Å2 for vinylmercaptan, and
-0.031 u Å2 for vinylselenol. The rotational transitions of the
sp conformer of vinyltellurol were fitted to a standard Watson
S-reduction Hamiltonian27 in Ir representation. In the centrifugal
distortion analysis, we have limited the value of Ka to 10. For
higher values of Ka there are some large residuals that cannot
be eliminated by inclusion of additional terms in the Hamiltonian. The rotational parameters obtained for the ground state
of the sp conformer of vinyltellurol are presented in the
Table 5.
An attempt has also been made to assign the first excited
torsional state, since for low-Ka transitions we observed satellite
lines, which were assigned with corresponding quantum numbers and fitted. However, the fit was successful only for Ka )
0 and Ka ) 1 transitions. It was therefore only possible to obtain
the values of the B and C rotational constants as well as DJ.
The assignments and the results of the fit for V ) 1 state of the
sp conformer are available in the Tables S14-S15 of the
Supporting Information.
Conclusions
This paper presents the results of the studies of the vinyltellurol millimeter-wave rotational spectrum, the first recorded and
assigned microwave spectrum of an unsaturated tellurol. Two
stable conformations have been identified: the sp conformer,
which has a planar structure, and the ac rotamer, which is most
obviously nonplanar as seen from the very large absolute value
of its inertial defect. An additional evidence for the nonplanarity
of ac is the torsional potential function, which was partially
determined from the observed rotational constants and from ab
initio calculations. In both cases, it has been shown that the
ground rotational state of ac lies below the torsional barrier,
that is, it is localized in a potential well whose minimum
corresponds to a C-C-Te-H dihedral angle of about 140°
from sp.
Concerning relative stability of the conformers, we cannot
make any quantitative conclusions from the experimental data.
However, qualitatively the observed and calculated relative
intensities of the ground state lines of both conformations agree
well as it can be seen from Figure 3. The intensities were
calculated on the basis of ab initio results, taking into account
dipole moments (µa ) 0.71 D for ac and µa ) 0.6 D for sp) and
relative energies (∆Eac ) 0.33 kJ/mol). One should note that
Rotational Spectrum of Vinyltellurol
J. Phys. Chem. A, Vol. 114, No. 46, 2010 12207
Supporting Information Available: Calculated molecular
structure and harmonic vibrational frequencies, rotational line
assignments, measured frequencies, experimental uncertainties,
and deviations from the final fits for studied isotopologues of
sp and ac conformers of vinyltellurol. This material is available
free of charge via the Internet at http://pubs.acs.org.
References and Notes
Figure 3. Example of calculated and observed relative intensities of
ground state rotational transitions of the vinytellurol conformers: (a)
ac 272,26 - 262,25 and sp 271,26 - 261,25; (b) ac 252,24 - 242,23 and sp
251,24 - 241,23.
the intensities of ac conformer were calculated with statistical
weight factor of 1 and not 2 as was suggested in ref 10. The
weight factor of 2 can be applied in case of very high barrier to
torsion between two equivalent configurations when the ground
state is considered as degenerate since no splitting can be
observed. When the barrier height is low enough or the
resolution of spectrometer is high enough, one can remove the
degeneracy and observe each ground state line split into V ) 0
and V ) 1 component. The intensities of each of these
components should now be taken with statistical weight factor
of 1.
Acknowledgment. Jean Demaison is gratefully acknowledged
for helpful discussions on fitting quartic-quadratic potential
function.
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