Experimental and theoretical studies of the vibrational spectra of cis-1-bromo-2-fluoroethene

Article abstract of DOI:10.1016/j.saa.2003.10.012

The gas-phase infrared spectrum of cis-1-bromo-2-fluoroethene has been studied at low resolution in the range 200-6500cm^-1, leading to a complete assignment of the fundamentals, except the lowest vibrational mode ν₉ predicted at 167cm^-1. The remaining vibrational structure has been mainly interpreted in terms of first overtone or two quanta combination bands. Isotopic ^79/81Br shift has been observed only in the ν₈ fundamental. The equilibrium structure and the quadratic force field have been investigated theoretically at CCSD(T) level of theory employing Dunning's correlation consistent triple-zeta basis set. Cubic and semidiagonal quartic force field have been calculated using second-order M?ller-Plesset perturbation theory and Ahlrich' split valence (SV) contracted basis set. After a minor scaled quantum mechanical (SQM) adjustment of the quadratic force constants, the vibrational analysis, based on the second-order perturbation theory, has been carried out with the calculated force constants.

Full text of DOI:10.1016/j.saa.2003.10.012

Spectrochimica Acta Part A 60 (2004) 1967–1975
Experimental and theoretical studies of the vibrational spectra
of cis-1-bromo-2-fluoroethene
a
a,∗
a
a
b
A. Baldacci , P. Stoppa , A. Pietropolli Charmet , J. Scaranto , A. Gambi
a
Università Ca’ Foscari di Venezia, DCF, D.D. 2137, Venezia I-30123, Italy
Università di Udine, DSTC, Via Cotonificio 108, Udine I-33100, Italy
b
Received 2 September 2003; received in revised form 15 October 2003; accepted 15 October 2003
Abstract
The gas-phase infrared spectrum of cis-1-bromo-2-fluoroethene has been studied at low resolution in the range 200–6500 cm−1, leading to
−
1
a complete assignment of the fundamentals, except the lowest vibrational mode ν
has been mainly interpreted in terms of first overtone or two quanta combination bands. Isotopic
9
predicted at 167 cm . The remaining vibrational structure
7
9/81
Br shift has been observed only in the ν
8
fundamental. The equilibrium structure and the quadratic force field have been investigated theoretically at CCSD(T) level of theory employing
Dunning’s correlation consistent triple-zeta basis set. Cubic and semidiagonal quartic force field have been calculated using second-order
Møller–Plesset perturbation theory and Ahlrich’ split valence (SV) contracted basis set. After a minor scaled quantum mechanical (SQM)
adjustment of the quadratic force constants, the vibrational analysis, based on the second-order perturbation theory, has been carried out with
the calculated force constants.
©
2003 Elsevier B.V. All rights reserved.
Keywords: Bromofluoroethene; Infrared spectra; Vibrational assignment; Ab initio calculations; Anharmonic force field
1
. Introduction
theoretical ab initio studies have also been carried out. High
level electronic structure calculations, CCSD(T), have been
employed to compute the molecular structure and the har-
monic force field at the optimised geometry. To take into
account the anharmonic effects, which are mandatory for
a comprehensive assignment of the infrared spectrum, cu-
bic and semidiagonal quartic force constants have also been
computed with the use of a finite difference procedure in the
space of the dimensionless normal coordinates.
Dihalogenated ethenes containing fluorine and chlorine
were extensively examined in the infrared region at low and
high resolution, while the heavier ones, with bromine or
iodine, were subjected to limited studies to dated. In this
context we have focused the attention on the cis isomer of
1
-bromo-2-fluoroethene recently examined for the first time
in the microwave region [1].
The present contribution reports a detailed vibrational as-
signment of this molecule which is here investigated at low
resolution in a large IR spectral region. The previous par-
tial assignment of Viehe et al. [2,3] is now completed pro-
viding the assignment of all the fundamentals occurring in
the region investigated, for ν9, expected in the far infrared,
we have estimated its value by means of difference of se-
lected band centres. In order to support the assignments of
the vibrational spectrum as well as to determine the molec-
ular equilibrium geometry of cis-1-bromo-2-fluoroethene,
2
. Experimental details
2.1. Synthesis of CHBr=CHF
CHBr=CHF was synthesised following the method of
Viehe [2] based on a two steps reaction:
SbF3, Br2
Br2CHCHBr2 −−−−→ Br2CHCHBrF + Br2CHCHF2
∗
Corresponding author. Tel.: +39-041-234-8513;
fax: +39-041-234-8594.
Zn, C H OH
2
5
Br2CHCHF2 −− −−− −→ CHBr=CHF
E-mail address: stoppa@unive.it (P. Stoppa).
1
386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2003.10.012

1
968
A. Baldacci et al. / Spectrochimica Acta Part A 60 (2004) 1967–1975
The reactant (Aldrich, 98%) was fluorinated with a
conjunction with the CCSD(T) level of theory, to search
the equilibrium geometry. The harmonic force field was
evaluated in a Cartesian coordinate representation at the
CCSD(T)/cc-pVTZ optimised geometry. The hessian ma-
trix is calculated numerically by finite differences [6]. In
addition, two other geometry optimisations and harmonic
force fields were carried out employing the Møller–Plesset
theory to second order (MP2) [7], using the GAUSSIAN
98 suite of programs [8] also implemented in the above
mentioned computer system. The basis sets employed in
these additional calculations were again cc-pVTZ and
6-311 + +G(3df, spd) with 212 cGTOs; the latter was used
mainly to monitor the extension of the set used in [1].
Spherical harmonics were used throughout, and all valence
electrons were correlated in MP2 and CCSD(T) calcula-
tions (frozen core approximation). MP2/cc-pVTZ harmonic
force field was evaluated by analytical second derivatives of
the energy; in addition the dipole moment derivatives were
also computed via analytical second-derivatives in order to
calculate the integrated infrared band intensities within the
double-harmonic approximation.
mixture of SbF3 and Br2 (Aldrich, 98.8 and 99%, respec-
◦
tively) in a closed steel reactor at 120–125 C for 23 h.
During the first step, the formation of the most volatile
1
,1-dibromo-2,2-difluoroethane was periodically checked
◦
by gas-phase IR spectroscopy. This compound (bp 107 C),
purified by fractionation, was treated with zinc powder
(
1
Aldrich, 98%) in ethanol at room temperature for about
0 min. The reaction mixture was then distilled and the
cis/trans sample of CHBr=CHF was collected between the
◦
boiling points of 25 and 39 C. The diastereoisomers were
next separated under vacuum at low temperature. The fi-
nal cis sample used in the analysis showed impurities of
the trans species and ethyl alcohol, both producing weak
absorptions easily recognised in the spectra.
2
.2. Spectra
The gas-phase infrared spectra were recorded at room
−
1
temperature in the range 200–6500 cm using the Bruker
Vector 22 (400–6500 cm 1) and the Nicolet Magna 750
−
−
1
(
200–500 cm ) FTIR spectrometers, at the resolution
down to 0.5 and 2.0 cm , respectively. In the region
00–500 cm , spectra were recorded using a 16 cm path
The cubic and quartic semidiagonal normal coordinates
force constants (φijk, φijkk) were determined with the use of
a finite difference procedure involving displacements along
the normal coordinates [9]. These displacements were cho-
sen in order to vary the potential energy of each vibra-
−
1
−
1
2
cell equipped with KRS-5 windows and a sample pressure
−
1
of 80 mbar. In the region 400–6500 cm , a 16 cm path cell
and a multipass cell, set to a path length of 22.5 m, both
equipped with KBr windows, were employed, sample pres-
sures were 13 and 40 mbar in the former and 7 mbar in the
latter case, respectively.
tional mode of 0.001 hartree. Since cis-CHBr
=
CHF has 58
electrons, to reduce the computational costs in terms either
of disk space or CPU’s time, the MP2/SV level of theory
was employed to calculate the cubic and quartic force con-
stants. The basis set employed in these calculations was the
fully optimised contracted split valence (SV) Gaussian ba-
sis set, [10], which computes 60 cGTOs basis functions:
(4s)/[2s] for H atoms, (7s4p)/[3s2p] for C and F atoms, and
(14s10p5d)/[5s4p2d] for Br atom.
A dedicated PC, using the GAMESS-UK program [11],
was used to calculate all the cubic and the semi diagonal
quartic normal coordinates force constants. To compensate
in part the limited basis set, all electrons were correlated.
The given band centre values are believed to have an ac-
−
1
curacy of ± 0.4 cm for well resolved absorption features;
however they cannot be usually evaluated with this preci-
sion in low resolution spectra. For the weaker and/or unre-
solved bands, the reported values have been estimated with
−
1
an accuracy not better than 1 cm
.
3
. Computational details
Ground state structural and vibrational parameters of
4. Results and discussion
cis-CHBr=CHF were computed in the harmonic approxima-
tion at coupled cluster singles and doubles level augmented
by a perturbative correction for connected triple excitations,
CCSD(T) [4]. These calculations were performed with the
4.1. Spectral data and assignment of fundamentals
1
Cis-1-bromo-2-fluoroethene is a planar molecule belong-
ing to the point group Cs, the 12 fundamental modes are
MOLPRO 2000 program package as implemented on a
SGI Origin 3800 computer system. The correlation consis-
tent triple-zeta basis set of Dunning [5], which comprises
61 contracted Gaussian-type orbital (cGTO), was used in
ꢀ
ꢀꢀ
distributed with nine of A species and three of A species.
This molecule is a near prolate asymmetric rotor with the
asymmetry parameter κ = −0.945 [1]. The a and b axes
lie in the molecular symmetry plane while the c-axis is
1
1
ꢀꢀ
MOLPRO is a package of ab initio programs written by H.-J. Werner,
perpendicular to this plane. Thus, vibrations of A species
are expected to appear as C-type bands while those of A
P.J. Knowles, with contributions from J. Almlöf, R.D. Amos, A. Berning,
D.L. Cooper, M.J.O. Deegan, A.J. Dobbyn, F. Eckert, S.T. Elbert, C.
Hampel, R. Lindh, A.W. Lloyd, W. Meyer, A. Nicklass, K. Peterson, R.
Pitzer, A.J. Stone, P.R. Taylor, M.E. Mura, P. Pulay, M. Schütz, H. Stoll,
and T. Thorsteinsson.
ꢀ
species should appear as A-/B-hybrid bands, though one of
the components might be much weaker than the other. A dis-
tinction between these two species is often possible on the

A. Baldacci et al. / Spectrochimica Acta Part A 60 (2004) 1967–1975
1969
a
b
1
00
5
0
0
ν8 ν ,ν7
ν2 ν1
11
5
00
1000
1500
2000
2500
3000
1
00
b
7
5
2
5
0
5
3
500
4000
4500
5000
5500
6000
-1
Wavenumber / cm
−
1
Fig. 1. The FTIR gas-phase survey spectrum of cis-CHBr=CHF at 1 cm resolution: (a) path length 16 cm, pressure = 13 mbar and (b) path length
2.5 m, pressure = 7 mbar. The labelled fundamentals are reproduced in more detail in the next figures.
2
respectively, for the former band the P,RQ rotational struc-
basis of the rotational contour. A- and C-type bands exhibit
a medium and strong Q-branch, respectively, while B-type
bands present a depression at their centres. Furthermore,
B- and C-type bands can show Q-clusters superimposed in
the P- and R-branches, in these cases the expected Q peaks
ture is again regularly spaced overall the P and R branches
while for the latter one the 7
9/81
Br isotopic splitting could
be readily detected as shown in Fig. 3.
The two C–C–H in plane bendings ν4 and ν give rise to
5
−
1
separation is about 0.86 cm by adopting the ground state
rotational constants of [1].
bands with a complicate structure. The ν4 band manifests
strong features irregularly displaced on both sides with re-
Survey spectra of the molecule, recorded at 1 cm 1 reso-
lution, are shown in Fig. 1. The labelled fundamentals are re-
produced in more detail in the next figures. Frequencies and
assignments of the observed bands are collected in Table 1.
Eleven fundamentals happen in the region investigated; the
ν9, expected to be very weak and out of the range, has been
evaluated by difference from assignment of hot and com-
bination bands. All assigned fundamentals were recognised
in the low pressure and short path length spectra; however
some of them are very weak or masked by stronger overlap-
ping absorptions. Among overtones and combinations the
−
spect to the band centre. The ν band shows two central
5
R
peaks of different intensity and a series of Q lines with
the expected rotational spacing; furthermore, the P-branch,
though partially overlapped, appears much stronger than the
R-branch. The third observed in plane deformation mode
ν7 (C–C–F) is almost completely obscured by the near
perpendicular fundamental ν11 and only the Q-branch is
detectable.
ꢀ
ꢀ
The three fundamentals of symmetry species A , two CH
out of plane bendings and the torsion, show the typical struc-
ture of C-type bands. However, ν10 is very weak and ap-
pears only in the form of a narrow spike at low pressure. In
Fig. 4 is reproduced the spectral region of ν11 which, apart
the presence of red shifted satellite peaks described later,
shows the Q-feature due to the ν7 band.
most intense ones are those observed near the ν3, ν4 and ν5
fundamentals. For these bands anharmonic vibrational res-
onance may be occurring.
The two C–H stretching modes ν1 and ν2, reproduced in
Fig. 2, show prevalent A- and B-type envelope, respectively.
Though partially overlapped, the weaker band assigned as
ν2 is unambiguous since no other bands are expected in this
4.2. Overtones, combination and hot bands
range. The C=C stretching ν3 produces a band with predom-
Once the fundamentals were characterised, most of the
additional bands were interpreted in terms of first overtone
or binary combinations. Some of these bands however have
an anomalous higher intensity than usually expected. Among
the absorptions observed near the fundamental bands it can
P,R
inant B-type envelope; the
Q rotational structure is clear
−
1
and well resolved with regular spacing of 0.85 ± 0.10 cm
in a long range. The two C–X (X = Br, F) stretching modes
ν and ν8 exhibit envelope of hybrid A-/B- and A-type bands,
6

1
970
A. Baldacci et al. / Spectrochimica Acta Part A 60 (2004) 1967–1975
1
00
8
6
4
2
0
0
0
0
ν2
ν1
3
080
3100
3120
3140
-1
Wavenumber / cm
Fig. 2. The FTIR spectral region of the ν1 and ν2 fundamentals of cis-CHBr=CHF ascribed to the C–H stretchings of the molecule showing the different
envelope of the bands (resolution = 0.5 cm− , path length 16 cm, pressure = 13 mbar).
1
be assessed that 2ν10, ν7 + ν8 and ν + ν9 enhance their
and combination bands related to the C–H stretchings are
expected to appear. Furthermore, the Fermi resonance oc-
curring in the regions of the fundamental bands is replicated
and occasionally identifications including three quanta were
performed. The proposed assignment, reported in Table 1,
6
intensity due to Fermi resonances with the near ν3, ν4 and
ν5, respectively.
Spectra recorded with the multipass cell allowed the inter-
−
1
pretation of many bands up to 6300 cm , where overtones
1
00
8
6
4
2
0
0
0
0
0
79
Br
81
Br
5
40
550
560
570
580
590
-1
Wavenumber / cm
Fig. 3. The FTIR spectral region of the ν8 band of cis-CHBr=CHF at 0.5 cm resolution showing the isotopic 79/81Br splitting (path length 16 cm,
−
1
pressure = 13 mbar).

A. Baldacci et al. / Spectrochimica Acta Part A 60 (2004) 1967–1975
1971
a
b
8
6
4
2
0
0
0
0
0
ν + 2ν - 2ν
9
1
1
9
ν + ν - ν
11
9
9
ν7
ν11
7
00
710
720
730
740
750
760
-1
Wavenumber / cm
−
1
Fig. 4. The FTIR spectral region of cis-CHBr=CHF at 0.5 cm
resolution showing the strong perpendicular ν11 band and the weaker obscured ν7
hybrid band. The weaker red-shifted Q-peaks of ν11 have been attributed to hot bands involving the lowest vibrational level ν9. (a) Path length 16 cm,
pressure = 13 mbar and (b) path length 16 cm, pressure = 40 mbar.
has been obtained taking into account harmonic approxima-
tion, relative intensity and the expected band contour derived
from symmetry species of the relative normal vibrations. In
addition, the anharmonicity constants determined from the
ab initio force field (see later) have been also considered.
Hot bands could also be detected. In particular the pres-
ence of satellite features observed in the fundamental band
regions were analysed in detail giving further information.
In terms of thermal distribution they can be attributed to the
structure optimisations, an accuracy of 0.005 Å for bond
lengths and 0.2 for angles can be assumed. The second
◦
column of Table 2 reports the optimised geometrical pa-
rameters using the analytical gradients as provided by the
MP2 level of theory and employing the same basis set. This
calculation was carried out mainly to have the analytical
derivatives of the dipole moment in order to compute the
integrated infrared band intensities. We have also tried to
improve the basis set quality used in [1] performing a ge-
ometry optimisation also at MP2/6-311 + +G (3df, 2pd)
level. The results, listed in the third column of Table 2, do
not show any improvement from the data reported in [1].
Assuming the CCSD(T)/cc-pVTZ optimised equilibrium
structure as the nearest to the true molecular geometry, the
harmonic force field was evaluated at the same level of
theory in a Cartesian coordinates representation. Since the
theoretical harmonic force field provided a very good over-
all description of the spectroscopic parameters, it was used
as the starting point for an additional empirical refinement
through a scaled quantum mechanical (SQM) adjustment
of the quadratic force constants. Employing the 12 scaling
factors associated to the respective internal coordinates, the
quadratic Cartesian force constants were first transformed
to the internal coordinates space and then scaled within the
SQM formalism [12]:
−
1
lower vibrational level ν9, expected at about 170 cm : tak-
ing the intensity of ␯i band as 100%, the expected relative
one of νi + ν9 − ν9 at room temperature is calculated to be
4
4%.
Seven of the fundamental bands, showing prevalent A-
or C-type contour, present features attributed to hot bands
having ν9 as lower level. One of these, namely ν +ν9 −ν9,
6
was used to calculate the lower vibrational energy level by
applying the Ritz principle.
4
.3. Equilibrium geometry and harmonic force field
Table 2 collects the set of chemically intuitive internal
coordinates R and the optimised values of the nine deter-
minable structural parameters which define the geometry of
cis-CHBr=CHF. The first column reports the results from
CCSD(T)/cc-pVTZ calculations where numerical gradi-
ents were employed using the default step-size. In general,
CCSD(T) calculations give very reliable results for equi-
librium geometry determinations and present systematic
errors. On the basis of the vast literature on these ab initio
√sisjFij.
scaled
ij
F
=
The experimental data consisted of rotational and centrifugal
distortion constants from [1] and the harmonised wavenum-
bers of the fundamental vibrations obtained employing the

1
972
A. Baldacci et al. / Spectrochimica Acta Part A 60 (2004) 1967–1975
Table 1
Tbale 1 (Continued ).
Observed band centres (cm 1) of the gas-phase IR spectra of cis-
-
Band
Envelope
Relative intensity
Wavenumbera
CHBr=CHF
ν
2
+ ν
6
A/B
A/B
A
A
A
A
A
A
A
w
w
vw
w
w
vw
w
vw
vw
vw
w
4134.5
Band
Envelope
Relative intensity
Wavenumbera
ν1 + ν6
4154 ± 1
4306 ± 1
4319 ± 1
4332 ± 1
4397 ± 1
4413 ± 1
4429 ± 2
4443 ± 1
4780 ± 2
6004 ± 1
6125 ± 2
b
ν9
(167.5 ± 0.6)
ν2 + ν5
ν12 + 2ν9 – 2ν9
ν12 + ν9 – ν9
ν12
413.8
ν1 + ν6 + ν9
ν1 + ν5
414.9
416.1
491.9
(567.0/565.5)c
(568.2/567.0)c
728.0
729.0
730.1
750.1
829.9
857.0
857.9
1042.8
1045.4
1144.3
1165.2
1209.1
1210.3
1222.0
1224.7
1273 ± 1
(1310.9/1309.8)c
1327.5
C
A
m
m
ν2 + ν7 + ν8
ν2 + ν4
ν5 – ν11
ν8 + ν9 – ν9
ν1 + ν7 + ν8
ν1 + ν4
ν8
ν11 + 2ν9 – 2ν9
ν11 + ν9 – ν9
ν11
ν1 + ν3
A/B
A/B
B
ν1 + ν2
C
A
A
s
w
vw
2
ν1
w
ν7
Abbreviations used are as follows: vs, very strong; s, strong; m, medium;
w, weak; vw, very weak.
2ν12
ν10 + ν9 – ν9
a
b
c
−1
The experimental error is ± 0.4 cm unless otherwise quoted.
ν10
C
vw
From hot and combination bands implying ␯6.
ν6 + ν9 – ν9
ν6
79/81
The two values indicate the
Br measurements.
A/B
A
C
vs
w
vw
ν11 + ν12
ν7 + ν12
ν6 + 2ν9 – ν9
ν6 + ν9
ν5
empirical procedure proposed by Duncan et al. [13] on the
base of the Dennison’s rule [14]. The 12 scaling factors were
then determined from a least squares fit of the observed data.
The rotational and quartic centrifugal distortion con-
A/B
A/B
m
s
ν5 + ν9 – ν9
ν10 + ν12
ν7 + ν8
ν4
B
A
A/B
A/B
A/B
B
vw
m
s
vw
vw
vw
m
79
81
stants for both isotopomers, cis-CH Br=CHF and cis-CH
Br=CHF, obtained with the scaled harmonic force constants,
are given in Table 3, and are compared with the experi-
mental ones [1]. There is a general overall good agreement
between the calculated and observed data. In the lower part
of the same table, the sextic centrifugal distortion constants
of the main species, which requires the cubic force field
(see later), are also included.
The elements of the bromine inertial nuclear quadrupole
coupling tensor, evaluated from an applied field gradient qij ,
are computed as:
ν6 + 2ν9
ν5 + ν9
1377.5
1392.1
2ν11
1457 ± 1
ν10 + ν11
A
1584.1
ν3
A/B
A/B
A
C
A
A
A/B
C
B
A
A
A
C
A
B
B
A
A
s
m
1651.8
1714.2
1818.0
1901.3
1959 ± 1
1971 ± 1
2087.2
2ν10
ν3 + ν9
vw
vw
vw
vw
w
vw
vw
w
ν6 + ν10
ν6 + ν7 + ν9
ν5 + ν7
2ν6
ν4 + ν10
2184.0
χij = e Q qij, i, j = a, b, c
ν3 + ν8
2217 ± 1
2
ν6 + ν9
2249.7
where χij is the ij-element of the nuclear quadrupole cou-
pling tensor, e the electronic charge, Q the nuclear electric
quadrupole moment of bromine nucleus and a, b, c are the
inertial axes. Since cis-1-bromo-2-fluoroethene is a planar
molecule, the inertial nuclear quadrupole tensor has only
one off-diagonal element, χab. The values of qij were com-
puted at HF/cc-pVTZ level and then converted into the χij
values by using the following expression:
ν5 + ν6
w
2263.7
2367.8
2376.5
ν4 + ν6
vw
vw
vw
w
ν3 + ν11
ν3 + ν7
2388 ± 1
2430 ± 1
2444 ± 1
2527 ± 1
2545 ± 1
2632 ± 1
2651 ± 2
2689.8
2854 ± 1
2865 ± 1
2948 ± 1
2967 ± 1
3091.8
3115.0
3116.5
3118.1
3289.4
3656 ± 2
3858 ± 2
3968 ± 1
ν5 + ν6 + ν9
2ν5
w
ν5 + ν7 + ν8
vw
vw
vw
vw
w
vw
vw
w
ν4 + ν5
ν4 + ν7 + ν8
A/B
A/B
A/B
A
A
A
2ν4
ν3 + ν6
χij(MHz) = 234.9647Q(barn)qij(amu)
ν3 + ν6 + ν9
ν3 + ν5
79
81
where Q ( Br) = +0.331 barn and Q ( Br) = +0.276
barn, both taken from [15]. These results, compared with the
experimental data obtained from the microwave spectra [1],
are given in Table 4. There is a very good agreement between
the calculated and observed values, only the off-diagonal
element χab exhibits a larger error which however does not
reach the 5%. The scaled quadratic force field, in terms of
the internal coordinates defined in Table 2, is reported in
Table 5.
ν3 + ν7 + ν8
ν3 + ν4
A/B
A/B
w
w
ν2
ν1 + 2ν9 – 2ν9
ν1 + ν9 – ν9
ν1
A
A/B
B
A/B
C
m
w
w
w
vw
2ν3
ν2 + ν8
ν1 + ν7
ν1 + ν10

A. Baldacci et al. / Spectrochimica Acta Part A 60 (2004) 1967–1975
1973
Table 2
Internal coordinates of cis-CHBr=CHF and theoretical equilibrium geometry computed at the levels of theory used to determine the harmonic force field
and the band intensities (see text)
CCSD(T)/cc-pVTZ
MP2/cc-pVTZ
MP2/6-311++G(3df, 2pd)
ꢀ
A
R1
R2
R3
R4
R5
C1–Br
C1–C2
C2–F
C1–H
C2–H
1.8823
1.3311
1.3336
1.0782
1.0818
1.8617
1.3282
1.3315
1.0765
1.0799
1.8652
1.3281
1.3306
1.0776
1.0802
R6
R7
R8
R9
∠BrC1C2
∠HC1C2
∠HC2C1
∠FC2C1
123.32
120.39
123.02
123.25
123.31
119.93
122.86
123.31
123.44
119.83
123.11
123.12
ꢀ
ꢀ
A
R10
R11
R12
C1–H out-of-plane bending
C2–H out-of-plane bending
Torsion
Distances in Å and angles in degree.
ative precision ε ≤ 10 6) at these displaced geometries. At
−
4
.4. Ab initio anharmonic force field
The cubic and quartic semidiagonal force constants were
the MP2/SV equilibrium geometry of cis-CHBr=CHF, the
−7
internal gradient components were less then 10 amu.
The anharmonic spectroscopic constants were then de-
termined from the theoretical normal coordinates cubic and
quartic force constants by applying the formulas based on
second-order perturbation theory [16]. The anharmonicity
calculated in the normal coordinates space with the use
of a finite difference procedure, as outlined in Section 3,
through the calculation of analytical second derivatives (rel-
79
constants xij of cis-CH Br=CHF, calculated using the theo-
retical force field determined so far, are reported in Table 6.
Since the anharmonic interactions between fundamentals
and overtones or combination bands may lead to indefinitely
large terms in the corresponding perturbational formulas,
it is necessary to exclude such contributions from the per-
turbational summations [17] and define therefore effective
constants. These effective anharmonicity constants were in-
troduced in the present work to account for the following
Fermi resonances: ν1/2ν3,ν3/2ν10, and ␯2/ν3 + ν4(xij values
marked with an asterisk in Table 6).
In Table 7 are summarised the vibrational fundamen-
tals computed with the scaled harmonic CCSD(T)/cc-pVTZ
force field together with the total energy distribution (TED)
values [18] in terms of the internal coordinates defined in
Table 2, and the corresponding approximate description of
the vibrational mode. Only for the vibrational mode ν8,
Table 3
Observed ([1]) and calculated rotational constants (MHz), quartic (kHz)
and sextic (in 10 Hz) centrifugal distortion constants for cis-CHBr=CHF
2
cis-CH79Br=CHF
cis-CH81Br=CHF
Observed
Calculated
Observed
Calculated
A0
B0
C0
15107.019
2494.8890
2139.3039
1.345
15209.1783
2445.7913
2106.9552
1.2876
−10.4705
62.1843
0.2466
4.2014
0.3301
−1.6058
−5.1539
33.2301
0.1088
15098.604
2474.7752
2124.3304
1.326
15200.6747
2426.0586
2092.1339
1.2683
∆
∆
∆
δJ
J
JK
K
−10.75
−10.60
−10.3652
57.7
56.2
61.9737
0.2643
4.95
0.2605
4.82
0.2413
4.1561
δK
ΦJ
ΦJK
ΦKJ
ΦK
φJ
φJK
φK
2.5766
71.0031
which corresponds to the C–Br stretching, the calculated
harmonic value shows 7
9/81
Br isotopic shift.
The integrated infrared band intensities were computed
with the formula:
Table 4
Observed ([1]) and HF/cc-pVTZ calculated inertial quadrupole tensor for
cis-CHBr=CHF
2
A = 42.25472 |∂µ/∂Q |
i
i
cis-CH79Br=CHF
cis-CH81Br=CHF
where Ai is in km/mol, and ∂µ/∂Qi in D/(Å amu1/2) are the
dipole moment derivatives evaluated via analytical deriva-
tives computed at MP2/cc-pVTZ level of theory.
Applying the anharmonic constants of Table 6, the cal-
culated wavenumbers are obtained and listed in the penulti-
mate column of Table 7. The wavenumbers of the first three
Observed
Calculated
Observed
Calculated
χaa
278.044
256.32
431.50
278.338
257.267
414.331
233.04
215.81
360.27
232.089
214.519
345.485
χbb−χcc
|χab|
All data are in MHz.

1
974
A. Baldacci et al. / Spectrochimica Acta Part A 60 (2004) 1967–1975
Table 5
Scaled quadratic force constants Fij
i
j
Fij
i
j
Fij
i
j
Fij
i
j
Fij
1
2
2
3
3
3
4
4
4
4
5
5
5
1
1
2
1
2
3
1
2
3
4
1
2
3
3.4787
5
5
6
6
6
6
6
6
7
7
7
7
7
4
5
1
2
3
4
5
6
1
2
3
4
5
0.0113
5.6508
0.1023
7
7
8
8
8
8
8
8
8
8
9
9
9
6
7
1
2
3
4
5
6
7
8
1
2
3
0.4889
0.9226
0.0563
9
9
9
9
9
9
4
5
6
7
8
9
0.0441
0.2371
9.4698
0.0560
0.4556
6.7534
0.0336
0.0059
−0.0232
5.7929
−0.0156
0.0450
0.1821
−0.1084
0.0471
0.1080
0.7179
1.8101
0.2664
0.1942
−0.0936
−0.0442
0.0263
−0.4872
−0.0023
0.0207
1.2654
0.0756
10
11
11
12
12
12
10
10
11
10
11
12
0.4190
−0.2034
0.5898
−0.3236
−0.0094
0.1988
1.1541
0.0691
0.0901
−0.0012
−0.0876
0.4012
0.1174
0.3313
−0.4075
0.5351
The units of the force constants are consistent with energy in aJ, bond length in Å and bond angles in radian.
Table 6
Anharmonicity constants xij (cm 1) of cis-CHBr=CHFa
−
i
j
1
2
3
4
5
6
7
8
9
10
11
12
1
2
3
4
5
6
7
8
9
0
1
2
−30.0
−118.4
−29.8
1.2*
−9.2
−7.9
−8.8
−8.6
−8.9
−0.5
−2.1
−1.8
−6.4
−3.0
−2.8
−2.5
0.3
0.3
0.0
−0.3
−0.3
−1.0
0.0
−8.8
−7.5
−8.0*
1.2
−2.3
−1.3
−0.7
−1.3
0.2
−7.8
−0.7
−0.5
−4.3
−0.3
−0.9
−0.3
−0.7
−1.1
0.3
−4.1*
−13.6*
−4.4*
−3.4
−1.0
−2.3
1.0
−0.9
−2.1
−1.1
−6.5
−4.2
0.2
−8.2*
−1.1
1.0
−1.5
−2.8
−2.8
−1.1
2.0
2.2
−1.0
−0.1
−1.5
0.3
−0.8
−0.8
−0.3
−0.3
−10.4
−2.8
1
1
1
−1.4*
−2.0
−2.3
−0.5
a
The constants which are affected by Fermi resonances are marked by asterisk.
Table 7
Calculated and observed gas phase infrared frequency fundamentals (cm 1) of cis-CHBr=CHF (see text for details)
−
Mode
Total energy
distributions
Approximate
description
Infrared
intensities
(Km mol 1)
Calculated
harmonic
wavenumber
Calculated
wavenumber
Observed
wavenumber
−
(TED) (terms
≥
20%)
ꢀ
A
ν1
ν2
ν3
ν4
ν5
89 R4
90 R5
76 R2
C1–H stretching
C2–H stretching
C=C stretching
∠C2C1H bending
∠C1C2H bending
C–F stretching
∠CCF bending
C–Br stretching
∠CCBr bending
7.6
5.9
3258
3209
1695
1352
1242
1062
758
3117a
3091a
1654a
1327
1222
1045
751
3118.1
3091.8
1651.8
1327.5
1222.0
1045.4
750.1
65.1
22.9
43.9
70.2
3.5
72 R8 + 27 R7
43 R7 + 34 R3
49 R3 + 30 R7
40 R9 + 29 R6 + 23 R1
71 R1 + 24 R9
69 R6 + 28 R9
␯
6
ν7
ν8
ν9
b
(568.2/567.0)b
167.5
14.6
1.3
(576/575)
168
568
167
ꢀ
ꢀ
A
ν10
ν11
ν12
96 R12
Torsion
C2H out-of-plane bending
C1H out-of-plane bending
0.5
40.8
11.8
882
751
423
859
730
416
857.9
730.1
416.1
62 R11 + 24 R10
84 R10 + 26 R11
a
Fermi resonance is taken into account.
b
79/81
The two values refer to
Br isotopomers.

A. Baldacci et al. / Spectrochimica Acta Part A 60 (2004) 1967–1975
1975
fundamentals (ν1, ν2 and ν3) were computed after diagonal-
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were taken into account. To facilitate the reading of the table,
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agreement between the calculated and observed values.
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A thorough study of the infrared vibrational spectrum
and spectroscopic properties of cis-1-bromo-2-fluoroethene
has been carried out both experimentally and theoretically.
All the fundamentals, many combination bands and over-
tones have been identified in a wide IR region and their
assignments have been supported by ab initio calculations.
The quantum mechanic computations provided accurate
values of equilibrium geometry, nuclear quadrupole tensor,
rotational and centrifugal distortion constants. In addition
a minor empirical adjusted force field has been deter-
mined, and employing second-order perturbation theory,
the fundamentals of cis-CHBr=CHF have been derived pre-
dicting band centres in agreement with the experimental
ones.
2001.
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6
. Supplementary materials
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Acknowledgements
[17] D. Papoušek, M.R. Aliev, Molecular Vibrational–Rotational Spectra,
Elsevier, Amsterdam, 1982, p. 160.
The authors thank Alessandro Baldan for the synthesis of
cis-CHBr=CHF and the MIUR, Rome, for financial support.
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