High-resolution FTIR study of the v2 fundamental of cis-CHF=CHF

Article abstract of DOI:10.1039/b102537f

The high-resolution FTIR spectrum at room temperature of cis-1,2-difluoroethylene has been analyzed in the v₂ fundamental region from 1670 to 1760 cm^-1. This vibration of A₁ symmetry, corresponding to the C=C stretching, gives rise to a strong b-type band approximately centered at 1719 cm^-1. The rovibrational analysis led to the assignment of many transitions in the P, Q and R branches with J′ ≤ 70, K_a′ ≤ 30, K_c′ ≤ 69. From a simultaneous fit of the ground state combination differences coming from the present work and the previously analyzed v₄ and v₁₀ fundamentals, together with a few literature microwave data, a set of ground state parameters, including all the quartic and four new sextic centrifugal distortion coefficients, was derived. Using the Watson's A-reduction Hamiltonian in the I^r representation, from the final fit of about 3600 assigned transitions, accurate rovibrational constants for the upper state were obtained with a standard deviation of about 8 × 10^-4 cm^-1.

Full text of DOI:10.1039/b102537f

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High-resolution FTIR study of the m fundamental of
2
cis-CHF2CHF¤
R. Visinoni,*a S. Giorgianni,a A. Baldana and G. Nivellinib
a Universita di V enezia, Dipartimento di Chimica Fisica, D. D. 2137, I-30123 V enezia, Italy.
E-mail: visinon=unive.it
b Universita di Bologna, Dipartimento di Chimica Fisica e Inorganica, V iale del Risorgimento 4,
I-40136 Bologna, Italy
Received 19th March 2001, Accepted 11th July 2001
First published as an Advance Article on the web 31st August 2001
The high-resolution FTIR spectrum at room temperature of cis-1,2-diÑuoroethylene has been analyzed in the
l fundamental region from 1670 to 1760 cm~1. This vibration of A symmetry, corresponding to the C2C
2
1
stretching, gives rise to a strong b-type band approximately centered at 1719 cm~1. The rovibrational analysis
led to the assignment of many transitions in the P, Q and R branches with J@ O 70, K@ O 30, K@ O 69. From a
a
c
simultaneous Ðt of the ground state combination di†erences coming from the present work and the previously
analyzed l and l fundamentals, together with a few literature microwave data, a set of ground state
4
10
parameters, including all the quartic and four new sextic centrifugal distortion coefficients, was derived. Using
the WatsonÏs A-reduction Hamiltonian in the Ir representation, from the Ðnal Ðt of about 3600 assigned
transitions, accurate rovibrational constants for the upper state were obtained with a standard deviation of
about 8 ] 10~4 cm~1.
splitter and a high sensitivity mercuryÈcadmium-telluride
(MCT) detector. The spectrum was obtained at room tem-
perature with a sample pressure of about 190 Pa using an 18
1. Introduction
Various halogenated ethylenes have been the subject of spec-
troscopic investigations owing to their signiÐcant role as air
pollutants.1,2 In this context high-resolution infrared spectra
of cis-1,2-diÑuoroethylene have been recently analyzed and
accurate constants for the ground and di†erent vibrational
states3,4 have been determined. As a part of a more complete
investigation on the spectroscopic characteristics of this mol-
ecule, the analysis has now been extended to the rovibrational
cm path cell. The calibration was made using H O lines as
2
internal standard.10 The accuracy of the measured wavenum-
bers, estimated from the ratio of the resolution and the signal
to noise ratio, was better than 0.0004 cm~1 as the signal to
noise ratio is not lower than 10.
The sample of cis-1,2-diÑuoroethylene was synthesized
according to the procedure described previously3 which
involves the following reactions:
structure of the l band near 1719 cm~1.
2
The microwave spectrum of this compound was investi-
LiAlH4
Mg
gated a long time ago5 and the results obtained concern the
determination of the rotational constants and the dipole
moment. Low-resolution infrared studies, published several
years ago6,7 led to the identiÐcation of all the fundamentals
and some combination bands. More recent spectroscopic
studies deal with ab initio calculations of the harmonic force
Ðeld and vibrational frequencies;8 measurements of gas phase
fundamental intensities were also performed.9
CFCl CFCl ÈÈÈÈ CHClFÈCHClF ÈÈÈÈ CHF2CHF
2
2
(Bu)2O
THF
The mixture of the cis- and trans-CHF2CHF isomers was
separated by fractional distillation under vacuum. The purity
of the cis-CHF2CHF, tested by gas chromatography, was
98.5%.
3. Results
In this paper the interpretation of the rovibrational details
3.1. Description of the spectrum
of the l fundamental of cis-CHF2CHF and accurate spectro-
2
scopic parameters are reported. In addition, ground state
The l vibration is one of the 12 fundamental modes of vibra-
combination di†erences obtained from the present analysis, l
2
tion of cis-1,2-diÑuoroethylene. This molecule is a planar
4
and l fundamentals, together with the available microwave
10
nearly prolate asymmetric top of C symmetry with the
data, were analyzed simultaneously to yield a new set of
2v
asymmetry parameter i \ [0.841; the molecular geometry
ground state constants including for the Ðrst time four sextic
together with the symmetry information and the statistical
weights of the rotational levels have already been reported in
ref. 3. The l mode (C2C stretching) has A symmetry and
distortion coefficients (U , U , U and r ).
J
KJ
K
J
2
1
gives rise to a strong b-type band centered at about 1719
cm~1. The infrared spectrum at low resolution (D0.25 cm~1)
of this fundamental, illustrated in Fig. 1, exhibits a gap at the
band center typical of a type-b envelope. At high resolution
the spectra show a rotational structure quite complex with the
Q-branch transitions, grouped into clusters, spaced by about
1.1 cm~1 and mingled with P and R branch features. Each
2. Experimental
The high-resolution (0.004 cm~1 unapodized) spectrum of the
cis-CHF2CHF was recorded in Bologna using a Bomem
DA3.002 FTIR spectrometer equipped with a KBr beam-
¤ Electronic Supplementary Information available. See http://
www.rsc.org/suppdata/cp/b1/b102537f/
series begins with a weak line having JA \ KA and the suc-
a
4242
Phys. Chem. Chem. Phys., 2001, 3, 4242È4246
This journal is ( The Owner Societies 2001
DOI: 10.1039/b102537f

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cessive lines di†er by one unit in J. Fig. 2 and 3 show the
Q-branch details which fall in the P- and R-branch regions,
respectively. In the PQ (J) group with medium K value, as
16
a
can be seen in Fig. 2, the J sequence does not follow a regular
pattern; initially the lines degrade to lower wavenumbers with
decreasing separation, producing a prominent unresolved
peak, and then gradually bend toward the higher wavenumber
side. Each labelled absorption is due to unresolved transitions
coming from even (KA ] KA \ JA) and odd (KA ] KA \ JA
a
c
a
c
] 1) levels. Di†erently, the Q-branch series in the R-branch
region regularly degrade from higher to lower wavenumbers;
a typical example is given by the RQ (J) manifold, reported in
14
Fig. 3. The asymmetry splitting is only evident in the groups
with low K values, where the odd component lies at lower
a
wavenumber.
Concerning the P- and R-branch transitions the spectrum is
characterized, for sufficiently large J and low K values, by
groups of intense lines which are typical of spectra of planar
molecules and, as already previously described in ref. 4 and
references quoted therein, are due to the near coincidence of
a
Fig. 1 IR spectrum at low resolution (+0.25 cm~1) of cis-
CHF2CHF in the l band region (room temperature, P + 1100 Pa, 16
2
cm cell).
Fig. 2 A portion of the spectrum of the cis-CHF2CHF l band around 1701 cm~1 showing the individual J-structure of the PQ (J) group. The
16
2
(52,53) clusters are also reported (room temperature, P + 190 Pa, 18 cm cell).
J
max
Fig. 3 A section of the spectrum of the cis-CHF2CHF l fundamental near 1733 cm~1 (room temperature, P + 190 Pa, 18 cm cell). The
2
individual J-structure of the RQ (J) series is labelled; the J (45È47) clusters are also indicated.
14
max
Phys. Chem. Chem. Phys., 2001, 3, 4242È4246
4243

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transitions involving the almost degenerate levels with K`
sitions having the ground state numbers (J ] 3, K [ 1, K
c
a
a
and (K ] 1)~. The manifolds are deÐned by a J
value and
] 4), (J ] 6, K [ 2, K ] 8), (J ] 9, K [ 3, K ] 12), . . .
a c a c
a
max
their separation is 2C + 0.31 cm~1; the series starts with a
line having JA \ KA and the successive lines have J decreasing
c
3.2. Assignment and ground state analysis
by 1 and K by 2. Three series of such groups with J
\
c
max
37È39, in the R-branch region around 1730.5 cm~1, are shown
The rotational energy levels in the lower and upper vibra-
tional states were computed using WatsonÏs A-reduced Hamil-
tonian up to the sixth order11 in the Ir representation:
in Fig. 4. As can be seen, the labelled features with K \ 0/1~,
a
1`/2~ are packed in the bandheads and the asymmetry split-
ting in the oblate top limit starts for K \ 3` and 4~ with the
a
H \ 1/2(B ] C)P2 ] [A [ 1/2(B ] C)]P2 [ D P4 [ D P2P2
even component at the higher wavenumber side.
a
J
JK
a
The features pertaining to eP([1,1)/oP([1,[1) and eR(1,1)
[D P4 ] U P6 ] U P4P2 ] U P2P4 ] U P6
K
a
J
JK
a
KJ
a
K
a
/oR(1,[1) subbranches are dispersed over a quite wide spec-
] [1/2(B [ C) [ 2d P2 ] 2r P4](P2 [ P2)
tral region and give rise, for high K values, to distinct groups
J
J
b
c
a
with peculiar patterns helpful in detecting lines with high J
] [([d P2 ] r P2P2 ] r P4), (P2 [ P2)]
,
K
a
JK
a
K
a
b
c
`
values. A section of the P-branch region around 1691 cm~1
which shows a series of PP (J) features is illustrated in Fig. 5.
where P is the operator of the total angular momentum, P ,
K
a
In each subgroup the line sequences have J increasing by 3,
P and P are its components along the principal inertial axes
b
c
K decreasing by 1 and K increasing by 4; thus the degrada-
in the molecular-Ðxed coordinate system and [] is the anti-
a
c
`
tion towards the higher frequency side proceeds with tran-
commutator.
Fig. 4 A section of the R-branch of cis-CHF2CHF l fundamental near 1730.5 cm~1 (room temperature, P + 190 Pa, 18 cm cell). The individual
2
transitions in the R (J) groups with J \ 37È39 are labelled.
K
max
Fig. 5 Details of the P-branch spectrum of cis-CHF2CHF l band near 1691 cm~1 (room temperature, P + 190 Pa, 18 cm cell). Distinct PP (J)
2
K
features degrading towards higher wavenumbers show a sequence of lines with J increasing by 3 and K decreasing by 1.
a
4244
Phys. Chem. Chem. Phys., 2001, 3, 4242È4246

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The assignment of the transitions was Ðrst performed in the
P- and R-branch regions where regular strong absorptions
due to eP(1,[1)/oP([1,[1) and eR(1,1)/oR([1,1) subbranches
are evident. In particular the assignment started from lines
correlation between the constants and to improve the stan-
dard deviation. The results achieved were not successful since
the quality of the Ðt remained invariable and the identical
correlation between d and r constants was obtained as in
J
J
with medium J (D20) and low K values. The identiÐed tran-
the A-reduced Hamiltonian.
a
sitions were Ðtted to obtain, keeping the ground state param-
eters of ref. 3 Ðxed, preliminary values for band origin and
upper state constants that were used to predict further P- and
R-transitions and to extend the assignment to the Q-branch
series. The analysis proceeded iteratively with identiÐcation of
new features and reÐnement of constants.
3.3. Upper state analysis
From the present analysis a total of 3613 lines were identiÐed
in the P, Q and R branches; the assignments comprise the
even transitions and only the odd split ones since no improve-
ment was obtained in the determination of the molecular
parameters by taking into account the unsplit lines. In the
reÐnement procedure blended or scarcely resolved lines were
weighted 1/10 with respect to apparently single lines. Band
origin and upper state constants, reported in column 3 of
Table 1, were obtained with a standard deviation of about
As higher J and K values were assigned, the set of ground
a
state constants became inadequate and the Ðt showed, for
transitions with higher quantum numbers, systematic devi-
ations between observed and calculated wavenumbers. Since
the maximum J and K values reached in the present analysis
a
(70 and 30, respectively) are higher than those employed to
obtain the previous ground state parameters3 a new Ðt was
performed in order to obtain a more complete set of ground
state constants including the sextic distortion terms. Therefore
the 2640 combination di†erences from the present study were
added to those from l 4 and l 3 and used, together with the
8 ] 10~4 cm~1 employing transitions with J@ O 70, K@ O 30,
a
K@ O 69. Some larger deviations in the Ðnal Ðt are produced
c
by weak features subject to overlapping or blended lines due
to the occurring of di†erent transitions at close wavenumbers.
Concerning the sextic distortion constants only the four
parameters U , U , U , r were determined with physical
4
10
available microwave data,5 in a simultaneous Ðt. Since the
microwave measurements have much higher precision than
the combination data, adopting the same choice of ref. 3, the
weights of all the combination di†erences were multiplied by a
factor of 10~5. The present analysis produced a well deter-
mined set of ground state constants (column 2 of Table 1)
obtained with a standard deviation of about 7 ] 10~5 cm~1
from the Ðt of 6254 data. Concerning the sextic distortion
parameters only four terms (U , U , U , r ) could be
J
KJ
K
J
meaning while the remaining terms were Ðxed to zero.
During the analysis some lines with medium J and low K
a
values could not be found in the calculated positions, being
the deviations probably due to weak interactions. Three
nearby states l ] 2l (D1741 cm~1) A , 2l ] l (D1744
3
5
1
7
12
cm~1) B and l ] 2l (D1749 cm~1) A are the possible
2
5
12
1
perturbers of the l level; the reported band centers were cal-
2
culated as harmonic values from vibrational data.7,9 A
J
KJ
K
J
obtained with physical meaning whereas the remaining three
coefficients led to poorly determined values which did not
a†ect the quality of the Ðt and therefore they were constrained
to zero.
detailed discussion on the possible resonances is not reliable
at this stage since the l perturbed features could not be
2
clearly identiÐed due to the high density of the lines in the
spectrum also arising from ““hotÏÏ band contributions.
A comparison between the present constants and those
obtained in ref. 3, included for completeness in Table 1, shows
On the whole, the values of the upper state parameters are
close to their ground state values thus indicating that the tran-
sitions used in the Ðt are almost unperturbed. To allow a
direct comparison with the observed details, simulations of
di†erent spectral regions were performed using the molecular
constants of Table 1. A small portion of the R branch around
a good agreement even if the d value has a larger uncertainty.
J
Owing to the high correlation of d with r the former term
J
J
could be obtained with a smaller uncertainty only by Ðxing r
J
to zero. Since in any case the quality of the Ðt is substantialy
invariable, the more complete set of constants was preferred.
It is worthwhile to note that, although the asymmetry
parameter (i \ [0.841) is rather far from the value of the
symmetric top, the data have also been Ðtted using the S-
reduced Hamiltonian in order to test a somewhat smaller
1750 cm~1 is shown in Fig. 6, where the distinct RR (J) fea-
K
tures with K \ 18 and 20 are also indicated. As can be seen,
a
the calculated spectrum reproduces quite faithfully the
observed one thus indicating the reliability of the obtained
molecular parameters; small discrepancies are due to weak
Table 1 Molecular parameters (cm~1) for the ground and the v \ 1 states of cis-CHF2CHFa
2
Ground state
Ref. 3
Present work
v \ 1
2
l
1718.838 91(5)
0
A
B
C
0.703 933 2(13)
0.703 934 54(75)
0.700 559 45(56)
0.197 808 96(28)
0.154 180 26(29)
0.243 41(20)
0.197 809 31(23)
0.154 180 69(26)
0.245 16(28)
[0.160 67(18)
0.488 31(36)
0.7299(11)
0.4373(48)
0.560(74)
[0.487(33)
0.1375(44)
0.228(38)
0.066
0.197 563 83(30)
0.153 893 36(25)
0.246 83(12)
[0.159 632(55)
0.479 65(16)
0.737 75(98)
0.4496(13)
0.472(32)
[0.5320(99)
0.1435(14)
0.218(23)
0.841
3613
D ] 106
J
JK
K
J
K
J
KJ
K
D
] 105
[0.158 14(23)
D ] 105
0.484(12)
d ] 107
0.722 56(77)
0.4675(86)
d ] 106
U ] 1012
U
] 1010
U ] 109
r ] 1012
J
p ] 103
0.025
799
No. of data
6254
J
59
25
70
31
70
30
max
K
a, max
a Uncertainties given are one standard deviation in units of the last signiÐcant digit.
Phys. Chem. Chem. Phys., 2001, 3, 4242È4246
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4. Conclusion
This paper reports the rovibrational analysis of the l funda-
2
mental of cis-CHF2CHF. From this investigation many tran-
sitions also with high quantum numbers were assigned in the
P, Q and R branches and accurate upper state molecular
parameters were derived.
By performing a simultaneous Ðt of the ground state com-
bination di†erences from the present analysis, l and l fun-
4
10
damentals, and the microwave transitions a new and more
complete set of ground state constants, including four terms of
the sextic order, was derived.
Acknowledgements
Financial support by MURST, Roma, is gratefully acknow-
ledged.
References
1
2
3
4
R. Visinoni, S. Giorgianni, A. Baldacci and S. Ghersetti, J. Mol.
Spectrosc., 1993, 162, 474.
R. Visinoni, S. Giorgianni, A. Baldacci and S. Ghersetti, J. Mol.
Spectrosc., 1997, 182, 371.
P. Stoppa, S. Giorgianni and S. Ghersetti, Mol. Phys., 1996, 88,
533.
R. Visinoni, S. Giorgianni, P. Stoppa and S. Ghersetti, Spectro-
chim. Acta, Part A, 2000, 56, 1887.
V. W. Laurie, J. Chem. Phys., 1961, 34, 291.
N. C. Craig and E. A. Entemann, J. Chem. Phys., 1962, 36, 243.
N. C. Craig and J. Overend, J. Chem. Phys., 1969, 51, 1127.
C. W. Bock, P. George and M. Trachtman, J. Mol. Spectrosc.,
1979, 76, 191.
R. O. Kagel, D. L. Powell, J. Overend, M. N. Ramos, A. B. M. S.
Bassi and R. E. Bruns, J. Chem. Phys., 1983, 78, 7029.
5
6
7
8
Fig. 6 Simulated (upper trace) and observed spectrum (lower trace;
room temperature, P + 190 Pa, 18 cm cell) of the cis-CHF2CHF l
2
vibration near 1750 cm~1. Distinct RR (J) features with K \ 18 and
K
a
20 are indicated.
9
hot-band features which could not be accounted for in the
simulation.
10 G. Guelachvili and K. N. Rao, in Handbook of Infrared Stan-
dards, Academic Press, Orlando, FL, 1986.
A complete list of all the assigned transitions can be found
as Electronic Supplementary Information.¤
11 J. K. G. Watson, in V ibrational Spectra and Structure, ed.
J. R. Durig, Elsevier, Amsterdam, 1977, vol. 6, p. 1.
4246
Phys. Chem. Chem. Phys., 2001, 3, 4242È4246