Spectroscopy of hexafluorothioacetone, a blue gas

Article abstract of DOI:10.1063/1.473753

In this study, the vibrational and electronic spectra of hexafluorothioacetone, a novel blue gas, were explored for the first time. Despite the emergence of several problems, complete analysis of the vibrational spectra has been obtained and the UV-visible absorption spectra has been successfully surveyed.

Full text of DOI:10.1063/1.473753

The spectroscopy of hexafluorothioacetone, a blue gas
Dennis J. Clouthier and Duck-Lae Joo
Citation: The Journal of Chemical Physics 106, 7479 (1997); doi: 10.1063/1.473753
View online: http://dx.doi.org/10.1063/1.473753
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The spectroscopy of hexafluorothioacetone, a blue gas
Dennis J. Clouthiera) and Duck-Lae Joo
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055
͑
Received 16 January 1997; accepted 5 February 1997͒
The vibrational and electronic spectra of hexafluorothioacetone, (CF ͒ CS, a novel blue gas, have
3
2
been studied. Ab initio calculations of the vibrational properties of CF COF, CF CSF, and
3
3
(
CF ͒ CO were used to establish the feasibility and effectiveness of using theoretical predictions in
3 2
the analysis of the spectra of perfluorinated compounds. These predictions have allowed us to obtain
revised interpretations of the spectra of trifluoroacetyl fluoride and trifluorothioacetyl fluoride that
are consistent with both experiment and theory and have allowed us to confirm a previous
theoretical and experimental study of the spectrum of hexafluoroacetone. Similar calculations on
hexafluorothioacetone predicted a ground state of C symmetry, with the CF groups staggered in
2
3
an antieclipsed configuration and a pattern of vibrational frequencies similar to that of
hexafluoroacetone. The gas phase and argon matrix infrared spectra and the Raman spectrum of
hexafluorothioacetone were analyzed with the aid of the ab initio predictions and 20 of the 24
fundamentals were assigned. The blue color of the compound originates from very weak T –S0
1
͑
800–675 nm͒ and S –S0 ͑725–400 nm͒ transitions in the visible due to the n–␲* electron
1
promotion. Promotion of an electron from the ␲ to the ␲* orbital gives rise to a very strong
electronic transition in the 230–190 nm region of the ultraviolet. No emission was observed on laser
excitation of hexafluorothioacetone in the visible. © 1997 American Institute of Physics.
͓
S0021-9606͑97͒00918-5͔
thioacetone15 by pyrolysis jet spectroscopy. Both compounds
are red colored liquids, unstable with respect to polymeriza-
tion at room temperature, and both exhibit gas phase phos-
I. INTRODUCTION
Thiocarbonyl compounds have a variety of spectroscopi-
cally interesting properties: They are often colored com-
pounds due to low-lying T –S0 and S –S (n–␲*) elec-
phorescence from the T state. The infrared spectra of these
1
1
1
0
compounds have not been studied because of their reactivity.
However, perfluorination dramatically decreases the reactiv-
ity, so that we were able to study the gas phase infrared and
Raman spectra of trifluorothioacetyl fluoride ͑TTAF͒, a
tronic absorption bands, they often fluoresce or phosphoresce
from these low-lying states and, in a variety of cases, they
also exhibit anomalous emission from the higher
1
–3
S (␲–␲*) state. The anomalous emission is in violation
16
2
stable yellow gas, many years ago.
4
of Kasha’s rule, which generalizes that emission or photo-
Perfluorinated thioacetone or hexafluorothioacetone
chemistry occurs from the lowest excited state of a given
multiplicity; violations of this rule have constituted impor-
tant tests of the theory of radiationless transitions. As empha-
sized in previous work, the thiones are often very different
from their carbonyl cousins, and study of their properties can
be rewarding.
Unfortunately, along with their brighter attributes, the
thiones also have a darker side. They often have foul odors,
are usually very reactive unless stabilized by bulky or very
electronegative substituents and have a strong tendency to
dimerize, trimerize, or polymerize, making them difficult to
isolate and study. For example, the prototypical compound,
thioformaldehyde, is a very reactive, transient species which
can only be studied in flow systems or matrices and poly-
merizes rapidly into a variety of odious products. Despite
these difficulties, thioformaldehyde has been very intensely
studied by spectroscopists and we now know a great deal
about the ground and excited electronic states.6
17
͑
HFTA͒ was first reported in 1961 by Middleton et al. as a
deep blue liquid with a boiling point of 8 °C. It was found to
be insensitive to water or oxygen but readily dimerized in the
presence of trace amounts of base. At that time, the com-
pound was prepared by the inconvenient reaction of bis͑per-
fluoroisopropyl͒mercury with refluxing sulfur, although py-
rolysis of hexafluorothioacetone dimer was also shown to
5
18
regenerate the monomer. Subsequent work showed that the
dimer was readily synthesized by the reaction of hexafluoro-
propene and sulfur in the presence of potassium fluoride. As
the dimer is a stable, easily handled compound, it is a con-
venient precursor to the monomer through simple vacuum
pyrolysis.
As part of our continuing investigations of thione spec-
troscopy, we were interested in hexafluorothioacetone both
as a candidate for infrared and Raman work and as a poten-
tial probe of methyl rotation dynamics in the S and T ex-
1
1
–13
cited states. The stability of the monomer and the likelihood
of being able to do laser excitation studies in the visible
made it an attractive subject for study. The fact that it is a
blue gas meant that the T –S and S –S electronic transi-
In recent years, we have reported studies of some of the
smaller alkyl thiocarbonyl compounds including laser
induced emission studies of thioacetaldehyde14 and
1
0
1
0
tions were even lower in energy than those of thioacetone. It
a͒Author to whom correspondence should be addressed.
also suggested that the S –S energy gap would likely be
2
1
J. Chem. Phys. 106 (18), 8 May 1997
0021-9606/97/106(18)/7479/12/$10.00
© 1997 American Institute of Physics
7479
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7480
D. J. Clouthier and D.-L. Joo: Spectroscopy of hexafluorothioacetone
very large, so that anomalous fluorescence or photodissocia-
dow. Low-resolution infrared spectra were taken intermit-
tently during the deposition and 0.5 cm spectra were taken
Ϫ1
tion from S might occur. Although the laser studies have
2
turned out to be unproductive, we have succeeded in a de-
tailed analysis of the vibrational spectrum, aided by ab initio
calculations, and we have surveyed the electronic absorption
spectrum throughout the UV-visible.
at the end of the deposition process.
UV-visible spectra of HFTA were taken with a Shi-
madzu UV-3101PC scanning spectrophotometer at a resolu-
tion of 0.5 nm with a digitization interval of 0.1 nm using a
1
0 cm quartz cell at pressures from 0.75 to 350 Torr.
II. EXPERIMENT
Hexafluorothioacetone ͑HFTA͒ was prepared by the va-
por phase pyrolysis of 2,2,4,4-tetrakis͑trifluoromethyl͒-1,3-
dithietane ͑HFTA dimer͒ in vacuum at a temperature of
III. AB INITIO CALCULATIONS
The vibrational analysis of the spectrum of hexafluo-
rothioacetone is complicated by the large number of normal
modes ͑24͒, the lack of an experimentally determined struc-
ture, the impossibility of isotopic substitutions on the fluo-
rine atoms, and the low molecular symmetry (C2). It was
anticipated that the normal modes would have substantial
contributions from more than one symmetry coordinate, so
that transferring vibrational frequencies or force constants
from much smaller molecules might be misleading. Our ap-
proach has been to use ab initio predictions of the molecular
structure, vibrational frequencies, normal coordinates, infra-
red intensities, and Raman depolarization ratios to aid in the
analysis. As a stringent test of these predictions, we initially
calculated the vibrational properties of trifluoroacetyl fluo-
ride (CF3COF), trifluorothioacetyl fluoride (CF3CSF), and
hexafluoroacetone ͓(CF3͒2CO͔, molecules whose spectra
have been previously analyzed.
The ab initio molecular orbital calculations were per-
formed with the GAUSSIAN 92 series of programs19 using a
6-311G(d) basis. The geometries were initially optimized
without constraints and later constrained to the nearest ap-
propriate point group for further calculations; no imaginary
frequencies were found for any of the reported structures.
The harmonic vibrational frequencies and absolute intensi-
ties were calculated at their ground state optimized geom-
etries using analytic second derivative methods and scaled
by the recently recommended factor20 of 0.9051 to predict
the vibrational fundamentals. The ab initio force field and
molecular structure were used as input for a normal coordi-
nate analysis, using the program of Hedberg and Mills,21 to
provide insight into the form of the normal modes from the
potential energy distribution ͑PED͒. The normal modes were
also visualized using HyperChem.22
700–725 °C. The dark blue gaseous product was collected
and stored at liquid nitrogen temperatures. Preliminary infra-
red investigations showed that the major impurities in the
sample were hexafluoroethane, silicon tetrafluoride, and car-
bon disulfide. The former two were almost completely re-
moved by pumping on the sample while cooling it to
Ϫ100 °C and monitoring the infrared spectrum. The HFTA
sample was always contaminated by small amounts of CS2
which proved very difficult to eliminate completely. HFTA
dimer, a colorless liquid which freezes at 24 °C, was synthe-
sized by the reaction of perfluoropropylene, sulfur, and po-
tassium fluoride according to the procedure of Dyatkin
1
8
et al.
Gas phase mid-infrared spectra were recorded on a
Bomem DA3-02 FTIR spectrometer equipped with a KBr
beamsplitter and an MCT detector. Far-infrared spectra were
Ϫ1
recorded down to 100 cm on the same instrument using a
variety of Mylar beam splitters. All the infrared spectra were
recorded in a 10 cm cell with either CsI or polyethylene
windows, as HFTA was found to dimerize readily when in
contact with metals so that many experiments involving mul-
tiple relection techniques, employing aluminum coated mir-
rors, and metal fittings, were not feasible.
Gas phase Raman spectra were obtained using a Spex
1403 double monochromator with a cooled RCA 31034 pho-
tomultiplier and photon counting detection system. The Ra-
man scattered radiation, excited by 100–400 mW of 514.5
nm laser light, was collected by a conventional 2-lens optical
system. The HFTA gas sample was contained in a 4 cm o.d.
Pyrex tube at a pressure of 400 Torr and the laser focused in
a single pass at the center of the tube. Depolarization ratios
were measured with a simple Polaroid-polarization scram-
bler combination. No fluorescence background was ob-
served, although the excitation laser wavelength coincided
with the S –S absorption band of HFTA ͑vide infra͒.
1
0
Infrared matrix isolation experiments were done using a
closed-cycle helium refrigerated system ͑Cryosystems LTS-
2
1
1͒ coupled to a high vacuum line maintained at
Ϫ7
0
Torr. Matrices were deposited on a CsI window cooled
Ϫ1
to 14 K and infrared spectra recorded at 0.5 cm resolution
using the Bomem FTIR described above. HFTA dimer/Ar
mixtures ͑1:1000͒ were prepared, stored, and deposited in an
all glass and quartz vacuum line. After passing through a
small leak, the gas mixtures were pyrolyzed in a dimpled
quartz tube ͑12 in.ϫ1 in. i.d.͒ electrically heated to 725 °C
before being deposited at an angle of 45° to the cold win-
FIG. 1. Diagram showing the atomic numbering and Cs symmetry of
CF CFO and CF CSF.
3
3
J. Chem. Phys., Vol. 106, No. 18, 8 May 1997
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D. J. Clouthier and D.-L. Joo: Spectroscopy of hexafluorothioacetone
7481
TABLE I. Geometry of trifluoroacetyl fluoride and trifluorothioacetyl fluoride.
CF CFO
3
CF CFS
Ab initio
3
Parametera
Ab initio
Expt.d
r(C1–X)
r(C1–C2)
r(C1–F1)
r(C2–F2)
1.154
1.529
1.302
1.300
1.310
125.9
109.9
110.3
109.7
1.158Ϯ0.006
1.525Ϯ0.006
1.324Ϯ0.002
1.324Ϯ0.002
1.324Ϯ0.002
129Ϯ2
109.6Ϯ0.5
109.6Ϯ0.5
109.6Ϯ0.5
1.578
1.526
1.302
1.301
1.311
127.8
107.8
112.2
109.3
b
r(C2–F3)
␪
␪
␪
␪
͑C2C1X͒
͑C2C1F1͒
͑C1C2F2͒
͑C1C2F3͒
c
a
Bond lengths in Å, bond angles in degrees, see Fig. 1 for numbering of atoms.
r(C2–F3)ϭr(C2–F4).
b
c
␪
͑C1C2F3͒ϭ␪͑C1C2F4͒.
d
Reference 23.
A. Trifluoroacetyl fluoride (TAF)
level of theory and not in good agreement with experiment,
even at the MP2/6-31G(d) level and suggested that further
experimental work may be necessary. We conclude that SCF
calculations with a 6-311G(d) basis give a reasonable ap-
proximation to the molecular structure, given the limitations
of the experimental geometry.
Unconstrained geometry optimization of TAF gave a
structure of C symmetry, as illustrated in Fig. 1. The oxy-
s
gen, two carbons, and fluorines 1 and 2 lie in a plane such
that F2 eclipses the oxygen atom. The optimized C structure
is presented in Table I along with the electron diffraction
structure obtained by Brake et al.23 who found a single av-
erage C–F bond length of 1.324 Å and assumed local C3v
s
The predicted fundamental vibrational frequencies of
TAF are collected in Table II. Under C symmetry, there are
s
ten in-plane modes of aЈ symmetry and five out-of-plane
modes of aЉ symmetry; all are IR and Raman active. Exami-
nation of the form of the normal modes shows that substan-
tial mixing of the symmetry coordinates occurs, particularly
in the case of the CF3 symmetric stretching and C–C stretch-
ing modes. This latter effect was noted previously by Tuazon
et al.25 who suggested that care must be exercised in making
symmetry for the CF group. Our results predict the C–F1
3
and C–F2 bond lengths to be about 1.300 Å, with slightly
longer ͑1.310 Å͒ out-of-plane C–F3 and C–F4 bond lengths.
The calculated CvO and C–C bond lengths and the various
angles are in excellent accord with experiment. In previous
24
theoretical work, Francisco and Williams found that the
C–F and CvO bond lengths were heavily dependent on the
TABLE II. Fundamental vibrational frequencies of trifluoroacetyl fluoride ͑in cmϪ1͒.
Ab initioa
Experimentb
Approximate
Mode
description
͑Int., depol. ratio͒
͑Int., depol. ratio͒
% diff.
aЈ species
␯1
␯2
␯3
␯4
␯5
␯6
␯7
␯8
␯9
CO str
1960 ͑308, 0.31͒
1377 ͑127, 0.60͒
1292 ͑394, 0.69͒
1122 ͑376, 0.72͒
814 ͑7, 0.04͒
697 ͑68, 0.74͒
591 ͑2, 0.74͒
427 ͑3, 0.71͒
1899 ͑108, 0.27͒
1340 ͑75, •••͒
3.2%
2.8%
3.0%
2.1%
1.0%
0.7%
Ϫ0.7%
0.2%
Ϫ2.3%
Ϫ0.9%
CC str/CF sym str
3
CF asym str
1254 ͑180, •••͒
1099 ͑239, •••͒
806 ͑33, 0.01͒
692c ͑109, 0.63͒
3
CF1 str/CF sym str
3
CF sym str/CC str
CF sym def/OCC def
3
3
c
CF asym def
595 ͑3.7, 0.78͒
3
OCC def/OCF1 def
ϳ426c ͑w, 0.72͒
390 ͑2.5, 0.27͒
228 ͑4.1, •••͒
CC str/CF sym def
381 ͑0.2, 0.44͒
226 ͑5, 0.73͒
3
␯10
aЉ species
␯11
␯12
␯13
␯14
␯15
OCF1 def/CF rock
3
CF asym str
1241 ͑348, 0.75͒
785 ͑31, 0.75͒
517 ͑14, 0.75͒
242 ͑9, 0.75͒
49 ͑0.8, 0.75͒
1214 ͑192, •••͒
2.2%
3.2%
Ϫ0.4%
0.0%
3
c
OCF1 inversion
761 ͑78, 0.15?͒
CF asym def
519c ͑37, 0.70͒
242 ͑m, 0.75͒
50 ͑0.25,•••͒
3
CF rock
3
torsion
2.0%
a6-311G(d,p) frequencies scaled by 0.9051; infrared intensities in km/mol; depol. ratio is the calculated Raman
spectrum depolarization ratio.
b
Experimental frequencies and depolarization ratios from Ref. 27; infrared intensities (10Ϫ3 cm /mol) from
2
Ref. 26.
c
Reassignment based on present work; see the text.
J. Chem. Phys., Vol. 106, No. 18, 8 May 1997
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7482
D. J. Clouthier and D.-L. Joo: Spectroscopy of hexafluorothioacetone
qualitative vibrational assignments in fluorine-containing
compounds with C–F bonds adjacent to a C–C single bond,
due to the extensive mixing of the C–F and C–C stretching
coordinates.
motions and ␯ to be a very intense band that is predomi-
3
nantly the CF asymmetric stretch, entirely consistent with
3
Ϫ1
the infrared bands at 1340 and 1254 cm , respectively. At
the MP2/3-21G* level, the frequency of the intense CF3
Ϫ1
The gas phase infrared spectrum of TAF was first stud-
ied by Loos and Lord,26 who reported vibrational frequen-
cies, intensities, band types, and assignments over the
asymmetric stretch is predicted to be 17 cm above the
weaker CC stretching/CF symmetric stretching mode, and
3
on this basis Francisco and Williams reversed the assign-
Ϫ1
27
Ϫ1
3
5–3900 cm range. Subsequently, Berney obtained new
ments so that the band at 1340 cm , although still ␯2 , was
infrared data along with liquid and solid phase Raman spec-
tra including depolarization ratios for the stronger bands and
reassigned some of the fundamentals. A few years later,
Redington analyzed matrix spectra of a variety of isotopi-
cally substituted trifluoroacetic acid species and proposed re-
assignments of the vibrational fundamentals of TAF to be
attributed to the CF3 asymmetric stretch. Experiment favors
the HF/6-311G(d) interpretation, as the weaker band clearly
occurs at higher frequency.
2
8
In summary, we conclude that the ab initio calculations
give a reasonable geometry and reliable predictions of the
scaled vibrational frequencies, relative infrared intensities
and Raman depolarization ratios for trifluoroacetyl fluoride.
The frequency scaling appears reliable for predicting the fun-
damentals to better than Ϯ5% and the infrared intensities and
Raman depolarization ratios have proven valuable in resolv-
ing discrepancies. A consistent interpretation of the available
experimental data can be obtained on the basis of these pre-
dictions.
consistent with those of similar CF -containing molecules.
3
Since the early experimental work, there have been three
reported ab initio studies of the vibrational frequencies of
TAF. Ottavianelli et al.29 calculated the frequencies at the
Hartree–Fock level with a 3-21G basis and found general
30
agreement with the assignments of Berney. Pacansky et al.
explored the sensitivity of the vibrational frequencies of TAF
and COF to various levels of theory and concluded that
2
scaled HF/6-31G* results gave excellent agreement with ex-
periment, without recourse to larger basis sets or correlated
wave functions. They did not advocate any changes in the
assignments of Berney. In the most detailed study, Francisco
B. Trifluorothioacetyl fluoride (TTAF)
There have not been any previously reported ab initio
studies of this molecule. Geometry optimization of trifluo-
rothioacetyl fluoride gave a Cs symmetry structure very
similar to that of trifluoroacetyl fluoride, as compared in
Table I. Although the structure of TTAF has not been deter-
mined experimentally, the ab initio CvS bond length of
2
4
and Williams predicted the vibrational frequencies at the
HF/3-21G and MP2/3-21G levels of theory and proposed a
variety of reassignments.
Our assignments of the experimental data of Berney27
are given in Table II. These are in general agreement with
the proposals of Francisco and Williams24 except for the val-
ues of ␯ and ␯ . Berney assigned ␯ , the CF symmetric
31
1.578 Å is comparable to that of F CvS ͑1.589 Å͒ and the
2
similarity of the rest of the parameters to those of TAF sug-
gest that the results are reasonable.
6
12
6
3
deformation, to a medium intensity infrared band at
Our ab initio predictions of the vibrational parameters
for TTAF are collected in Table III. Comparing the frequen-
cies to those of TAF, we see that sulfur substitution lowers
the CvX stretching frequency, allowing it to mix with the
TTAF. The aЉ and aЈ CF asymmetric stretching modes have
similar frequencies in both molecules, but the other aЈ CF
stretches differ substantially in both frequency and relative
intensity. These differences can be traced to more pro-
nounced mixing of the CC stretching coordinate with the CF
stretching coordinates in TAF. Most of the low frequency
modes are similar in the two molecules, although ␯12 is cal-
culated to be of medium intensity in TAF but very weak in
TTAF.
Ϫ1
7
61 cm , with a corresponding liquid phase Raman fre-
Ϫ1
quency of 770 cm and a depolarization ratio of 0.15. This
assignment is inconsistent with our ab initio prediction of a
Ϫ1
medium intensity band at 687 cm ͑a frequency error of
CC stretching mode, and diminishing the intensity of ␯ in
1
Ϫ10.8%͒ with a Raman depolarization ratio of 0.74. The
Ϫ1
stronger infrared band at 692 cm with a depolarization ra-
tio of 0.63 is readily reassigned as ␯ . The infrared band at
6
Ϫ1
7
61 cm may then be readily reassigned as ␯12 , consistent
with the ab initio frequency and intensity prediction. The
only discrepancy is the measured Raman depolarization
Ϫ1
value of 0.15 for the feature at 770 cm , which would have
to be an aЈ fundamental. However, comparison of theory and
experiment shows that a depolarization ratio of 0.15 is
clearly anomalous for fundamentals of TAF in the
We published the only experimental study of the vibra-
tional spectrum of TTAF several years ago, in which the
Ϫ1
16
8
00–600 cm region. The most likely explanation is that
Ϫ1
the Raman band at 758 cm ͑depolarization ratio unfortu-
nately not measured͒ corresponds to the IR fundamental at
assignments were made primarily by analogy to the TAF
assignments of Berney.27 As shown in Table III, most of the
frequency assignments and infrared intensities are in good
agreement with the present ab initio predictions. However,
the calculations suggest that the weak bands at 398 and
Ϫ1
Ϫ1
761 cm and that the Raman feature at 770 cm is not a
TAF fundamental.
The only other incongruity between the present assign-
ments and those of Francisco and Williams24 is in the de-
505 cm should be reassigned as ␯ and ␯ , respectively.
Ϫ1
8
13
scriptions of ␯2 and ␯ . At the HF/6-311G(d) level of
From these results, the 6-311G(d) basis set appears ad-
equate for providing predictions of the vibrational frequen-
cies, infrared intensities and Raman depolarization ratios of
3
theory, we find ␯ to be a medium intensity band described
2
as a mixture of CC stretching and CF symmetric stretching
3
J. Chem. Phys., Vol. 106, No. 18, 8 May 1997
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D. J. Clouthier and D.-L. Joo: Spectroscopy of hexafluorothioacetone
7483
TABLE III. Fundamental vibrational frequencies of trifluorothioacetyl fluoride ͑in cmϪ1͒.
Ab initioa
͑Int., depol. ratio͒
Experimentb
͑Int., depol.͒
Approximate
description
Mode
% diff.
aЈ species
␯1
␯2
␯3
␯4
␯5
␯6
␯7
␯8
␯9
CS str/CC str
1389 ͑83, 0.31͒
1293 ͑103, 0.35͒
1270 ͑850, 0.47͒
1061 ͑217, 0.58͒
765 ͑20, 0.06͒
581 ͑10, 0.66͒
546 ͑15, 0.21͒
402 ͑1, 0.73͒
1350 ͑m, p͒
1243 ͑s, •••͒
1227 ͑vs, •••͒
1049 ͑s, •••͒
764 ͑m, p͒
585 ͑w, •••͒
551 ͑m, p͒
398c ͑w, •••͒
328 ͑w, p͒
2.9%
4.0%
4.6%
1.1%
0.1%
Ϫ0.7%
Ϫ0.9%
1.0%
Ϫ3.0%
Ϫ4.5%
CF asym str
3
CF1 str/CF asym str
3
CF sym str/CF1 str
3
CF sym str/sym def
3
CF asym def
3
CS str/SCF1 def
SCF1 def/CF rock
3
CC str/CF rock
318 ͑0.3, 0.50͒
211 ͑2, 0.74͒
3
␯10
aЉ species
␯11
␯12
␯13
␯14
␯15
SCC def/SCF1 def
221 ͑w, •••͒
CF asym str
1230 ͑368, 0.75͒
685 ͑0.1, 0.75͒
508 ͑6, 0.75͒
232 ͑3, 0.75͒
47 ͑0.03, 0.75͒
1183 ͑s, •••͒
•••
505c ͑w, •••͒
228 ͑w, •••͒
Ͻ50
4.0%
•••
0.6%
1.8%
•••
3
SCF1 inversion
CF asym. def.
3
CF rock
3
torsion
a6-311G(d,p) frequencies scaled by 0.9051, infrared intensities in km/mol, depol. ratio is the calculated Raman
spectrum depolarization ratio.
b
Experimental frequencies, relative intensities, and depolarizations ͑pϭpolarized͒ from Ref. 16.
c
Reassignment based on present work; see the text.
the sulfur analog of trifluoroacetyl fluoride. The scaled fre-
quencies give predictions of the vibrational fundamentals re-
liable to about Ϯ5%.
the Boulet33 structure ͑Table IV͒, and less favorably to the
Hilderbrandt et al.32 geometry. In particular, the CvO bond
length of 1.246Ϯ0.014 Å in the latter structure is very long
compared to the 1.154 Å value in TAF.
The predicted vibrational frequencies of HFA are col-
lected in Table V. As in the simpler molecules TAF and
TTAF, the normal modes in HFA are often complicated,
involving substantial contributions from more than one sym-
metry coordinate; our approximate descriptions refer to the
dominant motions as given by the potential energy distribu-
tion. In previous work at the HF-SCF 3-21G level, Compton
et al.34 calculated the vibrational frequencies of HFA and
used the force constants to do a normal coordinate analysis
C. Hexafluoroacetone (HFA)
Geometry optimization of hexafluoroacetone gave a
structure of C symmetry as illustrated in Fig. 2 and Table
2
IV. The carbon atom framework and the oxygen atom lie in
a plane, with the CF groups twisted slightly away from each
3
other. Electron diffraction structures for HFA has been re-
ported by Hilderbrandt et al.32 and Boulet, assuming local
33
C3v symmetry for the CF groups. Our ab initio predictions,
3
34
and earlier HF-SCF 3-21G results, compare favorably to
FIG. 2. Diagram showing the atomic numbering and C2 symmetry of (CF ͒ CvO and (CF ͒ CvS.
3 2
3 2
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7484
D. J. Clouthier and D.-L. Joo: Spectroscopy of hexafluorothioacetone
TABLE IV. Geometry of hexafluoracetone and hexafluorothioacetone.
(
CF ͒ CO
3 2
(
CF ͒ CS
3 2
Parametera
Ab initio
Expt.d
Ab initio
r(C–X)
r(C–C)
r(C–F1)
r(C–F2)
r(C–F3)
1.168
1.540
1.301
1.311
1.312
121.4
110.2
109.6
110.6
18.6
1.185Ϯ0.020
1.527Ϯ0.015
1.321Ϯ0.007
1.321Ϯ0.007
1.321Ϯ0.007
120Ϯ4
110.3Ϯ1.0
110.3Ϯ1.0
110.3Ϯ1.0
0
1.582
1.532
1.303
1.314
1.313
122.9
112.0
109.8
110.5
19.5
b
b
b
␪͑CCX͒
c
c
c
␪
␪
␪
͑CCF1͒
͑CCF2͒
͑CCF3͒
␶
a
Bond lengths in Å, bond angles in degrees, see Fig. 2 for orientation of atoms.
r(C–F1) ϭ r(C–F4); r(C–F2) ϭ r(C–F5); r(C–F3) ϭ r(C–F6).
b
c
␪
͑CCF1͒ϭ␪͑CCF4͒; ␪͑CCF2͒ϭ␪͑CCF5͒; ␪͑CCF3͒ϭ␪͑CCF6͒.
d
Reference 33.
and obtain potential energy distributions for the various
classified as in-plane or out-of-plane, which is very useful in
the analysis and in using visualization programs, as we have
done, for describing the normal modes. Our results are in
good accord with their predictions.
modes. Although their calculations predicted a C structure,
2
it was found that coordinates that transform according to
different C2v symmetry species do not couple appreciably in
the C structure, in agreement with molecular symmetry
group arguments that favor classifying the nontorsional
The infrared and Raman spectra of HFA were first ana-
lyzed by Birney who concluded that the molecule had Cs
2
3
5
36
modes under C2v symmetry. This allows the modes to be
symmetry, based on Raman polarization data. At almost the
TABLE V. Fundamental vibrational frequencies of hexafluoroacetone ͑in cmϪ1͒.
Ab initiob
͑Int., depol. ratio͒
Experimentc
͑Int., depol.͒
Approximate
description
a
Mode
% diff.
a species
␯1
CO str
CF asym str͑ip͒
CF sym str/sym def
CF asym str͑op͒
CF sym str/sym def
CF asym def/rock͑ip͒
CF asym def͑op͒
CF asym def͑ip͒/CC str
1906 ͑96, 0.38͒
1313 ͑390, 0.67͒
1268 ͑248, 0.68͒
1206 ͑2, 0.74͒
775 ͑1, 0.00͒
631 ͑0.9, 0.46͒
538 ͑0.0, 0.75͒
472 ͑10, 0.39͒
316 ͑0.8, 0.34͒
264 ͑0.2, 0.71͒
145 ͑1, 0.12͒
1806 ͑s, p͒
1341 ͑s, dp͒
1254 ͑m, p͒
1169 ͑vw, dp͒
772 ͑m, p͒
631 ͑w, p͒
533 ͑m, dp͒
466 ͑m, p͒
317 ͑w, p͒
264 ͑m, dp͒
•••
5.5%
Ϫ2.1%
1.1%
3.2%
0.4%
0.0%
0.9%
1.3%
Ϫ0.3%
0.0%
•••
␯2
␯3
␯4
␯5
␯6
␯7
␯8
␯9
3
3
3
3
3
3
3
CF rock͑ip͒/CC str
3
␯10
␯11
␯12
b species
␯13
␯14
␯15
␯16
␯17
␯18
␯19
␯20
␯21
␯22
␯23
␯24
CF rock͑op͒
3
C–C–C bend
CF torsion
46 ͑0.0, 0.75͒
•••
•••
3
CC str/CF sym str
1375 ͑243, 0.75͒
1264 ͑647, 0.75͒
1238 ͑75, 0.75͒
971 ͑258, 0.75͒
800 ͑14, 0.75͒
717 ͑79, 0.75͒
531 ͑14, 0.75͒
505 ͑14, 0.75͒
370 ͑0.7, 0.75͒
277 ͑6, 0.75͒
1341 ͑s, dp͒
1274 ͑s, p͒
1220 ͑vs, p͒
971 ͑s, •••͒
800 ͑?, •••͒
717 ͑s, •••͒
530 ͑m, dp͒
508 ͑m, dp͒
368 ͑w, dp͒
273 ͑m, •••͒
192 ͑m, dp͒
•••
2.5%
Ϫ0.8%
1.5%
0.0%
0.0%
0.0%
0.2%
Ϫ0.6%
0.5%
1.5%
1.6%
•••
3
CF asym str͑op͒
CF asym str ͑ip͒/͑op͒
CF sym str/CO wag͑ip͒
3
3
3
CO wag͑op͒/CF rock͑op͒
3
CF sym def/str
CF asym def͑ip͒
CF asym def͑op͒
CO wag͑ip͒/CC str
3
3
3
CF rock͑ip͒
3
CF rock͑op͒/CO wag͑op͒
195 ͑11, 0.75͒
46 ͑0.4, 0.75͒
3
CF torsion
3
aApproximate descriptions using the symmetry coordinates defined in Ref. 34. Asymmetric coordinates are
labeled as ip or op to denote their symmetry with respect to the C–C–C plane.
b
6
-311G(d) frequencies scaled by 0.9051, infrared intensities in km/mol, depol. ratio is the calculated Raman
depolarization ratio.
c
Experimental frequencies from Ref. 34, IR gas phase intensities from Ref. 36 and Raman depolarizations
͑pϭpolarized, dpϭdepolarized͒ from liquid phase data of Ref. 34.
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D. J. Clouthier and D.-L. Joo: Spectroscopy of hexafluorothioacetone
7485
TABLE VI. Fundamental vibrational frequencies of hexafluorothioacetone ͑in cmϪ1͒.
Ab initiob
͑Int., depol. ratio͒
Experimentc
͑Int., depol.͒
Approximate
description
a
Mode
% diff.
a species
␯1
CS str/CC str
1381 ͑184, 0.34͒
1296 ͑88, 0.24͒
1199 ͑5, 0.60͒
1167 ͑505, 0.49͒
762 ͑15, 0.03͒
583 ͑0.5, 0.38͒
547 ͑0.0, 0.74͒
418 ͑7, 0.35͒
332 ͑0.2, 0.18͒
285 ͑0.8, 0.67͒
163 ͑0.7, 0.67͒
57 ͑0.0, 0.74͒
1324 ͑m, p͒
1262 ͑m, •••͒
•••
4.3%
2.7%
•••
␯2
␯3
␯4
␯5
␯6
␯7
␯8
␯9
CF asym str͑ip͒
3
CF asym str͑op͒
3
CF sym str/CS str
1159 ͑s, •••͒
764 ͑w, p͒
580 ͑•••, •••͒
550 ͑•••, p͒
417 ͑w, p͒
332 ͑vw, p͒
286 ͑vw, •••͒
159 ͑vw, •••͒
•••
0.7%
Ϫ0.03%
0.5%
Ϫ0.5%
0.2%
0.0%
Ϫ0.3%
2.5%
•••
3
CF sym str/sym def
3
CF asym def͑ip͒
3
CF asym def͑op͒
3
CF rock͑ip͒/CC str/CS str
3
CF rock͑ip͒/CC str
3
␯10
␯11
␯12
b species
␯13
␯14
␯15
␯16
␯17
␯18
␯19
␯20
␯21
␯22
␯23
␯24
CF rock͑op͒
3
C–C–C bend
CF Torsion
3
CC str/CF sym str
1316 ͑508, 0.75͒
1253 ͑697, 0.75͒
1217 ͑26, 0.75͒
937 ͑78, 0.75͒
729 ͑1, 0.75͒
695 ͑54, 0.75͒
537 ͑9, 0.75͒
501 ͑5, 0.75͒
1296 ͑s, •••͒
1202 ͑vs, •••͒
1174 ͑w, •••͒
941 ͑m, •••͒
•••
696 ͑m, •••͒
539 ͑w, •••͒
501 ͑w, •••͒
308 ͑vw, •••͒
237 ͑vw, •••͒
175 ͑vw, dp͒
•••
1.5%
4.2%
3.7%
Ϫ0.4%
•••
Ϫ0.1%
Ϫ0.4%
0.0%
0.3%
0.8%
3.4%
•••
3
CF asym str͑op͒
3
CF asym str͑ip͒
3
CF sym str/CC str
3
CS wag͑op͒/CF rock͑op͒
3
CF sym def
3
CF asym def͑ip͒
3
CF asym def͑op͒
3
CF rock͑ip͒
309 ͑1, 0.75͒
3
CS wag͑ip͒
CS wag͑op͒
239 ͑0.0, 0.75͒
181 ͑1.5, 0.75͒
40 ͑0.0, 0.75͒
CF Torsion
3
aApproximate descriptions using the symmetry coordinates defined in Ref. 34. Asymmetric coordinates are
labeled as ip or op to denote their symmetry with respect to the C–C–C plane.
b
6
-311G(d) frequencies scaled by 0.9051, infrared intensities in km/mol, depol. ratio is the calculated Raman
depolarization ratio.
c
Experimental frequencies, IR gas phase intensities and Raman depolarizations ͑pϭpolarized, dpϭdepolarized͒
from this work.
same time, Pace et al.37 reported an analysis of the infrared
group. The predicted vibrational frequencies and approxi-
mate descriptions of the normal modes, based on the domi-
nant contributions to the potential energy distribution, are
given in Table VI. Comparing the predicted frequencies of
HFA and HFTA we find that most of the modes are analo-
gous, with similar frequencies, PED’s and relative intensi-
ties. Of course, the CvS stretch is lower than the CvO
stretch and more heavily mixed with the C–C stretching co-
spectrum assuming C symmetry. Subsequently, Miller and
2
v
3
8
Kiviat remeasured the infrared and Raman spectra and re-
assigned a key vibration, negating the main reason for Ber-
34
ney’s conclusion of C symmetry. In 1984, Compton et al.
s
further refined the Raman spectrum of HFA and used ab
initio predictions of the vibrational frequencies to obtain a
nearly complete assignment of all the fundamentals. These
are given in Table V and are found to be in good agreement
with our calculated frequencies, with no indications of mis-
assignments. The only inconsistency is that the ␯1 funda-
mental is calculated to be 5.5% higher than the experimental
value, a somewhat larger error than would be anticipated on
the basis of the TAF and TTAF results.
ordinate, as was found for TTAF. The a symmetry CF sym-
3
Ϫ1
metric stretching mode is about 100 cm lower in HFTA, as
are the b symmetry in- and out-of-plane CS wagging modes.
These considerations will be used in assigning the vibra-
tional spectra of HFTA. Our studies of TAF, TTAF, and
HFA suggest that the scaled vibrational frequencies should
predict the fundamentals of HFTA to within Ϯ5% and that
the calculated infrared intensities and Raman depolarization
ratios can be used as qualitative tools to aid in making as-
signments.
D. Hexafluorothioacetone (HFTA)
There have not been any previously reported experimen-
tal or theoretical studies of the structure or vibrational fre-
quencies of hexafluoroacetone. Our ab initio structure of
HFTA ͑Table IV͒ is very similar to that of HFA except for
the CvS bond length, which is within 0.004 Å of that cal-
IV. SPECTROSCOPIC RESULTS AND ASSIGNMENTS
A. Vibrational spectra
culated for TTAF. The CF group torsional angles of HFA
3
and HFTA are both small ͑ϳ20°͒, suggesting that the vibra-
tional modes of HFTA may also be classified using the in-
plane and out-of-plane notation appropriate to the C2v point
The mid-infrared gas phase spectrum of hexafluorothio-
acetone is shown in Fig. 3. It consists of a set of very strong
bands in the 1350–1150 cm region, two weaker bands at
Ϫ1
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7486
D. J. Clouthier and D.-L. Joo: Spectroscopy of hexafluorothioacetone
FIG. 3. Mid-infrared spectrum of 3.0 Torr of (CF ͒ CvS in a 10 cm cell at
3
2
Ϫ1
a resolution of 1.0 cm
.
9
41 and 696 cmϪ1 and a few very weak bands. As contami-
FIG. 5. Mid-infrared matrix isolation spectrum of hexafluorothioacetone at
nation by HFTA dimer was always a concern in these ex-
periments, we show in Fig. 4 the gas phase spectrum of the
dimer. Although there are strong features in similar regions
Ϫ1
14 K and a resolution of 0.5 cm .
Ϫ1
in both spectra, the peak at 747 cm in Fig. 4 is diagnostic
should be between 1300 and 1350 cmϪ1 and would be of
medium intensity in the infrared and a medium intensity po-
for the presence of dimer and it was absent from all the gas
phase HFTA spectra. The infrared spectrum of HFTA in an
argon matrix, shown in Fig. 5, helped to untangle the over-
larized band in the Raman spectrum. The band at
Ϫ1
Ϫ1
1324 cm is the obvious candidate for ␯ , with a calculated
1
lapping bands in the 1350–1150 cm region of the gas
Ϫ1
Ϫ1
value of 1381 cm and a frequency of 1350 cm in TTAF.
The drop in CvO frequency in going from TAF to HFA is
paralleled in the sulfur compounds.
phase spectrum. Unfortunately, these spectra were unavoid-
ably contaminated by dimer absorptions due to the presence
of metal components in the matrix system. The gas phase
Raman spectrum of HFTA is illustrated in Fig. 6. The stron-
Ϫ1
gest feature in the spectrum occurs at 696 cm , coincident
2. C–F stretches
with one of the medium intensity bands in the infrared
spectrum. As expected, the CF stretching modes
(
trum but weak in the Raman spectrum. A compilation of all
of the observed features in the infrared and Raman spectra is
given in Table VII.
The six C–F bonds in HFTA should give rise to six
different, primarily C–F stretching modes in the
1
1300–1100 cmϪ1 region͒ are strong in the infrared spec-
350–1100 cmϪ1 region. The ab initio calculations predict
that four of these will be strong infrared bands, along with
the much weaker CF asymmetric out-of-plane stretch of a
3
symmetry and CF asymmetric in-plane stretch of b symme-
3
try. The gas phase infrared spectrum shows four strong ab-
B. Vibrational assignments
1
. CvS stretch
The CvS stretching mode is expected to occur in the
same region and with approximately the same intensity as
the CvS stretch in trifluorothioacetyl fluoride. This band
FIG. 4. Mid-infrared spectrum of 1.5 Torr of HFTA dimer in a 10 cm cell at
FIG. 6. Gas phase Raman spectrum of hexafluorothioacetone at a pressure
of 340 Torr and a resolution of 10 cm .
Ϫ1
Ϫ1
a resolution of 1.0 cm
.
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D. J. Clouthier and D.-L. Joo: Spectroscopy of hexafluorothioacetone
7487
TABLE VII. Observed vibrational spectra of hexafluorothioacetone ͑in cmϪ1͒.
IR ͑gas͒
IR ͑matrix͒a
cmϪ1
1328
Raman ͑gas͒
cmϪ1
int.
m
int.
cmϪ1
int.
25
Assignment
1324
m
m
m
m
vs
w
1324͑p͒
␯1
?
1304
1
1
1
296
262
202
s
m
vs
1293
1260
1193
␯13
␯2
␯14
␯15
␯4
C2F6
SiF4
␯16
␯5
␯18
?
1200
1155
3
7
1174
1
1
1
159
116
030
s
1154
vs
vw
vw
m
w
m
1023
940
763
696
vw
m
vw
m
9
7
6
6
41
64
96
39
765͑p͒
100
vw
5
5
80
50͑p͒
17
7
␯6
␯7
5
39
w
530
shoulder
10
␯19
dimer
␯20
␯8
CS2
SiF4
471͑p͒
501
417
397
389
372
332
308
286
vw
w
417͑p͒
74
vw
vw
vw
vw
vw
vw
334͑p͒
300
37
10
␯9
␯21
␯10
␯22
?
237
18
22
220
175
159
vw
vw
vw
175͑dp͒
␯23
␯11
aDimer absorption bands were also observed at 1268, 1256, 1216, 1212, 939, 934, 746, and 699 cmϪ1
.
sorption bands at 1296, 1262, 1202, and 1159 cmϪ1 which
shows up as a weak infrared band at 764 cm and as a
Ϫ1
Ϫ1
can be readily assigned as ␯13 , ␯ , ␯ , and ␯ , respec-
polarized, very intense Raman band at 765 cm . The calcu-
2
14
4
tively. The relative intensities and frequencies of these bands
fall in the same order as the theoretical predictions, so the
lated PED shows that these modes actually involve very sub-
stantial contributions from the CF symmetric stretches, as is
3
assignments are unambiguous. The infrared matrix spectrum
often found for compounds with C–F bonds adjacent to
carbon–carbon single bonds.
Ϫ1
25
shows a weak band at 1174 cm which we assign as ␯15
,
since it is calculated to be about five times stronger than the
weaker ␯ band which remains unassigned. It is likely that
4. C–F deformations
3
␯3
is coincident with ␯ or the much stronger ␯ band at
15 4
There should be six modes nominally describable as
C–F deformations. Examination of the PED shows that these
are the ␯ , ␯ and possibly ␯ a modes and the ␯ ␯19 and
Ϫ1
1
159 cm . Our Raman spectra were too weak in this region
to obtain reliable polarization data and only two bands at
200 and 1155 cmϪ1 could be positively identified.
6
7
8
18
1
␯20
b modes. Only ␯18 is predicted to be a strong infrared
band and it can readily be assigned to the feature at
3
. C–C stretches
Ϫ1
6
96 cm . The second most intense band in the Raman spec-
Ϫ1
Two nominally C–C stretching modes, ␯ of a symme-
trum of HFA is ␯9 at 316 cm , described as both CF3
5
try and ␯16 of b symmetry, are expected to occur in the
rock͑ip͒ and C–C stretch. The corresponding mode in HFTA
Ϫ1
Ϫ1
900–700 cm region. In HFA, the a mode is a medium
is ␯ , with a predicted frequency of 418 cm , coinciding
8
Ϫ1
intensity infrared band and the strongest Raman band in the
with the strong polarized Raman band at 417 cm . The two
spectrum, while the b mode is a strong infrared band and a
weak or missing band in the Raman spectrum, consistent
other b symmetry modes are predicted to be weak infrared
3
4
36
Ϫ1
bands at 537 and 501 cm corresponding to the observed
Ϫ1
Ϫ1
with our ab initio predictions for the IR intensities of ␯ and
features at 539 cm (␯ ) and 501 cm (␯ ). The ␯ and
5
19 20 6
␯16
. The C–C stretching mode of aЈ symmetry is also the
␯7 vibrations are calculated to give very weak infrared
most prominent Raman band in the spectra of TAF and
bands, which were not observed, but which could be as-
signed to Raman bands at 580 and 550 cm , respectively.
The other remaining fundamental in this region is ␯17 , which
Ϫ1
TTAF. In the HFTA spectra, the higher frequency ␯16 band
Ϫ1
occurs as a medium intensity infrared band at 941 cm with
no observed counterpart in the Raman spectrum and ␯5
involves a combination of CS out-of-plane wagging and
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7488
D. J. Clouthier and D.-L. Joo: Spectroscopy of hexafluorothioacetone
FIG. 7. Gas phase electronic absorption spectrum of hexafluorothioacetone
in the visible recorded at a pressure of 350 Torr, pathlength of 10 cm and
resolution of 0.5 nm.
FIG. 8. Gas phase electronic absorption spectrum of hexafluorothioacetone
in the ultraviolet recorded at a pressure of 0.8 Torr, pathlength of 10 cm and
resolution of 0.5 nm.
CF out-of-plane rocking; it is predicted to be a very weak
band at 729 cm and could not be found in the gas phase or
matrix infrared spectra.
to the ␲ antibonding orbital. Both transitions are expected to
be weak, as the former is spin-forbidden and the latter is
orbitally forbidden in molecules of C2v or higher symmetry.
The measured oscillator strengths of the two transitions are
3
Ϫ1
5. Low frequency modes
Ϫ6
Ϫ5
f ϭ 2.0 ϫ 10 for the T –S system and f ϭ 1.9 ϫ 10 for
1
0
The remaining nontorsional modes involve CF rocking,
3
the S –S0 system, comparable to the values measured
1
CS wagging and CCC skeletal bending motions. All of these
modes are calculated to be very weak in the infrared and, by
analogy with HFA data, are expected to have medium to
16
previously for the analogous band systems of TTAF. The
T –S transition shows some poorly resolved vibronic struc-
1
0
Ϫ1
ture with an average interval of about 140 cm . It is likely
that these low frequency intervals correspond to activity in
weak intensities in the Raman spectrum. The ␯ vibration,
9
Ϫ1
with a calculated frequency of 332 cm and a similar PED
the excited state CS out-of-plane wagging mode, similar to
to that of the strong Raman active ␯ mode of HFA, shows
14
8
that observed in the T –S spectra of CH CHS and
Ϫ1
1
0
3
up as a very weak IR band at 332 cm with a strong polar-
15
(
CH ͒ CS. Attempts were made to observe laser induced
Ϫ1
3 2
ized Raman line at 334 cm . The CF rocking modes ␯
3
10
Ϫ1
emission from HFTA vapor excited throughout the visible
absorption bands, in both static cells and using the pyrolysis
and ␯21 , with predicted frequencies of 285 and 309 cm
264 and 273 cm observed in HFA͒, are assigned to the
infrared bands at 286 and 308 cm . The CS wagging modes
␯22 and ␯ are assigned to the Raman lines at 237 and
75 cm ; the former is absent in the infrared as predicted
Ϫ1
͑
39
jet technique, but no signals were detected.
Ϫ1
In the ultraviolet, HFTA has a strong electronic transi-
tion in the 230–190 nm region as shown in Fig. 8, with
23
Ϫ1
1
Ϫ1
Ϫ1
␭max ϭ 207 nm and ⑀max ϭ 6200 l mol cm . The weak
by the ab initio results. Finally, the CCC bending skeletal
structure on the high energy side of this spectrum is due to
Ϫ1
mode, ␯11 , which is predicted to occur at 163 cm , was
1
ϩ
u
1
ϩ
g
the very strong ⌺ – ⌺ band system of CS present as an
2
tentatively identified as a weak shoulder on the side of the
impurity in the HFTA sample. The hexafluorothioacetone
Ϫ1
1
75 cm infrared band (␯ ).
23
transition is assigned as the S –S ͑␲–␲*͒ allowed electron
2
0
promotion. Although Rydberg transitions have been ob-
6
. Torsions
40,41
served in this region in the spectra of various thiones,
the
Ϫ1
The ab initio calculations predict two torsional vibra-
breadth of this transition ͑4000 cm FWHM͒ and measured
oscillator strength of 0.11 are more consistent with a
valence-shell electron promotion. Interference from strong
CS emission frustrated attempts to search for S –S emis-
Ϫ1
tions, both at 46 cm , a frequency range beyond the capa-
bilities of our instrumentation.
The results of the vibrational analysis are summarized in
Table VII and the assigned fundamentals are compared to the
ab initio predictions in Table VI.
2
2
0
sion in HFTA using laser induced fluorescence techniques.
V. DISCUSSION
C. Electronic spectra
In this work, we have explored the vibrational and elec-
tronic spectra of hexafluorothioacetone, a novel blue gas, for
the first time. Although our initial goal of studying the elec-
tronic spectra by LIF techniques has been thwarted by
mother nature, we have been able to obtain a rather complete
analysis of the vibrational spectra and have surveyed the
UV-visible absorption spectra.
As shown in Figs. 7 and 8, hexafluorothioacetone exhib-
its three electronic transitions in the UV-visible. The deep
blue color of liquid HFTA at low temperatures is a result of
the two overlapping band systems in the 800–400 nm region.
1
–3
By analogy with previous studies of the spectra of thiones,
3
these can be assigned as the a A–X A ͑800–680 nm͒ and
˜
˜
1
˜
1
˜
1
A A–X A ͑680–400 nm͒ electronic transitions, which in-
It is clear from our ab initio calculations of the vibra-
tional properties of TAF, TTAF, and HFA that the HF-SCF
volve promotion of a nonbonding electron on the sulfur atom
J. Chem. Phys., Vol. 106, No. 18, 8 May 1997
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D. J. Clouthier and D.-L. Joo: Spectroscopy of hexafluorothioacetone
7489
be extended to lower wave numbers to observe the torsional
fundamentals.
method with a 6-311G(d) basis, along with appropriate scal-
ing factors, gives predictions of the vibrational fundamentals,
relative infrared intensities and Raman depolarization ratios
which are sufficiently reliable as to be useful in the assign-
ment of spectra. These predictions have allowed us to finally
obtain interpretations of the spectra of trifluoroacetyl fluoride
and trifluorothioacetyl fluoride that are consistent with both
experiment and theory. Our calculations also confirm the ear-
lier experimental and ab initio attack on the spectrum of
hexafluoroacetone by Compton et al.34 in which 21 of the 24
vibrational fundamentals were observed and assigned. In the
present work, we have succeeded in identifying 20 of the
vibrational fundamentals of hexafluorothioacetone, with no
significant discrepancies between theory and experiment. As
anticipated, the vibrational spectra of HFA and HFTA are
very similar, with very strong CF3 and CvO or CvS
ACKNOWLEDGMENTS
The authors wish to thank Linda Osborne for recording
the Raman spectra, Dr. J. Laboy and Jeff York for recording
various matrix spectra and James Vollmer for his help with
the ab initio calculations. Many useful discussions with
Roger Grev are also gratefully acknowledged. Acknowledg-
ment is made to the donors of The Petroleum Research Fund,
administered by the American Chemical Society, for support
of this research.
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Ϫ1
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Ϫ6 16
Ϫ6
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(
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͑
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Ϫ5 16
Ϫ5
TTAF (5.3ϫ10 ), HFTA (1.9ϫ10 ), and H CS (ϳ4
2
Ϫ5 42
ϫ10
)
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Ϫ4 44
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2
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20
21
2
1
Ϫ1
2
1
9 700 cm , more than double the S –S separation of
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1 0
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Ϫ1
4
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0
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