Semiempirical Calculation of the Rotational Barrier
and Valence Force Constants in Fluorocarbonylsulfenyl Chloride
A. H. Jubert*, C. O. Della Védova**, E. L. Varetti ***, O. E. Piro***,
and P. J. Aymonino***
Cätedra de Quimica Inorgänica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata,
La Plata, R. Argentina
Z. Naturforsch. 38 a, 61 - 6 3 (1983); received August 23, 1982
The rotational barrier about the C - S bond in fluorocarbonylsulfenyl chloride [FC(0)SC1] is
investigated using the C N D O method. The results confirm the existence of p l a n a r eis and trans
conformers and the higher stability of the latter, as suggested by previous vibrational results.
The valence force constants in F C ( 0 ) S C 1 were also calculated and the values obtained
compare favourably with results from a previous normal coordinate calculation and with those of
related compounds.
Introduction
The infrared spectra of gaseous fluorocarbonylsulfenyl chloride [FC(0)SC1] show that it exists
as a conformational mixture as indicates by the
existence of two CO stretching bands and the temperature dependence of their relative intensities
[1,2].
The barrier to rotation about formal single bonds
in different types of molecules has been investigated
very throughly by spectroscopic techniques [3]. In
F C ( 0 ) S C 1 the rotational process of interest is the
rotation about the C - S bond. In this work we have
investigated this process by a semiempirical quantum calculation using the C N D O / 2 method [4],
Valence force constants were estimated from the
C N D O data by a simple method derived from a
point charge model for the chemical bond [5]. The
present results are compared with results obtained
previously by a normal coordinate analysis [1] and
with those of related compounds.
Computational Procedure
Molecular wave functions of the different conformers were calculated using the C N D O / 2 method
* M e m b e r of the Carrera del Investigador, CIC, Prov.
d e Buenos Aires.
** Predoctoral fellow of C O N I C E T , R. Argentina.
*** M e m b e r of the Carrera del Investigador, C O N I C E T ,
R. Argentina.
Reprint requests to A. H. Jubert, Cätedra de Quimica
Inorgänica, Facultad de Ciencias Exactas, U N L P , 47
esq. 115, 1900 La Plata, R. Argentina.
employing the original parametrization given by
Pople et al. [4],
As was already pointed out [1], no structural
study of FC(0)SC1 is still available but the molecular structure has been ascertained from the vibrational spectra and by comparison with related compounds.
A planar conformation should be expected as
observed for methyl haloformate [6] and methyl
thiofluoroformate ([7] and references therein). The
planar models for the molecule are supported by the
observed band envelopes in the IR spectrum of the
gaseous substance [1]. A trans conformation of
halogen atoms seems to be the most stable one as
deduced from the band contour analysis of the most
intense carbonyl band in the gas phase infrared
spectrum and by comparison with related molecules. The proposed molecular geometry was used
to calculate the separation between P and R
branches of parallel bands [1]. The calculation was
based in the following molecular parameters (in A
or angular degrees): C - F : 1.37; C = 0 : 1.19; C - S :
1.76; OCS: 126; FCS: 110 (from a microwave study
of methyl thiolfluoroformate [7]); S - C l : 2.01; CSC1:
103 (from a microwave study of SC12 [8]). (It is
worth mentioning that such a value for the CSC1
angle is compatible with a more stable trans conformation. This would not be the case if a smaller
angle were assumed, for example 98.9° by comparison with CF 3 SC1 [9]). The results of such a
calculation were practically coincident with the
observed PR separations [1] and gave us an ap-
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A. H. J u b e r t et al. • Valence F o r c e Constants in Fluorocarbonylsulfenyl C h l o r i d e
62
proach to the best
calculation.
molecular
T h e following three
sidered:
O
CI
O
\
/
X.
C
/
/
A
tp= 0°. trans
geometry
conformations
0
S —CI
B
^ = 90°. gauche
for
were
the
con-
X
C
cp= 180°. eis
T h e geometry of the c o n f o r m e r s was f o u n d by
m i n i m i z i n g the total energy with respect to the
geometry variables d e f i n i n g the structure. This was
d o n e by interpolation between energies calculated
for a n u m b e r of specific geometries a r o u n d the
m i n i m u m (local) energy. With the o p t i m i z e d structural parameters, density and o v e r l a p matrices, total
and bond energies and dipole m o m e n t s for A, B and
C were d e t e r m i n e d .
T h e calculations were p e r f o r m e d on a IBM 4331
c o m p u t e r with a Q C P E program using S T O as basis
set. T h e calculations described were carried out
with an extensively m o d i f i e d version of the
C N D O / 2 Q C P E P r o g r a m N ° 141 written by Pople
and D o b o r c h [ 4 a - c ] and completely d o c u m e n t e d
by Pople and Beveridge [4d]. I n t e r m e d i a t e n u m b e r s
were carried in d o u b l e precision for all m o l e c u l a r
orbital calculations. T h e p r o g r a m was m o d i f i e d to
suit local i n p u t - o u t p u t r e q u i r e m e n t s and to facilate
the change of the d i m e n s i o n statements.
Calculations were also p e r f o r m e d with M I N D O / 3
[10] and M N D O [11] using their s t a n d a r d forms, but
in both cases the resulting o p t i m i z e d g e o m e t r y was
far apart from the proposed c o n f i g u r a t i o n and both
m e t h o d s failed in predicting the trans c o n f o r m e r as
the most stable one. T h i s c o n f i r m s the importance of
including in the calculation d-orbitals for heavy
atoms.
T h e nature of the a t o m s in the studied molecule
m a d e it inconvenient to use a s t a n d a r d a b - i n i t i o
procedure.
Results and Discussion
T h e c o n f o r m a t i o n a l energies involved in the rotation of the SCI g r o u p around the C - S bond were
calculated for d i f f e r e n t values of the a z i m u t h a l
a n g l e s formed by the S - C l b o n d when rotated
around the r axis defined by the C - S bond. All
other atoms were considered fixed on the xz plane.
T h e results c o n f i r m that the planar eis and trans
configurations have the lowest energies and that the
trans c o n f o r m e r is the m o r e stable one. T h e predicted height of the rotational barrier is 4.39 k c a l m o l - 1 .
T h e calculated energy difference between the conformers (0.75 kcal m o l - 1 ) is lower than the experimental value (ca. 2.0 kcal m o l - 1 [2]).
T h e C N D O calculation shows f r o m the localized
molecular orbitals that the lone pairs on the sulphur
and chlorine a t o m s are m o r e localized in the g a u c h e
form than in the p l a n a r c o n f o r m a t i o n s ; thus it can
be accepted that the origin of the barrier is the
decrease in d e r e a l i z a t i o n for the gauche c o n f o r m e r .
In the discussion of the barrier to rotation a b o u t
the C - S single b o n d it is expedient to study the
changes in charge density and bond indices. T h e
changes in charge densities caused by a cis-gauche
rotation involve that the carbonyl oxygen and carbon
atoms lose a considerable a m o u n t of negative and
positive charge, respectively, in the gauche form.
T h e carbonyl U bond becomes less polarized and the
bond index has its m a x i m u m value in the g a u c h e
form, at tp = 9 0 ° . T h e bond index of the C - S bond
in planar configurations is larger than in the g a u c h e
one, showing an increase of d e r e a l i z a t i o n .
In spite of reports on the failure of the C N D O / 2
m e t h o d to predict correctly the most stable conf o r m e r in molecules showing some d e r e a l i z a t i o n
along the rotation axis [ 1 2 - 14], our results correctly
predict that the trans c o n f o r m e r is m o r e stable t h a n
the eis. a l t h o u g h the value of the energy d i f f e r e n c e
differs by a factor of three f r o m experimental
result [2].
T a b l e 1. C N D O
FC(0)SC1.
a t o m i c charges and
bond
indices
Conformation
gauche
trans
Atomic charge
C
+0.4175
O
-0.2066
F
-0.1889
S
-0.0013
CI
-0.0207
+0.4058
-0.1818
-0.1918
-0.0058
-0.0264
+0.4142
-0.2064
-0.1850
Bond index ( ß A B )
C=0
1.6698
C-F
0.9048
C-S
1.0036
S-Cl
1.5045
1.6792
0.9062
0.9049
1.5032
1.6717
0.9041
1.0242
1.5044
+0.0011
-0.0239
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for
63
A. H. J u b e r t et al. • Valence Force Constants in Fluorocarbonylsulfenyl C h l o r i d e
Table 2. C N D O , stepwise coupling force constants
F C ( 0 ) S C 1 and force constants of related compounds.
Bond
/AB f r o m F C ( 0 ) S C 1
this
work
for
/ A B from related
compounds
15.4
13.10
C-F
C-S
S-Cl
5.7
3.0
3.2
5.03
4.32
3.37
15.2
13.7
5.64
3.05
2.99
a
(COF,)
(COFCl)b
(COFCl)b
(CF3SH)c
(CF3SCl)de
As m e n t i o n e d a b o v e , the v a l e n c e force constants
w e r e e s t i m a t e d w i t h t h e e x p r e s s i o n [5]
i
- 2
^A
2 Z
A
- Q
factor
IFAB, for d o u b l e b o n d s # A B =
and
for
triple
bonds
5AB =
inB.
(Z
A
-2)/Z
A
,
the
so
called
atomic
A
possesses l o n e e l e c t r o n p a i r s s u c h as P, N . h a l o g e n s ,
etc.
Our
results are
those obtained
listed
in T a b l e 2 t o g e t h e r
from a normal coordinate
with
analysis
p r e v i o u s l y p e r f o r m e d [1] u s i n g a s t e p w i s e c o u p l i n g
m e t h o d [16]. T h e c o m p a r i s o n o f i n t e r n a l f o r c e c o n stants shows that t h e values o b t a i n e d for F C ( 0 ) S C 1
are reasonable. T h e C = 0 and C - F stretching force
constants are rather high w h e r e a s a rather low force
constant
that
the
results for the C - S
CNDO
stretching
underestimates
the
indicating
conjugation
effect.
Acknowledgements
To C O N I C E T
A
(Programa
QUINOR),
SUBCYT
and CIC Provincia de Buenos Aires, Argentina, for
R3
w h e r e Z A = c h a r g e o f a t o m A a s u s e d in t h e
approximation,
WnA^+(\/2)2ulW^
The
J. L. Brema and D. C. Moule, Spectr. Acta 28, 809
(1972).
b
H. O b e r h a m m e r , J. C h e m . Phys. 7 3 , 4 3 1 0 (1980).
c
J. Borrajo, E. L. Varetti, and P. J. Aymonino, J. Mol.
Struct. 29, 163 (1975).
d
D. Bielefeldt and H. Willner, Spectr. Acta 36 A, 929
(1980).
e
A. Ben Altabef, E. L. Varetti, a n d P. J. Aymonino,
XV. Argentine C h e m i c a l Meeting, San Miguel de Tucum ä n 1980.
A
F o r s i n g l e b o n d s BAB=
dipole contribution, occurs only when the atom
a
Z
i n d e x c a l c u l a t e d f r o m t h e W i b e r g b o n d i n d e x [15],
^ a b + (2/3) 21/2
stepwise
coupling [1]
force constants
C=0
net c h a r g e o n a t o m B. R = b o n d d i s t a n c e , Z?AB = b o n d
support.
CNDO
QA = g r o s s c h a r g e o n a t o m A , g B =
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W e t h a n k Dr. H o r a c i o G r i m b e r g for useful discussions.
[9] H. O b e r h a m m e r , W. G o m b l e r , and H. Willner, J.
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2449 (1967).
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