Conformational behavior of CH3OC(O)SX (X = CN and SCN) pseudohalide congeners. A combined experimental and theoretical study

Article abstract of DOI:10.1021/jp912044r

Pure methoxycarbonylsulfenyl cyanide, CH₃OC(O)SCN (I), and methoxycarbonylsulfenyl thiocyanate, CH₃OC(O)SSCN (II), were prepared by reacting liquid CH₃OC(O)SCl with either AgCN or AgSCN, respectively. Compounds I and II were characterized by ¹H NMR, CG-MS, and vibrational (FTIR and FT-Ranian) techniques. The conformational properties have been studied by using vibrational spectroscopy [infrared (gaseous, liquid, and Ar matrix isolated), Raman (liquid) spectroscopy] together with quantum chemical calculations at the B3LYP and MP2 methods with the extended 6-311++G** and aug-cc-pVTZ basis sets. Compound I exhibits a conformational equilibrium at room temperature having the most stable form Cs symmetry with a synperiplanar (syn) orientation of the carbonyl double bond (C=O) with respect to both the CH₃O- and -SCN groups (syn-syn). Several bands assigned to a second conformer have been observed in the IR matrix spectra. This rotamer presents an antiperiplanar orientation of the thiocyanate group (syn-anti). Evaluating the equilibrium compositions at different temperatures by quenching the gas phase mixtures as Ar matrices allowed us to determine the conformational enthalpy difference ΔH⁰ = H _(syn-anti)⁰ - H_syn-syn⁰ = 0.80(18) kcal mol^-1. A similar conformational behavior has been determined for compound II. Thermodynamic properties were also computed at the high-level G2MP2 and G3 model chemistry methods. The importance of mesomeric (resonance) and anomeric (hyperconjugation) electronic interaction in the conformational behavior is evaluated by using the NBO approach for both species.

Full text of DOI:10.1021/jp912044r

J. Phys. Chem. A 2010, 114, 3703–3712
3703
Conformational Behavior of CH OC(O)SX (X ) CN and SCN) Pseudohalide Congeners. A
3
Combined Experimental and Theoretical Study
†
,†
‡
§
Sonia Torrico-Vallejos, Mauricio F. Erben,* Mao-Fa Ge, Helge Willner, and
,
†,|
Carlos O. Della V e´ dova*
CEQUINOR (CONICET-UNLP), Departamento de Qu ´ı mica, Facultad de Ciencias Exactas, UniVersidad
Nacional de La Plata, C.C. 962, 47 esq. 115, La Plata (B1900AJL), Buenos Aires, Rep u´ blica Argentina, State
Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100080, China, Fachbereich C-Anorganische Chemie, Bergische UniVersit a¨ t,
Wuppertal Gausstrasse 20, 47097 Wuppertal, Germany, and Laboratorio de SerVicios a la Industria y al
Sistema Cient ´ı fico (LaSeISiC) (UNLP-CIC-CONICET), Camino Centenario e/505 y 508,
(1903) Gonnet, Rep u´ blica Argentina
ReceiVed: December 21, 2009; ReVised Manuscript ReceiVed: February 1, 2010
Pure methoxycarbonylsulfenyl cyanide, CH
3
OC(O)SCN (I), and methoxycarbonylsulfenyl thiocyanate,
OC(O)SCl with either AgCN or AgSCN,
CH OC(O)SSCN (II), were prepared by reacting liquid CH
3
3
1
respectively. Compounds I and II were characterized by H NMR, CG-MS, and vibrational (FTIR and FT-
Raman) techniques. The conformational properties have been studied by using vibrational spectroscopy [infrared
(
gaseous, liquid, and Ar matrix isolated), Raman (liquid) spectroscopy] together with quantum chemical
calculations at the B3LYP and MP2 methods with the extended 6-311++G** and aug-cc-pVTZ basis sets.
Compound I exhibits a conformational equilibrium at room temperature having the most stable form C
symmetry with a synperiplanar (syn) orientation of the carbonyl double bond (CdO) with respect to both the
CH
s
3
O- and -SCN groups (syn-syn). Several bands assigned to a second conformer have been observed in
the IR matrix spectra. This rotamer presents an antiperiplanar orientation of the thiocyanate group (syn-anti).
Evaluating the equilibrium compositions at different temperatures by quenching the gas phase mixtures as Ar
matrices allowed us to determine the conformational enthalpy difference ∆H ) H (syn-anti) - H (syn-syn)
0
0
0
0
)
-1
.80(18) kcal mol . A similar conformational behavior has been determined for compound II. Thermodynamic
properties were also computed at the high-level G2MP2 and G3 model chemistry methods. The importance
of mesomeric (resonance) and anomeric (hyperconjugation) electronic interaction in the conformational behavior
is evaluated by using the NBO approach for both species.
Introduction
Recently, we reported the synthesis of the novel com-
Since thiocyanates (R-SCN) are important reagents for the
preparation of sulfur-containing organic compounds, they are
5–8
highlighted in several reviews. To introduce the thiocyanate
1
2
3 3 3
pounds CH OC(O)SNCO and CH OC(O)SSCF derived from
group into an organic molecule, a convenient synthetic route
involves the reaction of metal thiocyanates with organic halides.
The thiocyanate group is thermally unstable and simple chro-
matography or a prolonged heating over 50 °C can cause an
intramolecular rearrangement to the thermodynamically favored
CH OC(O)SCl. A complete structural study was conducted
3
mainly by focusing in the conformational behavior around the
sulfenylcarbonyl moiety [-C(O)S-]. The presence of two
conformers could be clearly determined at room temperature,
the syn form (the CdO double bond in synperiplanar orientation
with respect to the S-NCO and S-S bonds, respectively) being
the preferred conformation, with an important contribution of
9
isothiocyanate isomers. This interconversion between these
10
linkage isomers has been known since 1873 when Gerlich and
11
Billeter independently observed that allyl thiocyanate isomer-
3 3
the anti form, of 19(5)% in the case of CH OC(O)SSCF .
izes during distillation. Correspondingly, the reaction between
Moreover, the isocyanate species was found to be a very
CH
derivative [CH
CH OC(O)SCN, as reported by Liotta and Engel.
3
OC(O)Cl with KSCN yields mainly the isothiocyanate
3
versatile reagent for the synthesis of carbamates and N,N′-
3
OC(O)NCS], with only a minor contribution of
4
12
disubstituted ureas. Thus, following this work, we became
3
interested in the two thiocyanate derivatives CH
and CH OC(O)SSCN, which offer the possibility of comparing
the effect that pseudohalide substituents exert on the
3
OC(O)SCN
Thiocyanate compounds with an SCN group attached directly
to the sulfur atom, XSSCN, are well-known. The first thiocy-
anate containing a formal single S-S bond, NCSSCN, was
3
CH
3
OC(O)S- group.
13
synthesized by S o¨ derb a¨ ck in 1918. Halogenated derivatives,
such as FC(O)SSCN and ClC(O)SSCN, were also reported by
*
To whom correspondence should be addressed. Tel/Fax: +54-221-
Haas1
4,15
and Jochims. However, these compounds received
little attention until recently, when the electronic properties of
16
4
25-9485. E-mail: erben@quimica.unlp.edu.ar (M.F.E.) and carlosdv@
quimica.unlp.edu.ar (C.O.D.V.).
†
Universidad Nacional de La Plata.
Chinese Academy of Sciences.
Bergische Universit a¨ t.
Laboratorio de Servicios a la Industria y al Sistema Cient ´ı fico.
17
18
XC(O)SSCN (X ) F and CH O- ) were studied by photo-
3
‡
§
electron spectroscopy and quantum chemical calculations. Both
compounds were obtained in situ by the heterogeneous reaction
|
1
0.1021/jp912044r  2010 American Chemical Society
Published on Web 02/18/2010

3
704 J. Phys. Chem. A, Vol. 114, No. 10, 2010
Torrico-Vallejos et al.
SCHEME 1: Representation of the Conformers of
CH OC(O)SCN
3
between solid AgCN and vapors of the corresponding sulfenyl
chloride [XC(O)SCl, X ) F and CH O].
In this work, the synthesis, isolation, and characterization of
two pseudohalide derivatives, i.e., CH OC(O)SCN (I) and
CH OC(O)SSCN (II), are presented. Gas and liquid phase
infrared measurements as well as extensive quantum chemical
calculations have been performed. For CH OC(O)SCN, Ar-
3
3
3
3
Figure 1. Calculated potential function (B3LYP/6-31G*) for internal
matrix IR spectra have been also recorded at different temper-
atures of the deposition nozzle to evaluate possible conformer
mixtures in the gas phase. The vibrational analysis is completed
with the Raman spectra of both compounds in the liquid phase.
Thus, the structural and vibro-conformational properties were
determined for both molecules. Finally, the NBO population
analyses were applied to rationalize the effect of the electronic
interactions on these properties.
rotation around the C-S single bond in CH OC(O)SCN.
3
kcal mol- (MP2/6-311++G**) corresponds to a conformer
with anti orientation of the δ(OdCsSC) dihedral angle. A third
form, the anti-syn conformer in Scheme 1, was found to be a
stable conformer with calculated energies higher than 6 kcal
1
-
1
mol with respect to the minimum (see Table 1) and is not
expected to be detected under the conditions used in our
experiments. On the other hand, the anti-anti rotamer does not
correspond to a minimum, probably because of steric repulsions
between the methyl and thiocyanate groups. Figure 2 shows
the molecular models for the two main forms of
Results and Discussion
Quantum Chemical Calculations. To the best of our
knowledge theoretical studies on the CH OC(O)SCN molecule
3
have not been reported. In principle, the compound may be
present in at least four conformers with a planar skeleton,
depending on the orientation around the OsC and CsS
single bonds (see Scheme 1). Taking into consideration stru-
ctural studies previously reported for the methoxycarbonyl
CH
3
OC(O)SCN.
For compound II, a theoretical study at the B3PW91/6-31+G*
18
level of approximation was recently reported by Du et al. The
main objective was the assignment of the photoelectron spectrum
through the determination of molecular orbital characters.
Interestingly, two conformations were found to be very close
in energy. The most stable conformer has synperiplanar orienta-
tion of both O-C and S-S single bonds with respect to the
CdO double bond (syn-syn), while the corresponding form
having the CdO and S-S bonds in mutual antiperiplanar
19–25
3
CH OC(O)- moiety,
a syn orientation of the δ(COsCdO)
dihedral angle is preferred, the anti form being higher in energy.
The conformational properties of sulfenylcarbonyl compounds,
with the general formula XC(O)SY, have been studied and the
preference of a synperiplanar conformation around the C-S
1
9,26–28
-1
single bond was well-established.
Furthermore, in a few
orientation (syn-anti) lies only 0.81 kcal mol higher in energy
examples, it has been experimentally established that the
antiperiplanar conformation appears as a second stable form at
ambient temperature.2 Therefore, the potential energy function
for internal rotation around the δ(OdCsSC) dihedral angle was
calculated at the B3LYP/6-31G* level by allowing geometry
optimizations with the dihedral angle varying from 0 to 180°
in steps of 30°. The potential energy curve is shown in
Figure 1.
(B3LYP/6-311G*) than the most stable form. The PES was
assigned on the basis of the exclusive presence of the most stable
conformer.
9,30
From the conformational point of view, such a small energy
difference between both conformers of compound II is of prime
interest.3
1–34
Thus, full geometry optimizations and frequency
calculations were performed for each of the most stable
structures of CH OC(O)SSCN with the B3LYP and the second-
3
Two structures correspond to minima in the potential energy
curve, corresponding to the syn-syn and syn-anti forms shown
in Scheme 1. The most stable one displays a syn orientation of
the CdO double bond with respect to the S-C bond
order perturbation theory methods and the 6-31G* and
6-311++G** basis sets. Our results are in perfect agreement
with those obtained recently by Ge et al.18 One of the important
structural features of this compound is the dihedral angle around
the S-S bond, with a computed value of 87.4° at the B3LYP/
6-311++G** level of approximation. Predicted relative ener-
gies, ∆E° (corrected by zero-point energy) are listed in Table
S1, in the Supporting Information.
[
δ(OdCsSC) ) 0°] while the anti form (syn-anti), with
δ(OdCsSC) ) 180°, is higher in energy by only ca. 0.4 kcal
-1
mol . Subsequently, full geometry optimizations and frequency
calculations were performed for each of the most stable
structures with the B3LYP and MP2 methods and the 6-31G*,
3
Vibrational Analysis of CH OC(O)SCN. The vibrational
6
-311++G**, and aug-cc-pVTZ basis sets. Predicted relative
3
and conformational properties of CH OC(O)NCS were studied
energies, ∆E° (corrected by zero-point energy) are listed in
Table 1.
by Campbell et al.35 On the other hand, due to the inherent
instability and the difficulties to isolate the thiocyanate, little is
The two computational methods predict the structure with
syn orientation around both O-C and C-S single bonds to be
known about the linkage isomer CH
The IR (liquid) and Raman (liquid) spectra of CH
are shown in Figure 3. A tentative assignment of the observed
bands was carried out by comparison with calculated wave-
3
OC(O)SCN.
3
OC(O)SCN
the most stable conformer of CH
3
OC(O)SCN. The second stable
form, higher in energy by 0.31 (B3LYP/6-311++G**) or 0.56

Conformational Behavior of CH3OC(O)SX
J. Phys. Chem. A, Vol. 114, No. 10, 2010 3705
TABLE 1: Calculated Relative Energies Corrected by Zero-Point Energy (kcal mol-1) and Vibrational Frequency of the CdO
-
1
-1
Stretching Mode (cm ) with IR Intensities (km mol , in Parentheses) for Three Conformers of CH
syn-syn syn-anti
ν(CdO) ν(CdO)
3
OC(O)SCN
anti-syn
method of calculation
∆E°
∆E°
∆E°
ν(CdO)
a
B3LYP/6-31G*
B3LYP/6-311++G**
B3LYP/aug-cc-pVTZ
MP2/6-31G*
MP2/6-311++G**
MP2/aug-cc-pVTZ
G2MP2
0.00
1870(221)
1840(272)
1829(249)
1848(180)
1824(225)
0.42
0.31
0.24
0.70
0.56
0.44
0.37
0.39
0.54
1849(326)
1813(421)
1801(387)
1831(267)
1801(351)
6.64
6.68
1899(315)
1872(380)
b
0.00c
0.00
d
0.00e
7.48
1865(275)
0.00
0.00f
g
0.00
0.00
h
G3
CCSD(T)/6-311++G**
0.00i
a
b
c
d
e
E° ) -719.421522 hartree. E° ) -719.547432 hartree. E° ) -719.588356 hartree. E° ) -717.969509 hartree. E° ) -718.171574
hartree. f E ) -718.4966263 hartree, zero-point correction at the MP2/6-311++G** level. G2MP2 energy ) -718.564018 hartree. G3
energy ) -719.159740 hartree. E ) -718.3057792 hartree, optimized geometry and zero-point correction at the MP2/6-311++G** level.
g
h
i
Conformational Equilibrium
The occurrence of two bands in the region of the CdO group
in the IR (gaseous an Ar matrix isolated) spectrum (Figure 4)
suggests that an equilibrium of two conformers in similar
concentrations is present at room temperature. It is well-known
that the ν(CdO) normal mode of carbonyl compounds is very
3
5,40
sensitive to conformational properties.
The carbonyl stretch-
ing region (1900-1700 cm ) of the infrared spectra of
CH OC(O)SCN in the vapor phase and isolated in an Ar-matrix
are shown in detail in Figure 4. Two intense absorptions are
-
1
3
-1
evident; one band in the gaseous spectrum occurs at 1794 cm
-
1
and the other at 1765 cm . The well-defined vibrational-
rotational contour of the blue-shifted band allows assigning this
absorption to the syn-syn form in a straightforward manner.
The almost parallel orientation of the carbonyl oscillator (dipole
derivative vector) with respect to the principal axis of inertia
B, a B-type band is expected for the ν(CdO) normal mode of
the syn-syn form. In the same way, from the band contour of
the absorption at 1765 cm , the syn-anti form is proposed.
Because of the orientation of the carbonyl oscillator with respect
to the principal axis of inertia A and B, an AB hybrid band
type is expected for the ν(CdO) normal mode of the syn-anti
conformer. The inset of Figure 4 shows the principal axis of
Figure 2. Molecular models for the two main conformers of
CH OC(O)SCN.
3
-
1
numbers and on comparison with spectra of related molecules,
1
4,36,27
specially XC(O)SNCO (X ) Cl, F and CH
3
O)
and XSCN
37
(
X ) H, Cl, Br). Furthermore, the potential energy distribution
PED) associated with each normal vibrational mode was
(
calculated under the harmonic assumption. Experimental and
calculated [MP2/6-311++G**] frequencies, intensities, and
their tentative assignments are given in Table 2. This table also
lists the data obtained from the isolated Ar-matrix infrared
spectrum.
inertia for both conformers.
Following the classical procedure proposed by Seth-Paul,41
a semiquantitative analysis of the band contours expected for
both prolate asymmetric tops syn-syn and syn-anti rotamers,
42
has been further performed. The observed P-R separations
-
1
-1
The characteristic mode of vibration of thiocyanate com-
for the B-type band at 1975 cm (10 cm ) and for the AB-
-
1
-1
pounds corresponds to a band with weak intensity located at
type band at 1765 cm (11 cm ) are good reproduced by the
2
176 cm- assigned to the ν(SCtN) stretching mode in the
1
values calculated for the syn and anti forms of CH OC(O)SCN
whose values are 9 and 11 cm , respectively. The detailed
procedure for the calculation of P-R separation is given as
Supporting Information.
3
-
1
-1
liquid-phase IR spectrum, with its counterpart at 2178 cm in
the liquid Raman spectrum as the most intense signal.
3
8,39
This
fundamental is absent in the gas-phase spectrum and is observed
as a very low intensity absorption in the Ar-matrix spectrum.
Characteristic absorptions corresponding to the stretching mode
Additionally, the calculated wavenumber difference [B3LYP/
6-311++G**] between the CdO stretching modes of the two
-
1
of vibrations of OCH
3
and CdO groups appear at 2970 and
conformers is 27 cm (Table 1), in good agreement with the
-1
-1
1
794/1765 cm , respectively.
experimentally observed value of 29 cm in both the Ar-matrix
and gas-phase spectra. This correspondence is observed in Figure
Good agreement is obtained when the experimental vibra-
4
, where the simulated spectrum from the MP2/6-311++G**
tional frequencies and the calculated values are compared. From
the PED analysis, the characteristic fundamentals are clearly
established, but many other vibrations are mixed. It becomes
apparent that absorptions observed in the gas IR spectrum and
in the Ar-matrix spectrum can be assigned, in principle, to the
calculated frequencies is shown, assuming a mixture of the
syn-syn and syn-anti forms in a 75:25 ratio.
Variable Temperature Conformational Equilibria
Quenched by Matrix Isolation
3
presence of a second conformer of CH OC(O)SCN at room
temperature. This topic will be investigated in more detail in
the following section.
Taking into account the experimental evidence coming from
the gas-phase infrared spectrum about the presence of more than

3
706 J. Phys. Chem. A, Vol. 114, No. 10, 2010
Torrico-Vallejos et al.
3
Figure 3. Liquid IR (top) and liquid Raman (bottom) spectra of CH OC(O)SCN.
one conformer, and that rich conformational equilibria are
envisaged by the quantum chemical calculations, further ex-
perimental evidence is desirable to fully characterize the
conformational properties of compound I. If the barrier between
0.39 cal/(K mol)], the standard free energy difference value of
0
-1
∆G ) 0.68 kcal mol is obtained. From this value, an
abundance of 24% for the less stable anti form at 298 K is
calculated.
-1
the rotamers is <10 kJ mol , it was demonstrated that the gas
The two computational methods used through this work, even
using the triple-ꢀ quality basis sets expanding with diffuse and
polarization functions yield energy difference values which are
phase equilibrium is not disturbed by quenching of the molecular
beam as a matrix.4
3,44
Thus, matrix-isolation IR spectroscopy
0
-1
is reported as a technique to investigate the conformational
definitively too low, with ∆E of 0.31 and 0.56 kcal mol at
the B3LYP and MP2 methods with the 6-311++G** basis sets,
respectively (Table 1). Thus, high-level quantum chemical
calculations have been performed, including the G2MP2 and
G3 composite methods and the coupled-cluster CCSD(T)/
6-311++G** level of approximation. Similar energy values are
computed at the G2MP2 and G3 methods, which are too low
4
5–47
preferences of volatile compounds.
Infrared spectra of Ar-matrix isolated CH
3
OC(O)SCN at
different temperatures of the spray-on nozzle have been
measured. When the mixture in Ar is passed through the heated
spray-on nozzle and the resulting mixture is deposited as a
matrix, some IR matrix bands initially present in the room
temperature spectrum increase in their relative intensity. These
bands can be assigned with confidence to the higher energy
syn-anti conformer of compound I and are also listed in Table
-
1
(0.37 and 0.39 kcal mol , respectively) when compared with
0
the experimentally determined ∆H value. Better agreement is
obtained with the CCSD(T)/6-311++G** calculation, with a
0
-1
2
.
∆E ) 0.54 kcal mol (zero point energy and thermal energy
Because the syn-anti conformational change involves sig-
computed at the MP2 level).
nificant variations mainly in the CdO bond strength, the
integrated absorbance ratios of the carbonyl stretching bands
belonging to both conformers have been plotted on a logarithmic
scale as a function of the reciprocal absolute temperature. These
ratio values correspond closely to the concentration ratios of
CH
3
OC(O)SSCN
The IR and Raman (liquid) spectra of CH
shown in Figure 6. A tentative assignment of the observed bands
was carried out by comparison with calculated wavenumbers,
as well as on comparison with spectra of related molecules.
Experimental and calculated [B3LYP/6-311++G**] frequen-
cies, intensities, and their tentative assignments are given as
3
OC(O)SSCN are
48
40,49
the two conformers. Other fundamental modes, for instances
the 1195/1201 cm- pair of absorptions, show a very similar
behavior. Such van’t Hoff plots represented in Figure 5 yielded
1
0
a mean value of standard enthalpy difference ∆H ) 0.80 (18)
Supporting Information (Table S1). C
for the feasible rotamers of CH OC(O)SSCN. Thus, the 27
1
symmetry is expected
kcal mol- for the syn T anti conformational equilibrium of
1
3
CH
3
OC(O)SCN. Taking into account the calculated (MP2/6-
normal modes of vibration are both infrared and Raman active
modes.
0
3
11++G**) entropy difference for both conformers [∆S )

Conformational Behavior of CH3OC(O)SX
J. Phys. Chem. A, Vol. 114, No. 10, 2010 3707
3
OC(O)SCN
-1
TABLE 2: Observed and Calculated Vibrational Data (cm ) for CH
experimental
calculatedc
IR (gas)a
IR (Ar matrix)
MP2/6-311++G**
tentative assignment
(PED)/symmetry
mode syn-syn syn-anti syn-syn syn-anti IR (liquid) Ramanb
syn-syn
syn-anti
ν
1
ν
2
ν
3
ν
4
ν
5
3022 vw
2970 w
2857 vw
3019
2969
2847
2183
1787
3056 vw 3241 (1)
2966 m 3207 (2)
2844 vw 3108 (4)
2178 vs 2124 (<1) 2118 (<1) ν(CtN) (90) + ν(SsCN) (10)/A′
1772 vw 1824 (41) 1801 (100) ν(CdO) (100)/A′
3242 (1)
3209 (3)
3109 (6)
ν
ν
ν
as(CH
as(CH
(CH ) (100)/A′
s 3
3
3
) (100)/A′
) (100)/A′′
2964 w
2176
1758
2176 w
1875 s
1799 R
1770 R
1765 Q
1
789 P
1
760 P
ν
ν
ν
ν
6
7
8
9
1444 vw
1438 vvw
1330 vvw
1198 m
1465
1447
1351
1195
1451
1436
1453 vw 1520 (2)
1505 (2)
1518 (3)
1504 (3)
1493 (4)
δ
δ
δ
as(CH
as(CH
3
) (85) + F
s
(CH
) (95) + Fas(CH
3
) (10)/ A′
) (5)/ A′
) (5)/A′
) (60) + νas(CsOsC) (25) +
as(CH ) (5)/A′
as(CH ) (90) + δas(CH
1433 w
1328 vw
1201 s
3
3
1494 (2)
1235 (41) 1240 (87)
s
(CH
(CH
3
) (95) + δas(CH
3
1202 m
1201
1171
F
s
3
δ
3
ν
ν
10
1154 vs
1151
1156
1147 vs
1153 vw 1195 (100) 1205 (85)
F
3
3
) (5)/A′′
(CH ) (20) +
11
1193 (<1) 1192 (<1)
ν
as(CsOsC) (60) + F
s
3
F(CdO) (20)/A′
(CsOsC) (85) + νas(CsOsC) (10)/A′
ν
ν
12
962 vvw
812 m
934 vw
814 w
935 w
815 m
998 (1)
831 (9)
1006 (1)
836 (17)
ν
s
13
813
817
F(CdO) (40) + δ(CsOsC) (20) +
ν(SsCN) (20)/A′
ν
ν
ν
14
15
16
697
663
688
661
697 vw
662 w
518 vw
719 (1)
661 (2)
531 (1)
716 (1)
662 (3)
496 (1)
ν(SsCN) (90) + ν(CtN) (5)/A′
oop(CdO) (90) + τ(OsC) (5)/A′′
δ(CSC) (45) + ν(SsCN) (25) +
F(CdO) (15)/A′
666 vw
523 vw
698 w
519 vw
ν
ν
ν
ν
17
18
19
20
481 vvw
435
494 vvw
488 vvw 489 vvw 398 (1)
430 (<1)
ν(CsS) (40) + δ(SCN) (20) +
δ(CSC) (20)/A′
416 vw
393 m
364 (3)
355 (3)
δ(CdO) (35) + δ(CsOsC) (30) +
δ (CdO) (15)/A′
316 (<1)
313 (<1)
254 (1)
τ(SsC)/A′′
3
64 vw
259 vw
263 (3)
F(CdO) (45) + δ(CsOsC) (35) +
δ(SCN) (15)/A′
ν
ν
ν
ν
21
22
23
24
157 (<1)
127 (1)
116 (<1)
58 (<1)
156 (<1)
136 (1)
118 (1)
47 (1)
3
τ(CH sO)/A′′
τ(OsC)/A′′
134 w
85 w
δ(CsSC) (60) + δ(SCN) (35)/A′
τ(CsS)/A′′
a
b
Band intensity: vs ) very strong, s ) strong, m ) medium, sh ) shoulder, w ) weak, vw ) very weak, vvw ) very very weak. Liquid
c
at room temperature. In parentheses relative band strengths for the two most stable forms, IR intensities [100% ≡ 554 km/mol for the
syn-syn form] and [100% ≡ 351 km/mol for the syn-anti form].
-
1
The general features of the vibrational spectra of
CH OC(O)SSCN can be interpreted on the basis of the presence
of the syn-syn conformer. However, the carbonyl stretching
region of the liquid phase infrared spectrum shows the presence
of a very broad absorption, which could indicate more than one
band with medium intensity located at 2158 cm , assigned to
the ν(SCtN) stretching mode in the IR spectrum, with its
counterpart in the Raman spectrum as the most intense signal,
3
-1
50,51
also centered at 2158 cm , in agreement with related species.
-
1
The most intense liquid-phase IR band at 1148 cm is assigned
to the antisymmetric stretching mode of the CH sOsC
and the stretching mode ν(SsCN) is assigned to a
medium intensity band in the IR at 666 cm (668 cm in
Raman).53 It is well-known that the ν(SsS) stretching mode
is usually observed in the Raman spectrum and can be
used to characterize disulfide containing compounds. For
conformer present in liquid CH
3
OC(O)SSCN. The calculated
3
group,1
9,27,52
wavenumber difference (B3LYP/6-311++G**) for this mode
-1
-1
-1
between the two conformers is 30 cm , with the ν(CdO) mode
of the most stable syn-syn conformer located at high wave-
numbers in comparison with the ν(CdO) of the second stable
syn-anti form. The low vapor pressure of compound II makes
the measurement of either gas-phase or matrix infrared spectrum
difficult. Considering that at least two conformations are very
close in energy, further calculations at a higher level of
approximation have been conducted. Thus, the G2MP2 and G3
composite methods were used to compute the energy difference
3
CH OC(O)SSCN the medium intensity signal appearing at 526
cm is assigned to ν(SsS), in agreement with reported values
-
1
for other disulfide compounds.3
4,54
NBO Analysis
3
between the two main conformers of CH OC(O)SSCN. Ac-
cording to these calculations, the syn-syn conformer is more
stable than the syn-anti form by 0.81 and 0.91 kcal/mol, for
the G3 and G2MP2 methods, respectively.
Additionally to the CdO normal mode of vibration, other
absorptions also are important to describe. The structure of the
title disulfide compound can be understood as formed by
disulfide and thiocyanate groups. So, one of the characteristic
mode of vibration of this type of compounds corresponds to a
For a better understanding of the conformational preferences
of the compounds here examined, a natural bond orbital (NBO)
analysis at DFT level of theory has been performed. NBO
method makes possible to examine hyperconjugative interactions
due to electron transfers from either bonding or lone pair filled
orbitals (donors) to empty antibonding (acceptor) orbitals.
Significant donor-acceptor NBO interactions and their second-
-
1
order perturbation stabilization energies (in kcal mol ), cal-

3
708 J. Phys. Chem. A, Vol. 114, No. 10, 2010
Torrico-Vallejos et al.
Figure 4. IR spectrum in the carbonyl stretching region for gaseous and for Ar-matrix isolated CH
3
OC(O)SCN, together with the simulated
spectrum from the calculated frequencies at the MP2/6-311++G**. Principal moments of inertia are shown for molecular models of both syn-syn
and syn-anti conformers (the C-axis is perpendicular to the plane formed by the A- and B-axes).
culated in vacuum at B3LYP/6-311++G** level of theory for
the main conformers of compounds I and II are showed in
Table 3.
For compound I, the NBO population analysis shows the
presence of two lone pairs formally located in the divalent sulfur
atom, i.e., lpπ(S) and lpσ(S), with occupancies of 1.78e and
π*(CdO) is observed. These interactions are responsible for
the high occupancy of the two acceptor orbitals π*(CdO)
(0.28e) and π*(CtN) (0.11e), which contributes to the stabi-
lization by resonance (mesomeric effect), favoring the local
planar structure around the -OC(O)SCN moiety. These interac-
tions are mostly not affected by the syn or anti orientation around
the CsS bond. However, the second lone pair formally located
at the sulfur atom [lpσ(S)] interacts in a different way depending
on the conformation of the molecule. Thus, lpσ(S) mainly
interacts with the σ*(CdO) orbital in the syn form [lpσ(S) f
σ*(CdO)] or with the σ*(OsC) orbital of the anti form [lpσ(S)
f σ*(OsC)] (anomeric effect). The syn form is favored by
1
.96e, respectively, indicating that the electron donor capacity
is mainly due to the lpπ(S). They interact preferably with the
CdO and CtN antibonding orbitals, the most intense interac-
tions are due to the donation of the HOMO orbital lpπ(S) to
the LUMO orbital π*(CdO) [lpπ(S) f π*(CdO)] and to the
π*(CtN) [lpπ(S) f π*(CtN)]. Also electron donation from
the lpπ(O) orbital in the methoxycarbonyl group into the
-
1
2.3 kcal mol (see Table 3). Obviously, differences between

Conformational Behavior of CH3OC(O)SX
J. Phys. Chem. A, Vol. 114, No. 10, 2010 3709
respect to the S-S single bond is suggested, the syn-syn
conformer being the most important form.
Experimental Section
Synthesis. Following the early method reported for related
15
species by Haas and Reinke and the recent work of Ge et
1
8
al. CH
3
OC(O)SSCN and CH
3
OC(O)SCN have been synthe-
OC(O)SCl and the
sized by the methathesis reaction of CH
3
corresponding silver salts according to eq 1:
CH OC(O)SC1(1) + AgX(s) f CH OC(O)SX(1) +
3
3
AgC1(s)
X ) CN, SCN (1)
Physical Properties and Spectroscopic Characterization.
Compounds I and II are colorless liquids, with the characteristic
overpowering sulfenylcarbonyl odor. In the liquid state,
3
CH OC(O)SSCN is stable for several hours at air and room
Figure 5. van’t Hoff plots using the absorbance ratios of the IR Ar-
matrix band pairs at 1787/1758 (b) and 1201/1195 (0) cm-1 obtained
temperature. The thiocyanate compound isomerizes after several
hours at room temperature to the respective isothiocyanate
after quenching the rotamer equilibria of CH
deposition temperatures.
3
OC(O)SCN at different
isomer [CH
3
OC(O)NCS].55
Isolation of both compounds from the reaction mixture can
1
be performed by vacuum distillation. The H NMR spectra of
these conformations are not limited to hyperconjugation effects
since the bonds around the -C(O)S- group are quite polar.
NBO calculations predict net atomic charges about -0.54e and
compounds I and II only shows singlet signals located at δ )
3
4.0 ppm that corresponds to the CH O protons. Previous studies
for CH
3
OC(O)SCN reported a value of 4.2 ppm for the proton
+
0.27e on the O and S atoms, respectively, whereas the C atom
12
13
signal. The C NMR spectrum of the CH
3
OC(O)SSCN shows
carry a +0.68e charge.
two quartet and a singlet signals. Thus, chemical shifts at δ )
For compound II, the resonance interaction involving the
π*(CdO) and π*(CtN) acceptor orbitals is disrupted by the
presence of the disulfide bond and the concomitant loss of local
planarity. Thus, the occurrence of an extended π system
including both the -OC(O)S- and -SCN groups is precluded.
Nevertheless, the anomeric interactions that account for the
syn-anti conformational equilibrium are very similar to those
observed for compound I, as listed in Table 3. For comparison,
equivalent second-order donor-acceptor interaction for
3
1
64.7 ppm (q, JCH ) 3.7 Hz), δ ) 110.3 ppm (s) and δ ) 57.4
1
ppm (q, JCH ) 150.3 Hz) are assigned to the CdO, CtN, and
CH carbons, respectively. These chemical shift values and
3
coupling constants are in good agreement with reported data
16,19,27,56,57
for related compounds.
The GC chromatogram of compound II shows a peak at a
retention time of 6.1 min (measured in CHCl
3
). In the electron
impact GC-MS spectra the compound shows the molecular ion
•+
(
M ) peak as a very weak intensity signal at m/z 149. Several
3
CH OC(O)SCl are also included in Table 3. Very similar values
ionic fragments arising from logical ruptures appear at m/z
are observed for the three molecules. This fact denotes that both
+
[
relative intensity, fragment] at 118 [10, C(O)SSCN ], 90 [15,
-
CN and -SCN groups and the chlorine atom have a similar
+
+
+
+
SSCN ], 64 [70, SS ], 60 [30, SCO ], 59 [100, CH
3
OC(O) ],
effect, acting thus, as pseudohalogenide groups.
+
+
3
2 [15, S ], and 15 [70, CH ]. This fragmentation pattern is
3
in agreement with the reported photoionization (He I) mass
spectrum acquired together with the photoelectron spectrum of
II, where peaks at m/z 149, 118, and 90 were observed.
Conclusions
1
8
From the analysis of the vibrational spectra of CH
3
OC(O)SCN,
the existence of two conformers in the gas phase is well-established.
These forms differ in the relative orientation of the CdO double
bond respect to the SsC single bond being the syn-syn conformer
the most stable form, having a planar skeleton structure with the
CdO double bond in mutual synperiplanar orientation with respect
to both the methoxy and thiocyanate groups. According to NBO
analysis, resonant (mesomeric) interactions seem to be responsible
for the planar structure, while hyperconjugative interactions tend
to stabilize the syn-syn conformer with respect to the syn-anti
one. From studies of the Ar-matrix spectrum at variable temper-
Additional evidence for identifying both compounds comes
from the analysis of their IR and Raman (liquid) spectra, as
commented above. In summary, the complement of both
techniques allows for an easy characterization of these thiocy-
anate species. The characteristic mode of vibration of the
thiocyanate group ν(SCtN) present in both molecules is
observed as the most intense signal in the Raman spectra at
-
1
2178 and 2158 cm for I and II, respectively, with their
-
1
counterparts at 2176 (weak) and 2158 cm (medium) in the
IR spectrum of the liquid substances. The most intense band
observed in the IR(liquid) spectra of both compounds, respec-
0
0
atures the conformational enthalpy difference ∆H ) H (anti-syn)
-
0
-1
-1
H
(syn-syn) ) has been determined to be 0.80(18) kcal mol .
tively, is centered at 1147 and 1154 cm assigned to the
Quantum chemical calculations, even at high levels of approxi-
mationm yield energy differences that are fairly lower than the
experimental values; the better agreement is obtained with the MP2
and CCSD(T) methods with the extended 6-311++G** basis sets,
with an estimated ∆E value of 0.56 and 0.54 kcal mol ,
respectively.
antisymmetric stretching mode (νas CsOsC) in the methoxy-
carbonyl group. For compound II, the characteristic disulfide
stretching mode is observed in the Raman (liquid) spectrum as
-
1
a strong signal at 526 cm .
0
-1
General Procedure and Reagents. In a Carius tube adapted
with a Young’s valve kept in a cold bath at -25 °C, 20 mmol
of CH OC(O)SCl was condensed on 32 mmol of finely
3
For compound II, the coexistence of also two conformers that
differ in the relative orientation of the carbonyl group with
powdered AgSCN or AgCN, which previously were dried in

3
710 J. Phys. Chem. A, Vol. 114, No. 10, 2010
Torrico-Vallejos et al.
Figure 6. Liquid IR (top) and Raman (bottom) spectra for CH
3
OC(O)SSCN.
-1
TABLE 3: Stabilization Energies (kcal mol ) for Orbital Interactions for Syn and Anti Conformers of Compounds I and II
and the Related CH OC(O)SCl Species, Using the B3LYP/6-311++G** Approximation
3
CH
syn-syn
3
OC(O)SCN
syn-anti
CH
3
OC(O)SSCN
syn-anti
CH
syn-syn
3
OC(O)SCla
syn-anti
interaction
syn-syn
lpπS f π*CtN
lpπS f π*CdO
lpπO f π*CdO
lpσS f σ*CdO
lpσS f σ*OsC
27.56
23.18
49.57
4.64
27.97
22.75
52.52
28.00
22.20
49.12
4.47
28.15
20.10
48.03
23.95
47.13
4.51
24.61
51.56
2.35
2.30
2.45
a
Similar values are reported in ref 19 by using the B3LYP/6-31+G* level of approximation.
vacuum at 80 °C for 2 h. The reaction mixture was kept with
stirring for 1 h at low temperature and one more hour at room
temperature. The reaction is followed by observing the vanishing
were stored in flame-sealed glass ampules under liquid nitrogen
in a long-term Dewar vessel. CH OC(O)SCl (97% Aldrich) was
purified by fractional trap-to-trap condensation. AgCN and
AgSCN were purchased from commercial source (Aldrich,
estimated purity better than 99%).
3
3
of the pale yellow color (due to CH OC(O)SCl) of the reaction
mixture. The completion of the reaction was followed by
infrared spectroscopy and was confirmed mainly checking the
Instrumentation. (A) Vibrational Spectroscopy. Gas and
liquid phase infrared spectra were recorded with a resolution
-1
band at 550 cm due to the νS-Cl stretching mode of
OC(O)SCl.40 After this, the volatile components were
of 1 and 2 cm , respectively, in the range 4000-400 cm on
the Bruker EQUINOX 55 FTIR spectrometer. FT-Raman
spectrum of the liquid substances were run with a Bruker 66
-1
-1
CH
3
fractionated under dynamic vacuum trap-to-trap distillation
through traps held at -20, -60, and -196 °C. A 2.0 g (17
-
1
mmol) and 2.80 g (19 mmol) amount of pure CH
3
OC(O)SCN
with FRA 106 Raman accessory at a resolution of 2 cm ;
samples in a 4 mm glass capillary was excited with 5 mW of
a 1064 nm Nd:YAG laser.
and CH
the -20 °C trap, representing a yield of ca. 85 and 95%,
respectively. Only minor quantities of OCS and CO were
3
OC(O)SSCN were collected as a colorless liquids in
2
(B) Matrix Spectroscopy. In a stainless steel vacuum line
observed as decomposition products in the U-trap at -196 °C.
3
(1.1 L volume), a small amount of CH OC(O)SCN (ca. 0.05
1
The final purity was checked by IR and H NMR spectroscopy.
mmol) was mixed with an 1:1000 excess of Ar. For each
experiment ca. 0.6 mmol of this mixture was passed via a
stainless steel capillary through a heated quartz nozzle, which
was placed directly in front of the matrix support. The
temperature of the matrix support was held at 14 K and the
nozzle temperature was adjusted at the several temperatures in
the range 20 < T < 340 °C. Details of the matrix apparatus have
been given elsewhere.58 Matrix IR spectra were recorded on an
Volatile materials were manipulated in a glass vacuum-line
equipped with PTFE valves (Young, London, U.K.) and
capacitance pressure gauge (680 A, Setra System) for the control
of the pressure. The vacuum line has a connection to an IR cell
(
optical path length 10 cm, Si windows 0.5 mm thick). This
arrangement allowed us to observe the course of the reaction
by FTIR spectroscopy (EQUINOX 55, Bruker). Pure compounds

Conformational Behavior of CH3OC(O)SX
J. Phys. Chem. A, Vol. 114, No. 10, 2010 3711
References and Notes
IFS66v/S FT spectrometer (Bruker, Karlsruhe, Germany) in the
reflectance mode with a transfer optic. A DTGS detector with
a KBr/Ge beam splitter in the region ν ) 4000-400 cm was
(
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-1
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(
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1
13
(
C) NMR Spectroscopy. The H and C NMR measurements
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00 and 50 MHz, respectively, for CH OC(O)SCN and with a
Bruker Avance DRX-300 spectrometer operating at 300 and
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of S.T.V., who is a Postdoctoral fellow of CONICET. C.O.D.V.
and M.F.E. are members of the Carrera del Investigador of
CONICET, Rep u´ blica Argentina. Financial support by the
Volkswagen-Stiftung and the Deutsche Forschungsgemeinschaft
is gratefully acknowledged. The Argentinean authors thank the
ANPCYT-DAAD for the German-Argentinean cooperation
Awards (PROALAR) and the DAAD Regional Program of
Chemistry for Argentina. They also thank the Consejo Nacional
de Investigaciones Cient ´ı ficas y T e´ cnicas (CONICET), to
Comisi o´ n de Investigaciones Cient ´ı ficas de la Provincia de
Buenos Aires (CIC), Rep u´ blica Argentina. They are indebted
to the Facultad de Ciencias Exactas, Universidad Nacional de
La Plata, for financial support.
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3
712 J. Phys. Chem. A, Vol. 114, No. 10, 2010
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