2300
J. Phys. Chem. A 2010, 114, 2300–2305
Microwave and Quantum Chemical Study of Propargyl Thiocyanate (HCtCCH2SCtN)
Harald Møllendal,*,† Alexey Konovalov,† and Jean-Claude Guillemin‡,§
Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, UniVersity of Oslo,
P.O. Box 1033 Blindern, NO-0315 Oslo, Norway, École Nationale Supérieure de Chimie de Rennes, CNRS,
UMR 6226, AVenue du Général Leclerc, CS 50837, 35708 Rennes Cedex 7, France, and
UniVersité européenne de Bretagne
ReceiVed: December 4, 2009; ReVised Manuscript ReceiVed: January 8, 2010
The MW spectrum of propargyl thiocyanate (HCtCCH2SCtN) has been investigated for the first time in
the 25-80 GHz spectral region at room temperature or at 0 °C. The spectra of the ground vibrational state
and of the first excited state of the C-S torsional vibration have been assigned for one conformer. This
rotamer, denoted ap, has a symmetry plane (Cs symmetry) and an antiperiplanar arrangement for the C-C-S-C
link of atoms. It has previously been claimed that a conformer that has a synclinal conformation for this
chain of atoms is present in the gas in approximately the same concentration as ap (∼50% of the gas), but
this is not supported by the present experiments, where it is shown that the synclinal rotamer, denoted sc,
cannot be present in a concentration exceeding 1/3 of the total. It is therefore concluded that ap must be at
least 3.0 kJ/mol more stable than sc. The spectroscopic work has been augmented by quantum chemical
calculations at advanced B3LYP/aug-cc-pVQZ, B3LYP/6-311++G(3df,3pd), and MP2/aug-cc-pVTZ levels
of theory. These theoretical calculations underestimate the energy difference between ap and sc and predict
values for the conformationally important C-C-S-C dihedral angle of the hypothetical synclinal form that
deviates by ∼10°.
Introduction
Organic thiocyanates, RSCtN, have a C-S-C angle of
∼100° and may therefore exist in different conformations, owing
to restricted rotation about the R-S bond. Relatively few gasphase studies of the conformational properties of thiocyanates
have been reported. Ethylthiocyanate (CH3CH2SCN) has been
studied by microwave (MW)1 spectroscopy in the gas phase
and by infrared (IR) spectroscopy using the matrix-isolation
technique.2 The C-C-S-C dihedral angle determines the
conformation of this compound. Only the sc conformer, in which
this angle is 58° from synperiplanar (sp; 0°), was identified in
the MW work.1 The ap rotamer, where the C-C-S-C dihedral
angle is 180° from sp, was identified in the matrix-isolation
study in addition to the sc form.2 The sc rotamer was determined
to be 4.2(3) kJ/mol more stable than the ap conformer.2
Two rotamers, ap and sc, exist for isopropyl thiocyanate
(CH3CH(SCN)CH3) according to an IR and Raman study.3 The
ap form is the more stable in this case; the enthalpy difference
in the gas phase was found to be 2.3(3) kJ/mol using matrixisolation techniques.3
Ethenyl thiocyanate (H2CdCHSCN) has been studied by
MW, Raman, and IR spectroscopy.4,5 This compound has a more
stable sp form (C-C-S-C dihedral angle ) 0°) and a less
stable anticlinal (ac) rotamer, where this angle is ∼130° from
sp.4,5 The energy difference is 3-6 kJ/mol in the gas according
to the MW study.4,5
Interestingly, a MW study6 of the conformational properties
of propa-1,2-dienyl thiocyanate (H2CdCdCHSCN) has shown
that they are strikingly different from the properties of the related
* To whom correspondence should be addressed. Tel: +47 2285 5674.
Fax: +47 2285 5441. E-mail: harald.mollendal@kjemi.uio.no.
†
University of Oslo.
‡
École Nationale Supérieure de Chimie de Rennes.
§
Université européenne de Bretagne.
ethenyl thiocyanate (H2CdCHSCN). Whereas H2CdCHSCN
prefers a sp conformer,4,5 H2CdCdCHSCN prefers an ac form,6
with a C-C-S-C dihedral angle ∼134° from sp. Substitution
of a vinyl (H2CdCH-) group with an allenyl group
(H2CdCdCH-) therefore has a remarkable effect on the
conformational preferences.
The examples above demonstrate that the conformational
properties of thiocyanates are very sensitive to the group to
which the thiocyanate group is attached. It was therefore decided
to extend these studies to propargyl thiocyanate
(H-CtC-CH2SCtN), where the thiocyanate group is attached
to a propargyl (H-CtC-CH2-) moiety. H-CtC-CH2SCtN
has previously been investigated by IR and Raman spectroscopy
and gas electron diffraction (ED).7 It was found that this
compound exists as a mixture of two forms, a less stable
C-C-S-C sc form and a more stable ap conformer. The ED
rR values for the C-C-S-C dihedral angle were 57(8)° for sc
and 171(12)° for ap. The energy difference was determined to
be 1.24 kJ/mol with ap as the more stable.7
The methods we have used are MW spectroscopy and highlevel quantum chemical calculations. MW spectroscopy was
chosen because of its high accuracy and resolution, making this
method especially suitable for conformational studies. The
spectroscopic work has been augmented by high-level quantum
chemical calculations, which were conducted with the purpose
of obtaining information for use in assigning the MW spectrum
and investigating properties of the potential-energy hypersurface.
Experimental Section
Synthesis. The synthesis of this compound has already been
reported7,8 and is repeated in the Supporting Information for
the convenience of the reader.
Microwave Experiment. The spectrum of propargyl thiocyanate was studied in the 25-80 GHz frequency interval by
10.1021/jp9115244  2010 American Chemical Society
Published on Web 01/26/2010
Microwave Spectrum of HCtCCH2SCtN
Figure 1. Models of the antiperiplanar (ap) and synclinal (sc)
conformers of propargyl thiocyanate. The microwave spectrum of ap
was assigned. No assignments could be made for sc, which is at least
3.0 kJ/mol less stable than ap.
Stark-modulation spectroscopy using the microwave spectrometer of the University of Oslo. Details of the construction and
operation of this device have been given elsewhere.9,10 This
spectrometer has a resolution of ∼0.5 MHz and measures the
frequency of isolated transitions with an estimated accuracy of
∼0.10 MHz. Radio frequency microwave double resonance
(RFMWDR) experiments were carried out as described by
Wodarczyk and Wilson.11 The compound has a vapor pressure
of roughly 80 Pa at room temperature. The experiments were
performed at room temperature, or at about 0 °C, with a pressure
of roughly 10 Pa. Lower temperatures than 0 °C would have
enhanced the intensity of the spectrum, but a study at such low
temperatures was not possible owing to insufficient vapor
pressure.
Quantum Chemical Methods. The present ab initio and
density functional theory (DFT) calculations were performed
employing the Gaussian03 suite of programs,12 running on
the Titan cluster in Oslo. Electron correlation was taken into
consideration in the ab initio calculations using Møller-Plesset
second-order perturbation calculations (MP2).13 Becke’s
three-parameter hybrid functional14 employing the Lee, Yang,
and Parr correlation functional (B3LYP)15 was employed in
the density functional theory (DFT) calculations. The
6-311++G(3df,3pd) and aug-cc-pVQZ16,17 wave functions
were employed in the B3LYP calculations, whereas the
aug-cc-pVTZ16,17 wave function was used in the MP2
calculations. The default convergence criteria of Gaussian
03 were observed in all calculations.
Results and Discussion
Quantum-Chemical Calculations. Rotation about the C4-S7
bond (see Figure 1 for atom numbering) may produce rotational
isomerism. The B3LYP/6-311(3df,3pd) electronic energy as a
function of the C3-C4-S7-C8 dihedral angle was first
calculated in steps of 10° of this dihedral angle using the scan
option of the Gaussian03 program. The resulting potential
function indicated that there are two minima corresponding to
sc and ap, which are depicted in Figure 1. The energies,
structures, dipole moments, rotational constants, Watson centrifugal distortion constants,18 and vibration-rotation constants
(the R’s)19 were calculated for sc and ap employing the B3LYP/
6-311(3df,3pd) procedure. Only positive vibrational frequencies
J. Phys. Chem. A, Vol. 114, No. 6, 2010 2301
Figure 2. B3LYP/6-311++G(3df,3pd) electronic potential function
for rotation about the C4-S7 bond in HCtCCH2SCtN. The values
of the C3-C4-S7-C8 dihedral angle in degree are given on the
abscissa, and the relative energies in kilojoules per mole are given on
the ordinate. ap has a C3-C4-S7-C8 dihedral angle of exactly 180°.
A dihedral angle of 69.1° is predicted for sc, which is calculated to be
1.52 kJ/mol less stable than the global minimum, ap. This potential
function has maxima at exactly 0° (11.79 kJ/mol above the energy of
ap and at 120.0° (6.06 kJ/mol above the energy of ap)).
were found in these two cases, which indicate that they are
minima (“stable conformers”) on the potential-energy hypersurface. The C3-C4-S7-C8 dihedral angle of ap was found
to be exactly 180° from synclinal (0°), resulting in Cs symmetry
for this form, whereas this angle was found to be 69.1° in sc.
ap was predicted to be 1.50 kJ/mol more stable than sc, after
correction for zero-point energies.
B3LYP/6-311(3df,3pd) calculations of the energies, geometries, and vibrational frequencies of the two transition states
were then performed. The C3-C4-S7-C8 dihedral angle was
found to be exactly 0 and 120.0°, respectively for the two
transitions states, with energies 11.79 and 6.06 kJ/mol above
the energy of ap. Each transition state was found to have one
imaginary vibrational frequency, which indicates that they are
first-order transition states. The full potential function could now
be drawn, as shown in Figure 2.
B3LYP calculations with the large aug-cc-pVQZ basis set
were also undertaken for ap and sc but limited to the calculations
of electronic energies, structures, and dipole moments of the
said two forms because these calculations are costly. Interestingly, only relatively small structural differences between the
aug-cc-pVQZ structure and the 6-311++G(3df,3pd) structure
were seen. The former structure is listed in Table 1, where the
ED rR-structure7 is also included for comparison (last column).
The bond lengths and bond angles of the ED structure7 are
averages of the structures of ap and sc. The B3LYP/aug-ccpVQZ electronic energy difference between sc and ap was
predicted to be 1.55 kJ/mol, with ap as the more stable rotamer.
Additional results of spectroscopic interest of the two B3LYP
calculations are shown in Table 2. The B3LYP/aug-cc-pVQZ
results have been listed in this Table when available. B3LYP/
6-311++G(3df,3pd) harmonic and anharmonic vibrational
frequencies as well as the rotation-vibration constants are
displayed in Tables S1-S4 of the Supporting Information.
MP2 calculations with large basis sets have been shown to
predict accurate structures.20 MP2/aug-cc-pVTZ calculations of
the structures, dipole moments, vibrational frequencies, and
quartic Watson centrifugal distortion constants18 of the two
2302
J. Phys. Chem. A, Vol. 114, No. 6, 2010
Møllendal et al.
TABLE 1: MP2/aug-cc-pVTZ and B3LYP/aug-cc-pVQZ
Geometries of the ap and sc Conformers of
HCtCCH2SCtNa
method:
conformer:
MP2
ap
C1-H2
C1-C3
C3-C4
C4-H5
C4-H6
C4-S7
S7-C8
C8-N9
Bond
106.2
121.5
145.0
108.8
108.8
184.2
169.2
117.8
C3-C4-H5
C3-C4-H6
C3-C4-S7
H5-C4-H6
H5-C4-S7
H6-C4-S7
C4-S7-C8
H2-C1-C3
C1-C3-C4
S7-C8-N9
111.6
111.6
107.3
109.9
108.1
108.1
96.9
179.2
178.3
179.7
C3-C4-S7-C8
H5-C4-S7-C8
H6-C4-S7-C8
H2-C1-C4-H5
C1-C3-C4-H5
H5-S7-C8-N9
ap
Length (pm)
106.2
106.2
121.5
119.8
144.7
144.6
108.8
108.8
109.1
108.8
183.9
186.5
169.0
169.0
117.8
115.5
Angle (deg)
111.8
111.7
112.4
108.8
108.1
103.6
97.6
179.9
178.0
178.6
Dihedral Angle
-180.0
60.5
59.5
-63.4
-59.5 -178.7
118.1
65.4
118.3
74.2
-158.2
162.2
method:
EDb
B3LYP
sc
TABLE 2: MP2 and B3LYP Spectroscopic Constants of
HCtCCH2SCtNa,b
conformer:
sc
106.1
119.8
144.4
108.8
109.0
186.0
168.9
115.5
111.6
111.6
108.5
109.5
107.8
107.8
98.8
180.0
179.2
178.4
112.0
111.5
114.1
108.2
107.6
102.8
100.2
179.3
179.4
179.6
(deg)
-180.0
59.0
-59.0
-61.5
118.7
-158.5
70.1
-54.9
-169.0
46.1
64.0
171.0
MP2
ap
B3LYP
sc
ap
sc
b
117.7(4)
142.8(4)
182.7(2)
167.8(2)
115.5(3)
113.7(6)
97.6(9)
57(8)
a
Entries for the ap conformer, whose MW spectrum was
assigned, are in boldface. b Electron-diffraction rR values.7
forms were therefore performed. Selected MP2 results are
displayed in Tables 1 and 2. It is noted that these calculations
corrected for zero-point vibrations predict sc to be more stable
than ap by 0.44 kJ/mol, which is the opposite energy order of
what was found in the B3LYP calculations.
The theoretical MP2/aug-cc-pVTZ and B3LYP/aug-cc-pVQZ
structures in Table 1 should be compared with one another and
with relevant experimental structures. Inspection of this Table
shows that there are several significant differences in the two
theoretical structures. The MP2 triple bonds C1tC3 and
C8tN9 are longer than their B3LYP counterparts by ∼2 pm.
The equilibrium triple bond in acetylene21 is 120.292(13) pm
long. This value is much closer to the B3LYP length (119.8
pm; Table 1) than to the MP2 prediction (121.5 pm) and to the
ED rR-result7 (117.7(4) pm; Table 1). The rs CtN triple bond
in CH3SCtN is 116.97(20) pm,22 which is somewhat closer to
the MP2 result (117.8 pm; Table 1) than to the B3LYP values
(115.5 pm) and the ED value7 (115.5(3) pm; Table 1). The
experimental C4-S7 bond length (187.7(2) pm) is shorter than
the MP2 values by ∼1.5 pm and shorter than the B3LYP result
by >3 pm. The MP2 and B3LYP S7-C8 bond lengths agree
well but are longer than the corresponding bond in CH3SCtN
(168.42(30) pm)22 and the ED result for the title compound.7
The theoretical values of the C4-S7-C8 bond angle differ by
up to 2.6° for the individual conformers (Table 1). The
corresponding angle is 99.0(1)° in CH3SCtN22 compared with
an average ED value of 97.6(9)° in propargyl thiocyanate.7
The ED C3-C4-S7-C8 dihedral angle7 of sc is not very
accurate (57(8)°) compared with 60.5 (MP2) and 70.1° (B3LYP/
aug-cc-pVQZ). The MP2 method therefore predicts a much
A
B
C
DJ
DJK
DK
d1
d2
Rotational Constants (MHz)
13 864.3
4301.5
14 276.0
1384.0
2339.9
1361.5
1268.5
1646.0
1252.7
Quartic Centrifugal
0.166
-4.37
190.0
≈0.0
≈0.0
4756.1
2048.6
1557.1
Distortion Constants (kHz)c
4.58
0.151
3.46
-14.4
-3.98
-18.5
21.5
197.1
42.1
-2.13
-0.0227
-1.52
-0.217
-0.00105
-0.127
Principal-Axes Dipole Moment Components (10-30 C m)d
µa
13.9
8.5
13.6
9.7
µb
0.2
10.7
0.1
9.4
µc
0e
0.7
0e
0.8
∆E (kJ/mol)f
0.40
0.0
0.0
1.55
a
Basis set aug-cc-pVTZ has been used in all MP2 calculations.
In the B3LYP calculations, the basis set aug-cc-pVQZ has been
used to calculate the rotational constants, dipole moment, and
energy difference, whereas the 6-311++G(3df,3pd) basis set has
been used to calculate the quartic centrifugal distortion constants.
Entries for the ap conformer, whose MW spectrum was assigned,
are in boldface. c S-reduction Ir-representation.18 d 1 debye )
3.33564 × 10-30 C m. e For symmetry reasons. f MP2 energy
differences have been corrected for zero-point vibrational energies,
whereas the B3LYP energies are electronic energies. MP2 energy of
sc corrected for zero-point energy: -1591 409.25 kJ/mol. Total
electronic B3LYP/aug-cc-pVQZ energy of the ap conformer:
-1 594 212.35 kJ/mol.
b
more compact structure than the B3LYP procedure does for
this conformer.
Microwave Spectrum and Assignment of ap. The quantum
chemical results summarized in Table 2 as well as the previous
spectroscopic and ED study7 would imply that both ap and sc
would be present in the spectrum, each in relatively large
concentration. The dipole moments of these conformations are
relatively large, as seen from Table 2. However, a strong MW
spectrum was not expected at the temperatures we could employ
because the partition functions of the two forms must be
relatively large, leaving a low population in each quantum state,
primarily because they have several low-frequency vibrational
fundamentals. In fact, seven fundamentals of each form are
calculated to have frequencies below ∼500 cm-1 (Tables 1S
and 3S in the Supporting Information). The fact that the nitrogen
atom has a spin of 1 leads to splitting of the rotational transitions
by quadrupole interactions. This splitting is not large and
unresolved but would lead to broadening of the lines and lower
peak intensities.
ap should have a relatively strong a-type R-branch spectrum,
whereas sc should have strong a- and b-type spectra, according
to the principal-axes dipole moments of the two forms. (See
Table 2.) Pile-up regions with a complicated fine-structure
consisting of aR-transitions of the ground and several vibrationally excited states separated by B + C ≈ 2.6 GHz were
expected for ap because this is nearly a prolate rotor (Ray’s
asymmetry parameter23 κ ≈ -0.98), whereas several series of
b-type Q-branch lines would be the predominating feature of
the spectrum of sc.
A comparatively weak spectrum was indeed observed in
accord with the predictions above. Prominent pile-up regions
separated by ∼2.65 GHz were readily observed. These regions
are rich in lines originating from the ground and several
Microwave Spectrum of HCtCCH2SCtN
J. Phys. Chem. A, Vol. 114, No. 6, 2010 2303
TABLE 3: Spectroscopic Constants of the ap Conformer of
HCtCCH2SCtNa,b
Figure 3. Microwave spectrum of the aR J ) 14 r 13 pile-up region
of ap taken at a Stark field of roughly 110 V/cm. g.s. denotes the
ground vibrational state, and T1, T2, and T3 indicate successively
excited states of the torsion about the C4-S7 bond. The g.s. line,
as well as most of the excited-state lines shown here, are actually
composed of unresolved transitions with K-1 ) 5-12. The line
marked with 4 is the K-1 ) 4 pair of coalescing lines, whereas the
two lines marked with 3 are the K-1 ) 3 resolved pair of transitions
of the ground state. The pile-up regions become increasingly more
complicated than shown in this Figure as J increases because
additional resolved splittings of the K-1 states become more
pronounced as a result of centrifugal distortion.
vibrationally excited states. A typical example is shown in
Figure 3. The strongest line of each vibrational state consists
of several unresolved pairs of coalescing K-1 lines, with K-1
generally larger than 3. Additional K-1 pairs become resolved
from the major peaks as J increases, which is largely a result
of the effect of the DJK centrifugal distortion constant.18 The
pile-up regions therefore become increasingly complicated as
J increases.
The theoretical centrifugal distortion constants (Table 2) were
very useful for a detailed assignment of the complicated pileup regions because they predict the patterns of the K-1
transitions rather well. A weighted least-squares fit was performed using Sørensen’s program Rotfit.24 Watson’s S-reduction
Ir-representation18 was employed in the fitting procedure. Only
the DJ and DJK quartic centrifugal distortion constants could be
determined. The remaining centrifugal distortion constants DK,
d1, and d2 were kept fixed at the B3LYP/6-311++G(3df,3pd)
values (Table 2) in the least-squares fit. Isolated lines were
weighted according to the experimental uncertainty ((0.10
MHz). Several K-1 lines of each J + 1 r J transition overlap
and have therefore been assigned much larger weights, as shown
in Table 5S in the Supporting Information, where the groundstate spectrum is listed. Attempts to find b-type transitions failed,
presumably because µb is almost negligible (Table 2). The
spectroscopic constants of the ground state are displayed in
Table 3, where it is seen that the pseudo-inertial defect ∆ ) Ic
- Ia - Ib ) -3.15(1) × 10-20 u m2. This value is typical for
compounds having a symmetry plane and two out-of-plane sp3hybridized hydrogen atoms. The ∆ values calculated from the
rotational constants in Table 2, using a conversion factor of
505 379.05 × 10-20 MHz u m2, are -3.20 (MP2) and -3.15 ×
10-20 u m2 (B3LYP), respectively, which is very similar to ∆
given in Table 3.
The rotational constants of the ground vibrational state (Table
3) should be compared with their counterparts in Table 2.
vib. state
groundc
first excited
torsional stated
A/MHz
B/MHz
C/MHz
∆/10-20 u m2f
DJ/kHz
DJK/kHz
DK/kHzg
d1/kHz
d2/kHz
rmsh
no. trans.i
14 134.0(39)
1382.5018(79)
1269.3008(79)
-3.15(1)
0.16763(87)
-4.356(17)
197
-0.0227
-0.00105
1.099
233
13 715e
1383.5566(95)
1272.3385(98)
-4.919(5)
0.1768(15)
-4.348(38)
197
-0.0227
-0.00105
1.199
96
a
Uncertainties represent one standard deviation. b S-reduction
Ir-representation.18 c Spectrum listed in Table 5S in the Supporting
Information. d Fixed, see text. e Spectrum shown in Table 6S in the
Supporting Information. f Conversion factor 505379.05 × 10-20
MHz u m2. ∆ ) Ic s Ia s Ib. g DK, d1, and d2 were fixed in the
least-squares fit. See the text. h Root-mean-square deviation of a
weighted least-squares fit. See the text. i Number of transitions used
in the fit.
Complete agreement cannot be expected because the effective
constants of Table 3 are defined differently from the theoretical
constants displayed in Table 2. It is seen that the experimental
constants agree to within 2%, or better, with both the MP2 and
the B3LYP results. This is a satisfactory result taking into
account the different definitions of the rotational constants. It
is therefore not possible to conclude from a comparison of the
rotational constants which theoretical method predicts the more
accurate structure. It is also seen that the B3LYP and experimental DJ and DJK centrifugal distortion constants agree well.
Vibrationally Excited States. The ground vibrational state
a
R-branch transitions were accompanied by several vibrationally
excited states. (See Figure 3.) The most prominent series is the
first excited state of the torsion about C4-S7 bond, denoted
T1 in this Figure, which has a frequency of 55 cm-1 according
to the B3LYP calculations (Table 1S in the Supporting
Information). Unambiguous assignments of the K-1-fine structure transitions of the excited-state transitions were very difficult
to obtain because of frequent overlaps and comparatively low
intensities. However, it was possible to assign several lines with
K-1 g 2 of T1. This was not sufficient to obtain an accurate
value of the A rotational constant because of the near-prolacy
of ap. The A rotational constant was therefore fixed at 13 715.0
MHz in the least-squares fit. This value was derived using the
vibration-rotation constant RA ) 419.0 MHz, as given in Table
2S in the Supporting Information. RA is defined by RA ) A0 A1,19 where A0 is the rotational constant of the ground state and
A1 is the corresponding constant of the first excited state of a
particular vibrational mode. The spectrum of the T1 state is
listed in Table 6S in the Supporting Information, whereas the
spectroscopic constants are listed in Table 3. The vibration-rotation
constants associated with the B and C rotational constants, RB
and RC are defined in a manner analogous to RA.19 The
experimental value obtained from the rotational constants of
Table 3 are RB ) -1.055(12) and RC ) -3.038(13) MHz, in
satisfactory agreement with the B3LYP values of 0.03, and
-2.84 MHz, respectively (Table 1S in the Supporting Information).
Searches for sc. The numerous absorption lines seen outside
the pile-up regions of ap are all relatively weak, which is one
indication that there is not a high concentration of sc because
this conformer should have relatively large µa and µb dipole
2304
J. Phys. Chem. A, Vol. 114, No. 6, 2010
moment components (Table 2) and a statistical weight of 2
relative to the weight of ap, which is 1 because there are two
mirror-image forms of sc.
Studies of the spectrum at comparatively low Stark field
strengths (∼100 V/cm) simplify the spectrum significantly
because a majority of the transitions are not modulated under
these conditions, and this was exploited in an attempt to assign
sc. However, the coalescing and near-coalescing pairs of the
highest K-1 aR-lines of both ap and sc should be completely
modulated at this field strength. Their intensities depend
critically on µa2, as well as on the statistical weight of each
conformer. µa of ap is larger than µa of sc (Table 2), but the
fact that latter form has a statistical weight of 2 compared with
1 for ap means that the intensities of these aR-lines would be
comparable if both forms were present in a concentration of
50% each, as claimed in the previous study.7
Many attempts to find the said aR-lines transitions of sc failed,
and there are no relatively strong unassigned transitions in the
regions predicted for these lines, which is another strong
indication that the gas composition is predominated by ap. The
RFMWDR technique11 was also employed in an attempt to find
selected aR-lines of sc, however, with negative results. It is
concluded that the spectrum of sc would have been assigned
provided that the concentration of this form were larger than
∼1/2 of the concentration of ap. Assumption of a Boltzmann
distribution of the energy and a statistical weight of 2 for sc
therefore leads to the conclusion that ap is at least 3.0 kJ/mol
more stable than sc. This result, which is considered to be
conservative, is at variance with the previous experimental result,
which reported an energy difference of only 1.24 kJ/mol, with
ap as the more stable form.7 Interestingly, the theoretical energy
calculations (Table 2) underestimate the energy difference by
more than 3.5 (MP2) and 1.6 kJ/mol (B3LYP).
Conclusions
The MW spectrum of HCtCCH2SCtN has been investigated for the first time. The spectra of the ground vibrational
state and of the first excited state of the C-S torsional vibration
have been assigned for one conformer denoted ap, which has a
symmetry plane (Cs symmetry) and an antiperiplanar arrangement for the C-C-S-C link of atoms.
A previous ED and vibrational spectroscopic study7 claims
that a conformer that has a sc conformation for this chain of
atoms is present in the gas in approximately the same concentration as ap (∼50% of the gas). Assuming the statistical weight
of sc to be 2, compared with 1 for ap, the energy difference
was estimated to be 1.2 kJ/mol, with ap as the more stable.7
Extensive searches have been undertaken in an attempt to assign
the spectrum of sc. However, intensity considerations, Starkmodulation patterns, and RFMWDR searches indicate that if
sc is present, then it has to be in a concentration significantly
<50%. In fact, sc cannot be present in a concentration exceeding
1/3 of the total. ap must therefore be at least 3.0 kJ/mol more
stable than sc.
The spectroscopic work has been augmented by quantum
chemical calculations at B3LYP/aug-cc-pVQZ, B3LYP/6311++G(3df,3pd), and MP2/aug-cc-pVTZ levels of theory. The
B3LYP calculations predict that ap is more stable than sc by
∼1.5 kJ/mol, whereas the MP2 calculations indicate the
opposite, namely, that sc is the more stable form by 0.44 kJ/
mol. These theoretical calculations therefore underestimate the
energy difference between these rotamers. However, spectroscopic constants of ap are well predicted in both the MP2 and
B3LYP calculations. It is therefore not possible to decide which
Møllendal et al.
method predicts the more accurate structure of ap from a
comparison of the experimental and theoretical rotational
constants. Interestingly, a significant difference is predicted in
the theoretical structures of the hypothetical sc form. The
conformationally important C-C-S-C dihedral angle of the
hypothetical sc form is calculated to be 60.5° in the MP2
calculation, ∼10° less than the B3LYP/aug-cc-pVQZ prediction
(70.1°). It is concluded that the theoretical calculations even at
the high levels of theory reported herein are still insufficient to
predict correct energy difference between the two forms and a
consistent structure for sc.
The present conformational findings for HCtCCH2SCtN
resemble the results for the selenium analogue,
HCtCCH2SeCtN, where a conformation similar to ap was
identified in the MW spectrum and is presumed to be the more
stable form of the molecule.25 No similar experimental information appears to be available for the oxygen member
(HCtCCH2OCtN) of this series of cyanates.
Acknowledgment. We thank Anne Horn for her skillful
assistance. The Research Council of Norway (Program for
Supercomputing) is thanked for a grant of computer time. A.K.
thanks The Research Council of Norway for financial assistance
through contract 177540/V30. J.-C.G. thanks the PID OPV
(CNRS) for financial support.
Supporting Information Available: The synthesis, results
of the B3LYP/6-311++G(3df,3pd) calculations and the microwave spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
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