PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
A microwave spectroscopic and quantum chemical study of
propa-1,2-dienyl selenocyanate (H2QCQCHSeCRN) and
cyclopropyl selenocyanate (C3H5SeCRN)w
Rajmund Mokso,a Harald Møllendal*a and Jean-Claude Guilleminb
Received 28th February 2008, Accepted 16th April 2008
First published as an Advance Article on the web 2nd June 2008
DOI: 10.1039/b803562h
The microwave spectra of propa-1,2-dienyl selenocyanate, H2CQCQCHSeCRN, and
cyclopropyl selenocyanate, C3H5SeCRN, are reported. The spectra of the ground and two
vibrationally excited states of the 80Se isotopologue and the spectrum of the ground state of the
78
Se isotopologue were assigned for one rotameric form of H2CQCQCHSeCRN. This
conformer is characterized by a C–C–Se–C dihedral angle of 129(5)1 from synperiplanar (01) and
is shown to be the global minimum of H2CQCQCHSeCRN. The spectra of the ground and of
three vibrationally excited states of the 80Se isotopologue, as well as of the ground state of the
78
Se isotopologue of one rotamer of C3H5SeCRN were assigned. This conformer has a
H–C–Se–C dihedral angle of 80(4)1 from synperiplanar and is at least 3 kJ mol 1 more stable than
any other form of the molecule. The microwave study has been augmented by quantum chemical
calculations at the B3LYP/6-311++G(3df,3pd) and MP2/6-311++G(3df,3pd) levels of theory.
1. Introduction
Very few studies of the structural and conformational properties of organic selenocyanates (X–Se–CRN) in the gaseous
state have been performed. In one such study, the structure,
dipole moment and barrier to internal rotation of the methyl
group of methyl selenocyanate (CH3SeCN) were investigated
by Sakaizumi et al.1,2 employing microwave (MW) spectroscopy. The conformational properties of ethyl selenocyanate
(CH3CH2SeCN) was the theme of another MW study.3 The
spectrum of only one rotamer, which has a synclinal (obsolete
‘‘gauche’’) conformation for the C–C–Se–C link of atoms, was
identified in this investigation, whereas the spectrum of
the antiperiplanar (‘‘trans’’) form was not observable. Very
recently, the MW spectrum of etheneselenocyanate,
H2CQCHSeCN, was reported revealing the existence of two
rotameric forms of this compound. The dihedral angle formed
by the C–C–Se–C chain of atoms is 01 in the synperiplanar
(‘‘cis’’) conformer and takes the unusual value of 166(3)1 in the
second anticlinal (‘‘skew’’) rotamer of this molecule.4 The
synperiplanar rotamer of this compound is more stable by
4.5(4) kJ mol 1.4
Recently, several new selenocyanates have been synthesized5–8 and this has made it possible to investigate their
physical properties. The syntheses and photoelectron spectra
of several allenyl selenocyanates, including the two title compounds H2CQCQCHSeCN7 and cyclopropylselenocyanate
(C3H5SeCN),8 have already been reported together with
quantum chemical calculations.7,8
In H2CQCQCHSeCN, the selenocyanate group is attached
to an allenyl group, whereas the selenocyanate group is
attached to a cyclopropyl ring in C3H5SeCN. Rotation about
the C––Se bonds may produce rotational isomerism in these
two cases and interactions between the selenocyanate group
and the allenyl, or between the cyclopropyl group and the
selenocyanate group, will determine the conformational properties of these two molecules.
The methods we have used in this study are MW
spectroscopy and high-level quantum chemical calculations.
MW spectroscopy was chosen because of its extremely
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 the use in assigning the MW
spectra and investigating properties on the potential-energy
hypersurface.
2. Experimental
a
Centre for Theoretical and Computational Chemistry (CTCC),
Department of Chemistry, University of Oslo, P. O. Box 1033
Blindern, NO-0315 Oslo, Norway.
E-mail: harald.mollendal@kjemi.uio.no; Fax: +47 2285 5441;
Tel: +47 2285 5674
b
Sciences Chimiques de Rennes, École Nationale Supe´rieure de
Chimie de Rennes-CNRS, F-35700 Rennes, France
w Electronic supplementary information (ESI) available: Syntheses of
H2QCQCHSeCRN and (C3H5SeCRN) and their microwave spectra. See DOI: 10.1039/b803562h
4138 | Phys. Chem. Chem. Phys., 2008, 10, 4138–4146
Synthesis of selenocyanic acid propa-1,2-diene ester and
selenocyanic acid cyclopropane ester
The synthesis of both compounds have been reported recently.7,8 The three-step syntheses are described in Schemes 1
and 2, respectively. A detailed experimental procedure is given
in the electronic supplementary information (ESI) for the
convenience of the reader.w
This journal is
c
the Owner Societies 2008
Scheme 1
.
Fig. 1 The synperiplanar (sp) and anticlinal (ac) rotamers of
H2CQCQCHSeCN. Atom numbering is shown on sp. The microwave spectrum of ac was assigned.
Scheme 2
.
Microwave experiment
The MW spectrum was recorded in the 48–80 GHz spectral
region in the case of H2CQCQCHSeCN and in the 38–80
GHz range for C3H5SeCN, using the Stark-modulated spectrometer of the University of Oslo. Details of the construction
and operation of this spectrometer, which has a 2 m HewlettPackard Stark cell, have been given elsewhere.9,10 The spectrum of H2CQCQCHSeCN was taken with the cell cooled to
roughly 0 1C, whereas C3H5SeCN was recorded at about
10 1C. A reduction of the temperature from room temperature was done in order to enhance the intensity of the spectral
lines. Lower temperatures, which would have increased the
intensity of the spectra even more, could not be achieved,
owing to insufficient vapour pressures of both
H2CQCQCHSeCN and C3H5SeCN. The spectral lines of
these two compounds were found to be broad and were
measured with an estimated accuracy of E0.10 MHz.
Radio-frequency microwave double resonance experiments
(RFMWDR), similar to those performed by Wodarczyk and
Wilson,11 were also conducted to assign unambiguously particular transitions.
function has two minima at about 0 and 1301 of the
C1–C2–Se7–C8 dihedral angle, and maxima at about 50
and 1801.
Separate calculations of the structures, energies, dipole
moments, vibrational frequencies, Watson’s A-reduction centrifugal distortion constants16 and vibration–rotation constants (the a’s)17 were then undertaken for the two
conformers. The starting values of the C1–C2–Se7–C8 dihedral angles were chosen to be close to 0 and 1301, respectively.
Full geometry optimizations with no symmetry restrictions
were made employing the default convergence criteria of
Gaussian 03. The C1–C2–Se7–C8 dihedral angle was found
to be exactly 01 (synperiplanar) for one of the conformers,
denoted sp, and 130.91 (anticlinal) for the other rotamer, which
is henceforth called ac. Only positive values were found for the
vibrational frequencies of each of these conformers. The
B3LYP structures found in these calculations are listed
in Table 1.
The electronic energy difference between the two rotamers
was predicted to be 3.34 kJ mol 1, with ac as the more stable
Table 1 B3LYP and MP2 geometriesa.b of the sp and ac conformers
of H2CQCQCHSeCN
3. Results
Method:
B3LYP
Quantum-chemical methods
Conformer:
sp
The present ab initio and density functional theory (DFT)
calculations were performed employing the Gaussian 03 suite
of programs,12 running on the 64 processor HP ‘‘superdome’’
computer 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 functional15 (B3LYP) was employed in
the DFT calculations. The 6-311++G(3df,3pd) basis set was
employed because this basis set has been optimized for
selenium.
Bond length/pm
C1–C2
C1–C4
C2–H3
C2–Se7
C4–H5
C4–H6
Se7–C8
C8–N9
Angle/1
C1–C2–H3
C1–C2–Se7
C1–C4–H5
C1–C4–H6
C2–Se7–C8
Se7–C8–N9
C2–C1–C4
Dihedral angle/1
C1–C2–Se7–C8
H3–C2–Se7–C8
H3–C2–C4–H5
H3–C2–C4–H6
Se7–C2–C4–H5
Se7–C2–C4–H6
C4–C2–C3–Se7
C1–Se7–C8–N9
Calculations for H2CQCQCHSeCN
A model of this compound with atom numbering is shown in
Fig. 1. Rotation about the C2–Se7 bond may produce rotational isomerism. B3LYP calculations were performed in an
attempt to predict which rotameric forms are minima
(‘‘stable’’) of the potential-energy hypersurface. Calculations
of energies were performed for the 0 to 1801 interval in steps of
101 of the C1–C2–Se7–C8 dihedral angle, employing the scan
option of the Gaussian 03 program, allowing all remaining
structural parameters to vary freely. The resulting potential
This journal is
c
the Owner Societies 2008
a
MP2
ac
sp
ac
129.3
130.0
108.3
194.4
108.3
108.3
183.8
115.5
129.6
130.0
108.1
194.0
108.3
108.3
184.7
115.5
130.2
130.9
108.2
192.3
108.2
108.2
182.8
117.6
130.6
130.8
108.2
191.6
108.2
108.2
183.8
117.6
123.9
126.1
121.3
121.3
98.9
177.5
178.9
124.1
120.1
121.4
121.0
97.4
177.8
179.6
123.6
124.1
120.8
120.8
96.1
179.5
177.3
123.2
119.5
120.8
120.5
95.9
178.5
179.3
0.0
180.0
90.4
90.4
89.6
89.6
1.8
179.7
130.9
53.7
89.9
90.0
84.9
95.2
20.6
160.5
0.0
180.0
90.6
–90.6
–89.4
89.4
0.3
179.4
126.0
58.7
89.8
90.3
85.2
94.7
1.5
156.8
Basis set: 6-311++G(3df,3pd).
b
Atom numbering is given in Fig. 1.
Phys. Chem. Chem. Phys., 2008, 10, 4138–4146 | 4139
Table 2 B3LYPa and MP2a parameters of spectroscopic interest of
the sp and ac conformers of H2CQCQCH80SeCN
Method:
B3LYP
Conformer: sp
Experimentalb
MP2
ac
sp
ac
Rotational constant/MHz
A
3347.3
5624.7
3461.4
5213.0
B
1879.1
1300.2
1943.7
1346.8
C
1213.5
1114.3
1255.5
1137.5
Quartic centrifugal distortion constantc/kHz
0.505
0.592
0.436
0.820
DJ
4.51
13.5
4.82
14.8
DJK
3.96
190
2.46
169
DK
dJ
0.172
0.157
0.134
0.249
2.95
0.993
2.80
0.291
dK
Dipole momente/10 30 C m
3.8
14.8
2.1
15.4
ma
11.8
5.6
12.0
6.0
mb
f
f
0.0
1.3
0.0
1.2
mc
12.4
15.9
12.2
16.6
mtot
Energy differenceg/kJ mol 1
DE
3.1
0.0h
0.0i
1.3
5487.4(56)
1325.453(81)
1129.332(89)
0.6970(41)
16.141(21)
190d
0.299(17)
0.993d
Basis set: 6-311++G(3df,3pd). b This work. c A-reduction.16 d
Fixed at the B3LYP value. e 1 debye = 3.33564 10 30 C m. f For
symmetry reasons. g Corrected for zero-point vibrational energy.
h
Electronic energy: 6853 911.11 kJ mol 1. i Electronic energy:
6848 389.20 kJ mol 1.
a
form. This energy difference becomes 3.10 kJ mol 1 when the
effects of the harmonic zero-point vibrational energies are
taken into consideration. This value for the energy difference
is listed in Table 2, together with the rotational and quartic
centrifugal distortion constants for the most abundant isotopologue H2CQCQCH80SeCN (80Se occuring with 49.8%
relative abundance), and principal-axes dipole moment
components.
The two maxima of the potential function were explored
next using the transition-state option of Gaussian 03. The first
maximum was found for a value of 49.61 of the
C1–C2–Se7–C8 dihedral angle and an electronic energy that
is 4.86 kJ mol 1 higher than the energy of ac. The second
maximum was found at exactly 1801 (2.27 kJ mol 1 above the
energy of ac). Each of these maxima has one imaginary
vibrational frequency associated with the torsion about the
C2–Se7 bond, which indicates that they are first-order transition states. The full electronic potential function for rotation
about the C2–Se7 bond could now be drawn, as shown in
Fig. 2.
MP2/6-311++G(3df,3pd) calculations of the structure,
dipole moment, vibrational frequencies and quartic centrifugal
distortion constants were repeated for the sp and ac rotamers,
because we wanted to compare results obtained by the MP2
procedure with the counterparts calculated by the B3LYP
method and with the relevant experimental findings.
Selected MP2 results are included in Tables 1 and 2. The sp
form is found to have exact Cs symmetry in both procedures,
whereas the MP2 C1–C2–Se7–C8 dihedral angle was predicted
to be 126.01 in the ac, about 61 less than found in the B3LYP
calculations. The MP2 CQC bond distances are typically 1 pm
longer in MP2 than in the B3LYP calculations. The C–H bond
lengths are practically the same in both methods. The C2–Se7
distances are over 2 pm longer in the B3LYP than in the MP2
4140 | Phys. Chem. Chem. Phys., 2008, 10, 4138–4146
Fig. 2 B3LYP/6-311++G(3df,3pd) electronic potential function for
rotation about the C2–Se7 bond in H2CQCQCHSeCN. The values of
the C1–C2–Se7–C8 dihedral angle in degree are given on the abscissa
and the relative energies in kJ mol 1 are given on the ordinate. sp has a
C1–C2–Se7–C8 dihedral angle of 01. A dihedral angle of 130.91 is
predicted for ac. sp is calculated to be 3.34 kJ mol 1 less stable than ac.
This potential function has maxima at 49.61 (4.86 kJ mol 1 above the
energy of ac), and at 1801 (2.27 kJ mol 1 above the energy of ac).
calculations, whereas the B3LYP Se7–C8 bond lengths are
longer by about 1 pm. There is also a significant difference in
the predicted C8RN9 bond length, which is about 2 pm
longer in the MP2 calculations. Bond angles calculated with
the two procedures generally agree to within 11, with the
exception of the C2–Se7–C8 angle, which is predicted to be
1.5–2.71 larger in the B3LYP calculations.
The energy difference corrected for zero-point vibrational
effects was predicted to be 1.24 kJ mol 1, with sp as more
stable in the MP2 case. This is the opposite energy order of
what was found in the B3LYP calculations above. The B3LYP
and MP2 results for the energy difference between the two
forms therefore vary by 4.34 kJ mol 1, which are not unexpected given the large number of electrons (66) involved in
these calculations and the inherent approximations of the two
methods.
MP2 and B3LYP calculations of the electronic energy
difference between sp and ac have been reported previously.7
In this case, the cc-pVTZ wave function18 was used. ac was
found to be preferred by 0.76 kJ mol 1 in these B3LYP
calculations, whereas the opposite was the case in the MP2
calculations (0.11 kJ mol 1 in favour of sp). These calculations
therefore predict a smaller energy difference between the sp
and ac than was found in the present calculations.
Calculations for C3H5SeCN
Rotation about the C1–Se9 bond (atom numbering in Fig. 3)
may produce rotational isomerism in this compound. The
H4–C1–Se9–C10 dihedral angle is conveniently used to define
the rotational isomerism in this case. B3LYP and MP2
calculations analogous to those described above for
H2CQCQCHSeCN were performed.
This journal is
c
the Owner Societies 2008
Table 3 MP2 and B3LYP geometriesab of the sc and ap conformers
of C3H5SeCN
Fig. 3 Model of the synclinal (sc) and antiperiplanar (ap) conformers
of cyclopropyl selenocyanate with atom numbering given on sc. The
MW spectrum of sc was assigned. This rotamer is at least 3 kJ mol 1
more stable than ap.
The B3LYP electronic-energy potential function for rotation about the C1–Se9 bond, which is shown in Fig. 4, has
minima at 76.0 synclinal and 180.01 antiperiplanar and maxima
at 0 and 146.01 for the H4–C1–Se9–C10 dihedral angle. The
rotamers corresponding to these two minima are henceforth
denoted sc and ap, respectively. The electronic energy of ap is
predicted to be 6.73 kJ mol 1 higher than that of sc. The two
‘‘stable’’ rotamers, ap and sc, are sketched in Fig. 3. The
maximum at 01 is 12.49 kJ mol 1 higher in energy than sc, and
the maximum at 146.01 is 7.58 kJ mol 1 higher. The energy
difference between ap and sc is calculated to be 6.18 kJ mol 1,
when contributions from zero-point vibrational effects are
taken into consideration. The structures of the two forms
are listed in Table 3, and selected parameters of spectroscopic
interest are found in Table 4.
Method:
B3LYP
Conformer:
sc
Bond length/pm
C1–C2
C1–C3
C1–H4
C1–Se9
C2–C3
C2–H5
C2–H6
C3–H7
C3–H8
Se9–C10
C10–N11
Angle/1
C2–C1–H4
C2–C1–Se9
C3–C1–H4
C3–C1–Se9
H4–C1–Se9
C1–C2–H5
C1–C2–H6
C3–C2–H5
C3–C2–H6
H5–C2–H6
C1–C3–H7
C1–C3–H8
C2–C3–H7
C2–C3–H8
H7–C3–H8
C1–Se9–C10
Se9–C10–N11
Dihedral angle/1
H4–C1–C2–H5
H4–C1–C2–H6
Se9–C1–C2–H5
Se9–C1–C2–H6
H4–C1–C3–H7
H4–C1–C3–H8
Se9–C1–C3–H7
Se9–C1–C3–H8
C2–C1–Se9–C10
C3–C1–Se9–C10
H4–C1–Se9–C10
H5–C2–C3–H7
H5–C2–C3–H8
H6–C2–C3–H7
H6–C2–C3–H8
C1–Se9–C10–N11
a
Fig. 4 B3LYP/6-311++G(3df,3pd) electronic potential function for
rotation about the C1–Se9 bond of C3H5SeCN. The values of the
H4–C1–Se9–C10 dihedral angle in degree are given on the abscissa
and the relative energies in kJ mol 1 are given on the ordinate. sc is
calculated to have a H4–C1–Se9–C10 dihedral angle of 76.01, whereas
a dihedral angle of exactly 1801 is predicted for ap. sc is calculated to
be 6.73 kJ mol 1 more stable than ap. This potential function has
maxima at 01 (12.49 kJ mol 1 above the energy of sc), and at 146.01
(7.58 kJ mol 1 above the energy of sc).
This journal is
c
the Owner Societies 2008
MP2
ap
sc
ap
150.5
149.8
107.9
194.3
150.3
108.1
108.1
108.0
108.1
184.7
115.5
149.3
149.3
108.2
196.2
151.3
108.1
108.1
108.1
108.1
184.0
115.6
150.0
150.8
108.0
191.5
150.2
107.9
107.9
108.0
107.9
183.8
117.6
149.5
149.5
108.2
193.4
151.2
108.0
108.1
108.1
108.0
182.9
117.7
118.2
116.9
118.7
120.9
112.5
117.0
118.3
119.2
117.5
114.5
118.1
116.6
117.8
118.7
114.7
98.0
178.1
118.5
123.9
118.5
123.9
106.1
117.5
118.2
119.2
116.8
114.7
118.2
117.5
116.8
119.2
114.7
97.9
178.9
118.3
119.2
118.0
116.5
114.3
116.3
117.3
118.2
117.7
115.7
117.6
116.6
117.3
118.6
115.6
96.2
179.3
118.2
122.5
118.2
122.5
108.3
117.3
117.5
118.7
116.4
115.7
117.5
117.3
116.4
118.7
115.7
95.2
179.2
1.1
144.4
138.4
4.9
144.4
1.6
2.4
145.3
142.2
72.6
76.0
145.9
0.0
0.2
145.6
177.7
0.8
145.2
137.4
7.1
145.2
0.8
7.1
137.4
37.6
37.6
180.0
144.9
0.0
0.0
144.9
178.7
1.3
144.3
145.6
2.5
144.6
0.9
2.8
140.8
135.3
66.6
81.6
146.5
0.0
0.4
146.9
147.0
0.6
145.7
139.2
5.9
145.7
0.6
5.9
139.2
36.8
36.8
180.0
145.5
0.0
0.0
145.5
0.8
Basis set: 6-311++G(3df,3pd).
b
Atom numbering is given in Fig. 3.
It was found again in the subsequent MP2 calculations that
ap and sc are minima on the potential-energy hypersurface,
with the results summarized in Tables 3 and 4. The
H4–C1–Se9–C10 dihedral angle was predicted to be exact
1801 in ap and 81.61 in sc. The MP2 energy difference corrected
for zero-point vibrational effects was calculated to be 4.53 kJ
mol 1, with sc as the preferred form relative to ap, compared
to 6.18 kJ mol 1 found in the B3LYP calculations.
The equilibrium structure of the prototype compound
cyclopropane is known.19 The equilibrium bond lengths of
C–C and C–H are 151.007(77) and 107.415(98) pm, respectively,19 which are close to their counterparts in Table 3.
Comparison of the bond lengths and bond angles (Table 3)
of the B3LYP and MP2 structures reveals differences similar
Phys. Chem. Chem. Phys., 2008, 10, 4138–4146 | 4141
Table 4 B3LYPa and MP2a parameters of spectroscopic interest of
the sc and ap conformers of C3H580SeCN
Method:
B3LYP
Conformer: sc
Experimentalb
MP2
ap
sc
ap
Rotational constant/MHz
A
4341.1
3198.0
4095.4
3140.1
4176.4(19)
B
1729.0
2124.4
1846.3
2268.4
1812.965(96)
C
1306.1
1421.3
1346.8
1470.8
1338.271(99)
Quartic centrifugal distortion constantc/kHz
0.966
0.724
1.31
0.601
1.2605(29)
DJ
8.17
1.12
9.59
2.37
9.5769(87)
DJK
30.7
0.665
28.4
2.41
30.7d
DK
dJ
0.407
0.285
0.567
0.229
0.407d
dK
0.696
0.274
0.643
0.789
0.696d
Dipole momente/10 30 C m
14.9
10.1
15.0
8.9
ma
5.4
10.3
5.7
11.4
mb
f
0.8
0.0
1.0
0.0f
mc
mtot
15.8
14.4
16.1
14.5
Energy differenceg/kJ mol 1
DE
0.0h
6.2
0.0i
4.5
Basis set: 6-311++G(3df,3pd). b This work. c A-reduction.16
Fixed at this value in the least-squares fit. e 1 debye = 3.33564 10 30 C m. f For symmetry reasons. g Corrected for zero-point vibrational energy. h Electronic energy: 6857 160.41 kJ mol 1. i Electronic energy: 6851 642.59 kJ mol 1.
a
d
to those discussed above for H2CQCQCHSeCN. The dihedral angles are also similar in the two calculations with the
exception of the dihedral angles related to the selenocyanate
group, whose orientation relative to the ring, as expressed by
the H4–C1–Se9–C10 dihedral angle, differs by 5.61 in the two
methods of calculation.
ap has a symmetry plane (Cs symmetry) with the selenocyanate group lying in the symmetry plane. Interestingly, the C–C
bonds of the ring adjacent to this group (the C1–C2 and
C1–C3 bonds) are shorter by about 2 pm (Table 3) than the
bond length opposite to the selenocyanate group (the C2–C3
bond) in both methods of calculation. This difference in bond
lengths is typical for substituted cyclopropanes with Cs symmetry.20 It is noted that the C–C bond lengths of sc, which has
no symmetry, do not show (Table 3) this kind of variation.
fundamentals. The quantum chemical calculations above indicate that there is one fundamental below 50 cm 1 (the C–S
torsion) and six additional between 100 and 500 cm 1 (not
given in Table 1 or 2) both for ac and for sp.
Another factor that contributes negatively to the intensity is
the fact that selenium has six naturally occurring isotopes, of
which five are relatively abundant (76Se (9.0%), 77Se (7.6%),
78
Se (23.5%), 80Se (49.8%), and 82Se (9.2%)), which means
that the intensity is reduced accordingly. The presence of
relatively large concentrations of more than one rotameric
form would have a similar effect on the intensity. Finally, the
14
N nucleus (99.6% abundance) has a spin = 1, and the
nuclear quadrupole coupling effects will therefore split each
transition into several components, which were not resolved in
our experiment, but will broaden the lines and reduce peak
intensities.
The theoretical predictions shown in Table 2 indicate that
the parent 80Se isotopologue of the ac rotamer is a prolate
asymmetrical top with Ray’s asymmetry parameter21 k E
0.91, with ma as its major dipole moment component
(Table 2). Accumulations of aR-branch transitions separated
by roughly B + C E 2.4 GHz were therefore expected in the
48–80 GHz spectral region. The high-K 1 members of these
series would be modulated at comparatively low Stark fields,
which would facilitate their assignments.
Series of accumulations were readily seen to protrude from
the background of weaker transitions, when a relatively low
Stark field was applied. A typical example is the accumulation
associated with the ground vibrational state of the J = 28 ’
27 a-type transitions of the 80Se species, which is shown
in Fig. 5.
These accumulations were the key to the assignment of the
MW spectrum of ac. It was found that pairs of aR-lines with
Microwave spectrum and assignment of the spectrum of the ac
sof H2CQCQCH80SeCN
Survey spectra taken at about 0 1C revealed a comparatively
weak and dense spectrum, with relatively broad absorption
lines occurring every few MHz throughout the 48–80 GHz
spectral interval.
Several factors contribute to the intensity of a MW spectrum. The intensity is proportional to the square of the
principal-axis dipole moment components and inversely proportional to the partition function.17 sp has a comparatively
large mb, and ac has a large ma (Table 2). The fact that a
relatively weak spectrum was observed must therefore indicate
that the partition function is relatively large at 0 1C, which
causes a low population in each quantum state, and consequently a weak spectrum. The large value of the partition
function is primarily caused by relatively small rotational
constants (Table 2) and several low-frequency vibrational
4142 | Phys. Chem. Chem. Phys., 2008, 10, 4138–4146
Fig. 5 This portion of the MW spectrum shows the a-type J = 28 ’
27 of the ground vibrational state of the H2CQCQCH80SeCN
isotopologue of the ac conformer. The values of the coalescing
K 1—lines are shown above each assigned line. The K 1 = 12 and
14, and the K 1 = 11 and 15 overlap. The spectrum was recorded
applying a Stark-modulation field strength of approximately
120 V cm 1.
This journal is
c
the Owner Societies 2008
Table 5
Spectroscopic constantsa,b of ac of H2CQCQCHSeCN
Species:
H2CQCQCH80SeCN
H2CQCQCH78SeCN
Vibrational state:
Ground
1st ex. torsion
1st bending
Ground
A/MHz
B/MHz
C/kHz
DJ/kHz
DJK/kHz
DK/kHz
dJ/kHz
dK/kHz
FdJK/Hz
Rmse
No. transitionsf
5487.4(56)
1325.453(81)
1129.332(89)
0.6970(41)
16.141(21)
190c
0.299(17)
0.993c
0.234(13)
1.6350
156
5614.39(81)
1334.609(86)
1130.704(87)
0.8551(43)
17.819(20)
190c
0.160(18)
0.993c
0.148(14)
1.1929
173
5638(14)
1316.25(16)
1125.08(17)
0.6549(56)
15.648(31)
190c
0.134(22)
0.993c
0.133(21)
1.6092
139
5536.2c
1326.358(38)
1132.194(44)
0.7200(40)
16.679(54)
190c
0.229c
0.993c
0.272(37)
1.2503
62
a
A-Reduction, Ir-representation.16 Spectra are found in the ESI, Tables 1S–4S. b Uncertainties represent one standard deviation. c Fixed; see
text. d Further sextic constants preset at zero. e Root-mean-squares deviation. f Number of transitions used in the weighted least-squares fit.
identical K 1 Z 7 coalesce in this spectral region, because k
E 0.91, and that transitions with different values of K 1
frequently overlap. A total of 156 aR-lines with K 1 Z 5 were
ultimately assigned. Definite assignments of transitions with
K 1o5 could not be made unambiguously, primarily because
these transitions have slow Stark effects and are therefore
difficult to modulate, and they are often overlapped. Searches
for b-type lines were also made because a mb component of
about 6 10 30 C m is predicted for ac (Table 2), but no such
transitions could be assigned. This is not surprising because
the b-type transitions should be considerably weaker than the
already weak aR-lines because ma is calculated to be more that
twice as large as mb (Table 2).
The aR-spectrum of ac was fitted to Watson’s A-reduction
Hamiltonian using the Ir-representation16 employing Sørensen’s program Rotfit.22 The inverse squares of the estimated
uncertainties of the transitions were used as weights in the
least-squares fitting procedure. The spectrum is shown in
Table 1S in the ESIw and the spectroscopic constants of the
80
Se isotopologue are listed in Table 5, and repeated in Table 2
for convenient comparison with the theoretical predictions.
The assigned aR-lines provide insufficient information for an
accurate determination of the DK and dK quartic centrifugal
distortion constants for this near-prolate rotor. These two
centrifugal distortion constants were held fixed at the B3LYP
values (Table 2) in the weighted least-squares fit. One of the
sextic centrifugal distortion constants (FJK) could also be
determined, as shown in Table 5.
The assignment of the ground-state spectrum of the 78Se
isotopologue was straightforward. The spectrum consisting of
62 transitions is listed in Table 3S in the ESIw and the
spectroscopic constants are given in Table 5. The A rotational
constant was fixed at 5536.2 MHz in this case. This value was
estimated from the B3LYP structure.
Attempts to assign the spectra of the 76Se (9.0%) and 82Se
(9.2%) species were made, but these spectra were found to be
so weak that a detailed assignment could not be achieved.
Excited states of the
80
Se isotopologue of ac
The MW spectra of the first excited state of the torsion about
the C2–Se7 bond and the first excited state of the lowest
This journal is
c
the Owner Societies 2008
bending vibration were assigned for the 80Se species. The
spectra are listed in Tables 2S and 3S in the ESI,w respectively,
and the spectroscopic constants are displayed in Table 5.
It is possible to calculate the vibration–rotation interaction
constants (the a’s)17 defined by aX = X0 X1, where X0 are
the ground-state rotational constants and X1 are the rotational
constants of a particular excited vibrational state. The entries
in Table 5 yield aA = 127(6), aB = 9.16(12), and aC =
1.37(12) MHz for the torsional state, and aA = 151(14), aB
= 9.20(17), and aC = 4.25(19) MHz for the bending vibration.
The corresponding B3LYP values are –40.84, –3.93, and –1.72
MHz, respectively, for the torsion, and –159.14, 8.23, and 4.16
MHz, respectively, for the bending vibration. The agreement is
far from perfect, but the a’s are derived by taking the third
derivatives of the potential energy at the minimum, which is
not expected to be very accurate in the present case.
Only rough relative intensity measurements of the two
lowest vibrational fundamentals could be performed in this
case owing to the density of the spectrum. These measurements yielded 62(30) and 87(40) cm 1 for the torsion and
bending vibrations, respectively. The B3LYP values corrected
for anharmonicity are 29 and 107 cm 1 for these vibrations.
Structure of ac
It is of interest to compare the experimental rotational constants (Tables 2 and 5) of the 80Se isotopologue with the
theoretical counterparts (Table 2). The B3LYP rotational
constants A, B, and C differ from the experimental rotational
constants of the ground vibrational state by 2.5, 1.9, and
1.3%, respectively, whereas the MP2 rotational constants
differ by 5.0, 1.6, and 0.2%, respectively. The theoretical
rotational constants are approximations of the equilibrium
rotational constants, whereas the experimental rotational
constants are effective constants. This makes a comparison
somewhat difficult. However, an agreement to within 2%, or
better, is expected. The largest differences, namely 2.5%
(B3LYP) and 5.0% (MP2), are seen for the A rotational
constant. This constant depends much on the value of the
C1–C2–Se7–C8 dihedral angle. The B3LYP value of 130.91 for
this dihedral angle (Table 1) seems to be too large because the
rotational constants obtained from this structure indicate that
Phys. Chem. Chem. Phys., 2008, 10, 4138–4146 | 4143
the B3LYP structure is more prolate than the experimental
rotational constants imply. Similarly, the MP2 structure with a
dihedral angle of 126.01 seems too oblate. Our best estimate is
129(5)1 for this dihedral angle. This value is very different from
the corresponding value of the C–C–Se–C dihedral angle in
the anticlinal conformer of H2CQCHSeCN, which is 166(3)1.4
Interestingly, the anticlinal conformer of the sulfur analogue,
H2CQCQCHSCN, has a C–C–S–C dihedral angle of 1341,23
almost the same as that found in ac (129(5)1).
Searches for sp
This rotamer is predicted to be favoured in the MP2 calculations, whereas the opposite was found using the B3LYP
procedure (Table 2). The statistical weight of sp is 1 compared
to 2 for ac. The b-type spectrum of sp was expected to be much
stronger than the a-type spectrum, because mb is the predominating dipole moment component (Table 2). The b-type
spectra are generally more difficult to assign than the a-type
spectra. Extensive searches were made for both the a- and the
b-type MW spectra of this form, but they were not found. It is
likely that this rotamer is a high-energy form of the molecule.
Interestingly, a similar conclusion was made in the case of the
sulfur counterpart, H2CQCQCHSCN, where the anticlinal
form is at least 2 kJ mol 1 more stable than the synperiplanar
rotamer.23
The reduced stability of the synperiplanar form of
H2CQCQCHSCN relative to the anticlinal conformer was
explained as largely caused by non-bonded repulsion between
the p electrons of the H2CQC double bond and the p electrons
of the triple CRN bond.23 It is possible that a similar kind of
interaction dictates a lower stability of sp compared to ac in
the present case of H2CQCQCHSeCN.
dominated by transitions originating from sc. This prolate
rotamer (k E 0.66) is predicted (Table 4) to have its major
dipole-moment component along the a-axis, and accumulations of aR-lines, similar to those seen for H2CQCQCHSeCN
and separated by roughly B + C E 3.2 GHz, were therefore
expected.
The observed spectrum was indeed comparatively weak and
consisted of accumulations of lines, as predicted. Only very
weak lines were seen between the accumulation regions, which
have a complicated fine structure extending over several
hundred MHz because the ground-vibrational and excitedstate spectra contribute together with the spectra of the
isotopologues.
The ground-state aR-spectrum was readily assigned. The btype lines are predicted to be much weaker because mb E 1/3 ma
(Table 4), and none of them were identified unambiguoulsy. A
total of 161 transitions were assigned for the ground state,
whose spectrum is given in Table 5S in the ESI,w while the
spectroscopic constants are listed in Table 6 and repeated in
Table 4 for convenient comparison with the theoretical counterparts. Only two of the quartic centrifugal distortion constants (DJ and DJK) were determined, whereas the remaining
quartic constants were kept fixed at the B3LYP values. No
sextic constants were fitted in this case.
The assignment of the ground-state spectrum of the 78Se
isotopologue was straightforward. The spectrum consisting of
99 transitions is listed in Table 9S in the ESIw and the
spectroscopic constants are given in Table 6.
Attempts to assign the spectra of the 76Se (9.0%) and 82Se
(9.2%) species were made, but these spectra were found to be
so weak that a detailed assignment could not be achieved.
Excited states of the
Microwave spectrum and assignment of the spectrum of sc of
C3H5SeCN
The MW spectrum of this compound was expected to be
relatively weak and to consist of broad spectral lines for
reasons
similar
to
those
discussed
above
for
H2CQCQCHSeCN. The B3LYP and MP2 calculations predict (Table 4) that sc is more stable than ap by 6.2 and 4.5 kJ
mol 1, respectively. The statistical weight of sc is twice that of
ap. It was therefore expected that the MW spectrum would be
Table 6
80
Se isotopologue of sc
The MW spectra of the first excited state of the torsion about
the C2–Se9 bond and the first excited state of the lowest
bending vibrations were assigned for the 80Se species. The
spectrum of an additional excited state, which may be the
second lowest bending vibration, was also assigned. The
spectra are listed in Tables 6S–9S in the ESIw and the spectroscopic constants are displayed in Table 6.
It was not possible to determine experimentally the A
rotational constants of the vibrationally excited states. These
Spectroscopic constantsa,b of sc of C3H5SeCN
Species:
C3H580SeCN
C3H578SeCN
Vibrational state:
Ground
1st ex. torsion
Lowest bending
Low bendingc
Ground
A/MHz
B/MHz
C/kHz
DJ/kHz
DJK/kHz
DK/kHz
dJ/kHz
dK/kHz
Rmse
No. transitionsf
4176.4(19)
1812.965(96)
1338.271(99)
1.2605(29)
9.5769(87)
30.7d
0.407d
0.696d
1.6787
161
4171.62d
1810.869(31)
1340.066(42)
1.1490(60)
8.983(20)
30.7d
0.407d
0.696d
2.2360
85
4160.7d
1810.556(38)
1339.289(51)
1.2890(73)
9.664(28)
30.7d
0.407d
0.696d
2.1992
89
4176.4d
1820.090(84)
1335.63(10)
1.106(23)
9.842(54)
30.7d
0.407d
0.696d
3.2402
32
4219.7(69)
1814.67(33)
1341.93(34)
1.2662(63)
9.723(21)
30.7d
0.407d
0.696d
2.2214
99
A-Reduction, Ir-representation.16 Spectra are found in the ESI, Tables 4S–9S. b Uncertainties represent one standard deviation. c Tentative
assignment; see text. d Fixed; see text. e Root-mean-squares deviation. f Number of transitions used in the weighted least-squares fit.
a
4144 | Phys. Chem. Chem. Phys., 2008, 10, 4138–4146
This journal is
c
the Owner Societies 2008
constants were therefore kept fixed in the least-squares fit. An
estimate of the A rotational constants of the first excited state
of the torsion and of the lowest bending vibration were made
by adding the aA’s obtained in the B3LYP calculations to the
A rotational constant of the ground vibrational state. The
B3LYP values were aA = –4.75 MHz for the torsion and
15.73 MHz for the lowest bending vibration. The A rotational constant of the ground state was employed for the last
vibration, because we are not sure what excited state this is.
Rough relative intensity measurements yielded 65(30) and
92(40) cm 1 for the torsion and bending vibrations, respectively. The B3LYP values corrected for anharmonicity are 53
and 109 cm 1, respectively, for these two vibrations.
Structure of sc
The B3LYP rotational constants A, B, and C differ from the
experimental rotational constants of the ground vibrational
state by 3.9, 4.6, and 2.4%, respectively, whereas the MP2
rotational constants differ by 1.9, 1.8, and 0.6%, respectively. Presumably, this reflects the fact that the MP2 structure
is somewhat more accurate than the B3LYP structure. The
rotational constants depend much on the orientation of the
selenocyanate group. The H4–C1–Se9–C10 dihedral angle is
assumed to be closer to the MP2 value (81.61; Table 3) than to
the 76.01 obtained in the B3LYP calculations. Our best
estimate of this angle is 80(4)1.
Searches for the spectrum of ap
This very asymmetric (k E 0.1) conformer is predicted to
have a relatively large ma and searches were consequently made
for its aR-type lines, which would be stronger than any other
transitions. There are practically no strong lines between the
accumulation regions, where many of the lines belonging to
the hypothetical ap should occur. It is therefore concluded that
this rotamer must be a high-energy form of the molecule,
which must be at least 3 kJ mol 1 less stable than sc. This is in
agreement with the theoretical predictions (Table 4).
The preferred conformer of C3H5SeCN has a synclinal
orientation of the H–C–Se–C chain of atoms. There are few
examples that can be used for comparison with the present
conformational results for C3H5SeCN, but it is noted that the
preferred form of cyclopropaneselenol, C3H5SeH, has a
synclinal conformation for the H–C–Se–H link of atoms,
according to our MW study.24
methods agree that one of the conformers has an exact
synperiplanar conformation (the C–C–Se–C dihedral angle
= 01). The other anticlinal form is predicted to have a
C–C–Se–C dihedral angle of 126.01 (MP2), or 130.91
(B3LYP). The synperiplanar conformer is slightly less stable
by 1.3 kJ mol 1, according to the MP2 calculations. The
B3LYP method finds the opposite, namely that the synperiplanar form is less stable than the anticlinal rotamer by
3.1 kJ mol 1. The anticlinal form was found in the MW
spectrum and is shown to be the preferred form of the
molecule, in agreement with the B3LYP predictions. The
C–C–Se–C dihedral angle was determined to be 129(5)1 by
comparison of the theoretical and experimental rotational
constants.
The
conformational
behaviour
of
H2CQCQCHSeCN is strikingly different from its congener
H2CQCHSeCN, which prefers a synperiplanar form by 4.5(4)
kJ mol 14 over an anticlinal form and whose C–C–Se–C
dihedral angle is more than 301 larger than in
H2CQCQCHSeCN.
For C3H5SeCN, the two theoretical methods predict that
the conformer having a synclinal (E 801) orientation of the
H–C–Se–C chain of atoms is preferred by several kJ mol 1
over an antiperiplanar form. The MW spectrum of the synclinal conformer has been assigned. There is no indication in the
spectrum for substantial amounts of a second antiperiplanar
form, which must be less stable than the synclinal rotamer by
at least 3 kJ mol 1. The H–C–Se–C dihedral angle was
determined to be 80(4)1 in the identified synclinal conformer.
A comparison with the oxygen (R-OCN) and sulfur
(R-SCN) analogues of the two title compounds would have
been of interest. However, only one such sulfur analogue,
H2CQCQCHSCN, is known to date, and this compound has
conformational properties very similar to those of
H2CQCQCHSeCN, as pointed out above.
Acknowledgements
We thank Anne Horn for her skilful assistance. The Research
Council of Norway (Program for Supercomputing) is thanked
for a grant of computer time. R.M. thanks the Research
Council of Norway for financial assistance through Contract
177540/V30. J.-C.G. thanks the University of Oslo for a travel
grant and the ‘‘PID-OPV’’ (CNRS) for financial support.
Notes and references
4. Conclusions
The microwave spectra of H2CQCQCHSeCN and
C3H5SeCN have been investigated for the first time. The
ground vibrational state spectrum of the 80Se and 78Se isotopologues of one conformer of each of these two compounds
has been assigned together with several vibrationally excitedstate spectra of their 80Se isotopologues. The spectroscopic
work has been augmented by B3LYP and MP2 quantum
chemical calculations employing the 6-311++G(3df,3pd)
basis set.
The B3LYP and MP2 methods both predict the existence of
two conformers for H2CQCQCHSeCN, which can be characterized by the value of the C–C–Se–C dihedral angle. Both
This journal is
c
the Owner Societies 2008
1. T. Sakaizumi, Y. Kohri, O. Ohashi and I. Yamaguchi, Bull. Chem.
Soc. Jpn., 1978, 51, 3411.
2. T. Sakaizumi, M. Obata, K. Takahashi, E. Sakaki, Y. Takeuchi,
O. Ohashi and I. Yamaguchi, Bull. Chem. Soc. Jpn., 1986, 59,
3791.
3. T. Sakaizumi and T. Itakura, J. Mol. Spectrosc., 1994, 163, 1.
4. H. Møllendal and J.-C. Guillemin, J. Phys. Chem. A, 2007, 111,
7073.
5. E. H. Riague and J.-C. Guillemin, Organometallics, 2002, 21, 68.
6. G. Bajor, T. Veszprémi, E. H. Riague and J.-C. Guillemin,
Chem.–Eur. J., 2004, 10, 3649.
7. J.-C. Guillemin, G. Bajor, E. H. Riague, B. Khater and
T. Veszprémi, Organometallics, 2007, 26, 2507.
8. B. Khater, J.-C. Guillemin, G. Bajor and T. Veszprémi, Inorg.
Chem., 2008, 47, 1502.
9. H. Møllendal, A. Leonov and A. de Meijere, J. Phys. Chem. A,
2005, 109, 6344.
Phys. Chem. Chem. Phys., 2008, 10, 4138–4146 | 4145
10. H. Møllendal, G. C. Cole and J.-C. Guillemin, J. Phys. Chem. A,
2006, 110, 921.
11. F. J. Wodarczyk and E. B. Wilson Jr, J. Mol. Spectrosc., 1971, 37,
445.
12. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr,
T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam,
S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada,
M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida,
T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li,
J. E. Knox, H. P. Hratchian, J. B. Cross, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala,
K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain,
O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford,
J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill,
4146 | Phys. Chem. Chem. Phys., 2008, 10, 4138–4146
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople,
in GAUSSIAN 03, revision B.03, Pittsburgh, PA, 2003.
C. Møller and M. S. Plesset, Phys. Rev., 1934, 46, 618.
A. D. Becke, Phys. Rev. A, 1988, 38, 3098.
C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785.
J. K. G. Watson, in Vibrational Spectra and Structure, ed.
J. R. Durig, Elsevier, 1977.
W. Gordy and R. L. Cook, in Techniques of Chemistry, Vol XVII:
Microwave Molecular Spectra, ed. A Weissberger, John Wiley &
Sons, 1984.
T. H. Dunning Jr, J. Chem. Phys., 1989, 90, 1007.
Y. Endo, M. C. Chang and E. Hirota, J. Mol. Spectrosc., 1987,
126, 63.
R. E. Penn and J. E. Boggs, J. Chem. Soc., Chem. Commun., 1972,
666.
B. S. Ray, Z. Physik, 1932, 78, 74.
G. O. Sørensen, in ROTFIT. Personal communication, 1972.
H. Møllendal, G. C. Cole and J.-C. Guillemin, J. Phys. Chem. A,
2007, 111, 2542.
R. Mokso, H. Møllendal and J. C. Guillemin, J. Phys. Chem. A,
2008, submitted.
This journal is
c
the Owner Societies 2008