Article
pubs.acs.org/JPCA
Microwave Spectra and Barriers to Internal Rotation of Z- and E‑1Propenyl Isocyanide
Svein Samdal,† Harald Møllendal,*,† 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
‡
Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, Avenue du Général Leclerc,
CS 50837, 35708 Rennes Cedex 7, France
S Supporting Information
*
ABSTRACT: A synthetic procedure yielding a mixture of Zand E-1-propenyl isocyanide (CH3CHCHNC) is described.
The microwave spectrum of this mixture has been recorded in
the 12−100 GHz spectral range, and the spectra of the Z and E
isomers have been assigned for the first time. Most transitions
of the Z form were split into two components of equal
intensity due to tunneling of the methyl group, which allowed
the barrier to internal rotation of this group to be determined
as 4.0124(12) kJ/mol by fitting 568 transitions with a
maximum value of J = 46 using the computer program
Xiam. This fit had a root-mean-square deviation as large as 4.325. The same transitions were therefore fitted anew using the more
sophisticated program Erham. This fit has a rms deviation marginally better (4.136) than the Xiam fit. No split MW lines were
found for E-1-propenyl isocyanide. The absence of splittings is ascribed to a barrier to internal rotation of the methyl group that
is significantly higher than the barrier of the Z isomer. It is concluded that the barrier must be larger than 6 kJ/mol for the E
form. The experimental work was augmented by quantum chemical calculations at CCSD/cc-pVTZ, B3LYP/cc-pVTZ, and
MP2/cc-pVTZ levels of theory. The CCSD method predicts rotational constants of the Z and E forms well. The B3LYP barriers
to internal rotation of a series of substituted propenes were calculated and found to be in good agreement with experiments.
Calculations of the quartic centrifugal distortion constants of the two 1-propenyl isocyanides by the B3LYP and MP2 methods
were less successful.
■
INTRODUCTION
Organic isocyanides have an interesting and unique chemistry.1−3 However, the literature dealing with the physical
properties of these compounds is not rich. Our two laboratories
have therefore started synthetic, microwave (MW) spectroscopic, and theoretical studies of members of this interesting
class of molecules.
MW studies were already available for several isocyanides
including hydrogen isocyanide (HNC), 4 − 9 methyl
(CH 3 NC), 1 0 − 1 4 ethyl (CH 3 CH 2 NC), 1 5 − 1 9 ethynyl
(HCCNC), 2 0 vinyl (H 2 CCHNC), 2 1 propargyl
(HCCCH2NC),22 propynyl (H3CCCNC),23 cyclopropyl
(C3H5NC),24 phenyl (C6H5NC),25 and trifluoromethyl isocyanide (CF3NC)26 when we started our investigations. This
list has very recently been extended to include our
contributions, namely, allenyl isocyanide
(H2CCCHNC),27 2-fluoroethyl isocyanide
(FCH2CH2NC),28 and 2-chloroethyl isocyanide
(ClCH2CH2NC).29
This time, the first MW spectra of two additional isocyanides,
Z- and E-1-propenyl isocyanide (CH3CHCHNC) are
reported. The methyl groups of propenes perform large
amplitude vibrations, which makes it possible to derive accurate
© 2012 American Chemical Society
values for the barriers to internal rotation of this group. The
methyl group barriers of several such compounds have been
determined in the past. A selection of propene barriers of E and
Z isomers is shown in Table 1. Several trends are seen from this
table. The first is that the barriers of E forms are roughly twice
as large as barriers of the corresponding Z isomers. The second
trend is that the barriers of Z forms depend strongly on the
substituent in the 1-position, whereas the barriers are much less
dependent on the substituent for the E isomers. The present
study of isocyanides should give a more complete picture of this
interesting barrier variation.
Another interest of ours has been the chemistry of the
interstellar medium and of planetary atmospheres and the
possible role played by isocyanides in these environments.
Interstellar nitriles are relatively abundant,30 but several of their
isocyanide isomers including HNC,6 HCCNC,20 and
CH3NC,31 which are significantly less stable in a thermodynamic sense than their nitrile isomers,32 have nevertheless been
found in space. A recent theoretical study of possible reactions
Received: May 2, 2012
Revised: July 27, 2012
Published: July 27, 2012
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■
Table 1. Methyl Group Barriers of E- and Z-Isomers of Some
Substituted Propenes
CH3CHCH266
Z-CH3CHCHCH367
Z-CH3CHCHCCH62
Z-CH3CHCHF53
Z-CH3CHCHCl68
Z-CH3CHCHBr68
Z-CH3CHCHCN69
Z-CH3CHCHNC70
E-CH3CHCHCH367
E-CH3CHCHCCH71
E-CH3CHCHF72
E-CH3CHCHCl73
E-CH3CHCHBr74
E-CH3CHCHCN64
E-CH3CHCHNC70
a
B3LYP/cc-pVTZ calculations.
available.
b
experimental
theorya
8.276(21)
3.06b
4.77b
4.42(21)
2.59b
1.76b
5.804(4)
4.0124(12)
c
7.96(21)
9.20b
9.08(42)
8.87(84)
8.033(82)
>6.0
8.27
4.31
5.07
4.75
3.28
3.19
6.17
4.55
8.20
8.03
8.92
8.47
8.37
8.02
8.41
No error estimate given.
c
EXPERIMENTAL SECTION
Synthesis. Z-1-Propenyl isocyanide was prepared more
than 40 years ago by rearrangement of the 2-propenyl
isocyanide with cuprous oxide.34 1H and 13C NMR data have
been reported for this isomer.35 We prepared this compound
by the same approach but using powdered potassium hydroxide
as a reagent. When allyl isocyanide36 (10 mmol) was mixed for
10 min with KOH (3 g), Z-2-propenyl isocyanide was obtained
with traces of the E-isomer (yield: 60%) However, a
concentration range varying from 8:1/Z:E to 3:1/Z:E was
observed after 1 h with KOH.
NMR data (qc: quadrupolar coupling). Z isomer: 1H NMR
(CDCl3, 400 MHz) δ 1.84 (dd, 3H, 3JHH = 7.0 Hz, 4JHH = 1.8
Hz, CH3), 5.68 (ddt, 1H, 3JHHcis = 8.4 Hz, 4JHH = 1.8 Hz, 2JHNqc
= 2.0 Hz, CH-N), 5.77 (m, 1H, 3JHHcis = 8.4 Hz, 3JHH = 7.0 Hz,
3
JHNqc = 1.3 Hz, MeCH); 13C NMR (CDCl3, 100 MHz) δ 13.2
(1JCH = 127.4 Hz (q), CH3), 112.5 (1JCH = 192.2 Hz (d), 1JCNqc
= 12.3 Hz (t), CH−N), 132.1 (1JCH = 160.6 Hz (d), CH−Me),
165.8 (1JCNqc = 5.8 Hz (t), NC).
E isomer: 1H NMR (CDCl3, 400 MHz) δ 1.71 (dd, 3H, 3JHH
= 7.0 Hz, 4JHH = 1.8 Hz, CH3), 5.67 (m, 1H, 3JHHtrans = 14.1 Hz,
4
JHH = 1.8 Hz, CH−N), 6.12 (dqt, 1H, 3JHHtrans = 14.1 Hz, 3JHH
= 7.0 Hz, 3JHNqc = 2.6 Hz, MeCH); 13C NMR (CDCl3, 100
MHz) δ 14.9 (1JCH = 128.4 Hz (q), 3JCNqc = 2.2 Hz, CH3),
113.3 (1JCH = 186.3 Hz (d), 1JCNqc = 12.7 Hz (t), CH−N),
134.1(1JCH = 157.7 Hz (d), CH−Me), 161.5 (1JCNqc = 6.5 Hz
(t), NC).
Spectroscopic Experiments. The synthesis above yielded
a mixture of Z- and E-1-propenyl isocyanide. The microwave
spectrum of the fumes of this mixture was studied using the
Stark-modulation MW spectrometer of the University of Oslo
operating in the 7−120 GHz spectral range. Details of the
construction and operation of this device have been given
elsewhere.37−39 This spectrometer has a resolution of about 0.5
MHz and measures the frequency of isolated transitions with an
estimated accuracy of ≈0.10 MHz. Radio-frequency microwave
double-resonance experiments (RFMWDR), similar to those
performed by Wodarczyk and Wilson,40 were conducted to
unambiguously assign particular transitions, using the equipment described elsewhere.37 Measurements were performed in
the 12−100 GHz frequency interval. The spectra were recorded
at room temperature, or at about −5 °C, with pressures in the
5−10 Pa range and processed by employing the Grams/AI
program.41 Relative intensity measurements performed on
Methyl group barrier (kJ/mol)
compound
Article
Not
in the atmosphere of Saturn’s moon Titan33 indicate that
isocyanides, including the two studied in this work, may be
formed there. Future searches for interstellar and extraterrestrial
compounds should therefore not overlook the potential
existence of isocyanides. Isocyanides normally have comparatively large dipole moments and therefore relatively strong
rotational spectra. This is an advantage for a potential
extraterrestrial discovery because the vast majority of
interstellar compounds have been detected by means of their
rotational spectra30 and high intensity is a definite advantage. It
should be possible to use the MW spectra of Z, and E
CH3CHCHNC presented herein in an attempt, for example,
by radio astronomy, to detect these two compounds anywhere
in the Universe.
The present spectroscopic work has been augmented by
high-level quantum chemical calculations, which were undertaken to obtain information for use in assigning the MW
spectrum and investigating properties of the potential-energy
hypersurface.
Figure 1. Models of Z- and E-1-propenyl isocyanide.
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Article
spectral lines indicate that the Z:E ratio was roughly 3:1 in the
sample used in this study.
Quantum Chemical Methods. The present quantum
chemical calculations were undertaken employing the Gaussian
0942 suites of programs running on the Titan cluster in Oslo.
Møller−Plesset second-order perturbation calculations
(MP2),43 density functional theory (DFT) calculations using
the B3LYP method,44,45 and coupled cluster calculations with
singlet and doublet excitations, CCSD,46−48 were performed.
Peterson and Dunning’s correlation-consistent cc-pVTZ basis
set,49 which is of triple-ξ quality, was chosen for these
calculations, where the frozen-core approximation was
employed.
Table 2. CCSD/cc-pVTZ Structures and Dipole Moments of
Z- and E-Propenyl Isocyanide
Z-1-propenyl isocyanidea
C1C2
C1H3
C1C5
C2H4
C2N9
C5H6
C5H7
C5H8
N9C10
■
RESULTS AND DISCUSSION
Quantum Chemical Calculations. A model of Z- and E-1propenyl isocyanide with atom numbering is shown in Figure 1.
Both compounds were assumed to have a symmetry plane (Cs
symmetry) in the present calculations and the fact that force
field calculations produced no imaginary vibrational frequencies
confirmed this assumption. CCSD/cc-pVTZ calculations are
expected to produce structures that are close to the equilibrium
structures for compounds containing only elements from the
first (H) and second period (C and N), and this method was
therefore used to compute the electronic energies, structures,
and dipole moments of the Z and E isomers with the results
shown in Table 2. The dipole moment components shown in
this table have been transferred to the principal inertial axis
system from the Gaussian 09 standard orientation system using
the program Axis by Bailey.50 The rotational constants
calculated from the two structures in Table 2 are listed in
Tables 3 (Z) and 4 (E) together with their respective
experimental counterparts. The electronic energies of the two
forms are given in the footnote of Table 2 and it is seen there
that the CCSD calculations predict the Z form is more stable
than E by 2.6 kJ/mol. The comparatively small energy
difference predicted for these two forms could be part of the
explanation why so much of the E isomer is obtained together
with the major Z form in the synthetic procedure described
above.
The CCSD bond lengths are very similar in the two isomers
(Table 2), as expected. Fortunately, accurate equilibrium bond
lengths are available for related molecules and can be used for a
comparison with the present results. Equilibrium NC bond
lengths have, for example, been determined experimentally to
be 116.83506(16) pm in HNC,8 and 116.9(1) in CH3NC.14
These results are almost the same as the CCSD predictions
(117.2 pm). The equilibrium bond length of the CC double
bond is 133.05(10) pm in ethylene,51 very similar to those
reported for the CC bonds in Table 2.
The isocyanide group is able to conjugate electrons with the
double bond π-electrons in the title compounds, but this
appears to affect their CC and NC bond lengths little,
because the values of these bond lengths (Table 2) are close to
their experimental counterparts in HNC,8 CH3NC,14 and
H2CCH2,51 as remarked above.
The CCSD bond angles of Z and E are very similar in most
cases, with the C2C1C5 angle as an exception. This angle
opens up from 123.3° in E to 126.0° in Z, possibly because of
nonbonded repulsion between the H6 atom of the methyl
group and the isocyanide group. Comparison of the nonbonded
distances and the van der Waals distances indicates that a slight
repulsion may exist in the Z form. In the present case, one has
C2C1H3
C2C1C5
H3C1C5
C1C2H4
C1C2N9
H4C2N9
C1C5H6
C1C5H7
C1C5H8
H6C5H7
H6C5H8
H7C5H8
C2N9C10
C2C1C5H7
C2C1C5H8
C3C1C5H7
C3C1C5H8
μa
μb
μtot
Δ
E-1-propenyl isocyanidec
Bond Length (pm)
133.3
108.2
149.5
108.7
139.0
108.8
109.1
109.1
117.2
Bond Angle (deg)
116.2
126.0
117.9
122.7
122.8
114.5
111.6
110.3
110.3
108.5
108.5
107.2
178.4b
Dihedral Angle (deg)
120.9
−120.9
−59.1
59.1
Dipole Momente (10−30 C m)
10.48
5.10
11.65
Energy Differencef (kJ/mol)
0.0
133.2
108.3
149.6
108.0
138.9
108.9
109.1
109.1
117.2
118.5
123.3
118.2
123.0
122.4
114.6
111.4
110.6
110.6
108.4
108.4
107.2
177.8d
120.7
−120.7
−59.3
59.3
12.86
1.91
13.00
2.61
Electronic energy: −550572.53 kJ/mol. bBent away from the methyl
group. cElectronic energy: −550569.92 kJ/mol. dBent toward H4. e1
Debye = 3.33564 × 10−30 C m. fRelative to the energy of the Z isomer.
a
that the CCSD nonbonded distance (not given in Table 2)
between H6 and N9 is 257 pm and the nonbonded distance
between H6 and C10 is 304 pm, compared to the sum (290
pm) of the Pauling van der Waals distances52 of hydrogen (120
pm) and the half-thickness of an aromatic molecule (170 pm).
This nearness of the methyl and isocyanide groups could
have several consequences other than the opening up of the
C2C1C5 angle. One consequence could be that the H6 may be
repelled by the isocyano group, resulting in a smaller barrier in
the Z than in the E isomer. A similar view was advocated by
Beaudet and Wilson53 in the case of the Z-CH3CHCHF to
explain why this barrier is only about half the barrier to its E
isomer (see also Table 1). Another result of this closeness
could be that a nonbonded stabilization between the whole
methyl group and the isocyanide group could occur at the same
time. It is possible that this stabilization effect is more
important than repulsion between H6 and the isocyanide
group leading to the preference of Z over E by 2.6 kJ/mol
found in the CCSD calculations.
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Table 3. Spectroscopic Constantsa of Z-1-Propenyl
Isocyanide
A (MHz)
B (MHz)
C (MHz)
ΔJ (kHz)
ΔJK (kHz)
ΔK (kHz)
δJ (kHz)
δK (kHz)
ΦJ (Hz)
δ (deg)f
Iα (10−20 u m2)
V3 (kJ/mol)
ε1 (MHz)
ε2 (MHz)
B001 (MHz)
B020 (kHz)
B200 (kHz)
no. of transg
rmsh
Xiam fitb
Erhamz fitb
theoryc,d
12195.798(11)
3734.1544(39)
2906.4080(37)
3.137(12)
−22.462(29)
82.81(35)
1.0272(16)
5.311(44)
5.6(6)
75.419(39)
3.18e
4.0124(12)
12195.810(11)
3734.0997(35)
2906.4406(34)
3.119(11)
−22.557(27)
84.52(33)
1.0150(11)
5.275(41)
0.0e
75.96(52)
3.157(37)
12350.7
3690.8
2892.2
2.31
−21.9
45.6
0.683
9.52
568
4.235
spectral range. The more stable Z isomer has its major dipole
moment component of about 10.5 × 10−30 C m (Table 2)
along the a-inertial axis. The spectrum of this form would
therefore be dominated by comparatively strong aR-branch
transitions, which were first searched for using the theoretical
spectroscopic constants listed in Table 3. Watson’s A-reduction
Hamiltonian54 was used to predict the approximate frequencies
of these a-type R-branch transitions. Searches for these lines
soon met with success. The fact that many of these transitions
were split into doublets of equal intensities separated by a few
megahertz was very useful for making the initial assignments.
These doublets were assumed to be caused by tunneling of the
methyl group and belong to the A and E symmetry species.
Further splitting caused by nuclear quadrupole coupling of the
nitrogen nucleus was not observed. This was expected because
this nucleus has relatively small quadrupole coupling constants
in isocyanides.55
A rough value of the barrier was first obtained using our
computer program MB10.56 A least-squares fit using the
program Xiam by Hartwig and Dreizler57 obtained from a
database maintained by Kisiel,52 was then performed. This
program is based on the so-called internal axis method
(IAM).58,59 The transitions were weighted by the inverse
squares of their uncertainties.
Having assigned the aR-transitions, the weaker b-type lines
were searched for and soon identified. The tunneling splittings
were again useful for making assignments. Finally, aQ-branch
transitions were added to the fit. A total of 568 transitions with
a maximum value of J = 46 listed in Table 4S in the Supporting
Information were used to determine the spectroscopic
constants shown in Table 3. It is seen from this table that
accurate values were obtained for the A-reduction54 rotational
and quartic centrifugal distortion constants. Significant values
could only be obtained for one sextic centrifugal distortion
constant (ΦJ). Further sextic constants were preset at zero in
the fit. The moment of inertia of the methyl top could not be
determined accurately and was preset at 3.18 × 10−20 u m2. The
weighted fit has a root-mean-square (rms) deviation as large as
4.235 (Table 3) and a standard deviation of the fit of 0.471
MHz (Table 4S, Supporting Information). It is thought that the
fact that the rms is much larger than 1.0 reflects that the IAM
method lacks interaction terms with vibrational modes.
We have also performed a least-squares fit of A-species lines
using Sørensen’s program Rotfit60 to facilitate predictions of the
frequencies of spectral lines not listed in Table 4S (Supporting
Information), which could, for example, be convenient for
astrophysical searches for this compound in interstellar spece.
Watson’s A-reduction Hamiltonian54 was used for this purpose.
This fit, which has a rms deviation of 1.51, is presented in Table
5S in the Supporting Information.
The comparatively large rms found using the Xiam program
indicated that a more advanced fitting procedure than that
offered by Xiam should be preferred in the present case. The
Erham program by Groner61 has interaction terms not
implemented in Xiam and has been successfully employed in
many cases.52 A version of this program, Erhamz, was
downloaded from Kisiel’s web page.52 The same 568 transitions
used with Xiam were now fitted using Erhamz with the results
shown in Table 6S (Supporting Information). The spectroscopic constants obtained in this case are listed in Table 3
together with the Xiam values and the theoretical constants.
Interestingly, the Erhamz fit is only marginally better than
the Xiam fit, as judged by the rms value, which is 4.136 in the
71.87
4.55
−242.6(39)
−2.39(86)
−0.93(32)
−20.9(95)
7.40(85)
568
4.136
A-reduction, Ir-representation.54 The Xiam fit is found in Table 4S,
and the Erham fit is found in Table 6S of the Supporting Information.
b
Uncertainties represent one standard deviation. cThe theoretical
rotational constants and δ were obtained in CCSD/cc-pVTZ
calculations. dThe theoretical centrifugal distortion constants and the
barrier to internal rotation were found in B3LYP/cc-pVTZ
calculations. eFixed. fAngle between the methyl top and the a-inertial
axis. gNumber of transitions. hRoot-mean-square deviation of a
weighted fit.
a
Another hypothesis would be that the increased C2C1C5
angle in Z is a result of a slight rehybridization of the C1 atom,
but it is unlikely that this effect alone could result in the
reduced methyl group barrier of this isomer.
Calculation of vibrational frequencies and Watson’s quartic
centrifugal distortion constants54 of Z and E, which are useful
for the spectroscopic work could not be undertaken using the
comprehensive CCSD method. A less costly procedure had to
be employed for these purposes. Our choice was to undertake
B3LYP/cc-pVTZ calculations of these parameters. The B3LYP
structures are listed in Table 1S of the Supporting Information,
whereas the harmonic vibrational frequencies and centrifugal
distortion constants in the A-reduction form54 are listed in
Tables 2S (Z isomer) and 3S (E form).
It turned out in the course of this work that some of the
quartic centrifugal distortion constants are not well predicted in
the B3LYP/cc-pVTZ calculations. MP2/cc-pVTZ calculations
of these parameters were therefore undertaken to investigate
how these constants depend on the method of calculations.
The barriers to internal rotation of the methyl groups of the
E and Z isomers of the propenes listed in Table 1 were
calculated using the B3LYP method, because CCSD calculations would have been too expensive. The B3LYP barriers
agree with the experimental values to within about 1 kJ/mol.
The striking feature of the Z barriers is that they are 3−5 kJ/
mol lower than the E barriers (Table 1), and this is well
reproduced in the B3LYP calculations.
Microwave Spectrum and Assignment of the Z
Isomer. The microwave spectrum of the mixture of the two
isomers is comparatively strong and very dense with absorption
lines occurring every few megahertz throughout the whole
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Table 4. Spectroscopic Constantsa of E-1-Propenyl
Isocyanide
Erhamz case, compared to 4.235 found with Xiam. The Erhamz
standard deviation of the fit is 0.431 MHz (Table 6S,
Supporting Information), slightly better than 0.471 MHz
found using Xiam. There may be several reasons for the
unexpectedly large rms found in the Erhamz fit. The data set
involves many transitions with relatively large values of J, which
means that interactions with vibrational modes may occur,
resulting in a poorer fit than expected.
In Table 3, it is shown that the Xiam barrier to internal
rotation is 4.0124(12) kJ/mol. This barrier is in satisfactory
agreement with the B3LYP value (4.55 kJ/mol; Table 1). The
barrier of Z-1-propenyl isocyanide (4.0124(12) kJ/mol) is
similar to the barriers of two other isoelectronic Z-propenes,
namely, Z-CH3CHCHCCH (4.77 kJ/mol),62 and ZCH3CHCHCN (5.804(4) kJ/mol).63
The CCSD rotational constants (Table 3) agree with the
experimental rotational constants to within about 1%.
Deviation of this order of magnitude is to be expected because
the CCSD rotational constants are derived from the
equilibrium structure, whereas the experimental constants are
effective constants (r0-constants).
The B3LYP quartic centrifugal distortion constants (Table 3)
deviate much from their experimental counterparts except for
ΔJK, where good agreement is seen. The large differences found
in this case prompted calculations of the quartic constants using
the MP2/cc-pVTZ procedure, which yielded +2.62, −5.61,
+46.0, +0.816, and +8.80 kHz, respectively, for these constants.
These values are not in better agreement with the experimental
values (Table 3) than the B3LYP quartic constants. The MP2
value of ΔJK (−5.61 kHz) is in very poor agreement with the
values of the two fits (about −22.5 kHz) and the B3LYP
prediction (−21.9 kHz). The B3LYP and MP2 procedures are
both obviously not sophisticated enough to derive accurate
centrifugal distortion constants in this case.
The angle between the methyl top and the a-inertial axis (δ,
Table 3) was also obtained from the least-squares fits. This
Xiam value of this angle is 75.419(39)°, whereas the Erhamz fit
yielded 75.96(52)°, compared to 71.97° calculated from the
CCSD structure shown in Table 2.
Microwave Spectrum and Assignments of the E
isomer. The theoretical spectroscopic constants listed in
Table 4 were used to predict the frequencies of strong aR-lines,
because μa is the largest dipole moment component (Table 2).
The CCSD B and C rotational constants are close to the
experimental counterparts, as shown in Table 4, and this made
the assignment of these lines straightforward. Confirmation of
several assignments was obtained by RFMWDR experiments.
None of these aR-lines displayed resolved A−E splittings due to
tunneling of the methyl group, which means that the splittings
must be less than the resolution of our spectrometer, which is
about 0.5 MHz. Searches for b-type lines, which have much
larger A−E splittings, were then made, but these transitions
were not identified, presumably because μb is as small as about
1.9 × 10−30 C m (Table 2), producing insufficient intensities for
these transitions. The resolution limit of 0.5 MHz must
therefore be used to estimate a lower limit for the barrier. The
barrier height was varied systematically using our program
MB10,56 and it was found that the lower limit is 6 kJ/mol. The
barrier to internal rotation is therefore much higher in E than in
Z, and this is in agreement with the barriers obtained for the
other Z−E pairs, as shown in Table 1. Interestingly, the barrier
is 8.033(82) kJ/mol in the isoelectronic compound ECH3CHCHCN.64
A (MHz)
B (MHz)
C (MHz)
ΔJ (kHz)
ΔJK (kHz)
ΔK (kHz)
δJ (kHz)
δK (kHz)
ΦKJ (Hz)
no. of trans.f
rmsg
experimentalb
theoryc,d
39057(40)
2433.2823(67)
2322.4229(68)
0.3062(12)
−16.322(34)
335e
0.00108e
13.2e
2.28(17)
111
1.575
39337
2431.6
2322.8
0.217
42.3
335
0.00108
13.2
a
A-reduction, Ir-representation.54 bUncertainties represent one standard deviation. cThe theoretical rotational constants were obtained in
CCSD/cc-pVTZ calculations. dThe theoretical centrifugal distortion
constants and the barrier to internal rotation were obtained in B3LYP/
cc-pVTZ calculations. eFixed. fNumber of transitions. gRoot-meansquare deviation of a weighted fit.
Finally, a total of 111 aR-lines listed in Table 7S in the
Supporting Information were fitted to Watson’s A-reduction
Hamiltonian54 using Sørensen’s program Rotfit.60 It was only
possible to get significant values for two quartic, ΔJ and ΔJK,
and one sextic, ΦKJ, centrifugal distortion constants, due to the
fact that only aR-lines were assigned for this very prolate
asymmetric rotor (Ray’s asymmetry parameter65 κ = −0.9940).
The three remaining quartic centrifugal distortion constants
ΔK, δJ, and δK were preset at their B3LYP values in the leastsquares fit.
The resulting spectroscopic constants are listed in Table 4,
where it is seen that there is very good agreement between the
observed and theoretical CCSD B and C rotational constants.
The A rotational constant is too uncertain to warrant
comparison. The B3LYP value of the centrifugal distortion
constant ΔJ is much smaller than the experimental constant,
whereas even the sign is wrong in the case of ΔJK. The MP2/ccpVTZ values for the five quartic constants are 0.230, 37.0, 300,
0.00459, and 12.5 kHz, respectively, which is not in much
better agreement with experiment than the B3LYP constants.
The B3LYP and MP2 procedures are therefore obviously not
able to produce reliable values for all the centrifugal distortion
constants in the case of E-1-propenyl isocyanide as well.
■
CONCLUSIONS
A synthetic procedure is described in which Z- and E-1propenyl isocyanide are formed simultaneously. The MW
spectrum of this mixture has been analyzed and assigned. Most
of the Z-isomer MW transitions are split into two components
of equal intensities, which is assumed to arise from internal
rotation of the methyl group. These splittings were used to
derive a barrier to internal rotation of 4.0124(12) kJ/mol from
a fit of 568 transitions using the computer program Xiam.57
This barrier height is similar to those of two other isoelectronic
Z propenes, namely, Z-CH3CHCHCCH (4.77 kJ/mol)62
and Z-CH3CHCHCN (5.804(4) kJ/mol).63
However, the root-mean-square deviation was found to be as
large as 4.235 for the Xiam fit. The Erham program61 contains
interaction terms not included in Xiam and a fit using this
program was undertaken yielding a marginal improvement to
4.136 of the rms deviation.
8837
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The Journal of Physical Chemistry A
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No resolved MW lines due to internal rotation of the methyl
group were found for E-1-propenyl isocyanide. The absence of
such splittings is ascribed to a barrier to internal rotation of the
methyl group that is significantly higher than the barrier of the
Z isomer. It is concluded that the barrier must be larger than 6
kJ/mol for this form. This is in accord with barriers of other
substituted E propenes, which are in the 8−9 kJ/mol range. It is
pointed out that a repulsive interaction between one of the
hydrogen atoms of the methyl group on the one hand and the
isocyanide group on the other, may be responsible for the
smaller barrier in the Z isomer.
CCSD/cc-pVTZ, B3LYP/cc-pVTZ, and MP2/cc-pVTZ
calculations were performed with mixed results. CCSD
calculations predict rotational constants well. The barriers to
internal rotation of substituted propenes is well reproduced in
the B3LYP calculations, whereas the quartic centrifugal
distortion constants obtained in the B3LYP and MP2
calculations deviate much from their experimental counterparts
in most cases. The CCSD method predicts Z to be more stable
than E by 2.6 kJ/mol, possibly as a result of a nonbonded
stabilization between the methyl and isocyanide groups, which
are in close proximity in this isomer.
■
ASSOCIATED CONTENT
S Supporting Information
*
Results of the theoretical calculations and the microwave
spectra including bond distances and angles, rotational
constants, harmonic frequencies and quartic centrifugal
distortion constants, microwave spectral data, and A-species
lines. This material is available free of charge via the Internet at
http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*Tel: +47 2285 5674. Fax: +47 2285 5441. E-mail: harald.
mollendal@kjemi.uio.no.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank Anne Horn for her skillful assistance. The Research
Council of Norway (Program for Supercomputing) is thanked
for a grant of computer time. J.-C.G. thanks the Centre
National d’Etudes Spatiales (CNES) and PCMI (INSU-CNRS)
for financial support.
■
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■
NOTE ADDED IN PROOF
The quartic centrifugal distortion constants were calculated
assuming Cs symmetry for the two compounds. After the Just
Accepted version of this manuscript appeared, Prof. W. C.
Bailey, Kean University, informed us that much better
agreement with experiment is obtained if this symmetry
restriction is lifted.
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