Article
pubs.acs.org/JPCA
Microwave Spectrum and Conformational Properties of 4‑Isocyano1-butene (H2CCHCH2CH2NC)
Svein Samdal,† Terje Grønås,† 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
‡
Institut des Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, 11 Allée de
Beaulieu, CS 50837, 35708 Rennes Cedex 7, France
S Supporting Information
*
ABSTRACT: The microwave spectrum of 4-isocyano-1-butene (H 2 C
CHCH2CH2NC) has been investigated in the 35−80 GHz spectral region.
Selected measurements have also been made outside this spectral range. Rotation
about the −CH−CH2− and −CH2−CH2− single bonds may produce rotational
isomerism resulting in five conformers. The spectra of three of them, denoted I, III,
and IV, have been assigned. In conformer I, the CC−C−C link of atoms is
+anticlinal and the C−C−C−N chain is antiperiplanar. In III, the two links of
atoms are +anticlinal and +synclinal, whereas in IV, the two chains are
synperiplanar and antiperiplanar, respectively. Conformer I was found to have
the lowest energy of the three forms by relative intensity measurements. These
measurements yielded EIII − EI = 1.1(7) kJ/mol, and EIV − EI = 2.9(7) kJ/mol for
the internal energy differences. The microwave study was augmented by quantum
chemical calculations at the CCSD/cc-pVQZ and MP2/cc-pVTZ levels of theory.
Good agreement between experimental and theoretical results was seen in most cases.
■
INTRODUCTION
Isocyanides are an interesting class of compounds possessing a
unique chemistry that has been investigated comparatively
little.1−3 We have therefore synthesized several isocyanides,
which have subsequently been investigated by UV photon
electron spectroscopy,4 microwave (MW) spectroscopy,5−9 and
high-level quantum chemical calculations.4−9 We have already
reported MW spectra of allenyl isocyanide (H2CC
CHNC),5 2-fluoroethyl isocyanide (FCH2CH2NC),6 2-chloroethyl isocyanide (ClCH2CH2NC),7 E- and Z-1-propenylisocyanide (CH3CHCHNC),8 cyclopropylmethyl isocyanide
(C 3 H 5 CH 2 NC), 9 and 4-isocyano-1-butyne (HC
CCH2CH2NC).10 Our MW studies of H2CCCHNC5
and E- and Z-CH3CHCHNC8 were undertaken because of
their potential astrochemical interest, while conformational
properties were the focus of our investigations of 2-fluoroethyl,6
2-chloroethyl,7 cyclopropylmethyl isocyanide,9 and 4-isocyano1-butyne.10
In this work, our studies are extended to include the first
MW investigation of the structural and conformational
properties of 4-isocyano-1-butene (H2CCHCH2CH2N
C). This compound has two single bonds and rotational
isomerism is therefore possible. Five typical conformers are
depicted in Figure 1 and given Roman numerals for reference.
Atom numbering is shown on I. The C1C2C3C4 and
C2C3C4N5 dihedral angle can conveniently be used to
describe the conformations of the five forms. The
C1C2C3C4 dihedral angle is anticlinal (about 120° from
© 2014 American Chemical Society
synperiplanar) in rotamers I−III, and synperiplanar (approximately 0°) in IV and V. The C2C3C4N5 dihedral angle is
antiperiplanar (roughly 180°) in I and IV, −synclinal (about
−60°) in II and V, and +synclinal (ca. 60°) in III. Mirror-image
forms, which can be obtained by adequate changes of the signs
of the dihedral angle(s), exist for all conformers but IV.
4-Isocyano-1-butene is a 4-substituted 1-butene, H2C
CHCH2CH2X, where X is the substituent. The structural and
conformational properties of this class of compound, which are
analogous to that of H2CCHCH2CH2NC, have received
considerable attention in the past. One example is 4-fluoro-1butene (H2CCHCH2CH2F), which has been studied several
times.11−13 A conformer corresponding to III has been found
to be the lowest energy form.11−13 Two additional forms with
slightly higher energies were found in the gas-phase in a MW
work,12 while all five forms were identified in a far-infrared
study of a krypton solution.13 A gas electron diffraction (GED)
and molecular-mechanics investigation of H 2 C
CHCH2CH2Cl and H2CCHCH2CH2Br again showed that
forms analogous to III are the most stable ones in these two
cases and that there are minor contributions from other
conformers.14 The preferred form of H2CCHCH2CH2OH
corresponds to II, and it is stabilized by an intramolecular
hydrogen bond between the hydroxyl group and the πReceived: December 13, 2013
Revised: January 27, 2014
Published: January 29, 2014
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Article
been investigated prompted the present research. Our
experimental method of investigation is MW spectroscopy
due to its superior accuracy and resolution, thereby making this
method ideal for conformational and structural studies. The
experimental work has been augmented by theoretical
calculations, which were performed at a much higher level of
theory than previously4 with the purpose of obtaining
information for use in assigning the MW spectrum and of
investigating properties of the potential-energy hypersurface.
■
EXPERIMENTAL SECTION
Synthesis. 4-Isocyano-1-butene was synthesized starting
from the corresponding formamide as described previously
(Scheme 1).4 A yield of 76% was obtained starting from 10
Scheme 1
mmol of N-3-buten-1-yl-formamide. 4-Isocyano-1-butene is a
colorless liquid at room temperature with a vapor pressure of
roughly 100 Pa.
Spectrometer and Experimental Details. The Starkmodulated spectrometer of the University of Oslo was used in
this study. This instrument has a resolution of about 0.5 MHz
and measures the frequency of isolated transitions with an
accuracy of 0.1 MHz. The spectrometer has a 50 kHz homebuilt Stark generator. The Stark cells are a 2-m-long HewlettPackard and homemade 2- and 3-m-long brass cells. The
microwave radiation is generated using a 1730B Systron
Donner frequency synthesizer operating in the 2−26.5 GHz
frequency range. Several Millitech frequency multipliers are
used to generate radiation in the 26.5−120 GHz spectral
interval. The lock-in amplifier is a Perkin-Elmer model 5209.
Double resonance radiofrequency microwave experiments
(RFMWDR) similar to those performed by Wodarczyk and
Wilson20 are performed using a Rohde & Schwarz SML01
signal generator operating in the 9 kHz to 1.1 GHz spectral
region as the radio frequency source. An EIN Model 503L
amplifier provides 3 W linear amplification of the radio signals
between 2 and 510 MHz. Mixing of the radio signal with the
Stark modulation signal is provided employing a HewlettPackard 10514 mixer. A National Instruments (NI) Compact
RIO programmable automation controller (PAC) model cRIO9076 was synchronized with the 1730B Systron Donner
Synthesizer for data acquisition. The PAC is a combination
of a real-time controller, reconfigurable input/output modules
(RIO), a FPGA module, and an Ethernet expansion chassis.
The RIO moduls consists of a 24-Bit Analog Input Modul NI
9239 and a TTL digital moduls NI 9401. A program developed
using LabVIEW runs on the PAC, triggers the synthesizer for
the step sweep mode, and executes a high-speed data
acquisition of multiple samples (16) for data averaging 12 ms
after each trigger pulse. The data are buffered and streamed out
to network. The LabVIEW FPGA Module makes it possible to
program FPGA for custom synchronization and fast real time
data decisions or analyses. The spectra were processed using
the Grams/AI spectroscopy program.21
The MW spectrum of 4-isocyano-1-butene was recorded
with the cell cooled to about −20 °C to enhance spectral
Figure 1. Five rotameric forms of H2CCHCH2CH2NC. Atom
numbering is indicated on conformer I. The C1C2−C3−C4 link of
atoms is ac in I−III, and sp in IV and V. The C2−C3−C4−N5 chain
is ap in I and IV, −sc in II, and V +sc in III.
electrons of the double bond.15,16 The GED investigation15
indicates that more rotamers coexist with the hydrogen-bonded
form. The MW spectra of three conformers of each of H2C
CHCH2CH2SH17 and H2CCHCH2CH2SeH18 have been
reported. The lowest-energy conformers of the thiol17 and
selenol18 are stabilized by an internal hydrogen bond, just as in
the case of H2CCHCH2CH2OH.15,16 The two additional
higher energy rotamers of the thiol and selenol are similar to I
and consequently not stabilized by this interaction. A total of
four rotameric forms were assigned for H 2 C
CHCH2CH2NH2 in a MW study.19 Two of these conformers
are analogous to II and stabilized with an intramolecular
hydrogen bond formed between one of the amino group
hydrogen atoms and the π-electrons of the double bond,
whereas two other forms correspond to I and they do not have
this interaction.
The physical properties of 4-isocyano-1-butene have been
subject to only one investigation in the past by Chrostowska et
al.,4 who performed a photoelectron spectroscopy study and
density functional theory (DFT) calculations employing the
CAM-B3LYP functional with the 6-311G(d,p) basis set. No
conformational analysis was reported in this study. The wide
variety of conformational properties of 4-substituted butenes
and the fact that the microwave spectrum and the conformational properties of the title compound have not previously
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Some of the theoretical results above warrant comments. The
CCSD isocyanide group (N5C6) triple bond length is 116.6
pm in all conformers (Table 1). Fortunately, some equilibrium
bond lengths of the isocyanide group exist. In HNC it is
116.83506(16) pm,29 and in CH3NC, a value of 116.9(1) pm
has been determined.30 These bond lengths are in good
agreement with the CCSD predictions (116.6 pm). The
theoretical C3C4 bond lengths vary between 152.8 (conformers II and III) and 152.1 pm (IV and V) similar to the
equilibrium CC bond length of ethane (152.2 pm).31 The
theoretical C1C2 double bond lengths are in the 132.8−
133.0 pm interval (Table 1), whereas the equilibrium CC
bond length of ethylene (H2CCH2) is 133.05(10) pm.32
There is an interesting variation in the C2C3C4 bond angle,
which is 110.9° in I, and 5° larger in V, which may reflect steric
repulsion in the latter conformer because of the close proximity
of the ethylene part and the CH 2 NC moiety. The
conformations of the five rotamers depend heavily on the
C1C2C3C4 and C2C3C4N5 dihedral angles. The C1C2C3C4
dihedral angle is exactly 0° in IV and −0.3° in V, and a few
degrees smaller than the “canonical” 120° in the remaining
three forms. The C2C3C4N5 dihedral angle is exactly 180° in
IV. In V, this angle deviates by 9.1° from 60°, possibly as a
result of the said repulsion between the ethylene and CH2NC
parts. The same dihedral angle deviates a few degrees from the
characteristic ±60° and 180° in I−III.
The CCSD electronic energy differences are shown in Table
2. It is seen that III is predicted to be the minimum-energy
conformer, followed by I (+0.27 kJ/mol), II (+1.22 kJ/mol),
IV (+2.09 kJ/mol), and V (+4.06 kJ/mol). No zero-point
corrections are available at this level of theory, as noted above.
The MP2 internal energy difference corrected for zero-point
vibrational frequencies derived from entries in the SI, Tables
1S−5S, were 0, +0.32, +1.11, +3.64, and +4.13 kJ/mol for III,
II, I, IV, and V, respectively. Both theoretical methods predict
that III is the global minimum, but the increasing energy order
of MP2 (III, II, I, IV, V) differs from that of CCSD (III, I, II,
IV, V). The energy span of the five conformers of about 4 kJ/
mol is also similar.
Microwave Spectrum and Pile-Up Series. The quantum
chemical calculations above indicate that there are comparatively small energy differences between the five conformers of
Figure 1. Survey spectra revealed a comparatively weak and
extremely rich MW spectrum, which is compatible with the
presence of several conformers separated by relatively small
energy differences, just as predicted in the theoretical
calculations above. Another factor that leads to low spectral
intensity is the fact that each of these rotamers has several lowfrequency vibrational modes. In fact, the MP2 calculations (SI,
Tables S1−S5) indicate that there are 6−7 fundamentals with
frequencies below 500 cm−1 for each of the five conformers,
three of which are typically lower than 200 cm−1. The two
lowest vibrational modes can best be described as torsional
vibrations about the C2−C3 and C3−C4 single bonds. Each of
these several vibrationally excited states will consequently have
a significant Boltzmann population and an associated MW
spectrum resulting in a very crowded MW spectrum and overall
reduction of intensity in agreement with observations.
The survey spectra also revealed two series of characteristic
and very rich pile-up series protruding from the weaker spectral
background. Such pile-up series are characteristic for highly
prolate rotors (Ray’s asymmetry parameter33 κ approaching
−1) with a large μa and involve a-type R-branch transitions. A
intensities using small portions of dry ice to cool the waveguide.
Selected measurements were also performed at room temperature. The pressure in the spectrometer cell was approximately
5−10 Pa during the measurements. The spectrum was recorded
in the whole 35−80 GHz frequency interval. Selected
measurements were also performed in other frequency regions.
RFMWDR experiments20 were performed to unambiguously
assign particular transitions.
■
RESULTS AND DISCUSSION
Quantum Chemical Methods. Several quantum chemical
methods were used in the present calculations, which were
performed using the Abel cluster of the University of Oslo.
Second order Møller−Plesset perturbation calculations
(MP2)22 were undertaken employing the Gaussian 0923
program package. Very high level ab initio coupled cluster
calculations with singlet and doublet excitation, CCSD,24 were
performed using the Molpro25 suite of programs. Peterson and
Dunning’s26 correlation-consistent cc-pVTZ and cc-pVQZ
basis sets, which are of triple- and quadruple-ζ quality,
respectively, were used.
Theoretical Calculations. Optimized structures, dipole
moments, vibrational frequencies, and Watson27 quartic
centrifugal distortion constants were first calculated at the
MP2/cc-pVTZ level of theory for the five conformers I−V
(Figure 1). The precautions of McKean et al.28 were observed
when computing the centrifugal distortion constants. The
results are shown in the Supporting Information (SI) Tables
1S−5S. Selected MP2 results are repeated in Table 2.
The structures of I−V were finally optimized at the CCSD/
cc-pVQZ level using the MP2 structures as starting points.
Calculations of CCSD vibrational frequencies, centrifugal
distortion constants, and zero-point vibrational corrections
are beyond our available computational resources. The CCSD
principal inertial axes coordinates of the five conformers are
listed in Table 6S of the SI together with their full geometrical
structures. The structural parameters not involving hydrogen
atoms are repeated in Table 1. The CCSD electronic energy
differences and the principal axes dipole moment components
are included in Table 2.
Table 1. CCSD/cc-pVQZ Structuresa of Five Conformers of
CH2CHCH2CH2NC
Conformer:
C1−C2
C2−C3
C3−C4
C4−N5
N5−C6
C1−C2−C3
C2−C3−C4
C3−C4−N5
C4−N5−C6
C1−C2−C3−C4
C2−C3−C4−N5
I
II
III
Bond Length (pm)
132.9
132.8
132.9
149.9
149.8
149.9
152.7
152.8
152.8
142.6
142.5
142.7
116.6
116.6
116.6
Angles (deg)
124.3
124.2
124.0
110.9
112.7
112.8
111.2
111.4
111.4
178.6b
178.3b
178.6b
Dihedral Angle (deg)
115.6
115.8
118.4
177.2
−65.4
62.5
IV
V
133.0
150.0
152.1
142.6
116.6
132.9
150.2
152.1
142.7
116.6
126.5
113.9
110.6
178.7b
126.6
115.8
112.3
180.2
0.0
180.0
−0.3
−69.1
a
Structural parameters involving hydrogen atoms have been omitted.
Full structures of the five conformers are given in Table 6S of the
Supporting Information. bBent toward C3.
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Table 2. CCSD/cc-pVQZ and MP2/cc-pVTZ Parameters of Spectroscopic Interest of Five Conformersa of H2C
CHCH2CH2NC
Conformer:
A
B
C
DJ
DJK
DK
d1
d2
I
17861.3
1502.9
1477.5
0.271
−12.9
423
−0.0198
−0.00244
μa
μb
μc
μtot
12.4
4.23
0.41
13.1
ΔE
0.27
II
III
Rotational Constantsb,c (MHz)
6091.9
8600.1
2571.7
2039.7
1972.4
1778.9
Quartic Centrifugal Distortion Constantsd (kHz)
5.22
1.60
−20.9
−20.6
31.4
97.7
−2.02
−0.356
−0.133
−0.0154
Dipole Momentb,e (10−30 C m)
6.51
8.66
11.1
9.28
0.75
1.67
12.9
12.8
Energy Differenceb,f,g (kJ/mol)
1.22
0.00
IV
V
14995.0
1693.7
1550.7
6130.0
2707.3
2135.2
0.181
−0.582
28.2
−0.0226
−0.00158
2.38
−8.50
15.8
−0.772
−0.0698
12.7
1.39
0.00
12.8
8.58
9.21
3.76
13.1
2.09
4.06
a
Minima on the potential energy hypersurface. bCCSD results. cRotational constants have been calculated from the structures in Table 1S of the SI.
d
MP2 results. S-reduction.27 e1 debye = 3.33564 × 10−30 C m. fElectronic energy. gElectronic energy of rotamer III: −653742.03 kJ/mol.
(2.97 GHz) showed that this series undoubtedly had to be
assigned to conformer I, while the weaker pile-up series (B + C
≈ 3.24 GHz) must belong to IV.
Assignment of the Spectrum of Conformer I. Detailed
description of the assignment of the spectrum of I is presented
here, while that of rotamer IV is discussed below. While the
identifications of the J quantum numbers of a aR-series are
obvious, the assignments of the K−1 lines belonging to a
particular pile-up are often problematic. Stark modulation
patterns and RFMWDR experiments were very helpful in this
respect. The MP2 centrifugal distortion constants were also
found to be very useful to identify the correct K−1 quantum
number.
The lines were fitted to Watson’s Hamiltonian in the Sreduction form using the Ir-representation27 employing
Sørensen’s program Rotfit.34 The assignments from the pileups produced accurate values for the B and C rotational
constants and the DJ and DJK quartic centrifugal distortion
constants. Rather inaccurate values of the A rotational constant
were obtained from these transitions. The K−1 = 1 pair of lines,
which are well separated from the pile-ups, is much more
sensitive to the A rotational constant than the other aR-lines.
Searches for these lines soon met with success and produced an
A rotational constant that was accurate to within roughly ±20
MHz (one standard deviation). A μb of 4.23 × 10−30 C m is
predicted for I (Table 2), and searches were made for the
strongest b-type transitions. These lines, which are predicted to
be much weaker than the aR-lines, were not found presumably
because of their weakness and the crowded nature of the
spectrum.
No transitions displayed a resolved hyperfine structure due
to quadrupole coupling of the 14N nucleus. This was expected
because the quadrupole coupling constants of 14N nuclei are
relatively small for isocyanides. The quadrupole coupling
constant of the 14N nucleus of CH3NC is, for example, only
0.4894(4) MHz.35 The MP2 14N quadrupole coupling
constants, which are indeed relatively small, are given at the
end of Table 1S.
typical example taken from the strongest pile-ups series is
shown in Figure 2. This figure demonstrates the characteristic
Figure 2. Portion of the MW spectrum taken at a field strength of
about 110 V/cm. This spectral region is dominated by absorption lines
mainly associated with the J = 23 ← 22 a-type transitions of I. Values
of the K−1 pseudo quantum number is listed above several peaks
belonging to the ground vibrational state. The two lines with K−1
quantum numbers 3 and 4 are not well resolved and well modulated.
Most of the remaining unlabeled transitions belong to vibrationally
excited states. The intensity (Y-axis) is in arbitrary units.
spectral richness of these pile-ups. Overlapping of spectral lines
of the ground and vibrationally excited states occurs frequently,
especially at the higher frequency end of the pile-up region.
Consecutive pile-ups are separated by almost exactly the sum
of the B and C rotational constants. It was found that B + C ≈
2.97 GHz for the stronger series, and ≈3.23 GHz for the
weaker series. There are two candidates, namely, rotamer I (κ =
−0.99 and B + C ≈ 2.98 GHz (Table 2)) and conformer IV (κ
= −0.92 and B + C ≈ 3.24 GHz (Table 2)), that have these
properties. The sum of B + C of the strongest pile-up series
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the frequencies of b-type transitions, which were readily
assigned. The assignments were then gradually extended to
include higher and higher values of the J quantum number.
Ultimately, 266 transitions with Jmax = 45 and K−1max (Table 8S
of the SI) were used to obtain the spectroscopic constants
shown in Table 4.
The assignments were gradually extended to include
transitions with J up to 29 and a maximum value of K−1 =
17. A total of 270 aR-transitions shown in Table 7S in the SI
were used to derive the spectroscopic constants shown in Table
3. Four of the quartic centrifugal distortion constants DJ, DJK,
Table 3. Spectroscopic Constantsa of the Ground
Vibrational State of Conformer I of
H2CCHCH2CH2NC
A (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
d1 (kHz)
d2 (kHz)
HKJc (Hz)
rmsd
Ne
Table 4. Spectroscopic Constantsa of the Ground and First
Vibrationally Excited States of Conformer III of H2C
CHCH2CH2NC
17558(20)
1496.8632(57)
1470.5239(57)
0.27292(70)
−13.407(11)
423.3b
−0.0151(35)
−0.0191(98)
−0.194(41)
1.588
278
a
S-reduction Ir-representation.27 Uncertainties represent one standard
deviation. The spectrum is listed in Tables 7S in the SI. bFixed. cSextic
constants other than HKJ were preset at zero in the least-squares fit.
d
Root-mean-square deviation defined as rms2 = Σ[(νobs − νcalc)/u]2/
(N − P), where νobs and νcalc are the observed and calculated
frequencies, u is the uncertainty of the observed frequency, N is the
number of transitions used in the least-squares fit, and P is the number
of spectroscopic constants. eNumber of transitions.
vibrational state:
ground
first excited torsion
A (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
d1 (kHz)
d2 (kHz)
HJb (Hz)
HKJ (Hz)
HK (Hz)
rmsc
Nd
8536.286(18)
2034.3132(32)
1770.7575(32)
1.5778(94)
−20.880(22)
150.21(70)
−0.34524(13)
−0.016291(51)
0.0358(87)
1.82(43)
−56(12)
1.875
266
8478.289(30)
2042.1809(32)
1777.2100(31)
1.676(11)
−21.072(60)
100.0(14)
−0.36697(27)
−0.01742(16)
0.023(11)
2.8(12)
−35(22)
1.683
177
a
S-reduction Ir-representation.27 Uncertainties represent one standard
deviation. The spectra are listed in Tables 8S and 9S in the Supporting
Information. bSextic constants other than HJ, HKJ, and HK were preset
at zero in the least-squares fit. cRoot-mean-square deviation defined as
rms2 = Σ[(νobs − νcalc)/u]2/(N − P), where νobs and νcalc are the
observed and calculated frequencies, u is the uncertainty of the
observed frequency, N is the number of transitions used in the leastsquares fit, and P is the number of spectroscopic constants. dNumber
of transitions.
d1, and d2 could be determined. The first two of these are quite
accurate. DK depends to a very minor degree on the aRtransitions and no significant value was obtained for this
centrifugal distortion constant. It was therefore preset at the
MP2-value in the least-squares fit. One sextic centrifugal
distortion constant, namely, HKJ, was also determined. The
remaining sextic constants were preset at zero in the leastsquares fit.
The experimental spectroscopic constants can now be
compared with their CCSD and MP2 counterparts. The
experimental B and C rotational constants (Table 3) deviate by
less than 0.4% and 0.5%, respectively, from the CCSD values
(Table 2), whereas A differs by about 1.7% from its CCSD
counterpart. Difference of this order of magnitude is to be
expected because the CCSD and experimental rotational
constants are defined differently. The experimental constants
are obtained from an effective structure, whereas the theoretical
constants are calculated from an approximate equilibrium
structure. However, the good agreement between the CCSD
(Table 2) and effective rotational constants (Table 3) is an
indication that the CCSD structure in Table 1 is accurate,
which is not surprising given the high computational level it is
based on.
There is very satisfactory agreement between the MP2 and
experimental quartic centrifugal distortion constants in the
cases of DJ and DJK (better than 4%). The uncertainties of the
experimental constants d1 and d2 are quite large. A comparison
with their MP2 equivalents is therefore not warranted.
Conformer III. Relatively strong a- and b-type spectra were
expected for this form due to the fact that μa = 6.51 and μb =
11.1 × 10−30 C m (Table 2). Searches for the aR-spectrum soon
met with success using the spectroscopic constants of Table 2
to predict the spectrum. The preliminary spectroscopic
constants obtained from these transitions were used to predict
It is seen from this table that accurate values have been
obtained for all five quartic centrifugal distortion constants.
Three sextic constants HJ, HKJ, and HK were also obtained, but
they have rather large standard deviations. Comparison of the
rotational constants (Table 4) with their theoretical counterparts (Table 2) reveals relatively small deviations of 0.7%, 0.3%,
and 0.5% in the cases of A, B, and C, respectively, whereas
deviations of 1.2%, 1.4%, 7.1%, 3.2%, and 5.5% were found for
DJ, DJK, DK, d1, and d2 for the centrifugal distortion constants,
which is considered to be very satisfactory.
The spectrum of one vibrationally excited state was also
assigned in the same manner as the spectrum of the ground
vibrational state. The spectrum, consisting of 177 transitions, is
found in Table 9S, whereas the spectroscopic constants are
listed in Table 4. Relative intensity measurements yielded 73
(25) cm−1 for this vibration, which is assumed to be the lowest
torsional mode, compared to 78 cm−1 obtained in the MP2
calculations (Table 3S).
Conformer IV. This very prolate rotor has a spectrum that
in principle is very similar to that of I but significantly less
intense, and it was assigned in the same manner as the
assignment of the spectrum of I. Only aR-transitions with Jmax =
29 and K−1max = 11 were assigned. The spectrum is listed in
Table 10S in the SI and the resulting spectroscopic constants
are given in Table 5. The DK, d1, and d2 centrifugal distortion
constants were held fixed at the MP2 values in the fit in this
case.
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Table 5. Spectroscopic Constantsa of the Ground
Vibrational State of Conformer IV of H2C
CHCH2CH2NC
A (MHz)
B (MHz)
C (MHz)
2Pccb (10−20 u m2)
DJ (kHz)
DJK (kHz)
DK (kHz)
d1 (kHz)
d2 (kHz)
HKJd (Hz)
rmse
Nf
could be one reason why we were not able to assign its
spectrum.
Energy Differences. The internal energy differences
between the three conformers were obtained from comparison
of the intensities of several selected transitions of the ground
states of the three rotamers using the procedure outlined by
Esbitt and Wilson.36 The dipole moment must be known in
order to derive the energy difference. Experimental dipole
moments are not available. The CCSD dipole moments (Table
2) were therefore used because dipole moments at this level of
theory are expected to be very close to their ground-state
counterparts. Conformer I was found to be the global energy
minimum. The results were EIII − EI = 1.1(7) kJ/mol and EIV −
EI = 2.9(7) kJ/mol. The statistical weight of IV was taken to be
1, whereas the statistical weight of I and III was assumed to be
2. The quoted uncertainty of ±0.7 kJ/mol is one estimated
standard deviation. The uncertainties of the dipole moment
components have been taken into consideration in this
estimate. These values differ a little from the CCSD results
(Table 2), which predict III to be the global minimum with I
0.27 and IV 2.09 kJ/mol higher in energy. The CCSD energy
differences are not corrected for zero-point vibrational effects.
The lower-level MP2 calculations corrected for these effects
predict I to be 1.11 and IV to be 3.64 kJ/mol higher in energy
than III.
14861.5(52)
1684.1114(73)
1541.8417(61)
6.316(14)
0.1818(13)
−0.536(43)
28.2c
−0.0226c
−0.0158c
0.77(37)
1.202
112
a
S-reduction Ir-representation.27 Uncertainties represent one standard
deviation. The spectrum is listed in Tables 10S in the SI. bDefined by
Pcc = (Ia + Ib − Ic)/2, where Ia, Ib, and Ic, are the principal moments of
inertia. Conversion factor: 505379.05 × 10−20 MHz u m2. cFixed.
d
Sextic constants other than HKJ were preset at zero in the leastsquares fit. eRoot-mean-square deviation defined as rms2 = Σ[(νobs −
νcalc)/u]2/(N − P), where νobs and νcalc are the observed and calculated
frequencies, u is the uncertainty of the observed frequency, N is the
number of transitions, and P is the number of spectroscopic constants
used in the fit. fNumber of transitions.
■
CONCLUSIONS
CCSD/cc-pVQZ and MP2/cc-pVTZ calculations predict that
4-isocyano-1-butene has five conformers, denoted I−V.
Rotamer III is computed to have the global minimum
electronic energy and V represents the maximum with an
energy that is 4.06 kJ/mol higher than that of III. The CCSD
structures of each of these forms reveal rather small deviations
from expected geometries, with the exception of V, where there
is indication of intramolecular repulsive interaction because the
C2−C3−C4−N5 dihedral angle opens up to −69.1° from the
typical −60° bringing the isocyano and vinyl groups further
apart.
The comparatively weak MW spectra of three of these
rotamers denoted I, III, and IV were assigned and
spectroscopic constants were obtained for each of them.
Rotamers II and V may coexist with the other three forms, but
extensive searches for their spectra did not result in
assignments. The experimental and CCSD rotational constants
of I, III, and IV are in very good agreement in all cases, which is
evidence that the CCSD structures are good estimates of the
equilibrium structures of the conformers. The MP2 quartic
centrifugal distortion constants are in good agreement with
their experimental counterparts in all cases where meaningful
comparison can be made.
Relative intensity measurements indicate that I is the global
minimum with an internal energy that is 1.1(7) kJ/mol less
than the energy of III and 2.9(7) kJ/mol less than the energy of
IV. The CCSD computations predict another energy order with
III as the global minimum and I and IV with 0.27 and 2.09 kJ/
mol, respectively, higher energies.
Comparison of the experimental rotational constants and
quartic centrifugal distortion constants (Table 5) and the
CCSD and MP2 equivalents (Table 2) reveals similar, relatively
small differences as found in the cases of I and III discussed
above.
Conformer IV has a symmetry plane and two sp3-hybridized
carbon atoms, C3 and C4 (Figure 1), with four out-of-plane
hydrogen atoms. The value of the planar moment, defined by
Pcc = (Ia + Ib − Ic)/2, where Ia, Ib, and Ic, are the principal
moments of inertia, depends only on these hydrogen atoms.
Table 5 reports the value of 2Pcc to be 6.316(14) × 10−20 u m2.
The CCSD value calculated from the structure in Table 1 is
6.19 (same magnitude and units). Interestingly, the corresponding value found for the antiperiplanar conformer of 4isocyano-1-butyne (HCCCH2CH2NC), which also has
four similar out-of-plane hydrogen atoms, is −6.278903(65) ×
10−20 u m2,10 close to that obtained for IV.
Searches for the Spectra of II and V. The assignments
presented above for the three rotameric forms comprise the
majority of the strongest transitions of the spectrum. However,
many relatively strong transitions remain unassigned, but it is
assumed that most of them belong to the spectra of unassigned
vibrationally excited states of the three conformers. Comprehensive searches for the spectra of II and V were undertaken
using both Stark and RFMWDR spectroscopies using the
spectroscopic constants of Table 2 to predict their spectra. We
did not succeed in assigning the two spectra. It is likely that the
theoretical spectroscopic constants of II and V are just as
accurate as their counterparts of I, III, and IV. The fact that no
assignments were achieved can indicate that they have
somewhat higher energies than the assigned three forms
making their spectra even weaker than the spectra of I, III, and
IV. The high CCSD energy of V (4.06 kJ/mol; Table 2) points
in this direction, whereas it is harder to understand why the
spectrum of II was not found. The fact that this conformer
would lack comparatively strong and characteristic aR-pile-ups
■
ASSOCIATED CONTENT
* Supporting Information
S
Results of the theoretical calculations, including
energies; molecular structures; dipole moments;
vibrational frequencies; rotational and centrifugal
constants; and 14N nuclear quadrupole coupling
1418
electronic
harmonic
distortion
constants.
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The Journal of Physical Chemistry A
Article
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■
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. This work has
been supported by the Research Council of Norway through a
Centre of Excellence Grant (Grant No. 179568/V30). It has
also received support from the Norwegian Supercomputing
Program (NOTUR) through a grant of computer time (Grant
No. NN4654K). J.-C. G. thanks the Centre National d’Etudes
Spatiales (CNES) for financial support.
■
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