Article
pubs.acs.org/JPCA
Microwave Spectrum and Conformational Properties of 4‑Isocyano1-butyne (HCCCH2CH2NC)
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
‡
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-1butyne (HCCCH2CH2NC) has been investigated in the
12.4−77.6 GHz spectral region. The spectra of two rotamers
denoted ap and sc were assigned. ap has an antiperiplanar
arrangement for the C−C−C−N chain of atoms, whereas sc
has synclinal conformation for this link. The ground state
spectrum and three vibrationally excited state spectra of the
lowest torsional vibration were assigned for ap, while the ground vibrational state spectrum was assigned for sc. The C−C−C−N
dihedral angle was found to be 64.5(30)° in sc and exactly 180° in ap. ap was determined to be 2.9(6) kJ/mol lower in energy
than sc from relative intensity measurements. The microwave study has been augmented with ab initio and DFT calculations
employing the CCSD(T), MP2, and B3LYP methods with the cc-pVTZ basis set. A Natural Bond Order analysis has also been
performed. Most, but not all, of the quantum chemical predictions agree satisfactorily with the experimental results.
■
INTRODUCTION
Isocyanides have interesting and 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
(ClCH 2 CH 2 NC), 7 E- and Z-1-propenylisocyanide
(CH 3 CHCHNC),8 and cyclopropylmethyl isocyanide
(C3H5CH2NC).9 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 and cyclopropylmethyl isocyanide
(C3H5CH2NC).9
In this work, our studies are extended to include the first
MW investigation of the structural and conformational
properties of 4-isocyano-1-butyne (HCCCH2CH2NC).
This compound can be regarded as a 1,2-substituted ethane,
(XCH2CH2Y), where X = CCH and Y = NC. These
substituted ethanes exist in a X−C−C−Y antiperiplanar
(obsolete “trans”) or synclinal (obsolete “gauche”) forms.
Models of the antiperiplanar and synclinal conformers of the
title compound with atom numbering are sketched in Figure 1.
The two rotamers are henceforth referred to as ap and sc,
respectively.
Only two 1,2-substituted isocyanides, namely,
FCH2CH2NC6 and ClCH2CH2NC,7 have previously been
© 2013 American Chemical Society
investigated by MW spectroscopy. A gas electron diffraction
(GED) and molecular mechanics study has been reported for a
third similar isocyanide, namely, 1,2-diisocyanoethane
(CNCH2CH2NC).10 The conformational properties of these
three isocyanides vary: The synclinal rotamer is lower in energy
than the antiperiplanar form by 1.4(5) kJ/mol in the
FCH 2 CH 2 NC, 6 whereas the opposite is seen for
ClCH2CH2NC where the antiperiplanar conformer is lower
in energy than the synclinal rotamer by 4.3(8) kJ/mol. The
study of CNCH2CH2NC at 150 °C found that the
antiperiplanar form is present in a concentration of
56.9(88)%.10 Molecular-mechanics calculations predicted the
enthalpy difference to be 2.9 kJ/mol.10 These findings are a
clear demonstration that the nature of the substituents
influences the conformational properties to a remarkable
degree.
The physical properties of 4-isocyano-1-butyne 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 calculations employing the CAMB3LYP functional with the 6-311G(d,p) basis set. No
conformational analysis was reported in this study. The fact
that the microwave spectrum and the conformational properties
of the title compound have not previously been investigated
prompted the present research. Our experimental method of
investigation is MW spectroscopy due to its superior accuracy
Received: July 18, 2013
Revised: August 14, 2013
Published: September 4, 2013
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Figure 1. Models of the ap and sc conformers of 4-isocyano-1-butyne. Atom numbering is given on the ap rotamer.
doublet excitation including noniterative triplet excitations,
CCSD(T),18 were performed using the Molpro19 suite of
programs. A Natural Bond Order (NBO)20 analysis was also
carried out. Peterson and Dunning’s21 correlation-consistent ccpVTZ basis set, which is of triple-ζ quality, were used in all the
calculations.
Theoretical Calculations. B3LYP and MP2 internal
energy potential functions for rotation about the C1−C2
bond (Figure 2) were first calculated. The C3−C2−C1−N5
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-butyne was synthesized starting
from the corresponding formamide as described previously
(Scheme 1).4
Scheme 1
4-Isocyano-1-butyne is a colorless liquid at room temperature
with a vapor pressure of roughly 110 Pa.
Spectroscopic Experiments. The MW spectrum was
recorded with the cell cooled to about −20 °C using small
portions of dry ice to cool the waveguide. The pressure was
roughly 7 Pa during the measurements. The MW spectrum was
studied with Stark-modulation spectroscopy using the microwave spectrometer of the University of Oslo. Details of the
construction and operation of this device have been given
elsewhere.11−13 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. The spectrum was recorded
in the whole 12.4−77.6 GHz frequency interval. Selected
measurements were also performed in other frequency regions.
Radio-frequency microwave double-resonance experiments
(RFMWDR), similar to those undertaken by Wodarczyk and
Wilson,14 were also conducted to unambiguously assign
particular transitions, using the equipment described elsewhere.11
Figure 2. B3LYP/cc-pVTZ (triangles) and MP2/cc-pVTZ (circles)
potential functions for rotation about the C1−C2 bond. The functions
were calculated by varying the C3−C2−C1−N5 dihedral angle in
steps of 10°. Both functions have a global minimum at 180°,
corresponding to ap, and a maximum at 0°. A second minimum,
corresponding to sc, is located at 66.1° (B3LYP) and 62.6° (MP2).
The sc conformers have higher energies by 4.3 (B3LYP), and 2.5 kJ/
mol (MP2) relative to ap.
dihedral angle was stepped in 10° intervals with the remaining
structural parameters varying freely. Minima and maxima of
these two functions, as shown in Figure 2, were computed using
the optimize and transition-state options of Gaussian 09.
Both potential functions have minima at 180° for the C3−
C2−C1−N5 dihedral angle, corresponding to the ap conformer. There is a second minimum corresponding to sc. The
B3LYP calculations predict the C3−C2−C1−N5 dihedral angle
to be 66.1° in sc and find this form to be 4.3 kJ/mol higher in
energy than ap. The corresponding MP2 values are 62.6° and
2.5 kJ/mol. The potential functions have two maxima
(transition states). The first of these is at 0° in both cases.
The B3LYP energy of this maximum relative to the 180°
minimum is 25.0 kJ/mol, whereas the corresponding MP2
value is 24.1 kJ/mol. The second maximum is predicted at
118.7° (B3LYP) and 119.2° (MP2). The corresponding
■
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)15 and density functional theory (DFT) calculations
using Becke’s three-parameter hybrid functional employing the
Lee, Yang, and Parr exchange-correlation functional (B3LYP)16
were undertaken with the Gaussian 0917 program package. Very
high-level ab initio coupled cluster calculations with singlet and
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energies are 15.3 (B3LYP) and 17.2 kJ/mol (MP2),
respectively, higher than the energies at 180°.
Vibrational frequencies, quartic and sextic centrifugal
distortion constants, and vibration−rotation constants (the
α’s)22 were calculated at the MP2 level of theory in addition to
the optimized structures and dipole moments. The precautions
of McKean et al23 were observed when computing the
centrifugal distortion constants and the vibration−rotation
constants. The results are shown in the Supporting Information
Tables 1S (ap) and 2S (sc). Selected MP2 results are repeated
in Tables 2 and 3 below together with their experimental
counterparts.
The structures of ap and sc were finally optimized at the
CCSD(T)/cc-pVTZ level using the MP2 structures as the
starting point. ap was assumed to have a symmetry plane in
these calculations to save computer time. The resulting
CCSD(T) structures are listed in Table 1, while the CCSD(T)
principal inertial axes coordinates of the atoms are listed in
Table 3S (ap) and 4S (sc) of the Supporting Information. The
electronic energy difference and the principal axes dipole
moment components are included in Table 1. The rotational
constants obtained from the CCSD(T) structures are shown in
the last columns of Tables 2 and 3 together with their
experimental equivalents.
ap has a symmetry plane (Cs symmetry) consisting of the
C6−N5−C1−C2−C3−C4−H7 plane of atoms. The planar
moment, defined by Pcc = (Ic − Ia − Ib)/2, where Ia, Ib, and Ic,
are the principal moments of inertia, varies when a vibrational
mode is excited and can be used to identify the nature of the
vibration.24,25 The theoretical Pcc is useful for comparison with
experiment and is therefore shown in the last column of Table
2. Computations of vibrational frequencies, centrifugal
distortion constants, and vibration−rotation constants could
not be made at this very high level of theory (CCSD(T)) due
to limited computational resources.
Some of the theoretical results above warrant comments. The
CCSD(T) isocyanide group (N5C6) triple bond length is
117.8 pm in both conformers (Table 1), compared to the MP2
values (118.1 pm; Tables 1S and 2S). The corresponding GED
bond length in CNCH2CH2NC is 117.2(3) pm.10 No
separation into individual bond lengths of antiperiplanar and
synclinal rotamers was possible in this case.10 The C3C4
bond length is 121.1 pm in both rotamers, almost the same as
the MP2 predictions (121.4 pm). The CCSD(T) C1−C2 bond
lengths are 153.5 pm (ap) and 153.4 pm (sc), compared to the
MP2 bond lengths, which are 153.1 and 153.0 pm, respectively.
This bond length is 152.9(6) pm in CNCH2CH2NC.
Fortunately, some equilibrium bond lengths of the isocyanide
group exist. In HNC it is 116.83506(16) pm,26 and in CH3NC
a value of 116.9(1) pm has been determined.27 These bond
lengths are about 1 pm shorter than the MP2 (Tables 1S and
2S) and CCSD(T) (Table 1) predictions. Moreover, the
equilibrium bond length of the triple bond in acetylene is
120.2958(7) pm,28 approximately 0.7 pm shorter than the
CCSD(T) results for HCCCH2CH2NC. The CCSD(T)
C1−C2 bond lengths are also roughly 1 pm longer than the
equilibrium C−C bond length of ethane (152.2 pm).29 The
differences between the CCSD(T) bond lengths and relevant
experimental equilibrium counterparts are larger than expected
and seem to indicate that even these comprehensive ab initio
calculations are unable to produce a highly accurate
approximation of equilibrium bond lengths in this case.
Table 1. CCSD(T) Structures, Dipole Moments, of the ap
and sc Conformers of HCCCH2CH2NC
conformer:
ap
Bond Distance (pm)
C1−C2
153.5
C1−N5
143.0
C1−H8
109.0
C1−H9
109.0
C2−C3
146.7
C2−H10
109.2
C2−H11
109.2
C3−C4
121.1
C4−H7
106.3
N5−C6
117.8
Angle (deg)
C2−C1−N5
110.4
C2−C1−H8
110.4
C2−C1−H9
110.4
N5−C1−H8
108.7
N5−C1−H9
108.7
H8−C1−H9
108.3
C1−C2−C3
110.6
C1−C2−H10
109.3
C1−C2−H11
109.3
C3−C2−H10
110.1
C3−C2−H11
110.1
H10−C2−H11
107.5
C2−C3−C4
178.3a
C3−C4−H7
179.3b
C1−N5−C6
178.4c
Dihedral angle (deg)
N5−C1−C2−C3
180.0
N5−C1−C2−H10
58.7
N5−C1−C2−H11
−58.7
H8−C1−C2−C3
59.9
H8−C1−C2−H10
−61.4
H8−C1−C2−H11
−178.8
H9−C1−C2−C3
−59.9
H9−C1−C2−H10
178.8
H9−C1−C2−H11
61.4
Energy Difference Relative to apd (kJ/mol)
0.0
Dipole Momentse (10−30 C m)
μa
9.90
μb
2.74
μc
0.0f
μtot
10.27
sc
153.4
142.8
109.0
109.1
146.6
109.2
109.3
121.1
106.3
117.8
111.4
110.3
109.8
108.4
108.2
108.7
112.4
109.2
108.0
109.8
109.6
107.6
178.8
179.5b
178.7c
64.5
−57.7
−174.4
−55.9
−178.0
65.2
−175.6
62.2
−54.5
3.25
5.88
10.30
2.30
12.08
a
Bent toward C1. bBent away from C1. cBent toward C2. dTotal
CCSD(T) electronic energy of ap: −650415.17 kJ/mol. eConversion
factor: 1 debye = 3.33564 × 10−30 C m. fFor symmetry reasons.
All three theoretical methods predict the N5−C1−C2−C3
dihedral angle of sc to be larger than the “canonical” 60°,
namely, 64.5° (CCSD(T)), 66.2° (B3LYP), and 62.6° (MP2).
ap is lower in energy by 3.3 kJ/mol (CCSD(T)), 4.3 kJ/mol
(B3LYP), and 2.5 kJ/mol (MP2). These values are internal
energy differences. The MP2 internal energy difference
corrected for zero-point vibrational frequencies is 2.6 kJ/mol.
The NBO calculations were performed to model stereoelectronic interactions in the two conformers. The focus was to
determine relative hyperconjugative electron distributions
between bonding and antibonding orbitals. These interactions
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Table 2. Spectroscopic Constantsa of the ap Conformer of HCCCH2CH2NC
vib. state:
ground
first torsion
second torsion
third torsion
theoryb
A (MHz)
B (MHz)
C (MHz)
Pccc (10−20 u m2)
DJ (kHz)
DJK (kHz)
DK (kHz)
dJ (kHz)
dK (kHz)
HJKe (Hz)
HKJ (Hz)
rmsf
Ng
24940.150(23)
1553.38424(69)
1489.36393(66)
−6.278903(65)
0.16849(60)
−9.4756(55)
342.0(31)
−0.016529(49)
−0.000425(14)
−0.0153(51)
0.078(10)
1.358
438
23090(9)
1554.2800(38)
1493.0552(38)
−8.5543(85)
0.1711(14)
−9.245(15)
342.0d
−0.0289(20)
−0.000425d
−0.036(14)
−0.651(44)
1.371
277
21608.4(57)
1555.2910(36)
1496.7867(37)
−10.687(6)
0.1757(15)
−8.699(26)
342.0d
−0.0236(20)
−0.000425d
−0.108(28)
−0.428(92)
1.129
182
20548(28)
1556.4360(58)
1500.5483(54)
−12.50(3)
0.1935(24)
−7.945(20)
342.0d
−0.0276(38)
−0.000425d
−0.0153d
0.078d
1.395
89
25072.5
1542.3
1479.6
−6.28
0.166
−9.36
332.9
−0.0163
−0.000394
−0.0418
2.48
a
S-reduction Ir-representation.34 Uncertainties represent one standard deviation. The spectra are listed in Tables 5S−8S in the Supporting
Information. bThe equilibrium rotational constants were calculated from the CCSD(T) structure in Table 1. The centrifugal distortion constants
were taken from Table 1S. cDefined by Pcc = (Ic − Ia − Ib)/2, where Ia, Ib, and Ic, are the principal moments of inertia. Conversion factor: 505379.05
× 10−20 MHz u m2. dFixed. eSextic constants other than HJK and HKJ were preset at zero in the least-squares fit. fRoot-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 used in the fit. gNumber of
transitions used in the fit.
Table 3. Spectroscopic Constantsa of the Ground
Vibrational State of the sc Conformer of HC
CCH2CH2NC
A (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
dJ (kHz)
dK (kHz)
HJKc (Hz)
rmsd
Ne
experiment
theoryb
6853.778(12)
2527.7819(71)
1976.9942(71)
3.792(23)
−26.350(33)
66.34(31)
−1.41473(84)
−0.09369(37)
−0.100(12)
1.436
107
6751.5
2510.9
1969.0
3.777
−24.08
53.26
−1.408
−0.0842
−0.109
of ap by 1−2° (Table 1) supports the steric repulsion
hypothesis. Electrostatic dipole−dipole repulsion may add to
the steric repulsion in sc, because the bond moments of the
acetylene group31 and of the isocyanide group32 have their
negative ends pointing in the direction toward H7 and C6,
respectively. Steric repulsion as well as dipole−dipole repulsion
must be a minimum in ap, which, not surprisingly, is predicted
to be the lower-energy conformer in the theoretical
calculations.
Microwave Spectrum and Assignment of ap. Survey
spectra of 4-isocyano-1-butyne revealed a relatively dense and
mostly relatively weak spectrum with some characteristic,
strong and very rich pile-up regions protruding from the weaker
spectral background. These pile-ups were separated by almost
constant frequency intervals corresponding to the sum of the B
and C rotational constants. A typical example of one of these
pile-ups is shown in Figure 3. This feature is compatible with an
a-type R-branch spectrum of a very prolate asymmetrical top,
which is indeed the case for ap, whose asymmetry parameter33
κ is ≈ −0.994, and the fact that its a-axis dipole moment
component is as large as 9.90 × 10−30 C m according to the
CCSD(T) calculations (Table 1).
The individual K−1 lines of these pile-ups were first assigned.
The MP2 centrifugal distortion constants were found to be very
useful in this respect. The assignments of several of the pile-up
transitions were confirmed by RFMWDR experiments.14 The
assigned lines were fitted to Watson’s Hamiltonian in the Sreduction form using the Ir-representation34 employing
Sørensen’s program Rotfit.35 These assignments produced
accurate values for the B and C rotational constants and the DJ
and DJK quartic centrifugal distortion constants. 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 aRlines. Their approximate frequencies were then estimated by
varying A in a trial and error manner until Pcc became
approximately −6.3 × 10−20 μm2 (Table 2). Searches for these
lines using the improved A rotational constant obtained in this
manner were soon met with success and produced an A
rotational constant that was accurate to within roughly ±10
a
S-reduction Ir-representation.34 Uncertainties represent one standard
deviation. The spectra are listed in Table 9S in the Supporting
Information. bThe equilibrium rotational constants were calculated
from the CCSD(T) structure in Table 1. The centrifugal distortion
constants were taken from Table 2S. cSextic constants other than HJK
were preset at zero in the least-squares fit. dRoot-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 used in the fit. eNumber of transitions used in the fit.
can contribute to the stabilization of individual conformers. sc
was calculated to be lower in energy than ap by 5.94 kJ/mol by
such interactions, opposite to what was found for the total
energy difference just quoted. Obviously, stereoelectronic
interactions must be countered by other forces since all the
three theoretical methods predict ap to be the lower-energy
form. Steric and dipole−dipole forces therefore seem to
outweigh stereoelectronic interactions. The nonbonded distance between C3 and N5 is 298 pm (not given in Table 1)
compared to 340 pm, which is twice the van der Waals radius of
the half-thickness of aromatic carbon,30 is an indication that
steric repulsion should be important in sc. The fact that the
C2−C1−N5 and C1−C2−C3 angles of sc are larger than those
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were assumed to belong to the spectra of vibrationally excited
states. The seven lowest normal modes of the ap conformer
have anharmonic frequencies of 83, 122, 199, 277, 340, 442,
and 482 cm−1, according to the MP2 calculations (Table 1S). It
is seen from Table 2 and Tables 6S−8S that the aR-spectra of
three successively excited states of the torsion about the C1−
C2 bond were assigned. The assignments were performed in a
manner analogous to that of the ground state. Searches for btype lines were performed, but they were not assigned
presumably because they are quite weak.
The intensity of the first excited state is roughly 60% of that
of the ground vibrational state. Relative intensity measurements
performed largely as described by Esbitt and Wilson37 yielded
92(20) cm−1 for this mode, whose MP2 anharmonic frequency
is 83 cm−1.
It is possible to compare the experimental and theoretical
vibration−rotation constants defined by αex = X0 − Xex,22 where
X0 is a rotational constant of the ground state and Xex is a
rotational constant of the excited state under consideration.
The experimental vibration−rotation constants derived from
the entries in columns 2 and 3 of Table 2 are αA = 1850(9), αB
= −0.8958(39), and αC = −3.6913(39) MHz, compared to the
MP2 values of 1954.3, −0.064, and −2.72 MHz, respectively
(Table 1S). Calculation of vibration−rotation constants is very
demanding involving, among other things, third derivatives of
the potential energy at the equilibrium structure. Deviations of
several MHz between experimental and theoretical constants,
similar to the present ones, therefore have to be expected for
MP2 calculations. The spectroscopic constants of the excited
states (Table 2) change in a regular fashion upon excitation,
which is typical for an essentially harmonic vibration.24 The
same is seen for the absolute values of the planar moment, Pcc.
Assignment of the Spectrum of sc. The major dipole
moment of this conformer is μb according to the CCSD(T)
calculations (Table 1). Searches were therefore first performed
for the bQ-spectrum, which was found after some searching.
This spectrum was much weaker than the aR-spectrum of ap.
Q-branch lines with a maximum J = 46 and K−1 were ultimately
assigned. The bR-transitions were found next using a trial and
error procedure. The frequencies of the aR-spectrum could now
be predicted accurately. Several of these transitions were
assigned, but they were quite weak and therefore not included
in the least-squares fit. A total of 107 b-type transitions (Table
9S) were finally used to determine the spectroscopic constants
shown in Table 3. No excited-state spectra could be assigned
presumably because of low intensity.
The experimental rotational constants (Table 3, second
column) are all larger than the CCSD(T) constants (third
column). The differences are not large. The largest discrepancy
is seen for A (1.5%), whereas the differences are 0.67% for B,
and 0.40% for C. The MP2 quartic centrifugal distortion
constants differ from the experimental ones by 0.40% in the
case of DJ, to 19.7% for DK.
Structures. It was remarked above in the computational
section that the CCSD(T) C1−C2, C3C4, and N5C6
bond lengths seem to be too long compared to equilibrium
values found for similar compounds. The rotational constants
of ap and sc were therefore recalculated by substituting the
CCSD(T) values with 152.2, 120.3, and 116.9 pm for the C1−
C2, C2C3, and N5C6 bond lengths, respectively, keeping
all the other structural parameters fixed at the CCSD(T) values
given in Table 1. The rotational constants of ap were now
calculated to be A = 25349.4, B = 1553.0, and C = 1490.4 MHz.
Figure 3. A 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 ap. Values
of the K−1 pseudo quantum number is listed above several peaks
belonging to the ground vibrational state. Lines with K−1 quantum
numbers between 4 and 6 are not well resolved. Most of the remaining
unlabeled transitions belong to vibrationally excited states.
MHz. A μb of 2.74 × 10−30 C m is predicted for ap (Table 1),
and searches were then made for bQ-branch transitions, which
are the strongest transitions of this variety. These lines, which
are much weaker than the aR-lines, were found after some trials.
No transitions displayed a resolved hyperfine structure due to
quadrupole coupling of the 15N nucleus. This was expected
because the quadrupole coupling constants of 15N nuclei are
small for isocyanides. The quadrupole coupling constant of the
14
N nucleus of CH3NC is, for example, only 0.4894(4) MHz.36
The comparatively small MP2 quadrupole coupling constants
are given at the end of Table 1S.
The assignments were gradually extended to include higher
and higher values of the principal quantum number up to J =
50. A total of 438 transitions shown in Table 5S in the
Supporting Information were used to derive the spectroscopic
constants shown in Table 2, second column. All five quartic
constants could be determined, whereas only two sextic
constants, HJK and HKJ, were derived. The remaining sextic
constants were preset at zero in the least-squares fit.
The experimental spectroscopic constants can now be
compared with their CCSD(T) and MP2 counterparts. The
ground-state rotational constants deviate by less than one
percent from the CCSD(T) values. A difference of this order of
magnitude is to be expected because the CCSD(T) and
experimental rotational constants are defined differently. The
experimental constants are obtained from an effective structure,
whereas the theoretical constants are approximations of the
equilibrium structure. There is very good agreement between
the MP2 and experimental quartic centrifugal distortion
constants. The largest discrepancy is seen for the theoretical
dK, which deviates by 7%. The uncertainties of the experimental
sextic constants are of the same order of magnitude as the
constants. A comparison with their MP2 equivalents is
therefore not warranted.
Vibrationally Excited States of ap. The ground-state aRtransitions were accompanied by series of weaker lines with
very similar Stark effects and RFMWDR patterns. These lines
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The Journal of Physical Chemistry A
Article
assigned a bond moment of 2.3 × 10−30 C m.31 Electrostatic
repulsion should consequently have a maximum value in N
CCH 2 CH 2 CN and a minimum value in HC
CCH2CH2CCH, while an intermediate value is expected
for HCCCH2CH2NC. The energy differences (Table 4)
indeed has its largest value for NCCH2CH2CN (6.3(12)
kJ/mol), which is significantly larger than 2.9(6) kJ/mol found
for 4-isocyano-1-butyne in the present study. The value
reported for HCCCH2CH2CCH is 4.2(16) kJ/mol. The
large uncertainty makes it difficult to decide whether the energy
difference is larger or smaller than in HCCCH2CH2NC.
The same applies to the energy difference of the two forms of
CNCH2CH2NC, since only the composition at one
temperature has been reported.
The B and C rotational constants are now in better agreement
with their experimental counterparts (Table 2), whereas a
poorer agreement is seen for A compared to that calculated
from the full CCSD(T) structure (Table 2).
The results for sc were A = 6772.6, B = 2530.2, and C =
1981.5 MHz. These three constants are all in better agreement
with the experimental rotational constants (Table 3) than the
rotational constants derived from the full CCSD(T) structure
shown in the same table.
The important N5−C1−C2−C3 dihedral angle is found to
be 64.5° in the CCSD(T) calculations (Table 1). The close
agreement between the experimental and CCSD(T) rotational
constants is a strong indication that the said dihedral angle is
indeed close to this value. The uncertainty of this dihedral angle
is hardly more than ±3°.
Energy Difference Between ap and sc. The internal
energy difference between these two forms was obtained from
comparison of the intensities of several selected transitions of
the ground states of the two conformers using the procedure
outlined by Esbitt and Wilson.37 The dipole moment must be
known in order to derive the energy difference. The
experimental dipole moments are not available. The CCSD(T)
dipole moments (Table 1) of the two forms were therefore
used. The result was Esc − Eap = 2.9(6) kJ/mol, which means
that ap is lower in energy than ap. The quoted uncertainty of
±0.6 kJ/mol is one standard deviation. The experimental value
(2.9(6) kJ/mol) is in excellent agreement with the CCSD(T)
(3.3 kJ/mol), B3LYP (4.3 kJ/mol), and MP2 (2.5 kJ/mol)
results.
■
Results of the theoretical calculations, including electronic
energies; molecular structures; dipole moments; harmonic and
anharmonic vibrational frequencies; rotational and centrifugal
distortion constants; rotation−vibration constants; and 14N
nuclear quadrupole coupling constants. Microwave spectra of
the ground and vibrationally excited states of two conformers.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
*Tel: +47 2285 5674; Fax: +47 2285 5441; E-mail: harald.
mollendal@kjemi.uio.no.
DISCUSSION
4-Isocyano-1-butyne is a member of a family of isoelectronic
XCH2CH2Y compounds, where X,Y = CCH, CN, or N
C. Table 4 summarizes experimental conformational findings
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 for financial support.
Table 4. Conformational Properties of Some XCH2CH2Y
Compounds
X
X
X
X
X
=
=
=
=
=
38
CCH, Y = CCH
CCH, Y = CN39
CN, Y = CN40
CCH, Y = NC41
NC, Y = NC10
77(10)°
75(8)
64.5(30)
68.4(49)
energy
differencea
(kJ/mol)
method
4.2(16)
−b
6.3(12)
2.9(6)
56.9(88)%c
GED
IR
GED
MW
GED
■
b
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AUTHOR INFORMATION
Corresponding Author
■
XCCY angle
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ASSOCIATED CONTENT
S Supporting Information
*
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