Article
pubs.acs.org/JPCA
Microwave Spectrum, Conformational Properties, and Dipole
Moment of Cyclopropylmethyl Isocyanide (C3H5CH2NC)
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, Avenue du
Général Leclerc, CS 50837, 35708 Rennes Cedex 7, France
S Supporting Information
*
ABSTRACT: The microwave spectrum of cyclopropylmethyl
isocyanide, C3H5CH2NC, has been investigated in the 25−75 GHz
spectral range. The spectra of two conformers were assigned.
The H−C−C−N chain of atoms is antiperiplanar in the
conformer denoted ap and synclinal in the sc rotamer. The sc
conformer tends to be slightly more stable than the ap form.
The internal energy difference was determined to be Eap − Esc =
0.2(7) kJ/mol from relative intensity measurements. The
spectra of the ground vibrational state and six vibrationally
excited states belonging to two different normal vibrations were
assigned for sc. The frequencies of these two modes were
determined by relative intensity measurements. The dipole moment of this conformer was determined to be μa = 12.16(6), μb =
5.91(4), μc = 0 (preset), and μtot = 13.52(6) × 10−30 C m [4.05 (2) debye]. The spectra of the ground and of two vibrationally
excited states belonging to the torsion and lowest bending vibration were assigned for ap. The microwave work was supported by
quantum chemical calculations at the CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory. Most, but not all, of the theoretical
predictions are in good agreement with experiment.
■
INTRODUCTION
Our two laboratories have recently become interested in organic
isocyanide compounds because this functional group has an
interesting and unique chemistry that has been comparatively little
investigated.1−3 Our isocyanide studies have concentrated on the
syntheses4 of several members of this class of compounds, which
have subsequently been investigated by UV photon electron
spectroscopy,4 microwave (MW) spectroscopy,5−8 and high-level
quantum chemical calculations.4−8 So far, we have reported
microwave spectra of allenyl isocyanide (H2CCCHNC),5
2-fluoroethyl isocyanide (FCH2CH2NC),6 2-chloroethyl isocyanide (ClCH2CH2NC),7 and E- and Z-1-propenylisocyanide
(CH3CHCHNC).8 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 in focus in our investigations of 2-fluoroethyl6
and 2-chloroethyl7 isocyanide.
In this work, our isocyanide studies are extended to the first
MW investigation of the conformational and structural
properties of cyclopropylmethyl isocyanide (C3H5CH2NC).
A model of two typical conformers of this compound with
atom numbering is shown in Figure 1. The orientation of the
H6−C2−C9−N12 chain of atoms can conveniently be used to
describe these rotamers. In the conformer denoted ap, the said
link of atoms are antiperiplanar and form a dihedral angle of 180°.
This conformer has a symmetry plane bisecting the cyclopropyl
© 2013 American Chemical Society
ring. In the second rotamer denoted sc, the H6−C2−C9−N12
chain has a synclinal orientation and has a dihedral angle of about
60°. Obsolete nomenclature, “cis” and “gauche” for ap and sc,
respectively, is often encountered for other methyl substituted
cyclopropane conformers.
Experimental conformational and structural studies using
MW, infrared, and Raman spectroscopy have been reported for
many substituted methylcyclopropane derivatives (C3H5CH2X)
including X = F,9−12 Cl,10,13−16 Br,10,14,16,17 I,10,18 OH,19 SH,20
SeH,21 NH2,22 PH2,23 CH3,24 SiH3,25−28 SiF3,25,29,30 CN,31,32
and CC−H.33,34 The last two compounds, C3H5CH2CN
and C3H5CH2CCH, are of special relevance for the title
compound because these three molecules are isoelectronic and
have one triple bonded substituent each attached to the methyl
group.
It has generally been found in these comprehensive studies of
substituted methylcyclopropane derivatives (C3H5CH2X) that the
sc form predominates and that this form becomes increasingly more
stable as the size of the substituent X increases. Steric effects are
therefore important for the conformational preferences of these
molecules. The gauche effect35 should also favor the sc orientation,
especially when X is electronegative. Intramolecular hydrogen
Received: April 5, 2013
Revised: May 16, 2013
Published: May 16, 2013
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Figure 1. Models of the H6−C2−C9−N12 antiperiplanar (ap) and synclinal (sc) conformers of cyclopropylmethyl isocyanide. sc was found to be
0.2(7) kJ/mol more stable than ap from relative intensity measurements.
bonding with the pseudo-π electrons36 of the cyclopropyl ring will
stabilize the sc forms, as discussed in the cases of X = OH,19 SH,20
SeH,21 NH2,22 and PH2,23 as well as in a review.37
However, there is one important exception to this preference
of the sc conformer, namely, C3H5CH2CCH, where ap is
0.77(36) kJ/mol more stable than sc.33 However, its isoelectronic
cyano derivative, C3H5CH2CN, follows the general trend and
only the MW spectrum of sc was assigned in this case, and this
rotamer is present in the gas phase in at least 85% concentration at
room temperature according to an infrared study.31 The two
examples, C3H5CH2CN and C3H5CH2CCH, demonstrate
that the conformational preferences of these isoelectronic
compounds are quite sensitive to the nature of the triple bond.
The question whether the corresponding isocyanide derivative
C3H5CH2NC behaves in a way similar to that of its isoelectronic cyano relative, C3H5CH2CN, or similar to its acetylene
congener, C3H5CH2CCH, was another motivation to undertake
the present first MW study of cyclopropylmethyl isocyanide.
Our choice of experimental method is MW spectroscopy, due
to its superior accuracy and resolution, which makes this method
ideal for conformational and structural studies. The MW
investigation has been augmented with high-level quantum
chemical calculations, which were conducted with the purpose
of obtaining information for use in assigning the MW spectrum
and investigation properties of the potential-energy hypersurface.
the microwave spectrometer of the University of Oslo. Details of
the construction and operation of this device have been given
elsewhere.38−40 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 investigated
in the whole 25−75 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,41 were also conducted
to unambiguously assign particular transitions, using the equipment
described elsewhere.38
■
RESULTS AND DISCUSSION
Quantum Chemical Methods. The present ab initio
coupled clusters singlet and double substitutions42−45 (CCSD)
and density functional theory (DFT) calculations were performed
employing the Gaussian 0946 program package running on the
Abel cluster in Oslo. Becke’s three-parameter hybrid functional
employing the Lee, Yang, and Parr exchange-correlation functional
(B3LYP)47 was employed in the DFT calculations. Peterson and
Dunning’s48 correlation-consistent cc-pVTZ basis set, which is of
triple-ζ quality, were used in all the calculations.
Quantum Chemical Calculations. B3LYP/cc-pVTZ
calculations of the optimized structures and dipole moments of
ap and sc were first performed. All structural parameters were
varied freely in these calculations with no symmetry restrictions.
The vibrational frequencies, Watson’s A-reduction quartic and sextic
centrifugal distortion constants,49 and the vibration−rotation
interaction constants50 (the αs) were then calculated using the
principal inertial axes coordinates of the optimized structures, which
were kept fixed in the calculations, as pointed out by McKean
et al.51 The results of these calculations are found in Tables 1S and
2S of the Supporting Information. Centrifugal distortion constants
are repeated in the last columns of Tables 2 and 4 for comparison
with their experimental counterparts.
It is seen from Tables 1S and 2S, Supporting Information, that
no imaginary harmonic normal vibrations were obtained for any of
the two forms, which is an indication that ap and sc are indeed
minima on the potential-energy hypersurface. ap was found to have
a symmetry plane consisting of the H6−C2−C9−N12−C13 link of
atoms. The important H6−C2−C9−N12 dihedral angle was
found to be 62.4° in sc. The B3LYP energy difference corrected
for the zero-point vibrational effect is 2.25 kJ/mol, with sc as the
more stable rotamer, whereas the electronic energy difference is
1.86 kJ/mol.
■
EXPERIMENTAL SECTION
Synthesis. Cyclopropylmethyl isocyanide has been synthesized starting from the corresponding formamide as described
previously4 (Scheme 1).
Scheme 1
Spectroscopic Experiments. Cyclopropylmethyl isocyanide is a colorless liquid at room temperature with a vapor
pressure of roughly 90 Pa. The MW spectrum was recorded at
room temperature or with the cell cooled to about −30 °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
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Table 1. CCSD/pVTZ Structures, Dipole Moments, and 14N Nuclear Quadrupole Coupling Constants of the ap and sc
Conformers of C3H5CH2NC
conformer
C1−C2
C1−C3
C1−H4
C1−H5
C2−C3
C2−H6
C2−C9
C3−H7
C3−H8
C9−H10
C9−H11
C9−N12
N12−C13
C2−C1−H4
C2−C1−H5
C3−C1−H4
C3−C1−H5
H4−C1−H5
C1−C2−H6
C1−C2−C9
C3−C2−H6
C3−C2−C9
H6−C2−C9
C1−C3−H7
C1−C3−H8
C2−C3−H7
C2−C3−H8
H7−C3−H8
C2−C9−H10
C2−C9−H11
C2−C9−N12
H10−C9−H11
H10−C9−N12
a
ap
Bond Distance (pm)
150.2
150.9
108.0
108.0
150.2
108.2
151.3
108.0
108.0
109.0
109.0
142.9
116.9
Angle (deg)
117.8
117.7
118.4
116.9
115.2
116.5
121.4
116.5
121.4
111.9
116.9
118.4
117.7
117.8
115.2
110.4
110.4
112.3
108.1
107.7
conformer
sc
ap
sc
Angle (deg)
H11−C9−N12
107.7
107.9
C9−N12−C13
178.8
178.7
Dihedral Angle (deg)
H4−C1−C2−H6
1.5
0.7
H4−C1−C2−C9
−140.9
−144.0
H5−C1−C2−H6
146.5
144.7
H5−C1−C2−C9
4.1
0.0
H4−C1−C3−H7
144.7
144.7
H4−C1−C3−H8
0.0
−0.1
H5−C1−C3−H7
0.0
−0.4
H5−C1−C3−H8
−144.7
−145.2
H6−C2−C3−H7
−146.5
−145.3
H6−C2−C3−H8
−1.5
−1.1
C9−C2−C3−H7
−4.1
−1.4
C9−C2−C3−H8
140.9
142.8
C1−C2−C9−H10
84.2
−157.2
C1−C2−C9−H11
−156.3
−37.6
C1−C2−C9−N12
−36.1
82.3
C3−C2−C9−H10
156.3
−87.6
C3−C2−C9−H11
−84.2
32.0
C3−C2−C9−N12
36.1
151.9
H6−C2−C9−H10
−59.7
57.2
H6−C2−C9−H11
59.7
176.9
H6−C2−C9−N12
180.0
−63.2
Electronic Energy Differencea (kJ/mol)
0.16
0.0
Dipole Momentb (10−30 C m)
μa
8.37
11.4
μb
8.47
5.42
μc
0.0c
0.86
μtot
11.9
12.6
Principal-Axis 14N Quadrupole Coupling Constants (MHz)
χaa
0.0596
0.1456
χbb
0.0427
−0.0353
150.1
150.8
107.9
108.1
150.4
108.1
150.6
108.1
107.9
109.0
109.0
143.2
116.9
117.9
117.4
118.2
117.8
114.8
117.0
118.9
117.2
118.2
114.7
117.7
113.3
117.7
117.9
114.7
111.2
110.1
111.6
108.0
107.9
The CCSD/cc-pVTZ electronic energy of the sc conformer: −653542.38 kJ/mol. The energy of the ap form is 0.16 kJ/mol higher than the energy
of the sc rotamer. b1 debye = 3.33564 × 10−30 C m. cFor symmetry reasons.
inertial axis components of nuclear quadrupole coupling tensor of
the 14N nucleus are included in Table 1. The rotational constants
obtained from the CCSD structures are shown together with their
experimental equivalents in the last columns of Tables 2 and 4.
ap has a symmetry plane (Cs symmetry) consisting of the
H6−C2−C9−N12−C13 plane of atoms. This plane bisects
the cyclopropyl ring. 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.52,53 The theoretical
Pcc is useful for comparison with experiment and is therefore
shown in the last column of Table 4.
Some of the CCSD results warrant further comments. The
CCSD electronic energy difference is only 0.16 kJ/mol with sc
as the more stable conformer, compared to the B3LYP value
(1.86 kJ/mol) given above.
The C−C bond lengths of the cyclopropyl ring varies between
150.1 to 150.9 pm (Table 1), compared to the equilibrium bond
length in cyclopropane, which is 151.0077(77) pm.54 The
variation of the bond lengths of the cyclopropyl ring is interesting.
The C1−C3 bond length, which is opposite to the methyl
The B3LYP potential function for rotation about the C2−C9
bond was also calculated using the scan option of Gaussian09. The
H6−C2−C9−N12 dihedral angle was stepped in 10° intervals in
these calculations. The function, which is drawn in Figure 2, has
maxima at 0 and 123.2°. The energies at these transitions states are
14.52 (0°) and 13.01 kJ/mol (123.2°) higher than the electronic
energy of sc. The transitions state structures and energies were
obtained employing the Gaussian09 transition-state option.
Finally, comprehensive CCSD/cc-pVTZ calculations of optimized structures, dipole moments, electronic energies, and nuclear
quadrupole coupling constants of the 14N nucleus of the ap and sc
were performed using the B3LYP structures in Tables 1S and 2S,
Supporting Information, as starting points. ap was assumed to
have a symmetry plane in these calculations to save computational
time. Unfortunately, it is not possible to calculate vibrational
frequencies and centrifugal distortion constants at the CCSD with
our present computational resources.
The resulting CCSD structures are listed in Table 1, while the
CCSD principal inertial coordinates of the atoms are listed in
Table 3S (ap) and 4S (sc) of the Supporting Information. The
electronic energy difference, the dipole moments, and the principal
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Table 2. Spectroscopic Constantsa of the sc Conformer of C3H5CH2NC
A (MHz)
B (MHz)
C (MHz)
ΔJ (kHz)
ΔJK (kHz)
ΔK (kHz)
δJ (kHz)
δk (kHz)
ΦJ (Hz)
ΦKJ (Hz)
ϕJd (Hz)
rmse
Nf
ground state
first ex. tors.
second ex. tors.
third ex. tors.
fourth ex. tors.
lowest bend.
comb. state
theoryb
10841.5285(84)
2134.7048(12)
1935.7963(12)
0.9055(24)
−9.350(11)
59.21(37)
0.00090(15)
3.4329(76)
−0.0196(12)
−2.15(19)
0.00090(15)
1.508
364
10772.923(12)
2137.9832(26)
1938.5463(25)
0.9519(96)
−9.197(14)
54.19(43)
0.19442(13)
3.5489(92)
0.024(11)
−1.90(24)
0.000974(31)
1.564
283
10707.747(14)
2141.1014(22)
1941.2142(21)
0.9741(41)
−9.020(14)
47.48(60)
0.19523(18)
3.616(15)
0.02387c
−1.897c
0.000698(38)
1.628
159
10646.417(29)
2144.0599(36)
1943.7890(36)
1.023(14)
−9.040(48)
47.2(11)
0.19800(72)
3.965(32)
0.096(17)
−1.7(10)
0.00124(29)
1.665
148
10589.865(40)
2146.8078(29)
1946.2808(28)
1.0432(58)
−8.936(28)
50.7(12)
0.15081(43)
9.050(60)
0.09644c
−1.65c
0.0012388c
1.844
94
10913.539(20)
2138.5945(27)
1938.2801(26)
0.9370(53)
−9.450(21)
65.57(97)
0.19509(25)
3.813(33)
−0.019554c
−2.148c
0.00090987c
1.977
170
10839.167(42)
2141.9373(24)
1941.1274(24)
0.9444(45)
−9.770(77)
71.9(17)
0.19666(52)
3.905(41)
−0.019554c
−5.7(16)
0.00141(17)
1.844
155
10939.3
2130.7
1935.0
0.791
−8.16
58.8
0.159
3.05
a
A-reduction Ir-representation.49 Uncertainties represent one standard deviation. The spectra are found in the Supporting Information in Table 5S
(ground state), Tables S6−S9 (successive excited states of the torsion), Table 10S (lowest bending vibration), and 11S (combination state of the first
excited state of the torsion and first excited state of the lowest bending vibration). bThe theoretical rotational constants have been calculated from
the CCSD structure in Table 1, whereas the centrifugal distortion constants were obtained in the B3LYP calculations. cFixed. dFurther sextic
constants preset at zero; see text. eRoot-mean-square deviation defined as rms2 = Σ[(νobs − νcalcd)/u]2/(N − P), where νobs and νcalcd 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 spectroscopic constants used in the fit. fNumber of transitions used in the fit.
canonical 60°. This CCSD angle is similar to 62.4° found in the
B3LYP calculations (Table 2S, Supporting Information).
Comparatively small values were calculated for the 14N
quadrupole coupling constants (Table 1). The small quadrupole
coupling constants obtained in these calculations are typical for
isocyanides. The quadrupole coupling constant of the 14N nucleus
of CH3NC is, for example, only 0.4894(4) MHz.57
Microwave Spectrum and Assignment of the Spectrum of sc. The MW spectrum was found to be of moderate
intensity and dense with absorption lines occurring every few
MHz in the whole investigated spectral range. A typical example is
shown in Figure 3. The high spectral density is understandable
Figure 2. B3LYP/cc-pVTZ barrier to internal rotation about the C2−C9
bond. This curve has minima at 62.4° (sc) and 180° (ap). The energy
difference between these two forms including zero-point vibrational
effects is 1.86 kJ/mol favoring sc, compared to 0.2(7) kJ/mol found
experimentally. The two maxima at 0 and 123.2° have energies that are
14.52 and 13.01 kJ/mol higher than the B3LYP energy of sc.
ioscyanide substituent, is slightly longer by 0.4−0.7 pm than the
adjacent C1−C2 and C2−C3 bond lengths. This asymmetry is
typical for substituted cyclopropanes.55 The NC bond length is
116.9 pm in both conformers, almost the same as the equilibrium
NC bond length in H−NC, which is 116.83506(16) pm.56
Comparatively small variations (∼1°) are seen for the
majority of the bond angles in the two forms. There is a notable
exception. The C1−C2−C9 and the C3−C2−C9 angles
are both 121.4° for symmetry reasons in ap. In sc, these
angles are 118.9 and 118.2°, respectively. Similar differences for
these angles were predicted in the B3LYP calculations (Table
1S and 2S, Supporting Information).
Interestingly, the H6−C2−C9−N12 dihedral angle, which in
a sense describes the conformational properties of this
compound, is 63.2° in sc, three degrees larger than the
Figure 3. Portion of the MW spectrum taken at a field strength of about
110 V/cm demonstrating the complexity of the spectrum. This spectral
region is dominated by absorption lines mainly associated with the J = 17 ←
16 a-type transitions. 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 8 and 12 coalesce at a
frequency of about 69250 MHz. Pairs with K−1 quantum numbers 7, 6,
and 5 coalesce, whereas the K−1 = 4 pair is split. Most of the remaining
unlabeled transitions belong to vibrationally excited states.
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Table 3. Stark Coefficientsa and Dipole Momenta of the sc
Conformer of C3H5CH2NC
since both ap and sc have comparatively large dipole moment
components along the a- and the b-inertial axes (Table 1)
according to the CCSD calculations above, as well as the fact that
spectra of vibrationally excited states of both rotamers might
contribute significantly to the spectrum because there are five
fundamental vibrational normal modes of ap and six of sc below
500 cm−1 (Tables 1S and 2S of the Supporting Information).
The quantum chemical calculations predict that sc is the
slightly more stable rotamer. This form has a statistical weight
that is twice that of ap because there are two mirror-image forms
of this conformer. Searches were therefore first made for aR-lines
of sc using the RFMWDR technique.41 The approximate
frequencies of these transitions were predicted using the CCSD
rotational constants and the B3LYP quartic centrifugal distortion
constants shown in the last column of Table 2. The RFMWDR
experiments met with success and secured the first unambiguous
assignments. An example of a RFMWDR spectrum is shown in
Figure 4. The transitions assigned in this manner were weighted
Stark coefficients ΔE−2/(10−6 MHz V−2)cm2
transitions
41,4 ← 31,3
41,3 ← 31,2
51,4 ← 41,3
61,6 ← 51,5
60,6 ← 50,5
71,7 ← 61,6
μa = 12.16(6)
a
|M|
0
1
2
1
1
0
1
1
3
1
Dipole Momentb
μb = 5.91(4)
obs
calcd
−3.52(6)a
14.3(3)
68.6(8)
−19.8(4)
−8.74(10)
−3.80 (6)
−2.72(4)
−2.49(5)
4.58(10)
−18.4(3)
(10−30 C m)
μc = 0c
−3.54
14.5
68.7
−19.8
−8.68
−3.94
−2.70
−2.35
4.52
−18.5
μtot = 13.52(6)
b
Uncertainties represent one standard deviation. Conversion factor:
1 debye = 3.33564 × 10−30 C m. cPreset at this value; see text.
coupling constants calculated for this nucleus (Table 1).
Ultimately, nearly 400 lines were assigned; 364 of them, shown
in Table 5S of the Supporting Information, were used to
determine the spectroscopic constants displayed in Table 2. The
maximum value of J is 64. Transitions involving even higher values
of J were searched for, but these lines were too weak to be
assigned unambiguously. Their weakness is presumably due to an
unfavorable Boltzmann factor.
It is seen from Table 2 (second column) that the dimensionless
root-mean-square deviation is 1.51 and that accurate values have
been obtained for the rotational constants and the quartic
centrifugal distortion constants. Three of the sextic centrifugal
distortion constants, ΦJ, ΦKJ, and ϕJ, were also determined.
Attempts to determine significant values for additional sextic
centrifugal distortion constants failed, and these constants were
therefore preset at zero in the least-squares fit.
The experimental rotational constants (first column; Table 2)
and the CCSD rotational constants (last column of the same
table) agree to within 1% or better. A difference this small is to be
expected because the experimental rotational constants referred to
an ef fective structure, whereas the CCSD rotational constants have
been obtained from the approximate equilibrium structure shown
in Table 1. The fact that the CCSD and experimental rotational
constants are so similar is reassuring and is an indication that the
CCSD structure is indeed very accurate, as expected for this
compound composed of elements of the first and second row of
the periodic table.
The experimental quartic centrifugal distortion constants
deviate comparatively much more from their B3LYP counterparts
than the rotational constants do. The smallest relative deviation is
found for ΔK (0.7% deviation), while the largest deviation is seen
for δJ (17%). The experimental sextic constants are not sufficiently
accurate to warrant a comparison with their theoretical counterparts.
Vibrationally Excited States of sc. The ground-state
transitions were accompanied by series of weaker lines with very
similar Stark effects and RFMWDR patterns. These lines were
assumed to belong to the spectra of vibrationally excited states.
The six lowest normal modes of the sc conformer have
anharmonic frequencies of 87, 148, 273, 309, 352, and 477 cm−1,
according to the B3LYP calculations (Table 2S of the Supporting
Information). It is seen from Table 2 that six excited states were
assigned. Four of these states belong to successively excited states
Figure 4. RFMWDR spectrum of the 154,12 ← 144,11 and 154,11 ←
144,10 transitions of the ground and vibrationally excited states of sc
using a RF of 10.15 MHz. These two transitions are split by about
10 MHz. The ground-state transitions are found at 61179.76 and
61190.14 MHz, respectively. Most of the vibrationally excited states
have higher frequencies. Brackets are used to indicate 154,12 ← 144,11
and 154,11 ← 144,10 pairs of the ground and several vibrationally excited
states.
according to their estimated uncertainties and least-squares fitted
to Watson’s Hamiltonian using the A-reduction Ir-representation,49 as implemented in Sørensen’s program Rotfit58 to obtain a
set of preliminary spectroscopic constants that were employed to
predict the approximate frequencies of additional a-type R-branch
lines, which were subsequently found with ease. Their Stark effects
and fit to Watson’s Hamiltonian confirmed their assignment. The
preliminary spectroscopic constants thus obtained were now used
to predict the frequencies of bQ-transitions. These transitions were
also found close to their predicted frequencies. The assignments
were then gradually extended to include additional a- and b-type
lines of higher and higher values of the principal quantum number
J and the pseudoquantum number K−1. Searches were also made
for c-type lines, but none were found, presumably because of the
small μc dipole moment (see below, Table 3). No lines obviously
split by nuclear quadrupole interactions of the 14N nucleus were
seen, which is in accord with the comparatively small quadrupole
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Assignment of the Ground State Spectrum of ap. This
conformer has a significant μa and the RFMWDR technique
was therefore employed to obtain the first unambiguous
transitions of the spectrum of ap, which was found to be
significantly less intense than that of sc. The assignments were
gradually extended to include additional a- and b-type
transitions in a manner very similar to that of sc, as described
above. Ultimately, roughly 550 transitions were assigned; 526
of which (Table 11S, Supporting Information) were used to
determine the spectroscopic constants shown in Table 4. The
maximum values of J and K−1 are 70 and 22, respectively.
Searches were made for transitions involving even higher values
of J and K−1, but they were too weak to be unambiguously
assigned, presumably due to an unfavorable Boltzmann factor.
All five quartic and six of the seven sextic centrifugal distortion
constants were determined. The exception is ϕJ, which was
preset at zero in the least-squares fit.
It is seen from Table 4 that the experimental and CCSD
rotational constants deviate by less than 1%, which is an indication
that the CCSD structure is very close to the equilibrium structure,
as anticipated. Moreover, the good agreement between the
experimental (−21.684855(25) × 10−20 u m2) and theoretical
(−21.59 × 10−20 u m2) planar moments (Pcc) is additional
evidence of the quality of the CCSD structure. Interestingly, the
values of Pcc found for the corresponding ap conformer of the
isoelectronic compound C3H5CH2CCH calculated from the
reported rotational constants33 is −21.67 × 10−20 u m2, very
similar to the corresponding value of the title compound
(−21.684855(25) × 10−20 u m2; Table 4).
The best agreement between the B3LYP quartic centrifugal
distortion constants and their experimental counterparts (same
table) is seen for ΔK (2%). The worst case is δJ (13%). The
experimental and theoretical sextic constants deviate so much
that a comparison is meaningless. Obviously, more sophisticated quantum chemistry approaches are needed to improve
agreement between theory and experiment, but this was not
possible with the computational resources available for us.
The dipole moment could not be determined because
relevant transitions had insufficient intensities to allow a
quantitative determination of their Stark effects.
Vibrationally Excited States of ap. This rotamer has five
anharmonic vibrational fundamentals at 85, 135, 273, 296, and
334 cm−1, according to the B3LYP calculations (Table 1S,
Supporting Information). The spectra of two vibrationally
excited states were assigned. Only aR-lines were found in these
cases. The spectra consisting of 81 and 73 transitions,
respectively, are shown in Tables 13S and 14S of the
Supporting Information, while the spectroscopic constants are
listed in Table 4. Only three centrifugal distortion constants, ΔJ,
ΔJK, and δJ, were varied in the least-squares fit of the spectra of
these two excited states, with the remaining centrifugal
distortion constants kept fixed at the ground-state values. The
result is shown in Table 4, columns 3 and 4.
Relative intensity measurements yielded 77(30) cm−1 for the
first of these modes, which is presumed to be the first excited
state of the torsion about the C2−C9 bond. This frequency
should be compared with the B3LYP anharmonic frequency of
85 cm−1 (Table 1S, Supporting Information). The vibration−
rotation constants calculated for the torsion from the entries in
Table 3 are αA = 20.37(64), αB = 16.179(28), and αC =
4.104(32) MHz, which compare rather poorly with the B3LYP
values of −11.58, 12.76, and 4.26 MHz, respectively (Table 1S,
Supporting Information). The largest discrepancy is seen for
of the torsion about the C2−C9 bond (columns 3−6), one to the
lowest bending vibration (column 7), and one was assigned as a
combination mode consisting of the first excited state of
the torsion and the first excited state of the bending vibration
(column 8). The assignments were performed in a manner
analogous to that of the ground state.
The spectrum of the most intense excited state (Table 6S,
Supporting Information) has an intensity of roughly 65% of
that of the ground vibrational state. Relative intensity
measurements performed largely as described by Esbitt and
Wilson59 yielded 73(20) cm−1 for this mode, which is assumed
to be the first excited state of the torsion about the C2−C9
bond, whose B3LYP anharmonic frequency is 87 cm−1.
It is possible to compare the experimental and theoretical
vibration−rotation constants defined by αex = X0 − Xex,50 where
X0 is the rotational constants of the ground state and Xex are the
rotational constants 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 = 68.6055(15),
αB = −3.2784(29), and αC = −2.7500(28) MHz, compared to
the B3LYP values of 71.68, −2.15, and −2.03 MHz, respectively
(Table 2S, Supporting Information). The agreement is
satisfactory given the approximate nature of the B3LYP
calculations.
Spectra of four successively excited torsional states were
assigned (Tables 6S−10S, Supporting Information); the spectroscopic constants are shown in Table 2. It is seen that the rotational
constants of these states change in a regular fashion upon
excitation, which is typical for an essentially harmonic vibration.52
An exited-state spectrum (Table 11S, Supporting Information) assumed to belong to the first excited state of the lowest
bending vibration was also assigned and its frequency was
determined to be 138(30) cm−1 by relative intensity measurements, compared to the theoretical anharmonic frequency,
which is 148 cm−1 (Table 2S, Supporting Information). The
vibration−rotation constants derived for this mode from
columns 2 and 7 of Table 2 are αA = −72.011(22), αB =
−3.8897(30), and αC = −2.4838(29) MHz, in reasonable
agreement with the B3LYP values of −82.61, −3.21, and −1.93
MHz, respectively (Table 2S, Supporting Information).
Finally, the spectrum of a combination mode of the torsion
and bending vibrations were assigned. The vibration−rotation
constants derived from columns 2 and 8 of Table 2 are αA =
2.362(43), αB = −7.2325(27), and αC = −5.3311(27) MHz,
close to −3.41, −7.17, and −5.23 MHz, respectively, obtained by
simple addition of the αs of the torsion and bending vibrations
given above.
Dipole Moment of sc. The dipole moment was determined
by least-squares fitting the second-order Stark coefficient shown in
Table 3. The weight of each Stark coefficient was taken to be the
inverse square of its standard deviation, which is also shown in the
same table. The theoretical values of the second-order Stark
coefficients were calculated as described by Golden and Wilson60
using program MB04.61 μc is small, and it was not possible to
obtain it experimentally. This dipole moment component was
therefore preset at zero in the least-squares fit. The experimental
dipole moment components are μa = 12.16(6), μb = 5.91(4),
μc =0.0 (preset), and μtot = 13.52(6) × 10−30 C m [4.05 (2)
debye], where the uncertainties represent one standard deviation.
The experimental values are in quite good agreement with the
calculated dipole moments at the CCSD/cc- pVTZ level of
theory (12.1, 5.67, 0.96, and 13.4 × 10−30 C m, respectively;
Table 1).
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Table 4. Spectroscopic Constantsa of the ap Conformer of C3H5CH2NC
A (MHz)
B (MHz)
C (MHz)
Pccc (10−20 u m2)
ΔJ (kHz)
ΔJK (kHz)
ΔK (kHz)
δJ (kHz)
δk (kHz)
ΦJ (Hz)
ΦJK (Hz)
ΦKJ (Hz)
ΦK (Hz)
ϕJ (Hz)
ϕJK (Hz)
ϕK (Hz)
rmse
Nf
ground state
first ex. tors.
lowest bend.
theoryb
7122.0063(35)
2827.5981(11)
2449.4752(11)
−21.684855(25)
1.7491(27)
−4.8509(37)
11.364(31)
0.42012(16)
−2.5210(54)
−0.0070(14)
−0.0741(63)
0.130(42)
−0.600(71)
0.0c
0.0811(28)
−1.04(13)
1.737
526
7101.69(64)
2811.422(28)
2445.371(32)
−22.125(5)
1.801(12)
−4.356(45)
11.364d
0.101(34)
−2.5210d
−0.0070d
−0.0741d
0.130d
−0.600d
0.0d
0.0811d
−1.04d
1.676
81
7160.62(42)
2829.264(21)
2446.291(18)
−21.306(4)
1.767(21)
−5.409(58)
11.364d
0.445(39)
−2.5210d
−0.0070d
−0.0741d
0.130d
−0.600d
0.0d
0.0811d
−1.04d
2.159
77
7180.2
2816.1
2445.4
−21.59
1.58
−4.50
11.6
0.366
−2.76
0.00118
−0.00199
−0.0791
0.127
0.000610
0.0280
0.527
a
A-reduction Ir-representation.49 Uncertainties represent one standard deviation. The spectra are listed in Tables 12S−14S in the Supporting
Information. bThe rotational constants were calculated from the CCSD structure in Table 1. 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. eRoot-mean-square deviation defined as rms2 =
Σ[(νobs − νcalcd)/u]2/(N − P), where νobs and νcalcd 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 spectroscopic constants used in the fit. fNumber of transitions used in
the fit.
αA. Calculation of the vibration−rotation constants requires the
third derivative of the energy with respect to the structure and
deviations such as those seen for αA are to be expected due to
the approximation involved in B3LYP/cc-pVTZ calculations.
The second excited state has a frequency of 127(30) cm−1 by
relative intensity measurements, compared to 135 cm−1 (B3LYP
value; Table 1S, Supporting Information) of the lowest bending
vibration. The vibration−rotation constants calculated from the
entries in Table 3 are αA = −38.64(47), αB = −1.666(25), and αC =
3.184(32) MHz, in satisfactory agreement with the B3LYP values
that are −34.26, −3.41, and 1.90 MHz, respectively (Table 1S,
Supporting Information).
The assignments referred to above include the vast majority
of the strongest lines and lines of intermediate intensity. The
remaining weaker unassigned lines are assumed to belong to
further vibrationally excited states. There is no evidence of the
existence of additional conformers.
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.59
The dipole moment must be known in order to derive the energy
difference. The experimental dipole moment of ap is not available.
The CCSD dipole moments (Table 1) of the two forms were
therefore used. The result was Eap − Esc = 0.2(7) kJ/mol, which
means that sc tends to be slightly more stable than ap by 0.2 kJ/mol.
However, the estimated uncertainty of ±0.7 kJ/mol is so large that
the opposite cannot be excluded. It is concluded that ap and sc have
nearly the same energy. The experimental value is in excellent
agreement with the CCSD result (0.16 kJ/mol) and in good
agreement with the B3LYP result (1.86 kJ/mol).
Polarity of the substituents seems to be important. The dipole
moment of the sc form of the isoelectronic compound
C3H5CH2CCH is 2.52(3) × 10−30 C m,33 whereas its ap
rotamer has a dipole moment of 2.13(3) × 10−30 C m,33 much
less than roughly 12 × 10−30 C m found for the two forms of
the title compound (Tables 1 and 3). The ap form is favored
0.77(36) kJ/mol in C3H5CH2CCH,33 while sc is favored by
0.2(7) kJ/mol in C3H5CH2NC. The corresponding nitrile
C3H5CH2CN, which must have a dipole moment similar to
C3H5CH2NC, prefers a sc form.31 The preference of the
nitrile and the isonitrile to take sc conformations is in accord
with the gauche effect,35 which predicts that electronegative
substituents prefer sc conformations.
Steric repulsive forces may also play a role in both conformers
of cyclopropylmethyl isocyanide. The C1−C2−C9 and C3−C2−
C9 angles are the same by symmetry and as large as 121.4° in ap
according to the CCSD calculations (Table 1). This is about 4−5°
larger than the C−C−H angles of the cyclopropyl ring and about
3° larger than the two angles in sc. The opening up of the
C1−C2−C9 and C3−C2−C9 angles seen in ap, but not in sc,
could reflect a repulsion between the cyclopropyl ring and the
isocyano group in the former rotamer.
Another repulsion seems to be present in sc. In the CCSD
calculations (Table 1), the H6−C2−C9−N12 dihedral angle
(Table 1) is calculated to be 63.2°, three degrees lager than the
canonical 60° angle possibly because of repulsion between the
pseudo-π electrons along the edge of the cyclopropyl ring36 and
the π-electrons of the isocyano group. The observed energy
difference of Eap − Esc = 0.2(7) kJ/mol must therefore be a
compromise of several intramolecular forces.
■
■
ASSOCIATED CONTENT
S Supporting Information
*
DISCUSSION
The conformational properties of cyclopropylmethyl isocyanide
are presumably dictated by several intramolecular interactions.
Results of the theoretical calculations, including electronic
energies; molecular structures; dipole moments; harmonic and
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Structure of Small Ring Compounds-LXI. IR and Raman Spectra,
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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. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*(H.M.) 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 of Space
Science (CNES) and the program PCMI (INSU-CNRS) for
financial support.
■
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