ARTICLE
pubs.acs.org/JPCA
Microwave Spectrum and Conformational Composition of
2-Fluoroethylisocyanide
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
‡
cole Nationale Superieure de Chimie de Rennes, CNRS, UMR 6226, Avenue du General Leclerc,
Sciences Chimiques de Rennes, E
CS 50837, 35708 Rennes Cedex 7, France
bS Supporting Information
ABSTRACT: The microwave spectrum of 2-fluoroethylisocyanide, FCH2CH2NtC, has been investigated in the whole
50 120 GHz spectral region. Selected portions of the spectrum
in the range of 18 50 GHz have also been recorded. The
microwave spectra of the ground state and vibrationally excited
states of two conformers have been assigned. Accurate spectroscopic constants have been derived from a large number of
microwave transitions. The F—C—C—N chain of atoms is antiperiplanar in one of these rotamers and synclinal in the second
conformer. The energy difference between the two forms was obtained from relative intensity measurements. It was found that the
synclinal conformer is favored over the antiperiplanar form by 0.7(5) kJ/mol. Quantum chemical calculations at the high CCSD/ccpVTZ and B3LYP/cc-pVTZ levels of theory were performed. Most, but not all, of the spectroscopic constants predicted in
these calculations are in good agreement with the experimental counterparts. The theoretical calculations correctly indicate that the
F—C—C—N dihedral angle in the synclinal form is about 67° but underestimate the magnitude of the gauche effect and
erroneously predict the antiperiplanar rotamer to be 1.3 1.6 kJ/mol more stable than the synclinal conformer.
’ INTRODUCTION
Our two laboratories have recently taken an interest in the
structural and conformational properties of isocyanides, which is
a functional group that has not been much investigated in the
past. Recently, we published the synthesis and microwave
spectrum of allenylisocyanide, H2CdCdCHNtC,1 which is a
compound of potential astrochemical interest.
There are very few, if any, experimental gas-phase studies of
conformational properties of isocyanides reported in the literature. 2-Fluoroisocyanide, F—CH2—CH2—NtC, was chosen
as our first example of a conformational problem involving the
isocyanide group. There can be two conformers of the title
compound, which differ by having a F—C—C—NC antiperiplanar (obsolete: trans), 180°, conformation, or a synclinal
(obsolete: gauche), 60°, conformation. A model of these two
forms with atom numbering is shown in Figure 1. These two
rotamers will henceforth be denoted ap and sc, respectively. ap
has a symmetry plane formed by the heavy atoms, whereas sc
exists as two mirror images.
The C—F bond and the isocyanide group are both very polar,
with bond moments of 4.7 10 30 and 10.0 10 30 C m,
respectively.2 The bond moments have their negative ends on the
fluorine end of the C—F bond and on the carbon end of the
NtC bond. It was observed a long time ago that 1,2-ethane
derivatives, XCH2CH2Y, with highly electronegative substituents often prefer synclinal conformations3 despite the significant
electrostatic repulsion that exists in these compounds. The best
r 2011 American Chemical Society
example of this so-called gauche effect is perhaps 1,2-difluoroethane, FCH2CH2F, where the synclinal form was found to be
preferred by 3.9(17) kJ/mol in one gas electron-diffraction
study,4 with 7.5(21) kJ/mol reported in a second such study.5
No experimental conformational studies are available for
FCH2CH2NC, but very recent density functional theory
(DFT) calculations at the B3LYP/6-311+G(d,p) and M052X/6-311+G(d,p)6 levels predicted 1.0 and 1.2 kJ/mol for this
energy difference, with sc as the high-energy form.6 The near
total lack of information on the conformational properties of
isocyanides in general and the interesting presence of the polar
C—F and CtN bonds in this ethane derivative prompted the
present investigation.
Microwave (MW) spectroscopy is an ideal method for the
study of conformational equilibria because of its superior accuracy and resolution. The fact that relative intensity measurements
can be performed on MW transitions to obtain accurate energy
differences is another advantage of this method. The spectroscopic work has been augmented by high-level quantum
chemical calculations, which were conducted with the purpose
of obtaining information for use in assigning the MW spectrum and investigating properties of the potential-energy
hypersurface.
Received: May 30, 2011
Revised:
June 30, 2011
Published: July 01, 2011
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The Journal of Physical Chemistry A
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Figure 1. Antiperiplanar (ap) and synclinal (sc) conformers of
FCH2CH2NC. Atom numbering is indicated on ap, which was found
to be 0.7(5) kJ/mol less stable than sc by relative intensity measurements performed on microwave transitions.
Scheme 1
’ EXPERIMENTAL SECTION
Synthesis of 2-Fluoroethylisocyanide.7 Into a 100 mL one-
necked round-bottomed flask equipped with a stirring bar
were introduced 2-fluoroethylformamide7,8 (2.3 g, 25 mmol),
p-toluenesulfonyl chloride (6.67 g, 35 mmol), and quinoline
(6.46 g, 50 mmol). (See Scheme 1.) The flask was attached to a
vacuum line equipped with two traps with stopcocks. The first
trap was immersed in a liquid nitrogen bath; the apparatus was
evacuated to 6 mbar and maintained at this pressure during the
reaction. The mixture was then slowly heated to 90 °C for about
1 h. 2-Fluoroethylisocyanide was evacuated from the reaction
mixture as it formed and condensed in the first trap. At the end of
the reaction, the second trap was immersed in a 80 °C cold
bath, and the first trap was allowed to warm to room temperature.
The low-boiling isocyanide was revaporized in vacuo and condensed in the second trap to give the expected product in 46%
yield (0.84 g, 11.5 mmol). 1H NMR (CDCl3, 400 MHz) δ 3.70
(dtt, 2H, 1JHF = 24.2 Hz, 3JHH = 7.5 Hz, 2JNH(quad coupl) = 2.4 Hz,
CH2—N), 4.56 (dtt, 2H, 2JHF = 46.6 Hz, 3JHH = 7.5 Hz,
3
JNH(quad coupl) = 2.5 Hz, CH2F). 13C NMR (CDCl3, 100 MHz)
δ 42.6 [1JCH = 144.6 (t), 2JCF = 22.5 Hz (d), 1JNC(quad coupl) =
8.0 Hz (t), CH2N], 80.1 [1JCH = 154.8 (t), 1JCF = 175.8 Hz (d),
CH2F], 158.9 [1JNC(quad coupl) = 5.5 Hz (t), NC]. 19F NMR
(CDCl3, 374.4 MHz) δ 223.2.
Microwave Experiment. The MW spectrum of 2-fluoroethylisocyanide was studied using the Stark-modulation MW spectrometer of the University of Oslo. Details on the construction and
operation of this device have been given elsewhere.9 11 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 whole 50 120 GHz frequency interval was
recorded. Selected regions of the 18 50 GHz were also investigated. Radio-frequency microwave double-resonance (RFMWDR)
experiments, similar to those performed by Wodarczyk and
Wilson,12 were conducted to unambiguously assign particular
transitions, using the equipment described elsewhere.9 The
spectra were measured at room temperature at a pressure of
roughly 10 Pa.
Quantum Chemical Methods. The present quantum chemical calculations were performed using the Gaussian 03 suite of
Figure 2. B3LYP/cc-pVTZ electronic potential function for rotation
about the C1—C2 bond of FCH2CH2NC. The N3—C2—C1—F5
dihedral angle is given on the abscissa, and the electronic energy relative
to the energy of ap is given on the ordinate. A dihedral N3—C2—C1—F5
angle of 0° corresponds to the synperiplanar conformation.
programs,13 running on the Titan cluster in Oslo. Becke’s threeparameter hybrid functional14 employing the Lee, Yang, and Parr
correlation functional (B3LYP)15 was employed in the density
functional theory (DFT) calculations. Coupled-cluster calculations with singlet and doublet excitations (CCSD)16,17 were also
undertaken. The CCSD calculations are very costly and were
accelerated by making use of a B3LYP force field that was calculated prior to the CCSD calculations. Peterson and Dunning’s18
correlation-consistent cc-pVTZ basis set, which is of triple-ζ quality,
was used in the calculations.
’ RESULTS AND DISCUSSION
Quantum Chemical Calculations. An electronic energy
potential function for rotation about the C1—C2 bond was
calculated at the B3LYP/cc-pVTZ level of theory using the scan
option of Gaussian 03. The energies were computed in steps of
10° of the F5—C1—C2—N3 dihedral angle. All remaining
structural parameters were optimized for each dihedral angle.
Separate calculations of the energies and optimized structures of
the conformers ap and sc and of the transition state near 120°
were also performed. The potential function based on the results
of these calculations is drawn in Figure 2. Its global minimum
occurs at the antiperiplanar (180°) conformation, corresponding
to the ap conformer. The sc rotamer has a F5—C1—C2—N3
dihedral angle of 68.1° and an electronic energy, which is
1.30 kJ/mol higher than that of ap. The two transition states
in Figure 2 at 0° and 121.2° have energies that are 26.3 and
12.5 kJ/mol, respectively, higher than the energy of ap. The fully
optimized B3LYP structures of ap and sc are listed in Table 1,
whereas the rotational constants calculated from these structures
are found in the footnotes of Tables 2 (ap) and 3 (sc).
Several additional B3LYP molecular parameters such as the
total electronic energies, dipole moments, fundamental normal
vibrational frequencies, quartic and sextic Watson S-reduction
centrifugal distortion constants,19 and vibration rotation R
constants20 were calculated for ap and sc. The electronic energies
are given in Table 1. The dipole moment components of these
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ARTICLE
Table 1. CCSD/cc-pVTZ and B3LYP/cc-pVTZ Structuresa,
Dipole Moments, and Energy Differences of FCH2CH2NC
CCSD
b
ap
Table 2. Spectroscopic Constantsa
Conformer of FCH2CH2NC
B3LYP
sc
ap
c
c
of the Antiperiplanar
experimental
vibrational state
sc
Bond Lengths (pm)
C1—C2
152.1
151.5
152.6
151.8
C1—F5
C1—H6
137.6
108.9
137.4
108.9
138.7
109.0
138.5
109.0
C1—H7
108.9
109.0
109.0
109.2
C2—N3
142.5
142.5
142.1
142.1
C2—H8
108.8
108.9
109.0
109.1
C2—H9
108.8
109.0
109.0
109.2
N3—C4
117.0
117.0
116.7
C2—C1—F5
108.0
109.9
C2—C1—H6
C2—C1—H7
110.7
110.7
F5—C1—H6
ν21 = 1
ground
theoretical
equilibrium
A (MHz)
26785.0(29)
25440.2(64)
27144.0
B (MHz)
2448.0699(56)
2447.2378(89)
2448.8
C (MHz)
Δd (10 20 u m2)
2308.2505(59)
6.3632(25)
2313.005(10)
7.8808(56)
2311.2
6.33
DJ (kHz)
0.5329(11)
0.5542(23)
DJK (kHz)
DK (kHz)
d1 (kHz)
21.508(10)
308.6e
0.0926(33)
22.063(36)
308.6e
0.0724(74)
0.429
3.143
308.6
0.0426
116.7
d2 (kHz)
0.0027(12)
0.0144(34)
HJ (Hz)
0.00039e
0.00039e
108.0
110.2
HJK (Hz)
HKJ (Hz)
0.060(10)
1.150(18)
0.097(38)
0.83(13)
110.8
109.3
111.0
111.0
111.0
109.3
HK (Hz)
0.598e
0.598e
0.598
h1 (Hz)
0.00010e
0.00010e
0.00010
109.0
108.5
108.6
108.2
h2 (Hz)
0.000023e
0.000023e
0.000023
F5—C1—H7
109.0
108.6
108.6
108.3
h3 (Hz)
0.000004e
0.000004e
0.000004
H6—C1—H7
109.5
109.8
109.6
109.8
RA (MHz)
1344.8
1361.5
C1—C2—N3
109.6
111.6
110.4
112.5
RB (MHz)
40.83
0.89
C1—C2—H8
110.1
109.8
109.8
109.6
RC (MHz)
rms f
4.34
1.44
4.76
1.67
no. transg
338
182
Angles (deg)
C1—C2—H9
110.1
109.4
109.8
108.9
N3—C2—H8
N3—C2—H9
109.2
109.2
108.9
108.7
109.3
109.3
108.8
108.8
H8—C2—H9
108.6
108.6
108.2
108.1
C2—N3—C4
177.6d
179.5e
177.9d
179.5d
180.0
68.1
Dihedral Angles (deg)
F5—C1—C2—N3
180.0
F5—C1—C2—H8
59.9
67.1
53.5
59.4
53.1
F5—C1—C2—H9
59.8
172.6
59.4
171.1
H6—C1—C2—N3
H6—C1—C2—H8
60.8
59.3
52.8
173.3
61.0
59.6
51.8
173.0
H6—C1—C2—H9
179.1
H7—C1—C2—N3
60.8
H7—C1—C2—H8
179.1
H7—C1—C2—H9
59.3
67.6
178.4
173.9
61.0
65.6
178.4
53.6
f
Dipole Moments (10
30
59.6
173.0
65.8
52.3
C m)
5.23
7.20
6.36
μb
0.03
12.36
2.23
12.15
μc
μtot
0.0g
5.23
0.87
14.33
0.0g
6.73
0.43
14.52
7.94
Electronic Energy Differenceh (kJ/mol)
1.57
0.0
0.00936
0.147
a
Experimental constants are Watson’s S reduction, Ir representation.19
Theoretical rotational constants were calculated from the CCSD
structure, whereas theoretical centrifugal distortion constants and
vibration rotation constants were obtained in B3LYP calculations.
The B3LYP rotational constants were A = 27405.8 MHz, B = 2426.6
MHz, and C = 2293.3 MHz. b Uncertainties represent one standard
deviation. c Spectra in Table 3S (ground state) and Table 4S (ν21; lowest
torsional vibration) of the Supporting Information. d Defined by Δ = Ic Ia
Ib, where Ia, Ib, and Ic are the principal moments of inertia. Conversion
factor = 505379.05 10 20 MHz u m2. e Fixed in the least-squares fit.
f
Root-mean-square of a weighted fit. g Number of transitions used in the fit.
68.9
μa
0.0
0.00505
0.00039
1.30
a
Atom numbering given in Figure 1. b Total electronic energy of ap =
711031.16 kJ/mol. c Total electronic energy of ap = 712473.31
kJ/mol. d Bent toward C1. e Bent away from C1. f Conversion factor:
1 debye = 3.33564 10 30 C m. g By symmetry. h Relative to the
energy of ap.
two forms were transformed from the standard orientation of
Gaussian 03 to the principal inertial axis system using Bailey’s
program axis.21 The principal dipole moments of the two forms
are listed in Table 1, the fundamental frequencies are listed in the
Supporting Information [Tables 1S (ap) and 3S (sc)], and the
centrifugal distortion constants are reported in Tables 2 (ap) and
3 (sc) together with experimental results. Finally, the R constants
are listed in the Supporting Information (Tables 2S and 4S). A
few of these are also included in Tables 2 and 3. The force field
obtained in these B3LYP calculations allowed the calculation of
the zero-point harmonic vibration energies. The energy difference between ap and sc corrected for this effect is 1.21 kJ/mol,
nearly the same as obtained above for the electronic energy
difference (1.30 kJ/mol; Table 1).
The B3LYP structures were used as starting points to calculate
optimized structures, dipole moments, and electronic field
gradients of ap and sc at the CCSD/cc-pVTZ level. CCSD
calculations are very costly, and the calculations were therefore
limited to these parameters.
The CCSD structures, dipole moments, and electronic energies are reported in Table 1. The rotational constants derived
from these structures are listed in Tables 2 and 3. The CCSD
method predicts that the electronic energy difference is 1.57 kJ/mol,
again favoring ap.
The electronic field gradients obtained in the CCSD calculations were used to calculate the principal-axis nuclear quadrupole
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The Journal of Physical Chemistry A
Table 3. Spectroscopic Constantsa
c
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of the Synclinal Conformer of FCH2CH2NC
experimental
theoretical
ground
ν21 = 1
ν20 = 1
A (MHz)
11876.4068(19)
11955.6950(48)
11859.3563(69)
11893.3
B (MHz)
3374.52794(56)
3376.3475(12)
3373.5715(27)
3391.8
C (MHz)
2864.01352(54)
2862.6175(12)
2864.4514(26)
2877.2
DJ (kHz)
4.57549(40)
4.5786(22)
4.4794(53)
4.22
vibrational state
DJK (kHz)
DK (kHz)
37.1813(34)
132.4869(24)
37.899(11)
36.679(11)
139.822(84)
128.386(89)
equilibrium
32.8
126
d1 (kHz)
1.40983(16)
1.42551(23)
1.42439(29)
1.29
d2 (kHz)
HJ (Hz)
0.090144(75)
0.019986(72)
0.097614(39)
0.02009(94)
0.091336(46)
0.0882(25)
0.0839
0.0123
HJK (Hz)
0.1061(19)
0.0812(50)
0.1006(51)
0.159
HKJ (Hz)
1.1637(20)
1.1637d
1.1637d
0.235
HK (Hz)
5.4779(50)
5.4779d
5.4779d
h1 (Hz)
0.00925(12)
0.009410(90)
0.01016(11)
0.0059
h2 (Hz)
0.001735(94)
0.001735d
0.001735d
0.00055
h3 (Hz)
0.000173(24)
0.0d
0.000173d
0.00014
RA (MHz)
RB (MHz)
79.28 ( 82.5)
1.82 ( 1.32)e
RC (MHz)
rms
f
no. of transg
1.40 (2.02)e
e
3.15
17.05 (18.2)e
0.96 ( 2.24)e
0.44 ( 3.26)e
1.60
1.71
1.73
1177
440
261
a
Experimental constants are Watson’s S reduction, Ir representation.19 The theoretical rotational constants were calculated from the CCSD structure,
whereas the theoretical centrifugal distortion constants and vibration rotation constants were obtained in B3LYP calculations. The B3LYP rotational
constants were A = 12031.0 MHz, B = 3320.4 MHz, and C = 2834.4 MHz. b Uncertainties represent one standard deviation. c Spectra in Table 7S
(ground state), Table 8S (ν21; lowest torsional vibration), and Table 9S (ν20; lowest bending vibration) of the Supporting Information. d Fixed in the
least-squares fit. e B3LYP values; see text. f Root-mean-square of a weighted fit. g Number of transitions used in the fit.
coupling constants of the nitrogen nucleus by means of Bailey’s
program nqc.21 The results were χaa = 0.066, χbb = 0.063, and
χab = 0.004 MHz for ap and χaa = 0.015, χbb = 0.021, and χab =
0.074 MHz for sc. These quadrupole coupling constants are
comparatively small and would not result in a resolved quadrupole hyperfine structure because the resolution of our spectrometer is about 0.5 MHz. The small quadrupole coupling constants obtained in these calculations are typical for isocyanides.
The quadrupole coupling constant of the nitrogen nucleus of
CH3NC, for example, is 0.4894(4) MHz.22
The results of these calculations warrant further comments.
Inspection of Table 1 reveals that there are only small differences
in the CCSD and B3LYP structures of the ap and sc conformers,
as the bond lengths agree to within better than 1 pm and the
angles and dihedral angles agree to within 1° or better. The
calculations of two of the structural parameters, namely, the C—F
and NtC bond lengths, are critical in quantum chemistry,
and a comparison with equilibrium structures of similar
compounds is in order. The equilibrium C—F bond length is
138.3(1) pm in CH3F,23 for example, and the equilibrium
NtC bond length is 116.83506(16) pm in H—NtC.24
These two bond lengths are close to their CCSD counterparts
in 2-fluoroethylisocyanide (Table 1), which testifies to the
quality of the calculations.
Interestingly, the CCSD and B3LYP dipole moments (Table 1)
differ somewhat despite the great similarity of the two theoretical
structures. The Mulliken charges obtained in the CCSD and
B3LYP calculations (not included in Table 1) are also quite
different and might explain the variation in the dipole moment.
ap has a symmetry plane (Cs symmetry), whereas the
F5—C1—C2—N3 dihedral angle in sc is 7 8° larger than
the ideal 60° of a synclinal conformer. The increase of this
dihedral angle by 7 8° might indicate a repulsive interaction
between the fluorine atom and the isocyanide group. Interestingly, the C1—C2—N3 angle is about 2° larger in sc than in ap,
which is also indicative of a slight repulsive interaction. Interestingly,
the F—C—C—F dihedral angle in the synclinal conformation of
1,2-difluoroethane is 71.0(3)°,25 11° larger than the ideal value.
The energy difference between the two forms is predicted to
be small, 1.6 kJ/mol in the CCSD calculations and 1.3 kJ/mol in
the B3LYP calculations. These energy differences are similar to
the results obtained in lower-level density theory calculations,6
where the B3LYP/6-311+G(d,p) calculations yielded 1.0 kJ/mol
and M05-2X/6-311+G(d,p) calculations predicted 1.2 kJ/mol
for this energy difference, with sc as the high-energy form.
Microwave Spectrum and Assignment of the Spectrum of
ap. The small theoretical energy difference between sc and ap
indicates that both of these forms should be present in the gas in
considerable quantities. ap has its major dipole moment component along the a axis, whereas sc has a predominating μb. The
perpendicular b-type spectra of prolate asymmetrical tops, such
as sc, are rich with absorption lines occurring throughout the
investigated spectral region, whereas a-type lines of highly
prolate rotors such as ap are primarily found in pileups of
R-branch regions separated by the sum of the B + C rotational
constants.
Survey spectra revealed a rich MW spectrum with absorption
lines occurring every few megahertz throughout the investigated
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The Journal of Physical Chemistry A
Figure 3. Microwave spectrum of a portion of the J = 23 r 22 pileup
region of ap taken at a Stark field strength of about 110 V/cm. The
numbers above the peaks indicate the values of the K 1 pseudoquantum numbers.
spectral range, which was taken as an early indication that both ap
and sc were present in significant concentrations.
The theoretical predictions discussed above indicate that ap
would perhaps be the preferred form of 2-fluoroethylisocyanide,
and searches were therefore first made for the spectrum of this
conformer. ap is predicted (Table 1) to have Ray’s asymmetry
parameter26 k ≈ 0.99 and a major μa value of about (5 6) 10 30 C m (Table 1). The pileups of this spectrum should be
separated by B + C ≈ 4.7 GHz and involve transitions K 1 pairs
with K 1 > 2. These high-K 1 transitions should have rapid Stark
effect caused by the near-degeneracy of the K 1 pairs. This pileup
feature was readily recognized in the survey spectra taken at a
Stark field strength of roughly 110 V/cm, where K 1 > 3
transitions are fully modulated whereas numerous other transitions are not seen at all. An example of a portion of one of these
pileups, J = 23 r 22, is shown in Figure 3. RFMWDR experiments were performed next, and unambiguous assignments of
several of the K 1 pairs were achieved in this manner.
The spectrum was fitted to Watson’s S-reduction Hamiltonian,19
which was chosen because ap is nearly a symmetrical rotor.
Sørensen’s program Rotfit27 was used to least-squares fit the
transitions. Accurate values of the DJK and d1 S-reduction quartic
centrifugal distortion constants19 are generally very useful in
facilitating the assignments of the further high-K 1 pairs in the
pileup regions. Unfortunately, the B3LYP values of these two
constants shown in Table 3 were too inaccurate to be helpful in
the present case, and the assignments were obtained by employing a trial-and-error procedure. The failure to predict DJK and d1
accurately might be due to a comparatively inaccurate B3LYP
force field.
The said assignments were gradually extended to include
additional aR transitions. b-type lines were sought but not
assigned, presumably because they are too weak, which is not
surprising given the small μb component (Table 1). A total of 338
a
R transitions, which are listed in Table 5S in the Supporting
Information, were ultimately used to determine the spectroscopic constants listed in Table 2. The inverse squares of the
uncertainties listed in Table 5S (Supporting Information) were
ARTICLE
used as weights in the least-squares fit. It was not possible to get
accurate values for all of the spectroscopic constants from the
selection of aR-branch lines assigned here, and several of them
were preset to the B3LYP values in the least-squares fit.
It is seen from Table 2 that it was possible to determine
accurate values for the rotational constants; the DJ, DJK, and d1
quartic centrifugal distortion constants; and one sextic constant,
HKJ. There is good agreement between the experimental and
B3LYP centrifugal distortion constant DJ. It is also noted
(Table 2) that Δ = Ic Ia Ib = 6.3632(25) 10 20 u m2,
where Ia, Ib, and Ic are the principal moments of inertia. This
value is characteristic of a compound having a symmetry plane
and two pairs of sp3-hybridized out-of-plane hydrogen atoms.
The three experimental rotational constants furnish insufficient information for a complete determination of the geometrical structure of ap. The experimental B and C rotational
constants are very close to the CCSD counterparts (Table 2).
There is also a good agreement for the A rotational constant. The
effective experimental rotation constants are associated with the
r0 structure, whereas the CCSD rotational constants were
derived from an approximate equilibrium structure. A direct
comparison of the two sets of constants is therefore not
warranted, but the two structures are in general similar. The
good agreement between the experimental and CCSD rotational
constants is therefore taken as an indication that the CCSD
structure of Table 1 is indeed close to the equilibrium structure.
Vibrationally Excited State of ap. The lowest fundamental
vibration (ν21) has a harmonic frequency of 106 cm 1 according
to the B3LYP results (Table 1S of the Supporting Information).
This mode is the torsion about the C1—C2 bond. A total of 182
transitions were assigned for the spectrum of the first excited
state of this mode in the same manner as described above for the
ground-vibrational-state spectrum. The spectrum of this excited
state is listed in Table 6S in the Supporting Information, whereas
the spectroscopic constants are included in Table 2.
The vibration rotation R constants of this vibrational mode
were calculated20 from RX = X0 X1, where X0 and X1 are the
rotational constants of the ground and vibrationally excited
states, respectively, with the results shown in Table 2. It is seen
that the agreement between the experimental and theoretical RA
and RC values are in quite good agreement, whereas a larger
discrepancy is found for RB.
The increase of the absolute value of Δ from 6.3632(25) in the
ground vibrational state to 7.8808(56) 10 20 u m2 for the first
excited state of ν21 (Table 2) is typical for an out-of-plane
vibration such as torsion28 about the C1—C2 bond. Relative
intensity measurements performed largely as described by Esbitt
and Wilson29 yielded 93(15) cm 1, compared to the B3LYP
harmonic and anharmonic frequencies of 106 and 114 cm 1,
respectively (Table 1S, Supporting Information).
Assignment of the Spectrum of sc. This rotamer has a
comparatively large μb value and a significant μa value, ∼12 10 30 and 7 10 30 C m, respectively, according to the
theoretical calculations (Table 1). The theoretical spectroscopic
constants listed in Table 3 were first used to predict the aR
spectrum of this rotamer and subsequently followed by
RFMWDR experiments, which provided the first assignments
of such transitions. Searches for strong bQ lines were then
undertaken and soon met with success. bR-branch lines were
assigned next. The assignments were then gradually extended to
include transitions with higher and higher values of the J
quantum number. Searches for c-type lines were made, but none
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The Journal of Physical Chemistry A
were found, presumably because μc is so small (Table 1),
producing insufficient intensities for these lines. Ultimately, a
total of 1177 a- and b-type lines with J values up to 70 and K 1
values up to 30 were assigned. These transitions were used to
determine the S-reduction spectroscopic constants listed in
Table 3 from the spectrum shown in Table 7S in the Supporting
Information.
Transitions with high J and high K 1 generally have large
centrifugal distortion contributions of several gigahertz (Table 7S,
Supporting Information). It was therefore possible to obtain
accurate values for not just the five quartic but also the seven
sextic centrifugal distortion constants. Comparison of the theoretical centrifugal distortion constants with the B3LYP counterparts (Table 3) reveals a better than 10% agreement in the case of
the quartic constants, whereas the sextic constants are seen to
have an order-of-magnitude agreement.
The CCSD and experimental rotational constants of Table 3
agree to within better than about 0.5%, which is again taken as an
indication that the CCSD structure in Table 1 is close to the
equilibrium structure.
Vibrationally Excited States of sc. The spectra of two
vibrationally excited states belonging to the first excited state
of the torsion about the C1—C2 bond (ν21) and the first excited
state of the lowest bending vibration (ν20) were assigned in the
same manner as described for the ground-state spectrum. A total
of 440 transitions with a maximum of J = 49 were assigned for the
first excited state of the torsion, whereas 261 transitions with a
maximum of J = 47 were assigned for the first excited state of the
lowest bending vibration. The spectra are reported in Tables 8S
and 9S of the Supporting Information, whereas the spectroscopic
constants are listed in Table 3.
The B3LYP calculations predict harmonic frequencies of 106
and 193 cm 1 for these vibrations (Table 4S, Supporting Information), which are the torsion about the C1—C2 bond and the
lowest bending vibration, respectively. Relative intensity measurements yielded 96(15) cm 1 for the torsion, and a rough
value of ca. 180 cm 1 was obtained for the bending vibration.
The experimental vibration rotation constants are also listed
in Table 3 and compared to the B3LYP values (Table 4S, Supporting Information). There is a satisfactory agreement in the
case of the lowest torsional vibration, whereas somewhat larger
deviations are seen for the lowest bending vibration.
Energy Difference. The energy differences between the
ground vibrational states of the sc and ap rotamers were obtained
by comparing the intensities of selected rotational lines observing
the precautions of Esbitt and Wilson.29 The energy differences
were calculated as described by Townes and Schawlow.30 ap was
assigned a statistical weight of 1 because of its symmetry plane,
whereas sc was assumed to have a statistical weight of 2 because
of the existence of two mirror forms. The CCSD dipole moments
were employed.
sc was found be 0.7(5) kJ/mol more stable than ap in the
present relative intensity measurements. The CCSD and
B3LYP calculations above, as well as the previous DFT
calculations,6 predict the opposite, namely, that ap is more
stable than ac by 1.2 1.6 kJ/mol, a result that differs by
approximately 2 kJ/mol from that obtained in this experiment
by relative intensity measurements. It is not surprising that the
quantum chemical calculations underestimate the magnitude
of the gauche effect in 2-fluoroethylisocyanide because the
quantum chemical methods used here involve approximations
of various kinds.
ARTICLE
’ DISCUSSION
The fact that sc is preferred by 0.7(5) kJ/mol over ap is clearly
a compromise of several intramolecular interactions. Steric
repulsive forces presumably play some role in destabilizing sc,
as the CCSD nonbonded distance between the fluorine and
nitrogen atom is 289 pm, compared to 305 pm, which is the sum
of the Pauling van der Waals radius of fluorine (135 pm)31 and
the half-thickness of an aromatic molecule (170 pm).31 Steric
forces should therefore slightly favor ap because of the comparatively short nonbonded distance between the fluorine atom and
the nitrogen atom in sc.
Another factor that would greatly favor ap over sc is dipole
dipole repulsion, which must be important in sc, because the
negative end of the very polar C—F bond and the isocyanide
groups come quite close in this rotamer. This interaction is
minimized in ap. It has been claimed6 that yet another force,
namely, electrostatic repulsion between the fluorine atom and
the p orbital of the triple bond, destabilizes sc.
It has been advocated that the forces that destabilize sc relative
to ap are countered by hyperconjugation6 that occurs between
the bonding σ orbital of the C2—H bond and the antibonding
σ orbital of the C1—F5 bond on one hand and between the σ
orbital of the C1—H bond and the antibonding σ orbital of the
C2—N3 bond, when these bonds are antiperiplanar to one another.
The hyperconjugation interactions must outweigh the steric and
electrostatic repulsions in sc, and this is possibly the most important
reason why sc is 0.7(6) kJ/mol more stable than ap.
’ ASSOCIATED CONTENT
bS
Supporting Information. Results of the theoretical calculations and microwave spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Tel.: +47 2285 5674. Fax: +47 2285 5441. E-mail: harald.
mollendal@kjemi.uio.no.
’ ACKNOWLEDGMENT
We thank Anne Horn for her skillful assistance. The Research
Council of Norway (Program for Supercomputing) is thanked
for a grant of computer time. J.-C.G. thanks the PID EPOV and
PCMI (INSU-CNRS) for financial support.
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