ARTICLE
pubs.acs.org/JPCA
Microwave Spectrum and Conformational Composition of
3-Fluoropropionitrile (FCH2CH2CN)
Harald Møllendal,*,† Svein Samdal,† 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 3-fluoropropionitrile, FCH2CH2CtN,
has been investigated in the whole 17 75 GHz spectral region. Selected portions of
the spectrum in the 75 95 GHz have also been recorded. The microwave spectra
of the ground state as well as of three vibrationally excited states of each of two
conformers have been assigned. The spectra of the vibrationally excited states
belong to the lowest torsional and bending vibrations. The F C C C chain of
atoms is exactly antiperiplanar in one of these rotamers and synclinal in the second conformer. The F C C C dihedral angle is 65(2)°
in the synclinal form. The energy difference between the two forms has been obtained from relative intensity measurements performed on
microwave transitions. It was found that the antiperiplanar conformer is more stable than the synclinal form by 1.4(5) kJ/mol. It is argued
that the gauche effect is a significant force in this compound. Quantum chemical calculations at the high CCSD(full)/cc-pVTZ,
MP2(full)/cc-pVTZ, and B3LYP/cc-pVTZ levels of theory have been performed. Most, but not all, of the theoretical predictions are in
good agreement with experiment.
’ INTRODUCTION
1,2-Ethane derivatives, XCH2CH2Y, exist as a mixture of
X C C Y antiperiplanar (obsolete: trans) and synclinal
(obsolete: gauche) conformers. It was observed a long time ago
that XCH2CH2Y compounds with highly electronegative substituents X and Y often prefers synclinal conformations in spite of the significant electrostatic repulsion between the X and Y substituents
that exists in these compounds.1 The best example of this so-called
gauche effect1 is perhaps 1,2-difluoroethane, FCH2CH2F,
where the synclinal form has been found to be preferred by
3.9(17) kJ/mol in one gas electron-diffraction study,2 while
7.5(21) kJ/mol was reported in a second such study.3 Recent
density functional theory (DFT) calculations4 at the B3LYP/
6-311+G(d,p) and M05-2X/6-311+G(d,p) levels of theory
yielded 3.4 and 2.8 kJ/mol, respectively, for this energy difference.
The present article deals with the conformational properties of
3-fluoropropionitrile (F CH2 CH2 CtN). The F C C C
dihedral angle can conveniently be used to characterize the conformational properties of this compound. A model of its antiperiplanar and synclinal 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.
3-Fluoropropionitrile contains the very electronegative fluorine atom (Pauling electronegativity: 3.985) and the electronegative cyano group. This electronegative tendency is also reflected
in the C F and the cyano group bond moments, which are 4.7
and 11.7 10 30 C m, respectively.6 The bond moments have
r 2011 American Chemical Society
their negative ends on the fluorine end of the C F bond and on
the nitrogen end of the CtN bond. The question is will the
gauche effect prevail leaving the synclinal conformer the more
stable, or will repulsive forces have the upper hand producing a
more stable antiperiplanar form?
Very recently we investigated the MW spectrum of 2-fluoroethylisocyanide, FCH2CH2NtC,7 which contains another very
polar and similar group, namely, the isocyanide group, NtC,
which has its negative end on the carbon atom. The synclinal
form was found to be the slightly more stable by 0.7(5) kJ/mol in
this case, a clear demonstration of the importance of the gauche
effect. The question is now will the gauche effect in the cyanide
FCH2CH2CtN be more or less important than in the isocyanide FCH2CH2NtC?
No experimental conformational studies of gaseous FCH2CH2CN
are available, but the compound in the liquid and solid states has
been subject to an infrared and Raman study more than half a
century ago.8 No attempt was made in this study8 to measure the
enthalpy difference of the two conformers in the liquid phase, but
a value close to zero was estimated, although the synclinal form
appeared to be the more stable conformer. B3LYP/6-311+G
(d,p) and M05-2X/6-311+G(d,p) calculations4 both predicted
2.7 kJ/mol for this energy difference of 3-fluoropropionitrile with
sc as the high energy form.
Received: November 14, 2011
Revised:
December 15, 2011
Published: December 15, 2011
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dx.doi.org/10.1021/jp210932k | J. Phys. Chem. A 2012, 116, 1015–1022
The Journal of Physical Chemistry A
ARTICLE
Figure 1. Antiperiplanar (ap) and synclinal (sc) conformers of FCH2
CH2CN. Atom numbering is indicated on ap, which was found to be
1.4(5) kJ/mol more stable than sc by relative intensity measurements
performed on microwave transitions.
The fact that no experimental information of conformational
properties in the gas phase exists for the title compound was
another reason to perform the present microwave (MW) investigation of this 1,2-ethane derivative. MW spectroscopy is an
ideal method for the study of conformational equilibria due to 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.
’ EXPERIMENTAL SECTION
Synthesis of 3-Fluoropropionitrile. The synthesis of 3-fluoropropionitrile has already been performed by dehydration of the
corresponding amide.9 This compound was found in the photodifluoramination of cycloalkanes10 and in the nitrosative decomposition of azido nitriles.11 We prepared this species on a
half-gram scale in a 92% yield by flash vacuum thermolysis of the
2-fluoroethylisocyanide7 at 650 °C as previously reported for
allylisocyanide.12 The product was collected in pure form in a
U-trap equipped with stopcocks and immersed in a 90 °C bath.
NMR data of 3-fluoropropionitrile: 1H NMR (CDCl3, 400 MHz)
δ 2.78 (dt, 2H, 3JHH = 6.9 Hz, 3JHF = 21.8 Hz, CH2CN); 4.63 (dt,
2H, 3JHH = 6.9 Hz, 2JHF = 46.0 Hz, CH2F). 13C NMR (CDCl3,
100 MHz) δ 19.7 (1JCH = 135.7 Hz, 2JCF = 24.0 Hz (d), CH2CN);
78.0 (t, 1JCH = 155.4 Hz (t), 1JCF = 175.8 Hz (d), CH2F); 116.7
(3JCF = 7.3 Hz (d), CN). 19F NMR (CDCl3) δ 216.9.
Microwave Experiment. The MW spectrum of 3-fluoropropionitrile was studied using the Stark-modulation MW spectrometer of the University of Oslo. Details of the construction and
operation of this device have been given elsewhere.13 15 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 17 75 GHz frequency interval was
recorded. Selected regions of the 75 95 GHz were also investigated. Radio-frequency microwave double-resonance experiments (RFMWDR), similar to those performed by Wodarczyk
and Wilson,16 were conducted to unambiguously assign particular transitions, using the equipment described elsewhere.13
The vapor pressure of the compound was roughly 60 Pa at room
temperature. The spectra were measured at room temperature at
a pressure of roughly 10 Pa.
Quantum Chemical Methods. The present quantum chemical calculations were performed employing the Gaussian09 suite
Figure 2. B3LYP/cc-pVTZ (circles; red) and MP2/cc-pVTZ (squares;
blue) electronic potential functions for rotation about the C1 C2 bond
of FCH2CH2CN. The F3 C2 C1 C6 dihedral angle is given on the
abscissa, and the electronic energies relative to the energy of ap is given
on the ordinate. Each curve has its global minimum at 180° for the
F3 C2 C1 C6 dihedral angle corresponding to ap. The sc rotamer is
calculated by the B3LYP method to have a dihedral F3 C2 C1 C6
angle of 66.3° and an electronic energy that is 2.76 kJ/mol higher
than the energy of ap. The corresponding MP2 values are 64.6° and
2.33 kJ/mol. The transition states are located at 0 and 119.2° in both
methods of calculation. The B3LYP energies of the first and second
transition state are 13.07 and 22.45 kJ/mol, respectively, relative to the
electronic energy of ap. The corresponding MP2 values are 15.32 and
23.58 kJ/mol.
of programs17 running on the Titan cluster in Oslo. Becke’s
three-parameter hybrid functional18 employing the Lee Yang
Parr correlation functional (B3LYP)19 were employed in the
DFT calculations. Møller Plesset second order perturbation
calculations (MP2)20 as well as coupled-cluster calculations with
singlet and doublet excitations (CCSD)21,22 were also undertaken. Both frozen-core and all-electron calculations [MP2(full))
and CCSD(full)] were performed with the last two methods.
The CCSD(full) calculations are very costly and were speeded up by making use of a B3LYP force field that was calculated
prior to the CCSD(full) calculations. Peterson and Dunning’s23
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 and MP2/cc-pVTZ levels of
theory employing the scan option of Gaussian09. The energies
were computed in steps of 10° of the F3 C2 C1 C6 dihedral
angle. All the 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 functions
based on the results of these calculations are drawn in Figure 2.
The global minimum of each of the two calculations occurs at the
antiperiplanar (180°) conformation, corresponding to the ap
conformer. The sc rotamer has a F3 C2 C1 C6 dihedral
angle of 66.3° and an electronic energy, which is 2.76 kJ/mol
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dx.doi.org/10.1021/jp210932k |J. Phys. Chem. A 2012, 116, 1015–1022
The Journal of Physical Chemistry A
ARTICLE
Table 1. CCSD(full)/cc-pVTZ and MP2(full)/cc-pVTZ
Structuresa of FCH2CH2CN
method
conformer
CCSD(full)
ap
sc
Table 2. CCSD(full)/cc-pVTZ and MP2(full)/cc-pVTZ
Spectroscopic Constants of FCH2CH2CN
MP2(full)
method
ap
sc
conformer
CCSD(full)
a
ap
Bond Length (pm)
MP2(full)
sc
ap
b
sc
Rotational Constants (MHz)
C1 C2
151.7
151.3
151.1
151.2
A (MHz)
27815.6
11721.2
27580.0
11580.8
C2 F3
C2 H4
137.3
108.5
137.2
108.5
137.7
108.4
137.6
108.4
B (MHz)
C (MHz)
2305.5
2186.8
3223.9
2738.2
2307.9
2187.5
3248.6
2748.5
C2 H5
108.5
108.5
108.4
108.4
C1 C6
145.8
146.0
145.1
145.3
C6 N7
115.2
115.1
116.6
116.6
C1 H8
108.5
108.6
108.5
108.5
C1 H9
108.5
108.7
108.5
Planar Moments (10
Pccc
Dipole Moment (10
Angle (deg)
F3 C2 C1
108.2
109.5
108.3
109.4
108.7
108.7
108.4
108.4
108.7
108.7
108.4
108.4
C1 C2 H4
110.9
111.1
110.8
111.0
C1 C2 H5
110.9
109.7
110.8
109.8
H4 C2 H5
109.4
109.6
109.4
109.7
C2 C1 C6
110.9
112.4
110.6
109.7
109.6
109.6
109.4
C2 C1 H9
109.7
109.4
109.6
109.3
H8 C1 H9
C6 C1 H8
108.1
109.2
108.0
108.9
108.1
109.4
107.9
109.1
C6 C1 H9
109.2
108.9
109.4
108.9
H8 C1 H9
108.6
108.6
108.1
107.9
C1 C6 N7
178.7b
179.9
178.3b
179.7
F3 C2 C1 C6
180.0
180.0
64.5
F3 C2 C1 H8
59.3
56.2
59.3
56.7
F3 C1 C2 H9
H4 C2 C1 C6
59.3
60.9
174.4
54.9
59.3
60.8
174.7
55.1
H4 C2 C1 H8
59.8
176.0
59.9
H4 C2 C1 H9
178.5
u m2)
3.14
30
7.67
C m)
μa
7.21
9.40
6.36
μb
0.50
13.23
2.33
13.38
μc
0.0e
0.66
0.0e
0.676
7.22
16.24
7.21
16.40
μtot
9.45
14
N Nuclear Quadrupole Coupling Constants (MHz)
112.1
C2 C1 H8
7.65
d
108.7
F3 C2 H4
F3 C2 H5
3.14
20
χaa
χbb
3.683
1.508
χcc
2.175
χab
2.307
χac
0.0d
2.074
0.035
2.109
3.170
0.878
3.261
1.283
1.978
1.817
0.063
1.880
2.074
2.832
0.0d
0.772
Electronic Energy Differencef (kJ/mol)
0.0
a
2.23
2.01g
0.0
b
Total electronic energy of ap: 711319.92 kJ/mol. Total electronic
energy of ap: 711265.78 kJ/mol. c Conversion factor: 505379.05 10 20 MHz u m2. d 1 debye = 3.33564 10 30 C m. e By symmetry.
f
Relative to the energy of ap. g The energy difference corrected for zeropoint vibrational energies: 2.14 kJ/mol.
Dihedral Angle (deg)
H5 C2 C1 C6
a
60.9
H5 C2 C1 H8
178.5
H5 C2 C1 H9
59.8
64.9
65.8
176.2
62.5
55.5
178.4
60.8
178.4
59.9
176.3
65.7
176.7
62.2
55.8
Atom numbering given in Figure 1. b Bent toward C2.
higher than that of ap according to the B3LYP predictions,
whereas the MP2 method indicates 64.6° and 2.33 kJ/mol for the
dihedral angle and energy difference. The two transition states
appear at 0 and 119.2° in both methods of calculations. The
energies of the first of these barrier heights (0°) are 22.45
(B3LYP) and 23.58 kJ/mol (MP2), respectively, relative to the
energy of ap, whereas the corresponding relative energies are
13.07 (B3LYP) and 15.32 kJ/mol (MP2) for the second (119.2°)
maximum.
B3LYP calculations are much less costly than MP2 or CCSD
calculations and this method was therefore used to calculate
several additional molecular parameters such as the harmonic
and anharmonic fundamental normal vibrational frequencies,
quartic and sextic Watson S-reduction centrifugal distortion constants,24 and the vibration rotation α constants25 for ap and sc.
The vibrational frequencies are found in Tables 1S (ap) and 3S
(sc) of the Supporting Information, while the vibration rotation
constants are listed in Tables 2S (ap) and 4S (sc). The B3LYP Sreduction24 quartic centrifugal distortion constants are shown in
Table 3 (ap), while quartic and sextic constants of sc are listed in
Table 4.
Calculations of the structures of ap and sc, their dipole moment components, and 14N nuclear quadrupole coupling constants
were repeated at the CCSD(full)/cc-pVTZ and MP2(full)/
cc-pVTZ levels of theory using the B3LYP geometries as starting
points. The harmonic vibrational frequencies for these two conformers were calculated only at the MP2(full) level of theory because similar CCSD(full) calculations are too costly. The structures of the two conformers are shown in Table 1, whereas the
rotational constants, dipole moments, nuclear quadrupole coupling
constants, and electronic energy differences are listed in Table 2.
The CCSD(full) rotational constants are repeated in Tables 3
and 4 for convenient comparison with the experimental counterparts. The planar moments, Pcc, defined by Pcc = 1/2(Ia + Ib Ic)
where Ia, Ib, and Ic, the principal moments of inertia, have been
calculated from the rotational constants. These moments, which
are sensitive to nonplanarity, are shown in Table 2.
The dipole moment components of these two forms were
transformed from the standard orientation of Gaussian09 to the
principal inertial axis system using Bailey’s program axis.26 The
force field obtained in these MP2(full) calculations allowed
the calculation of the zero-point harmonic vibration energies.
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Table 3. Spectroscopic Constantsa
c
ARTICLE
of the Antiperiplanar Conformer of FCH2CH2CN
exptl
vib state
theor
ν21 = 2
ν21 = 1
ground
ν13 = 1
equilibrium
A (MHz)
27009(11)
25941(10)
24964(18)
28076(19)
27815.6
B (MHz)
2292.0710(76)
2292.7496(43)
2293.7311(74)
2298.5657(69)
2305.5
2170.9069(76)
2175.6449(49)
2180.5390(86)
2173.8208(85)
2186.8
3.2028(44)
3.8089(38)
4.404(8)
2.692(6)
3.14
0.4600(25)
0.4684(22)
0.4671(33)
0.4541(31)
0.393
C (MHz)
Pccd (10
20
u m2)
DJ (kHz)
DJKe (kHz)
18.167(8)
17.432(9)
αA (MHz)
16.546(16)
18.537(13)
1068 [1178]f
αB (MHz)
αC (MHz)
2.98
1067 [1079]f
0.68 [ 0.94]f
4.74 [ 4.11]f
6.49[ 5.45]f
2.91[ 2.02]f
rmsg
1.653
1.468
1.702
1.453
no. transh
157
158
89
88
a
The experimental constants are Watson’s S reduction, Ir representation.24 The theoretical rotational constants are calculated from the CCSD
structure, whereas the theoretical centrifugal distortion constants and vibration rotation constants were obtained in the B3LYP calculations.
b
Uncertainties represent one standard deviation. c See spectra in Tables 5S (ground state), 6S (ν21 = 1), 7S (ν21 = 2), and 8S (ν13 = 1) (Supporting
Information). d Defined by Pcc = 1/2(Ia + Ib Ic) where Ia, Ib, and Ic are the principal moments of inertia. Conversion factor: 505379.05 10 20 MHz u m2.
e
Further quartic centrifugal distortion constants fixed in the least-squares fit at the B3LYP values DK = 307, d1 = 0.0388, and d2 = 0.00301 kHz; see text.
f
B3LYP values in parentheses; see text. g Root-mean-square deviation of a weighted least-squares fit. h Number of transitions used in the fit.
Table 4. Spectroscopic Constantsa
c
of the Synclinal Conformer of FCH2CH2CN
exptl
ν21 = 2
ν20 = 1
equilibrium
11729.819(15)
3176.1124(42)
11827.46(11)
3173.531(10)
11629.763(18)
3183.4204(75)
11721.2
3223.9
2705.7206(11)
2702.3524(43)
2698.7627(97)
2709.2852(72)
2738.2
3.90873(88)
3.861(13)
3.758(29)
4.069(29)
3.51
vib state
ground
A (MHz)
B (MHz)
11635.6040(46)
3178.2835(12)
C (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
theor
29.6286(82)
111.260(11)
ν21 = 1
30.252(37)
30.10(18)
113.85(76)
120(10)
29.77(13)
114.4(19)
d1 (kHz)
1.17322(54)
1.1713(11)
1.2293(43)
1.1944(23)
d2 (kHz)
0.07975(15)
0.08455(56)
0.0848(15)
0.08155(94)
HJ (Hz)
HJK (Hz)
HKJ (Hz)
0.01222(19)
0.0101d
24.2
98.9
1.03
0.0839
0.00649
0.0101
1.220(14)
0.815
HK (Hz)
5.196(35)
h1 (Hz)
0.00718(23)
0.00342
h2 (Hz)
0.000623d
0.000623
h3 (Hz)
0.000110d
αA (MHz)
3.49
0.000110
94.22 [ 95.36]e
αB (MHz)
αC (MHz)
5.50 [ 4.99]e
2.17 [3.24]e
3.37 [3.87]e
5.10[ 1.59]e
3.57[ 1.58]e
rmsf
1.604
1.907
2.151
1.682
no. transg
354
114
34
59
a
The experimental constants are Watson’s S reduction, Ir representation.24 The theoretical rotational constants are calculated from the CCSD
structure, whereas the theoretical centrifugal distortion constants and vibration rotation constants were obtained in the B3LYP calculations.
b
Uncertainties represent one standard deviation. c See spectra in Tables 9S (ground state), 10S (ν21 = 1), 11S (ν21 =2), and 12S (ν20 = 1) of the
Supporting Information. d Fixed in the least-squares fit; see text. e B3LYP values in parentheses; see text. f Root-mean-square deviation of a weighted
least-squares fit. g Number of transitions used in the fit.
The energy difference between ap and sc corrected for this effect
is 2.14 kJ/mol, nearly the same as obtained for the electronic
energy difference (2.01 kJ/mol; Table 2).
The results of these calculations warrant further comments.
Inspection of Table 1 reveals that there are only small differences
in the CCSD(full) and MP2(full) structures of the ap and sc
conformers. Interestingly, both methods predicts that the F3
C2 C1 C6 dihedral angle is practically 65° in sc, almost the
same as in the corresponding isonitrile, FCH2CH2NC, which has
a F C C N dihedral angle of 67° in its synclinal conformer.7
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The Journal of Physical Chemistry A
The present increase of the F3 C2 C1 C6 dihedral angle of
sc from the canonical 60 to 65° may indicate that repulsive forces
are of importance in this rotamer. The increase of the C2 C1
C6 angle by 1.5° in the sc form compared to the ap conformer
(Table 1) is pointing in the same direction. Interestingly, the F
C C F dihedral angle in the synclinal conformation of the
gauche-effect prototype, namely, 1,2-difluoroethane, is 71.0(3)°,27
11° larger than the ideal value.
A significant difference between the CCSD(full) and MP2(full) predictions is seen for the C6tN7 bond length, which is
calculated to be 1.5 pm longer in the MP2(full) calculations. The
equilibrium bond length of the CtN bond in CH3CN is
115.6(2) pm,28 which is somewhat closer to the CCSD(full)
values (115.2 and 115.1 pm; Table 1) than to the MP2 values
(116.6 pm; Table 1).
The calculation of the C F bond length is critical in quantum
chemistry. The equilibrium C F bond length is for example
138.3(1) pm in CH3F,29 values in the 137.2 137.7 pm range are
found in the present calculations; see Table 1. These theoretical
values are close to the experimental counterpart, which is reassuring.
There is not much difference in the theoretical rotational constants of the two calculations (Table 2), which is a consequence
of the similarity of the CCSD(full) and MP2(full) structures.
This is also the case for the dipole moments and 14N nuclear
quadrupole coupling constants.
The energy difference between the two forms is predicted to
be small, 2.23 kJ/mol in the CCSD(full) and 2.01 kJ/mol in the
MP2(full) calculations (Table 1). These energy differences are
similar to the results obtained in much lower-level DFT calculations,4 where the B3LYP/6-311+G(d,p) and M05-2X/6-311+G(d,p)
calculations both predict 2.7 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
indicated 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 pile-ups consisting of aRlines separated approximately by the sum of the B + C rotational
constants.
Survey spectra revealed a rich MW spectrum with absorption
lines occurring every few MHz throughout the investigated
spectral range, which was taken as an early indication that both
ap and sc were present in significant concentrations. The
theoretical predictions above indicate that ap would presumably
be the preferred form of 3-fluoropropionitrile, and searches were
therefore first made for the spectrum of this conformer. ap is
predicted to have Ray’s asymmetry parameter30 k ≈ 0.99 and a
major μa of 6 7 10 30 C m (Table 2). The a-type R-branch
spectrum of this conformer was therefore predicted to be comparatively strong with characteristic pile-ups separated by B + C
≈ 4.4 GHz. The members of these regions should involve transitions of K 1-pairs with K 1 g 3. These high-K 1 transitions
would have rapid Stark effect caused by the near-degeneracy of
the K 1-pairs. This pile-up 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. An example of a
portion involving mainly the ground-state lines of one of these
pile-ups involving J = 20 r 19 is shown in Figure 3. RFMWDR
ARTICLE
Figure 3. Microwave spectrum of a portion of the J = 20 r 19 pile-up
region of ap taken at a Stark field strength of about 110 V/cm. Most of
the absorption lines shown here belong to the ground vibrational state.
The numbers above the peaks indicate the values of the K 1 pseudoquantum numbers.
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,24
which was chosen because ap is nearly a symmetrical rotor. Sørensen’s
program Rotfit31 was used to least-squares fit the transitions. An accurate value of the DJK centrifugal distortion constant24 is generally
very useful in order to facilitate the assignments of high-K 1 pairs of
the pile-ups. Unfortunately, the B3LYP value of this constant shown
in Table 3 was too inaccurate to be helpful in the present case, and the
assignments were obtained employing a trial and error procedure.
The failure to predict DJK accurately may be due to a comparatively inaccurate B3LYP force field.
The said assignments were gradually extended to include
additional aR-transitions. The fact that the planar moment Pcc
should be approximately 3.20 10 20 u m2 for ac (Table 2) was
a useful aid in the assignment process of the low-K 1 R-branch
lines, which are much more sensitive to the value of the A rotational constant than the high-K 1 aR-lines are. None of the
assigned aR-transitions displayed a resolved quadrupole structure
caused by the 14N nucleus. Calculations performed by the MB09
program32 of the quadrupole hyperfine structure of these transitions using the quadrupole coupling constants shown in Table 2
revealed that the splittings of strong quadrupole components
would be less than resolution (0.5 MHz) of our spectrometer.
b-type lines were also searched for but not assigned presumably because they are too weak, which is not surprising given the
small μb component (∼0.50 10 30 C m; Table 2). A total of
157 aR-transitions, which are listed in Table 5S in the Supporting
Information, were ultimately used to determine the spectroscopic constants shown in Table 3. The inverse squares of the uncertainties listed in Table 5S of the Supporting Information were
used as weights in the least-squares fit. It was not possible to get
accurate values for all the spectroscopic constants from the selection of aR-branch lines assigned here, and DK, d1, and d2 were
preset to the B3LYP values in the least-squares fit. Inclusion of
sextic constants was also attempted, but no significant improvement of the fit was achieved in this manner. No such constants
were therefore retained in the final fit.
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The Journal of Physical Chemistry A
It is seen from Table 3 that the experimental B and C rotational
constants are very close (better than 1% agreement) to the
CCSD(full) counterparts. There is also a satisfactory agreement
for the A rotational constant. There is good agreement between
the experimental and B3LYP centrifugal distortion constant DJ,
while a larger discrepancy exists for DJK, for reasons mentioned above. Moreover, it is noted (Table 3) that the planar moment
Pcc = 3.2028(44) 10 20 u m2. This value is characteristic for
a compound having a symmetry plane and two pairs of
sp3-hybridized out-of-plane hydrogen atoms, and it is close to
the value of 3.14 10 20 u m2 obtained in the theoretical
calculations (Table 2). An effective value larger than the approximate equilibrium value of 3.14 10 20 u m2 has to be expected
due to out-of-plane vibrations.33
Vibrationally Excited State of ap. The lowest fundamental
vibration (ν21) has a harmonic frequency of 104 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 158
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, while
the spectroscopic constants are displayed in Table 3.
The vibration rotation α-constants of this vibrational mode
were calculated25 from αX = X0 X1, where X0 and X1 are the
rotational constants of the ground and of the first vibrationally
excited state of a fundamental vibration, respectively, with the
results shown in Table 3 (in parentheses). It is seen that the
agreement between the experimental and theoretical α values are
in quite good agreement.
The increase of the value of Pcc from 3.2028(44) of the ground
vibrational state to 3.8089(38) 10 20 u m2 for the first excited
state of ν21, (Table 3) is typical for an out-of-symmetry plane
vibration such as torsion33 about the C1 C2 bond. Relative intensity measurements performed largely as described by Esbitt
and Wilson34 yielded 114(20) cm 1, compared to the B3LYP
harmonic and anharmonic frequencies of 104 and 109 cm 1,
respectively (Table 1S, Supporting Information).
Eighty-nine transitions of the second excited state of ν21
were also assigned. This spectrum is shown in Table 7S of the
Supporting Information, and the spectroscopic constants are
listed in Table 3. It is seen from this table that the changes of the
rotational constants of this mode upon excitation is quite regular,
which is typical for an essentially harmonic vibration.35,36
Finally, 88 transitions belonging to the spectrum of the first
excited state of the lowest bending vibration (ν13) were assigned, as shown in Table 8S of the Supporting Information. The
spectroscopic constants of this excited state are listed in Table 3.
The value of Pcc is lower for this excited state than for the ground
state (Table 3), which is in accord with theory for excited states of
bending vibrations.33,35,36 Relative intensity measurement
yielded 152(25) cm 1 for the ν13 vibration, close to the B3LYP
value of 166 cm 1 (Table 1S, Supporting Information).
Assignment of the Spectrum of sc. This rotamer has a
comparatively large μb ≈ 13 and a significant μa ≈ 9.4 10 30 C m,
respectively, according to the theoretical calculations (Table 2).
The theoretical spectroscopic constants shown in Table 3 were
first used to predict the frequencies of strong bQ-lines, which were
found close to the values predicted for them. This was also the
case for the aR-branch transitions, which were initially assigned using the RFMWDR-method. bR-branch lines were assigned
next. The assignments were now gradually extended to include
ARTICLE
transitions with higher and higher values of the J quantum number. Searches for c-type lines were made, but none were found
presumably because μc is small (Table 2) producing insufficient
intensities for these lines. Ultimately, a total of 354 a- and b-type
lines with J up to 66 and K 1 up to 23 were assigned. These
transitions were used to determine the S-reduction spectroscopic
constants listed in Table 4 from the spectrum shown in Table 9S
in the Supporting Information. No fully resolved 14N quadrupole
hyperfine structures were observed for these transitions in accord
with calculations using the quadrupole constants listed in Table 2.
The transitions with high J and high K 1 generally have large
centrifugal distortion contributions of several GHz (Table 9S,
Supporting Information). It was therefore possible to get accurate values not just for the five quartic but even for four (HJ, HKJ,
HK, and h1) of the seven sextic centrifugal distortion constants.
The three remaining sextic constants (HJK, h2, and h3) were
preset at the B3LYP results. The results of this least-squares fit
are listed in Table 4.
The CCSD(full) and experimental rotational constants agree
to within better than about 1.2%; see Table 4. Comparison of the
experimental centrifugal distortion constants with the B3LYP
counterparts (Table 4) reveals a better than 20% agreement in
the case of the quartic constants, while the sextic constants are
seen to have an order-of-magnitude agreement.
Vibrationally Excited States of sc. The spectra of three
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 114 transitions were assigned for the spectrum of the first excited state of the torsion, 34 transitions were assigned for the
second state of this motion, while 59 transitions were assigned for
the spectrum of the first excited state of the lowest bending vibration. The corresponding spectra are shown in Tables 10S 12S of
the Supporting Information, while the spectroscopic constants
are listed in Table 4. Only quartic centrifugal distortion constants
were employed in the least-squares fitting procedure of these
three excited states.
The B3LYP calculations predict harmonic frequencies of 108
and 206 cm 1 for these two fundamentals (Table 3S, Supporting
Information). Relative intensity measurements yielded 115(25) cm 1
for the torsion and 188(25) cm 1 for the bending vibration.
The experimental vibration rotation constants are listed in
Table 4 (in parentheses) and compared to the B3LYP values
(Tables 4 and 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.
Experimental information from vibrationally excited states has
been used in the past to derive potential energy functions similar
to those shown in Figure 2.37 However, the present experimental
results refer only to the lower parts of the minima region of the
potential functions because only the MW spectra of the ground
and the two first torsional states have been assigned. The vibrational frequencies of the torsions are also rather inaccurate, typically (25 cm 1. A potential function derived from this information will be quite uncertain and presumably far inferior to the two
potential functions shown in Figure 2, which were obtained in the
high-level B3LYP and MP2 calculations.
Structures. The three experimental rotational constants obtained for each rotamer ap and sc furnish insufficient information
for a complete determination of their experimental geometrical
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The Journal of Physical Chemistry A
structures. The effective experimental rotation constants are associated with the r0-structure, whereas the CCSD(full) and MP2
(full) rotational constants are derived from approximate equilibrium structures. A direct comparison of the experimental and theoretical sets of constants is therefore not warranted, but the two
structures are in general similar. The good agreement between the
experimental and theoretical rotational constants is therefore
taken as an indication that the theoretical structures of Table 1
are indeed close to the equilibrium structures of ap and sc.
There is not much difference between the CCSD(full) and
MP2(full) structures, as remarked above. However, the CCSD(full) structures are calculated at a higher level of theory than
MP2(full), and the former structure is therefore our favorite. It is
assumed that the bond lengths of the CCSD(full) structure
hardly deviate by more than 0.5 pm from the equilibrium values.
The bond angles are presumed to vary by less than 0.5° from the
equilibrium values. Larger values probably apply for dihedral
angles, especially for sc. The important F3 C2 C1 C6 dihedral angle is assumed to be 65(2)° in sc.
Energy Difference. The energy difference 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.34 The energy differences
were calculated as described by Townes and Schawlow.38 ap was
assigned a statistical weight of 1 due to its symmetry plane, while
sc was assumed to have a statistical weight of 2 because of the
existence of two mirror forms. The CCSD(full) dipole moments
were employed.
ap was found to be 1.4(5) kJ/mol more stable than sc in the
present relative intensity measurements. This energy difference is
lower than the CCSD(full) and MP2(full) results above (2.2 and
2.0 kJ/mol, respectively) and the previous DFT calculations4
(2.7 kJ/mol).
’ DISCUSSION
The fact that ap is preferred by 1.4(5) kJ/mol over sc must be
a compromise of several intramolecular forces, whose sizes are
difficult to estimate quantitatively. Repulsive interactions destabilizing sc seems to be prominent in FCH2CH2CN. One of these
repulsions is the weak steric repulsion between the F3 and C6
atoms, which are separated by 290 pm in this rotamer according
to the CCSD(full) calculations, compared to 305 pm, which is
the sum of the Pauling van der Waals radii of fluorine (135 pm)39
and the half-thickness of an aromatic molecule (170 pm).39Another factor that would greatly favor ap over sc is dipole
dipole repulsion, which must be important in sc because of the
negative end of the very polar C F bond and the nitrile groups
come quite close in this rotamer. Yet another force, namely,
electrostatic repulsion between the fluorine atom and the
π-orbitals of the triple bond, may destabilize sc.
The fact that ap is preferred by only 1.4(5) kJ/mol relative to
sc in spite of all this repulsion is a clear indication that the gauche
effect plays a significant role in 3-fluoropropionitrile, but what are
the main causes of this effect? Theoretical arguments have been4
given that hyperconjugation is the major effect stabilizing the sc
rotamer. This interaction is suggested4 to occur between the
bonding σ-orbital of the C1 H bond and the antibonding
σ-orbital of the C2 F3, when these bonds are antiperiplanar
to one another. Hyperconjugation may also occur between
the σ-orbital of the C1 H and the antibonding σ-orbital of
the C1 C6 bond in a similar way. Hyperconjugation should
ARTICLE
therefore outweigh much of the repulsive interactions leaving ap
only 1.4(5) kJ/mol more stable than sc.
The sc form of the corresponding isocyanide, FCH2CH2NC,
was preferred by a marginal 0.7(5) kJ/mol compared to its ap
rotamer,7 whereas the opposite stabilities (1.4(5) kJ/mol preference of ap) were observed in the present nitrile case of
FCH2CH2CN. The reasons for this difference are not obvious.
However, the electrostatic repulsive interactions are presumably
less in the isocyanide because the bond moment of the isocyanide
group is significantly less than the bond moment of the nitrile
group.6 Moreover, the antibonding σ-orbital of the bond between the carbon atom and nitrogen atom of the isocyanide
group, C NC, in FCH2CH2NC seems to be more favorable for
hyperconjugation than the C CN bond of FCH2CH2CN
because of the significant electronegativity difference between
the carbon and nitrogen atoms. These variations between the
nitrile and isocyanide may perhaps explain at least part of the
conformational energy differences seen for FCH2CH2NC and
FCH2CH2CN.
’ ASSOCIATED CONTENT
bS
Supporting Information. Results of the theoretical calculations and the 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 PCMI program
(INSU-CNRS) for financial support.
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