ARTICLE
pubs.acs.org/JPCA
Microwave Spectrum and Conformational Composition of
2-Chloroethylisocyanide
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: 2-Chloroethylisocyanide (ClCH2CH2NtC) has
been synthesized, and its microwave spectrum has been investigated in the 20 97 GHz spectral region. The spectra of
35
Cl and 37Cl isotopologues of two conformers have been
assigned. The Cl C C N chain of atoms is antiperiplanar
in one of these rotamers and synclinal in the second. The energy
difference between the two forms has been obtained from
relative intensity measurements. It was found that the antiperiplanar conformer is favored over the synclinal form by 4.3(8) kJ/
mol. Quantum chemical calculations at the CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory have been performed. Most, but
not all, of the spectroscopic constants predicted in these calculations are in good agreement with their experimental counterparts.
The theoretical calculations correctly predict that the 2-chloroethylisocyanide exists as a mixture of an antiperiplanar and a synclinal
conformer, with the former about 3.5 kJ/mol more stable than the latter. Both methods of calculations find that the antiperiplanar
rotamer has a symmetry plane. The dihedral angle formed by the Cl C C N link of atoms of the synclinal form is 67° according
to the CCSD calculations. It is estimated from a comparison with the experimental rotational constants that this dihedral angle is
uncertain by (3°.
’ INTRODUCTION
Organic isocyanides are interesting compounds whose gasphase structural and conformational properties have received
comparatively little attention in the past. Our two laboratories
have therefore started synthetic, microwave (MW) spectroscopic
and theoretical studies of this class of compounds.
Previous MW studies including hydrogen isocyanide
(HNC),1 methyl- (CH3NC),2 ethynyl- (HCtCNC),3 vinyl(H2CdCHNC),4 propargyl- (HCtCCH2NC),5 propynyl(H3CCtCNC),6 cyclopropyl- (C3H5NC),7 phenyl- (C6H5NC),8
and trifluoromethylisocyanide (CF3NC)9 focused on the structural properties of these isocyanides. Very recently, our investigation of allenylisocyanide (H2CdCdCHNC),10 which is also
of potential astrochemical interest, was added to this list of
structural studies.
In a second article from our laboratories, the conformational
properties of 2-fluoroethylisocyanide (FCH2CH2NC) was investigated.11 This study appears to be the first experimental
conformational investigation ever of a gaseous isocyanide. It was
found in this study that two conformations, one with an
antiperiplanar (180°) and the second with a synclinal (67.1°)
F C C NC chain of atoms, exist for FCH2CH2NC. The
synclinal conformer was found to be 0.7(5) kJ/mol more stable
than the antiperiplanar form by relative intensity measurements
performed on MW lines. The preference of FCH2CH2NC of a
synclinal conformer over an antiperiplanar form is in accord with
r 2011 American Chemical Society
the so-called gauche effect,12 the tendency to prefer synclinal
conformations in 1,2-substituted ethane derivatives (XCH2CH2Y),
provided X and Y are electronegative elements or groups.12
Our conformational studies of isocyanides are now extended
to the first MW investigation of 2-chloroethylisocyanide
(ClCH2CH2NC). This compound differs from 2-fluoroethylisocyanide in one respect, namely, the exchange of the extremely
electronegative fluorine atom (Pauling electronegativity13 of
3.98) by another very electronegative atom, namely, chlorine
(electronegativity13 of 3.16). The energetic, spectroscopic, conformational, and structural consequences of this substitution are
the themes of the present investigation.
Two conformers can be envisaged for ClCH2CH2NC: the
antiperiplanar (obsolete trans) and synclinal (obsolete gauche)
forms, which are shown in Figure 1 with atom numbering. 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 that cannot be differentiated by MW
spectroscopy because their spectra are identical.
MW spectroscopy was chosen for this investigation since it is
an ideal method to study conformational equilibria because of its
superior accuracy and resolution. The fact that relative intensity
Received: September 6, 2011
Revised:
October 10, 2011
Published: October 11, 2011
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The Journal of Physical Chemistry A
ARTICLE
Figure 1. The antiperiplanar (ap) and synclinal (sc) conformers of
ClCH2CH2NC. Atom numbering is indicated on ap, which was found to
be 4.3(8) kJ/mol more stable than sc by relative intensity measurements
performed on MW transitions.
Scheme 1. Synthesis of 2-Chloroethylisocyanide
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 undertaken to obtain information for use in assigning the MW spectrum and investigating
properties of the potential-energy hypersurface.
’ EXPERIMENTAL SECTION
Synthesis of 2-Chloroethylisocyanide. The experimental
procedure we reported previously for the syntheses of allenylisocyanide10 and 2-fluoroethylisocyanide11 used quinoline as
a base. However, for compounds having a high boiling point
(>60 °C under 0.1 mbar), quinoline distills off during the synthesis
and is therefore more difficult to separate from the product. To
avoid this problem, we used a high boiling amine, trioctylamine
(see Scheme 1), as we will report shortly.14 Another advantage of
this reagent by comparison with quinoline is that it gives a much
less viscous mixture at the end of the reaction.
In a 100 mL one-necked round-bottomed flask equipped with
a stirring bar, 2-chloroethylformamide15 (2.3 g, 25 mmol), p-toluensulfonylchloride (6.67 g, 35 mmol), and trioctylamine (12.4 g,
35 mmol) were introduced. The flask was attached to a vacuum
line equipped with two traps with stopcocks. The first trap was
immersed in a 80 °C cold bath. The apparatus was evacuated to
0.1 mbar and maintained at this pressure during the reaction. The
mixture was then slowly heated to 90 °C for about 1 h. 2-Chloroethylisocyanide was distilled from the reaction mixture as it was
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 40 °C. 2-Chloroethylisocyanide was revaporized in vacuo and condensed in the second
trap to give the expected product in a 51% yield (1.14 g, 12.5
mmol). 1H NMR (CDCl3, 400 MHz) δ 1H NMR (CDCl3) δ
3.65 (m, 4 H). 13C NMR (CDCl3) δ 40.6 (t, 1JCH = 154.1 Hz,
CH2Cl); 43.3 (tt, 1JCH = 148.2 Hz, 1JNC(quad.) = 7.7 Hz, CH2N);
158.5 (t, 1JNC(quad.) = 5.1 Hz, (t) NC). IR (gas phase, 2 mbar,
25 °C, ν (cm 1)): 2965 (s), 2934 (s), 2879 (m), 2144 (s, νNC),
1269 (w), 1258 (w), 960 (w), 739 (m, νCCl).
Figure 2. The B3LYP/cc-pVTZ electronic potential function for
rotation about the C1 C4 bond of ClCH2CH2NC. The Cl7 C1
C4 N8 dihedral angle is given on the abscissa and the electronic
energy relative to the energy of ap is given on the ordinate. A dihedral
Cl7 C1 C4 N8 angle of 0° corresponds to a synperiplanar conformation.
MW Experiment. The MW spectrum of 2-chloroethylisocyanide was studied using the Stark-modulation MW spectrometer
of the University of Oslo operating in the 7 120 GHz spectral
range. Details of the construction and operation of this device
have been given elsewhere.16 18 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 20
97 GHz frequency interval was recorded. Radio-frequency microwave double-resonance experiments (RFMWDR), similar to
those performed by Wodarczyk and Wilson,19 were conducted
to unambiguously assign particular transitions, using the equipment described elsewhere.16 The vapor pressure of the title
compound is roughly 80 Pa at room temperature. The spectra
were measured at room temperature, or at 30 °C at a pressure of roughly 10 Pa. The recording of the spectrum at low
temperature was done because the spectrum is stronger the
lower the temperature.
Quantum Chemical Methods. The present quantum chemical calculations were performed employing the Gaussian 03 suite
of programs,20 running on the Titan cluster in Oslo. Becke’s
three-parameter hybrid functional21 employing the Lee, Yang,
and Parr correlation functional (B3LYP)22 was employed in the
density functional theory (DFT) calculations. Coupled-cluster
calculations with singlet and doublet excitations (CCSD)23,24
were also undertaken. The CCSD calculations are very costly and
were sped up by making use of a B3LYP force field that was
calculated prior to the CCSD calculations. Peterson and Dunning’s25
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 C4 bond was
calculated at the B3LYP/cc-pVTZ level of theory employing
the scan option of Gaussian 03. The energies were computed in
steps of 10° of the Cl7 C1 C4 N8 dihedral angle. All the
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Table 1. CCSD/cc-pVTZ and B3LYP/cc-pVTZ Structures,a
Dipole Moments, and Energy Differences of ClCH2CH2NC
method:
CCSD
conformer:
ap
b
B3LYP
sc
c
ap
sc
Bond Length (pm)
C1 H2
108.5
108.6
108.6
108.6
C1 H3
C1 C4
108.5
152.3
108.6
151.8
108.6
152.6
108.6
152.1
C1 Cl7
178.5
178.5
180.2
180.2
C4 H5
108.8
108.9
109.0
109.0
C4 H6
108.8
109.0
109.0
109.3
C4 N8
142.7
142.4
142.3
142.0
N8 C9
117.0
117.0
116.7
116.7
H2 C1 H3
109.6
109.7
109.7
109.7
H2 C1 C4
H2 C1 Cl7
111.0
108.0
110.9
107.6
111.2
107.5
111.3
107.0
H3 C1 C4
111.0
109.4
111.2
109.5
H3 C1 Cl7
108.0
107.5
107.5
106.9
C4 C1 Cl7
109.2
111.6
109.6
112.3
C1 C4 H5
110.4
110.4
110.3
110.3
C1 C4 H6
110.4
108.9
110.3
108.3
C1 C4 N8
109.6
111.9
110.1
112.9
H5 C4 H6
H5 C4 N8
108.5
108.9
108.5
108.7
108.1
109.1
107.9
108.9
Angle (deg.)
H6 C4 N8
108.9
108.3
109.1
108.4
C4 N8 C9
177.9d
179.4d
178.1d
180.0
173.7
Dihedral Angle (deg.)
H2 C1 C4 H5
59.0
174.1
59.0
H2 C1 C4 H6
179.0
66.8
178.3
H2 C1 C4 N8
61.1
52.9
61.3
51.6
H3 C1 C4 H5
H3 C1 C4 H6
179.0
59.0
64.7
54.4
178.3
59.1
64.8
53.1
H3 C1 C4 N8
61.1
174.1
61.3
Cl7 C1 C4 H5
60.0
54.2
59.6
Cl7 C1 C4 H6
60.0
173.2
59.6
Cl7 C1 C4 N8
180.0
67.0
180.0
e
35
Dipole Moment of ClCH2CH2NC (10
30
68.4
173.1
53.7
171.6
68.3
C m)
μa
4.70
3.55
6.07
μb
1.60
13.71
1.91
13.68
μc
μtot
0.0f
5.23
0.38
14.33
0.0f
6.73
0.69
14.39
4.40
Electronic Energy Differenceg (kJ/mol)
0.0
3.46
0.0
3.70
a
Atom numbering given in Figure 1. b Total CCSD electronic energy of
ap: 1658600.57 kJ/mol. c Total B3LYP electronic energy of ap:
1656219.73 kJ/mol. d Bent toward C1. e Conversion factor: 1 debye =
3.33564 10 30 C m. f By symmetry. g Relative to the energy of ap.
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 all 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
Cl7 C1 C4 N8 dihedral angle of 68.3°, and an electronic
energy that is 3.70 kJ/mol higher than that of ap. The two
transition states in Figure 2 at 0 and 120.0° have energies that are
30.3 and 15.7 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 B3LYP 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, the fundamental normal
vibrational frequencies, quartic, and sextic Watson S-reduction
centrifugal distortion constants,26 and vibration rotation αconstants27 were calculated for ap and sc. The electronic energies
are given in the footnote of Table 1. The dipole moment components species of these two forms were transformed from the
standard orientation of Gaussian 03 to the principal inertial axis
system using Bailey’s program axis28 (Table 1). The fundamental
frequencies are listed in the Supporting Information, Tables 1S
(ap) and 3S (sc), while the centrifugal distortion constants are
shown in Tables 2 (ap) and 3 (sc) together with experimental
results. Finally, the α-constants of the two forms are listed in the
Supporting Information, Tables 2S and 4S.
The force field obtained in these B3LYP calculations allowed
the calculation of the zero-point harmonic vibrations energies.
The energy difference between ap and sc corrected for this effect
is 3.76 kJ/mol, essentially the same as obtained above for the
electronic energy difference (3.70 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, principal-axes dipole moment components, and electronic energies are shown 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 3.46 kJ/mol, again favoring ap.
The electronic field gradients obtained in the CCSD calculations were used to calculate the principal-axes nuclear quadrupole coupling tensors of the 35Cl, 37Cl, and 14N nuclei, occurring
in 75.5, 24.5, and 99.6% natural abundance, respectively, by
means of Bailey’s program nqc.28 The results are shown in Table
11S of the Supporting Information. It is seen from this table that
the quadrupole coupling constants of the 14N nucleus is calculated to be comparatively small, which is typical for isocyanides.
The quadrupole coupling constant of the nitrogen nucleus of
CH3NC is, for example, only 0.4894(4) MHz.29 The small nuclear
quadrupole coupling constants of the nitrogen nucleus of the title
compound is therefore expected to be without noticeable influence
on the splitting patterns of the MW transitions given the resolution of about 0.5 MHz of our spectrometer.
The results of these calculations warrant further comments.
Inspection of Table 1 reveals that most of the B3LYP and CCSD
bond lengths agree to within 1 pm. There is one exception,
namely, the C1 Cl7 bond length, which is 1.7 pm longer in the
B3LYP calculations. There are several experimental determinations of the C Cl bond length in a related compound, ethyl
chloride. The r0 value30 is 179.08(30) pm, the substitution31
bond length32 is rs = 178.9(1) pm, whereas the rm method33
yielded34 178.88(18) pm for ethyl chloride. These values are
significantly closer to the CCSD results (178.5 pm; Table 1) than
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Table 2. Spectroscopic Constantsa,b,c,d,e of the Antiperiplanar Conformer of ClCH2CH2NC
35
species:
vib. state:
37
ClCH2CH2NC
ν21 = 1
ground
ClCH2CH2NC
ν20 = 1
ground
35
ClCH2CH2NC
equilibrium
A (MHz)
26625.8(83)
26148(12)
26735(15)
26639(16)
26984.8
B (MHz)
1595.0005(45)
1596.3840(91)
1597.6265(67)
1557.272(10)
1589.3
C (MHz)
1533.9330(45)
1536.3007(89)
1536.4291(67)
1498.8169(97)
1529.5
6.3668(64)
6.946(11)
6.303(12)
6.315(13)
6.33
0.1708(11)
0.1704(13)
0.171
Δf (10
20
u m2)
DJ (kHz)
0.18491(76)
0.1820(11)
DJK (kHz)
8.672(14)
8.574(28)
9.039(41)
8.384(18)
7.91
d1 (kHz)
0.0317(17)
0.0988(42)
0.0400(25)
0.4322(39)
0.0132
HJK (Hz)
HKJ (Hz)
0.0956(86)
0.030(50)
0.032(18)
0.260(87)
αAg (MHz)
478(15)
αBg (MHz)
1.38(1)
αCg (MHz)
2.37(1)
0.335(28)
1.50(21)
0.029(12)
0.437(69)
0.052
0.028
111(17)
2.63(1)
2.50(1)
rmsh
1.774
1.624
1.788
1.621
no. transi
312
194
150
256
a
The experimental constants are Watson’s S reduction, Ir representation.26 The equilibrium rotational constants (last column) are calculated from
the CCSD structure, whereas the equilibrium centrifugal distortion constants were obtained in the B3LYP calculations. The B3LYP rotational constants
were A = 27082.1, B = 1573.2, and C = 1514.9 MHz. b Uncertainties represent one standard deviation. c Spectra of 35ClCH2CH2NC in Tables 5S (ground
state), 6S (ν21; lowest torsional vibration), and 7S (ν20; lowest bending vibration) of the Supporting Information. d Spectrum of the ground state of
37
ClCH2CH2NC in Table 8S of the Supporting Information. e The centrifugal distortion constants held fixed in the least-squares fit were (see text): DK =
364 kHz, d2 = 0.00267 kHz, HJ = 0.000069 Hz, HK = 44 Hz, h1 = 0.00013 Hz, h2 = 0.0000012 Hz, and h3 = 0.00000010 Hz. f 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. g Calculated from αX = X0 X1, where
X0 is the rotational constant of the ground state, and X1 is the rotational constant of the first excited state.27 h Root mean square deviation of a
weighted fit. i Number of transitions used in the fit.
Table 3. Spectroscopic Constantsa,b,c of the Synclinal Conformer of ClCH2CH2NC
species:
35
37
ClCH2CH2NC
ClCH2CH2NC
ground
35
ClCH2CH2NC
vibrational state:
ground
equilibrium
A (MHz)
8534.417(17)
8473.554(96)
8518.9
B (MHz)
2463.7378(65)
2410.943(44)
2442.3
C (MHz)
2040.8006(59)
2001.379(44)
2032.6
DJ (kHz)
DJK (kHz)
2.7678(79)
23.140(35)
2.64(16)
23.79(18)
2.37
21.2
DK (kHz)
73.41(24)
73.41d
71.6
d1 (kHz)
0.8739(15)
0.8290(56)
0.715
d2 (kHz)
0.05025(65)
0.05025d
0.0384
HJKe (Hz)
0.354d
0.354(23)
HKJ (Hz)
2.43(46)
2.43d
rmsf
2.08
3.44
no. transg
95
21
0.028
0.75
a
The experimental constants are Watson’s S reduction, Ir representation.26 The equilibrium rotational constants are calculated from the CCSD
structure, whereas the equilibrium centrifugal distortion constants were obtained in the B3LYP calculations. The B3LYP rotational constants were A =
8668.2, B = 2361.9, and C = 1983.9 MHz. b Uncertainties represent one standard deviation. c Spectra in Tables 9S (35Cl-species) and 10S (37Cl-species).
d
Quartic centrifugal distortion constant fixed in the least-squares fit. e The sextic centrifugal distortion constants held fixed in the least-squares fit were
(see text): HJ = 0.0026 Hz, HK = 2.5 Hz, h1 = 0.0016 Hz, h2 = 0.00025 Hz, and h3 = 0.000043 Hz. f Root mean square deviation of a weighted fit.
g
Number of transitions used in the fit.
to the B3LYP predictions (180.2 pm; Table 1). However, the
exact influence of the isocyano group on the C1 Cl7 bond
length is unknown, but hardly large.
The calculation of two of the NtC bond length is also critical
in quantum chemistry. Interestingly, the equilibrium NtC bond
length is 116.83506(16) pm in H NtC,35 similar to 117.0 pm
(CCSD) and 116.3 pm (B3LYP) found for the title compound
(Table 1). Moreover, it is noted that the angles and dihedral
angles obtained in the two theoretical procedures agree to within
about 1°, or better.
ap is predicted to have an exact symmetry plane (CS symmetry), whereas the Cl7 C1 C4 N8 dihedral angle in sc is 7 8°
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Figure 3. The MW spectrum of a portion of the J = 24 r 23 pile-up
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. The K 1 = 4, 5, 6, and 7 are not fully modulated
at this field strength.
larger than the ideal 60° of a synclinal conformer (Table 1). The
increase of this dihedral angle by 7 8° may indicate repulsive
interaction between the C1 Cl7 bond (bond moment36 4.87 10 30 C m) and the isocyanide group (bond moment 10.0 10 30 C m, with the carbon atom as the negative end36). The
prediction that C4 C1 Cl7 and C1 C4 N8 angles are about
2 3° larger in sc compared to ap is also indicative of a slight
repulsive interaction in sc.
The energy difference between the two forms is predicted to
be about 3.5 kJ/mol in both methods of calculations, with ap as
the preferred conformer. It is also noted that the B3LYP dipole
moment components differ somewhat from the CCSD components (Table 1).
MW Spectrum and Assignment of the Spectrum of ap
(35Cl Species). The theoretical energy difference between sc and
ap of roughly 3 4 kJ/mol indicated that both these rotamers
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 every few megahertz throughout the investigated
spectral region, whereas a-type lines of highly prolate rotors
such as ap (Ray’s asymmetry parameter37 k ≈ 0.99) are primarily
found in pile-ups of R-branch transitions separated approximately by the sum of the B + C rotational constants.
Survey spectra revealed a very rich MW spectrum with mostly
weak absorption lines throughout the investigated spectral range,
which was taken as an early indication that the high-energy form
sc had to be present in significant concentrations. Moreover, the
characteristic R-branch pile-ups of ap were seen to protrude from
a rich background of weaker transitions. The strongest pile-ups
were separated by about 3.1 GHz, close to what is predicted for B +
C for the 35Cl isotopologue (Table 2). An example of a portion
of one of the pile-ups, J = 24 r 23, is shown in Figure 3. These
pile-up transitions were assigned first. RFMWDR experiments
were also performed, and several of the K 1-pairs were unambiguously assigned in this manner. The K 1 = 0 and 1 transitions,
which are well separated from the pile-ups, were finally assigned
ARTICLE
in a trial and error procedure. None of the aR-lines displayed a
resolved quadrupole splitting. This is in accord with calculations
with the computer program MB0938 using the nuclear quadrupole coupling constants shown in Table 11S. These calculations
indicate that none of the strong quadrupole components are split
by more than 0.5 MHz, which is the resolution of our spectrometer. b-type lines were searched for, but not assigned presumably because they are too weak, which is not surprising given the
small μb component (Table 1).
The spectrum (Table 5S, Supporting Information) was leastsquares fitted to Watson’s S-reduction Hamiltonian,26 which was
chosen because ap is nearly a symmetrical rotor. The inverse
squares of the uncertainties of the spectral lines were used as
weights in this procedure, which were performed using Sørensen’s
program Rotfit.39 It was not possible to obtain accurate values for
DK and d2 quartic centrifugal distortion constants. These two
constants were therefore held fixed at the B3LYP values of 364
and 0.00267 kHz, respectively, in the fit (footnote Table 2). It
was also found that two of the sextic centrifugal distortion
constants, HJK and HKJ, had to be employed to obtain a fit with
an acceptable root-mean square deviation. The remaining sextic
constants were fixed at the B3LYP values given in the footnote of
Table 2. A total of 312 aR-transitions (Table 5S), were used to
determine the spectroscopic constants shown in Table 2.
It is seen from this table that the B and C rotational constants
are very accurate, whereas the A rotational constant has one
standard deviation as large as 8.3 MHz. This is typical for a very
prolate asymmetrical top for which only aR-transitions have been
assigned.
The deviation between the CCSD (Table 2) and B3LYP
(footnote Table 2) rotational constants of the 35Cl species on the
one hand and the experimental rotational constants for the
ground state on the other is better than 1.3% in all cases, which
is very satisfactory. The agreement between the experimental DJ
and DJK centrifugal distortion constants and their B3LYP counterparts is also satisfactory. A larger discrepancy is seen for d1 and
the two sextic constants HJK and HKJ (Table 2). It is also noted
that the pseudoinertial defect Δ, defined by Δ = Ic Ia Ib =
6.3668(64) 10 20 u m2, where Ia, Ib, and Ic are the principal
moments of inertia, is very similar to the value ( 6.33 same
magnitude and units) calculated from the CCSD rotational
constants. This value is characteristic for a compound having a
symmetry plane and two pairs of sp3-hybridized out-of-plane
hydrogen atoms.
Vibrationally Excited State of ap (35Cl Species). The lowest
frequency fundamental vibration (ν21) has a harmonic frequency
of 96 cm 1 according to the B3LYP results (Table 1S, Supporting Information). This mode is the torsion about the C1 C4
bond. A total of 194 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, while the
spectroscopic constants are displayed in Table 2.
The vibration rotation α-constants of this vibrational mode
were calculated27 from αX = X0 X1, where X0 and X1 are the
rotational constants of the ground and of the vibrationally excited
state, respectively, with the results shown in Table 2. It is seen
that the agreement between the experimental (Table 2) and
theoretical α’s (Table 2S) is only at the order-of-magnitude level.
The increase of the absolute value of Δ from 6.3668(64) of the
ground vibrational state to 6.946(11) 10 20 u m2 for the
first excited state of ν21, (Table 2) is typical for an out-of-plane
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vibration such as torsion40 about the C1 C4 bond. Relative
intensity measurements performed largely as described by Esbitt
and Wilson41 yielded 98(20) cm 1, compared to the B3LYP
harmonic and anharmonic frequencies of 96 and 97 cm 1,
respectively (Table 1S).
The first excited state of the lowest bending vibration (ν20)
was also assigned. The spectrum (150 transitions) is found in
Table 7S, and the spectroscopic constants are listed in Table 2.
The pseudoinertial defect ( 6.303(12) 10 20 u m2) is seen
from this table to be about the same as that for the ground state.
The vibration rotation constants are again not very well predicted in the B3LYP calculations (values in Tables 2 and 2S).
Relative intensity measurements yielded 134(25) cm 1 compared to the B3LYP harmonic (144 cm 1) and anharmonic
(147 cm 1) (Table 1S).
Assignment of the Spectrum of the 37Cl Species of ap. The
CCSD structure was used to predict the shifts in the rotational
constants upon substituting the 35Cl atom with the 37Cl atom.
These shifts were added to the experimental rotation constants of
the 35Cl species and used to predict the spectrum of the ap
conformer of 37ClCH2CH2NC, whose spectrum (Table 8S) was
found very close to the prediction. The assignments were performed in a manner identical with that reported above for the
main 35Cl species. The spectroscopic constants of the 37Cl
species are listed in Table 2.
The rotational constants of the 35Cl and 37Cl species of Table 2
allow Costain’s substitution coordinates31 of the chlorine atom of
ap to be calculated. If the 35Cl species is taken as the parent
molecule, one finds that the absolute values of the substitution
coordinates are |a| = 198.6(2) and |b| = 9.4(32) pm, with |c|
assumed to be zero for symmetry reasons. The quoted uncertainties have been obtained as described by van Eijck.42 The
CCSD coordinates were |a| = 199.3 and |b| = 14.1 pm, and the
B3LYP coordinates were |a| = 200.3 and |b| = 2.9 pm. The
agreement between experiment and theory is better in the case of
the CCSD calculations.
Assignment of the Spectrum of sc (35Cl Species). This
rotamer has μa ≈ 4, and μb ≈ 14 10 30 C m, according to the
CCSD calculations (Table 1). The quadrupole splittings of both
the 35Cl and 37Cl species of this conformer were calculated from
the CCSD values (Table 2) employing the computer program
MB0938 and found to be not resolvable for the strongest aRtransitions. The CCSD rotational constants and B3LYP quartic
centrifugal distortion constants (Table 3) of 35ClCH2CH2NC
were first used to predict the aR-spectrum, which is generally
easier to assign than a b-type spectrum. Subsequent RFMWDR
experiments revealed no signals, which could undoubtedly be
attributed to this species, presumably because this spectrum is
too weak.
Searches for strong b-type lines were then undertaken. These
lines have in general much larger quadrupole splittings than the
a-type transitions. The CCSD nuclear quadrupole coupling
constants (Table 11S) indicate that many b-type lines would
have a resolved or a partly resolved quadrupole hyperfine structure
with a characteristic intensity pattern. Searches revealed no
obvious candidates for these transitions, presumably due to their
weakness. A trial and error procedure was then employed in an
attempt to assign the bQ-branch transitions of the 35Cl species
using the CCSD rotational and the B3LYP quartic centrifugal
distortion constants (Table 3) to predict the positions of comparatively strong transitions. This procedure was met with success,
and many Q-branch lines were assigned. Several bR-branch lines
ARTICLE
were subsequently assigned using a similar procedure. The identified b-type lines were often seen to have a nonsymmetrical
shapes, or partly resolved quadrupole hyperfine structures. It was
not possible to determine the quadrupole coupling constants
from these lines.
Ninety-five b-type transitions (Table 9S) were used to determine the S-reduction rotational and quartic centrifugal distortion
constants listed in Table 3. Two sextic constants, HJK and HKJ,
were also determined keeping the remaining sextic constants
fixed at the values shown in the footnote of Table 3. Rather large
uncertainties have been assigned to many of lines in the weighted
least-squares fit due to compensation for the quadrupole influence.
The CCSD and the experimental rotational constants of the
35
Cl species (Table 3) agree to within better than about 1%, while
the B3LYP rotational constants (footnote Table 3) show a
somewhat poorer agreement and deviate by up to about 4%
(B rotational constant). The B3LYP quartic centrifugal distortion constants (same table) are in fair agreement with the experimental results. The two sextic centrifugal distortion constants
that were fitted (HJK and HKJ) deviate considerably from the
theoretical values.
Assignment of the Spectrum of 37Cl Species of sc. The very
weak spectrum of the 37Cl species was assigned in a manner
analogous to the one described above for its ap counterpart. This
spectrum is shown in Table 10S, and the spectroscopic constants
are listed in Table 3.
The substitution coordinates31 of the chlorine atom of sc were
calculated from the rotational constants in Table 3 as |a| = 151.10
(2), |b| = 0.462(6), and |c| = 10.1(3) pm where the uncertainties
have been obtained as described by van Eijck.42 The corresponding CCSD coordinates were |a| = 153.3, |b| = 46.6, and |c| = 7.7
pm, while |a| = 156.6, |b|= 46.0, and |c| = 7.5 pm are the B3LYP
coordinates. It is therefore seen that the CCSD principal-axis
coordinates are somewhat closer to the substitution coordinates
than the corresponding B3LYP coordinates are for this rotamer
as well.
Structure. The six rotational constants obtained for ap and sc
are insufficient to determine the full experimental structure for
each of the two rotamers. The experimental rotational constants
of ap and sc reflect the r0-structure, whereas the CCSD and B3LYP
counterparts in Table 1 are approximations of their equilibrium
(re) structures. Comparison of the two differently defined structures must therefore be made with caution.
However, some of the structural features can be inferred from
the experimental findings with a high degree of certainty. For
example, the experimental values of the pseudoinertial defects of
the two isotopologues of ap (Table 2) show unambiguously that
this conformer has a symmetry plane. Moreover, the fact that the
experimental rotational constants of sc are significantly closer to
CCSD rotational constants than to the B3LYP constants indicates that the CCSD structure is superior to the B3LYP
structure in this case. The substitution coordinates of the two
forms (see above) also favor the CCSD structures. The same can
be said about the C1 Cl7 bond length, where the CCSD bond
length is found to be nearly the same as in ethyl chloride, where
very accurate experimental values are available (see above).
The CCSD structure predicts 67° for the important Cl7
C1 C2 N8 dihedral angle of sc. The good agreement between
the theoretical and experimental rotational constants for this
conformer (Table 3) makes it likely that this value is uncertain by
less than (3°.
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The Journal of Physical Chemistry A
The observations above that the CCSD structural parameters
are in better agreement with their experimental counterparts than
the B3LYP predictions were expected because DFT methods do
not contain any dispersion interactions, which CCSD calculations take into account.
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 with very
small quadrupole coupling patterns observing the precautions of
Esbitt and Wilson.41 The energy differences were calculated as
described by Townes and Schawlow.43 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 dipole moments were employed in the
calculation of the energy difference.
ap was found be 4.3(8) kJ/mol more stable than sc in the
present relative intensity measurements. The CCSD and B3LYP
calculations above predict a somewhat smaller difference of about
3.5 kJ/mol.
’ DISCUSSION
The following question should be answered: Why is ap preferred over sc in ClCH2CH2NC, whereas the opposite is the case
in FCH2CH2NC?
The fact that the ap conformer is preferred by 4.3(8) kJ/mol
over sc in the chlorine derivative is clearly a compromise of
several intramolecular interactions. Steric repulsive forces presumably play a role in destabilizing sc since the CCSD nonbonded distance between the chlorine atom and nitrogen atom is
318 pm, compared to 350 pm, which is the sum of the Pauling van
der Waals radii of chlorine (180 pm)44 and the half-thickness of
an aromatic molecule (170 pm).44 The fact that the nonbonded
distance is 32 pm shorter than the van der Waals distance has a
parallel in the synclinal form of FCH2CH2NC, where the corresponding CCSD nonbonded distance is 26 pm shorter than the
van der Waals distance.11 Steric forces should therefore favor
antiperiplanar forms in the title compound as well as in its
fluorine counterpart (FCH2CH2NC) by similar values.
Another factor that would greatly favor ap over sc is dipole
dipole repulsion, which must be important in sc, because the
negative ends of the polar C Cl bond (bond moment: 4.87 10 30 C m)36 and the isocyanide group (bond moment: 10.0 10 30 C m)36 come quite close in this rotamer. This repulsion is
minimized in ap, where the two bonds are as far apart as possible.
This situation too is quite similar to what is the case in FCH2CH2NC
and is likely to have a similar magnitude in the two molecules.
It has been claimed45 that yet another force, electrostatic
repulsion between the fluorine atom and the π-orbitals of the
triple bond destabilizes sc in FCH2CH2NC. A similar situation
should be present in the title compound, but the relative magnitudes
are difficult to estimate in the two cases.
It has been advocated that the forces that destabilize sc relative
to ap in FCH2CH2NC is countered by hyperconjugation45 that
occurs between the bonding σ-orbital of a C H bond and the
antibonding σ-orbital of the C F bond, when these bonds are
antiperiplanar to one another. Hyperconjugation should then
appear between the σ-orbital of the C1 Cl7 bond and the antibonding σ-orbital of one of the C4 H bonds of ClCH2CH2NC. The
size of the hyperconjugation depends on the electronegativity,
which is 3.16 for the chlorine atom, considerably less than the
3.98 of fluorine.13 The hyperconjugation interaction is therefore
ARTICLE
likely to be much less pronounced in ClCH2CH2NC than in
FCH2CH2NC, and this might largely explain why ap predominates by 4.3(8) kJ/mol in ClCH2CH2NC, while sc is preferred
by 0.7(6) kJ/mol in FCH2CH2NC.
’ ASSOCIATED CONTENT
bS
Supporting Information. Results of the theoretical calculations and the MW 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 Centre National
d’Etudes Spatiales (CNES) for financial support.
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