Article
pubs.acs.org/JPCA
Microwave Spectra, Planarity, and Conformational Preferences of cisand trans-N‑Vinylformamide
Harald Møllendal* and Svein Samdal
Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, P.O. Box 1033
Blindern, NO-0315 Oslo, Norway
S Supporting Information
*
ABSTRACT: The microwave spectra of a mixture of cis- and
trans-HNCO forms of N-vinylformamide, (H2C
CHNHC(O)H), have been measured at room temperature
in the 18−75 GHz spectral range. The spectra of two forms
were assigned. The first of these forms has a cis arrangement
for the HNCO chain of atoms, whereas the second
form has a trans arrangement. The C−C−N−C chain of atoms
is antiperiplanar (180°) in both forms. The inertial defect of the ground vibrational state of cis is −0.142(5) × 10−20 u m2,
whereas this parameter is −0.087098(26) × 10−20 u m2 for trans. It is concluded that the equilibrium structures of both cis and
trans are completely planar. The dipole moment determined from Stark effect measurements is μa = 9.96(8), μb = 2.22(3), μc = 0
(by symmetry), and μtot = 10.20(8) × 10−30 C m [3.06(2) D], for cis, and μa = 7.64(16), μb = 9.24(10), μc = 0 (by symmetry),
and μtot = 12.0(2) × 10−30 C m [3.59(5) D] for trans. The spectrum of one vibrationally excited state, presumably the first
excited state of the torsion about the C−N bond of cis, was assigned and the frequency of this state was determined to be 76(15)
cm−1 by relative intensity measurements. The spectra of two vibrationally excited states of trans were assigned. These states are
assumed to be the first excited state of the torsion about the CN bond, and a low bending vibration. Relative intensity
measurements yielded 101(20) and ca. 300 cm−1, respectively, for the frequencies of these normal vibrations. Accurate values of
the quartic centrifugal distortion constants, the dipole moments, and the vibration−rotation constants have been obtained for
both cis and trans. The experimental work has been augmented by high-level quantum chemical calculations at the B3LYP/ccpVTZ and CCSD(T)/cc-pVTZ levels of theory. The theoretical calculation performed without symmetry restrictions correctly
predict that cis and trans are both planar. The CCSD(T) rotational constants are in excellent agreement with their experimental
counterparts, whereas the B3LYP quartic centrifugal distortion constants and the vibration−rotation constants are in fairly good
agreement with experiments. The CCSD(T) dipole moments deviate more than expected from the experimental dipole
moments. It is estimated that further conformers of cis and trans must be at least 4 kJ/mol higher in energy.
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INTRODUCTION
The peptide linkage, −C(O)NH−, found in amides is an
essential feature of proteins. The properties of amides are
therefore of great biological as well as chemical interest.1−3
Amides are crystalline substances with comparatively low
sublimation pressures, or liquids of low volatility at room
temperature. Intermolecular hydrogen bonding is prevalent in
condensed phases of N-unsubstituted and N-monosubstituted
amides due to the proton accepting oxygen atom and the
hydrogen atom(s) attached to the nitrogen atom. Many of
these substances therefore form hydrogen-bonded networks in
the crystalline state.2 Extensive hydrogen bonding and crystal
effects may therefore obscure the genuine structural and
conformational properties of this class of compounds.
X-ray crystallography may provide excellent knowledge of
crystal structures and conformations of solid amides, but the
gas-phase properties may, for reasons already stated, be
significantly different from the condensed-phase properties.
Experimental studies of amides in the gas phase at low
pressures augmented with modern, high-level quantum
chemical calculations are therefore preferred to obtain the
© 2012 American Chemical Society
best possible insight in the true, unperturbed structural and
conformational preferences of this class of compound.
Gas electron diffraction (GED) and microwave (MW)
spectroscopy are the experimental methods of choice for gasphase investigations due to their high resolution. Unfortunately,
the low vapor pressures of amides and their tendency to
decompose when heated have made gas-phase studies of
amides less accessible than for compounds containing other
functional groups resulting in a relatively limited literature
about this class of compounds. The relative scarcity of studies
of gaseous amides and their great biological and chemical
interest motivated our laboratory to undertake investigations of
them using the GED method and MW spectroscopy in
combination with quantum chemical calculations. Our previous
studies include acetamide (CH3CONH2),4 2-fluoroacetamide
(CH2FCONH2),5,6 2-chloroacetamide (CH2ClCONH2),6,7 2iodoacetamide (CH 2 ICONH 2 ), 8 2,2-difluoroacetamide
Received: September 26, 2012
Revised: November 12, 2012
Published: November 16, 2012
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(CF2HCONH2)9 2,2-dichloroacetamide (CHCl2CONH2),10 2chloro-2,2-difluoroacetamide (CF2ClCONH2),11 2,2,2-trifluoroacetamide (CF 3 CONH 2 ), 12 2,2,2-trichloroacetamide
(CCl3CONH2),13 propionamide (CH3CH2CONH2),14 formic
hydrazide (H2NNHCHO),15 acrylamide (H2C
CHCONH2),16 methyl carbamate (CH3OCONH2),17,18 methoxyacetamide (CH3OCH2CONH2),19 and 2-azetidinone.20,21
Experimental contributions from other laboratories include, for
example, the prototype formamide (HCONH2),22−24 urea
(H2NCONH2),25 H−N−C−O-cis- and trans-N-methylformamide (HCONHCH3),26 N,N-dimethylformamide (HCON(CH3)2),27 H−N−C−O-trans-N-ethylformamide
(HCONHCH2CH3),28 H−N−C−O-cis-methoxyformamide
( H C O N H O C H 3 ), 2 9 H −N −C − O - t r a n s - f o r m a n il i d e
(C6H5NHCHO),30,31 acetamide,32−35 H−N−C−O-trans-Nmethylacetamide (CH3CONHCH3),36 H−N−C−O-trans-Nmethylpropionamide (CH3CONHCH2CH3)37 H−N−C−Otrans-acetanilide (CH 3 CONHC 6 H 5 ), 38 and alaninamide
(H2NCH2CONH2).39
These studies have revealed a number of interesting, and in
some cases, unexpected findings. It has, for example, often been
assumed that the amide group as a rule is completely planar,
which is consistent with a significant weight of the −OCN+
resonance structure. However, a study based on accurate MW
data and very high-level quantum chemical calculations40 of
several small amides concluded that only one of the investigated
compounds, namely formamide, is planar, whereas all the other
amides in this study were found to have nonplanar amide
groups in the free state. It therefore appears that a planar amide
group in gaseous amides is the exception rather than the rule.
The planarity problem is not the only interesting feature of
amides. Recent studies have revealed a number of unexpected
properties. Acetamide is one example of an amide displaying an
unusual behavior. First, the conformation of the methyl group
is exceptional in that one of the C−H bonds is almost
perpendicular to the heavy atom framework according to very
high-level ab initio calculations,4 which also found that its
amide group is nonplanar. Interestingly, the barrier to internal
rotation of this group is only 0.30466413(14) kJ/mol,35 much
lower than, for example, the methyl barrier in the isoelectronic
compound acetic acid (2.01258(20) kJ/mol).41 There are also
significant changes in the barrier heights of the methyl group
when the amide group hydrogen atoms are substituted with
deuterium atoms due to coupling of the methyl torsion with the
amide group inversion.35 Remarkably, the small perturbation
caused by deuterium substitution has a substantial influence on
the electronic structure of the peptide linkage.35
Moreover, although conformational mixtures in crystalline
amides are very rare, this phenomenon is expected to be much
more prevalent in the gas phase. However, so far acrylamide
appears to be the only amide for which more than one
rotameric form has been safely assigned by MW spectroscopy.16
In this work, our amide investigations are extended to
include the first MW study, augmented with high-level
quantum chemical calculations, of cis- and trans-N-vinylformamide (H2CCHNHC(O)H), where cis and trans
refer to the configuration of the HNCO chain of atoms.
In both the two title compounds, the two π-electron systems of
the amide and vinyl groups, respectively, are conjugated.
Moreover, the CN bond connecting the amide and vinyl groups
has presumably a substantial σ-character. This opens up for
rotational isomerism about the CN bond. There appears to be
no experimental gas-phase structural work reported for this
kind of vinylamide rotational isomerism, but the MW spectra of
the trans forms of formanilide30,31 and acetanilide,38 which have
some conformational resemblance to the title compounds, are
known.
The π-electron conjugation implies that planar, or nearplanar forms can be expected for both cis and trans. Four
representative forms, denoted I−IV, are depicted in Figure 1,
Figure 1. Models of four forms (I−IV) of N-vinylformamide found to
be minima on the potential energy hypersurface in the B3LYP/ccpVTZ calculations. Atom numbering is indicated on I. Note that the
H7N6C8O9 chain of atoms is cis in I and II, and trans in III and IV.
The C1C2N6C8 link of atoms is antiperiplanar (dihedral angle =180°)
in I and in III, and synperiplanar in II (∼20°) and in IV (0°). The MW
spectra of I and III are reported herein.
with atom numbering indicated on I. The H7N6C8O9 chain of
atoms is locked in a cis (forms I and II) or in a trans (III and
IV) configuration due to restricted rotation about the amide
N6C8 bond. Rotation about the C2N6 bond allows for
rotational isomerism in both cis and trans. Four representative
conformers, namely, I and II of cis, and III and IV of trans, are
indicated in Figure 1. The C1C2N6C8 chain of atoms is exactly
antiperiplanar in I and III, nearly synperiplanar in II, and exactly
synperiplanar in IV.
Although no GED or MW studies have been reported for the
two N-vinylformamides, a recent quantum chemical investigation exists.42 Using HF and B3LYP calculations with the 6311++G** basis set, it was predicted that I and III are both
planar and have approximately the same energy, whereas II and
IV were found to be several kJ/mol less stable.42 II was
predicted to be slightly nonplanar, whereas IV was found to be
completely planar in these calculations.
The interesting structural and conformational problems
presented by the two N-vinylamides motivated the present
MW research, which has been augmented with quantum
chemical calculations performed at a much higher level of
theory than previously employed42 to obtain information that
could be useful for the assignment of the MW spectra and for
investigating properties of the potential energy hypersurface.
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Article
RESULTS AND DISCUSSION
predicted to be 8.9 kJ/mol more stable than IV. The energy
differences corrected for harmonic zero-point-vibrational
interaction are 12.2 for the I−II pair, 9.7 for the III−IV pair,
and 0.3 kJ/mol for the difference between cis represented by I
and trans III, with cis as the more stable form.
The transition states of cis and trans were also explored. Two
transition states, at 0° and 99.5° of the C1C2N6C8 dihedral
angle were located for cis. The first of these transition states at
0° has an energy of 12.4 kJ/mol above the energy of I, or only
0.2 kJ/mol above the energy of II. The energy of the second
transition state at 99.5° is 31.5 kJ/mol higher in energy than the
energy of I. The trans form has only one transition state at
90.9° of the said dihedral angle. The energy of this state is 34.6
kJ/mol above the energy of III. The comparatively small
energies of the cis and trans transition states indicates that there
is little double bond character in the C2N6 bond. The data
above allow potential functions for rotation about the C2N6
bond to be drawn for cis and trans, as shown in Figures 2 and 3.
EXPERIMENTAL SECTION
Compound and MW Experiment. A commercial sample
of N-vinylformamide was used as received. The sample
contained roughly 50% of both cis and trans as judged by the
intensities of the MW transitions. No impurities were noted.
The MW spectra were recorded at room temperature. NVinylformamide is a liquid at room temperature and has a
vapor pressure of about 30 Pa at this temperature. The MW
spectrum was studied in the 18−75 GHz frequency interval by
Stark-modulation spectroscopy using the microwave spectrometer of the University of Oslo. Details of the construction and
operation of this device have been given elsewhere.43−45 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. Radio-frequency microwave double-resonance
experiments (RFMWDR), similar to those performed by
Wodarczyk and Wilson,46 were also conducted to unambiguously assign particular transitions, using the equipment
described elsewhere.43 The spectra were measured at room
temperature at a pressure of roughly 7 Pa.
Quantum Chemical Methods. The present ab initio
calculations were performed employing the Gaussian 0947 and
Molpro48 programs, running on the Titan cluster in Oslo.
Becke’s three-parameter hybrid functional employing the Lee,
Yang, and Parr correlation functional (B3LYP)49 was employed
in the density functional theory (DFT) calculations. Coupledcluster calculations with singlet and doublet excitations
including noniterative triplet excitations, CCSD(T),50 were
also performed. Peterson and Dunning’s51 correlation-consistent cc-pVTZ basis set, which is of triple-ζ quality was used in
all the calculations.
Quantum Chemical Calculations. The conformational
properties of the O9C8N6H7 cis and trans forms were first
explored. The variation of the energy with the C1C2N6C8
dihedral angle can conveniently be used for this purpose. The
energies were calculated while the dihedral angle was stepped in
intervals of 10° allowing all remaining structural parameters to
vary freely. The B3LYP method was employed for these
potential function calculations because CCSD(T) calculations
would have been far too expensive. The first calculations
indicate that cis has minima at 180° (I) and at about 20° (II),
whereas trans has minima at 180° (III) and 0° (IV) of the
C1C2N6C8 dihedral angle. Calculations of the structures,
dipole moments, vibrational frequencies, quartic and sextic
Watson A-reduction centrifugal distortion constants, and
vibration−rotation constants (the α's), were then undertaken
for the corresponding four conformers. Due precautions were
observed in the calculations of the vibration−rotation
interactions.52 All structural parameters were varied freely in
these calculations with no symmetry restrictions. No imaginary
vibrational frequencies were found for any of these four forms,
which indicate that they are minima on the potential energy
hypersurface. Selected results of these calculations are listed in
Tables 1S−4S in the Supporting Information.
It is seen from these tables that I, III, and IV are predicted to
be exactly planar, whereas II is nonplanar with a C1C2N6C8
dihedral angle of −20.6°. All four forms were predicted to have
completely planar amide groups. The electronic energy difference between I and II is calculated to be rather large (12.2 kJ/
mol) with I as the more stable conformer. Likewise, III is
Figure 2. B3LYP/cc-pVTZ potential function for rotation about the
C2N6 bond for the H7N6C8O9 cis configuration. This function has
minima at 20.6° of the C1C2N6C8 dihedral angle (form II) and at
180° (I) and transitions states at 0° and 99.5°. The B3LYP electronic
energy difference between II and I is 12.2 kJ/mol, with I as the more
stable conformer. The energy of the transition state at 0° is 12.4 kJ/
mol, and the energy of the transition state at 99.5° is 31.0 kJ/mol
higher than the energy of I.
Much more comprehensive CCSD(T)/cc-pVTZ calculations
of optimized structures, dipole moments, nuclear quadupole
coupling constants, and electronic energies of the four forms I−
IV were finally undertaken. It was important to investigate
whether these extensive calculations predict a planar amide
group. All starting conformations were therefore chosen to be
nonplanar. The optimizations were done without symmetry
restrictions employing the default convergence criteria of
Molpro. All four forms were predicted to have completely
planar amide groups. Moreover, conformers I, III, and VI were
calculated to be exactly planar in this case, whereas II was found
to be nonplanar with a C1C2N6C8 dihedral angle of −26.7°.
The resulting structures are listed in Table 1. The rotational
constants calculated from these structures are shown in Table 2.
The dipole moment components of the Molpro calculations
were transferred to the principal-axes dipole moment
components using Bailey’s program Axis53 with the results
listed in Table 2. The CCSD(T) electric field gradients were
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Table 1. CCSD(T)/cc-pVTZ Structures of Four Formsa of
N-Vinylformamide
I
C1−C2
C1−H3
C1−H4
C2−H5
C2−N6
N6−H7
N6−C8
C8−O9
C8−H10
C2−C1−H3
C2−C1−H4
H3−C1−H4
C1−C2−H5
C1−C2−N6
H5−C2−N6
C2−N6−H7
C2−N6−C8
H7−N6−C8
N6−C8−O9
N6−C8−H10
O9−C8−H10
Figure 3. B3LYP/cc-pVTZ potential function for rotation about the
C2N6 bond for the H7N6C8O9 trans configuration. This function has
minima at 0° (form III) and at 180° (IV) and a transition state at
90.9°. The B3LYP energy difference between IV and III is 8.9 kJ/mol,
with III as the more stable conformer. The energy of the transition
state is 34.6 kJ/mol above the energy of III.
calculated with Gaussian 09 using the structures in Table 2.
Results of these calculations were employed to calculate the
principal axis nuclear quadrupole coupling constants of the 14N
nucleus shown in Table 2 using the program Nqc.53 Calculation
of the centrifugal distortion constants by the CCSD(T) method
is beyond our computational resources. The B3LYP quartic
centrifugal distortion constants in the A-reduction form are
therefore included in Table 2.
The results in Tables 1 and 2 warrant further comments. The
nonplanarity of II may result from nonbonded repulsion
between the H4 and H10 atoms. This CCSD(T) distance (not
given in Table 1) is 228 pm, compared to 240 pm, which is
twice the Pauling van der Waals radius of hydrogen.3 A planar
conformation of II would have brought these two hydrogen
atoms into considerable closer proximity with increased
repulsion as a result. The effect of a maximal conjugation
stabilization of a completely planar II therefore appears to be
insufficient to offset the ensuing nonbonded repulsion in this
case.
The situation in the planar IV rotamer is different. There is a
short contact between H4 and O9 atoms in this case. The
CCSD(T) nonbonded distance is as short as 231 pm compared
to the Pauling van der Waals3 sum (260 pm) of hydrogen (120
pm) and oxygen (140 pm). It is possible that the overall effect
of this close contact results in stabilization of the planar form
due to the electronegative character of oxygen and the
electropositive property of hydrogen. Whether this contact
should be called an intramolecular hydrogen bond is a matter of
semantics.
The corresponding bond lengths (Table 1) of all forms are
remarkably similar, varying by less than 0.5 pm. Partial loss of
π-electron conjugation in II relative to the three other forms (I,
III, and IV) therefore seems to influence its C2N6 bond length,
as well as other bond lengths, to a minor degree.
The N6C8 bond lengths of I−IV vary between 137.0 and
137.3 pm (Table 1), compared to 135.47 pm found for the
equilibrium NC bond length in formamide.40 A lengthening of
this bond in the title compound compared to the
corresponding bond in formamide could perhaps be due to
H3−C1−C2−H5
H3−C1−C2−N6
H4−C1−C2−H5
H4−C1−C2−N6
C1−C2−N6−H7
C1−C2−N6−C8
H5−C2−N6−H7
H5−C2−N6−C8
C2−N6−C8−O9
C2−N6−C8−H10
H7−N6−C8−O9
H7−N6−C8−H10
II
Bond Length (pm)
133.9
134.0
108. 0
108.0
108.3
108.2
108.4
108.3
139.6
140.4
100.9
100.8
137.1
137.3
121.3
121.4
110.2
109.9
Angle (deg)
119.8
119.3
121.9
122.7
118.3
118.1
121.5
120.5
124.9
125.8
113.6
113.7
119.6
118.9
124.0
125.6
116.3
115.3
124.4
123.9
112.2
112.6
123.4
123.5
Dihedral Angle (deg)
0.0
−2.8
180.0
176.3
180.0
177.2
0.0
−3.8
0.0
156.7
180.0
−26.7
180.0
−24.3
0.0
152.4
180.0
177.7
0.0
−3.5
0.0
−5.5
180.0
173.2
III
IV
133.9
108.0
108.4
108.1
140.0
100.7
137.2
121.4
110.1
134.0
108.1
107.8
108.3
140.7
100.5
137.0
121.4
110.2
119.9
122.0
118.1
123.2
124.0
112.8
119.1
123.0
118.0
124.9
111.9
123.2
118.0
122.3
119.7
120.7
127.0
112.3
117.0
127.1
115.9
125.9
111.3
122.8
0.0
180.0
180.0
0.0
0.0
180.0
180.0
0.0
0.0
180.0
180.0
0.0
0.0
180.0
180.0
0.0
180.0
0.0
0.0
180.0
0.0
180.0
180.0
0.0
a
The four forms are depicted in Figure 1. The parameters of the forms
whose MW spectra were assigned, are in boldface.
delocalization of π-electron density of the amide group into the
vinyl part with an increased bond length as a result. The C8O9
bonds of I−IV should also be affected by conjugation, but their
lengths vary little and are about 0.4 pm longer than the
corresponding equilibrium bond length in formamide.40
Conjugation should also influence the C1C2 bond length in
the title compound. There is a report of the rz value
[133.91(13) pm] of the CC bond length in ethylene,54 which
is the same as 133.9−134.0 pm calculated for the C1C2 bond
lengths of the four conformers in Table 1.
It is noted that most corresponding bond angles in Table 1
are quite similar. The largest differences are seen for the
C1C2N6 and C2N6C8 angles of the III−IV pair, where the
larger angles (by 3−4°) are found in IV. The short nonbonded
contact between H4 and O9 mentioned above may have
resulted in a opening up of this angle due to a repulsive
interaction, which is effectively countered by electron
conjugation and hydrogen-bond like stabilization.
The energy differences of the rotamer pairs are interesting.
The CCSD(T) electronic energy of I is lower than that of II by
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Table 2. Theoretical Spectroscopic Contants of Four Formsa
of N-Vinylformamide
I
A
B
C
ΔJ
ΔJK
ΔK
δJ
δK
μa
μb
μc
μtot
χaa
χbb
χcc
ΔE
II
III
moment component is μa, predicted to be as large as (10−11)
× 10−30 C m (Table 2). A relatively intense series a-type Rbranch pile-ups of MW transitions separated by almost exactly
the sum of the B and C rotational constants was predicted for
its spectrum.
These predictions turned out to be correct, and the spectrum
of the ground vibrational state, which is shown in Table 5S in
the Supporting Information, was readily assigned. The
assignments of several of its transitions were confirmed by
RFMWDR experiments. Only aR-transitions with J values up to
15 with K−1 between 0 and 4 were used to determine the
spectroscopic constants listed in Table 1 because other K−1
transitions were frequently overlapped. b-type lines were
searched for, but not definitely identified presumably because
they are comparatively weak, which is consistent the fact that μa
is almost than 5 times larger than μb (see dipole moment
section below). The quadrupole hyperfine structure of the
assigned lines caused by the nitrogen nucleus were predicted
using the program MB0956 employing the nuclear quadrupole
coupling constants listed in Table 2. However, the splittings
were predicted to be too small to be resolved and this effect was
therefore not taken into consideration. A total of 57 transitions
were assigned and least-squares fitted to Watson’s A-reduction
Hamiltonian in the Ir-representation using Sørensen’s program
Rotfit.57 It was only possible to determine two quartic
centrifugal distortion constans ΔJ and ΔJK. The remaining
centrifugal distortion constants were preset at zero in the leastsquares fit with the resulting spectroscopic constants listed in
Table 3.
IV
Rotational Constantsb (MHz)
37516.9
19598.4
19817.0
10220.2
2407.3
2887.8
2960.8
4491.5
2262.2
2542.8
2575.9
3120.3
Inertial Defectc (10−20 u m2)
0.0
−2.00
0.0
0.0
Quartic Centrifugal Distortion Constantsd (kHz)
0.190
0.686
0.664
2.62
−3.81
−11.5
−9.10
−6.93
280
165
108
12.6
0.0162
0.0983
0.133
0.918
1.11
4.05
2.60
3.89
Dipole Momente (10−30 C m)
10.56
11.67
6.11
3.21
3.15
2.86
10.65
13.50
0.00
1.16
0.00
0.00
11.02
12.07
12.28
13.87
Principal Axis Quadrupole Coupling Constantsb (MHz)
2.05
2.27
1.96
2.04
1.98
1.63
2.14
2.03
−4.03
−3.90
−4.10
−4.07
Energy Difference (kJ/mol)
0.0f
9.6g
0.0h
2.4i
a
The four forms are depicted in Figure 1. The parameters of the forms
whose MW spectra were assigned, are in boldface. bCalculated from
the CCSD(T) structures given in Table 1. cConversion factor:
505379.0 × 10−20 MHz u m2. dB3LYP/cc-pVTZ values. eCCSD(T)
principal-axes dipole moment components and total dipole moment.
f
Total CCSD(T) electronic energy: −648187.60 kJ/mol. gRelative to
I. hTotal CCSD(T) electronic energy: −648206.31 kJ/mol. iRelative
to III.
Table 3. Spectroscopic Constantsa of Form I of cis-NVinylformamide
A (MHz)
B (MHz)
C (MHz)
Δc (10−20 u m2)
ΔJ (kHz)
ΔJKd (kHz)
rmse
Nf
9.6 kJ/mol (Table 2) compared to 12.2 kJ/mol obtained in the
B3LYP calculations above. A difference of this order of
magnitude found in the two methods is to be expected.
A much larger variance is found for the III−IV pair. The
CCSD(T) method yields 2.4 kJ/mol, whereas the B3LYP
method predicts 9.7 kJ/mol. A smaller energy difference was
expected for the III−IV pair than for the I−II pair because the
two conformers of the first pair are both planar and the full
effect of π-electron conjugation is preserved for both forms.
The nonbonded interaction between the oxygen atom O9 and
the hydrogen atom H4 could also be of importance for the
reduced energy difference for this pair compared to the I−II
pair.
Finally, the energy difference between trans and cis is also of
interest. The CCSD(T) electronic energy difference between
cis and trans represented by I and III is 2.8 kJ/mol, with I as
the global minimum, whereas the corresponding B3LYP
difference given above is 0.3 kJ/mol.
MW Spectrum and Assignment of the MW Spectrum
of Form I. The MW spectrum of the two N-vinylformamides
was found to be dense with absorption lines occurring every
few megahertz throughout the MW region. A high spectral
density was expected because both cis and trans forms
contribute to the spectrum.
The spectrum of I was first searched for. This rotamer was
predicted to be the preferred form of cis in the quantum
chemical calculations above. This conformer is very prolate
with Ray’s asymmetry parameter55 κ ≈ −0.99. Its major dipole
ground
first ex tors stateb
37001(14)
2419.1292(38)
2272.1181(40)
−0.142(5)
0.2042(75)
−3.39(23)
1.078
57
35540(11)
2424.3806(45)
2280.3965(48)
−1.058(5)
0.2057(81)
−2.79(18)
1.226
53
a
A-reduction Ir-representation.64 bFirst excited state of the torsion
about the C2N6 bond. cInertial defect. Conversion factor: 505379.01
× 10−20 MHz u m2. dFurther quartic constants preset at zero in the
least-squares fit. eRoot-mean-square deviation of a weighted fit defined
by rms2 = ∑[(νobs − νcalc)/u]2/(N − P), where νobs and νcalc are the
observed and calculated frequencies, u is the uncertainty of the
observed frequency, N is the number of transitions used in the leastsquares fit, and P is the number of spectroscopic constants used in the
fit. fNumber of transitions used in the least-squares fit.
Comparison of the experimental (Table 3) and CCSD(T)
(Table 2) rotational constants show that there is very good
agreement between the two sets of constants. The B and C
rotational constants differ by 0.5%, whereas the A rotational
constant differs by 1.4%. The ef fective rotational constants in
Table 3 are defined differently from the approximate
equilibrium rotational constants in Table 2. The differences
between the two sets of constants are expected to be relatively
small, similar to what has been found in the present case. This
indicates that the structure in Table 1 is very close to the
equilibrium structure. Interestingly, the inertial defect is
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−0.142(5) × 10−20 u m2 (Table 3). The inertial defect of a
completely planar molecule is zero. It is also noted that the
experimental values of the ΔJ and ΔJK quartic centrifugal
distortion constants (Table 3) are quite similar to their B3LYP
counterparts (Table 2).
There could be two main reasons for the observed, relatively
small inertial defect, zero-point vibrations and/or a nonplanar
amide group. The fact that the CCSD(T) calculations predict a
completely planar structure for I, including the amide group, is
a strong indication that this is indeed the case. The observed
inertial defect of −0.142(5) × 10−20 u m2 is therefore thought
to arise mainly from low-frequency out-of-plane vibration(s).
This inertial defect is similar to that of acrylamide
(−0.131300(34) × 10−20 u m2),16 which is isomeric with Nvinylformamide. Acrylamide is planar, or very nearly planar.16
The inertial defect of the planar formamide molecule is
+0.00862 × 10−20 u m2,22 whereas −0.60 × 10−20 u m2 has been
reported for formanilide, which is assumed to be planar.30,31
Vibrationally Excited State of I. The ground-state
spectrum was accompanied by several satellite lines presumably
originating from vibrationally excited states. One such state,
whose spectrum is shown in Table 6S of the Supporting
Information, was assigned in the same manner as described for
the ground state. Maximum value of J is 15 and maximum value
of K−1 is 4. The spectroscopic constants are listed in Table 3.
Attempts were made to assign further vibrationally excited
states, but complete unambiguous assignments were not
achieved.
Relative intensity measurements performed largely as
described by Esbitt and Wilson58 yielded 76(15) cm−1 for the
torsional vibration. It is seen from Table 3 that the inertial
defect is −1.058(5) × 10−20 u m2 for this state. The increase in
the absolute value of this quantity from the ground state
(−0.142(5) × 10−20 u m2) indicates that this state is an out-ofplane vibration,59 presumably the first excited state of the
torsion about the C2N6 bond. The B3LYP calculations yield
116 cm−1 (anharmonic value; Table 1S, Supporting Information) for this mode, significantly higher than the experimental
value.
It is possible to compare the experimental and theoretical
vibration−rotation constants defined by αex = X0 − Xex, where
X0 is the rotational constants of the ground state and Xex are the
rotational constants of the excited state under consideration.
The experimental vibration−rotation constants derived from
the entries in Table 3 are αA = +1461(18), αB = −5.25(1), and
αC = −8.28(1) MHz, compared to +1644, −4.74, and −7.27
MHz, respectively (Table 1S, Supporting Information). The
agreement between the two sets of vibration−rotation
constants is satisfactory.
Planarity of I. The three lowest normal vibrations have
anharmonic frequencies of 116, 222, and 241 cm−1 according to
the B3LYP calculations shown in Table 1S (Supporting
Information). Two semiempirical equations by Oka60 and by
Hanyu et al.61 can be employed to derive a fairly accurate value
for the lowest torsional vibration provided it is well isolated
from other vibrational modes. The B3LYP torsional vibration at
116 cm−1 is well separated from other vibrational modes.
According to Oka,60 the dependency of the inertial defect, Δ0,
of the ground vibrational state of planar molecules on the
torsional frequency, ν1, and the largest principal moment of
inertia, Icc, of the molecule is given by Δ0 = (−33.715/ν1) +
0.0186Icc1/2 in units of u Å2. Insertion of the inertial defect
(−0.142(5) × 10−20 u m2; Table 3) of the ground state and Icc
derived from the C rotational constant in the same table gives
ν1 = 80 cm−1, in good agreement with the experimental
torsional frequency of 76(15) cm−1.
An alternative equation by Hanyu et al.,61 ν1 is given by ν1 =
−67.4/Δ1, where Δ1 is the change in the inertial defect (in u Å2
units) upon excitation of an out-of-plane vibration. Using the
inertial defects of the ground and of the first excited state, one
gets a torsional frequency of 74 cm−1 in this manner. This value
is smaller than 80 cm−1 derived from the Oka60 formula. A
smaller value was expected because the equation by Hanyu et
al.61 neglects interaction terms.60
It is concluded that the low torsional frequency about the
C2N6 bond is the major contributor to the inertial defect of I,
whose equilibrium structure is completely planar. This
conclusion is strongly supported by the advanced CCSD(T)
calculations.
Dipole Moment of I. The second-order Stark coefficients
of the ground vibrational state shown in Table 4 were used to
Table 4. Second-Order Stark Coefficientsa and Dipole
Moment of Form I of cis-N-Vinylformamide
ΔνE−2/10−6 MHz V−2 cm2
transition
41,3
50,5
51,4
51,4
51,4
51,5
51,5
51,5
←
←
←
←
←
←
←
←
31,2
40,4
41,3
41,3
41,3
41,4
41,4
41,4
μa = 9.96(8)
a
obs
= |1|
= |1|
=0
= |1|
= |2|
=0
= |1|
= |2|
Dipole Moment
μb = 2.22(3)
M
M
M
M
M
M
M
M
calc
−15.8(4)
−2.10(4)
−1.57(3)
−3.90(3)
−10.9(5)
−0.548(10)
2.48(4)
12.4(4)
(10−30 C m)
μc = 0.0b
−16.3
−1.97
−1.59
−3.81
−10.6
−0.554
2.63
12.1
μtot = 10.20(8)c
b
Uncertainties represent one standard deviation. By symmetry; see
text. cIn debye units: 3.06(2) D. Conversion factor 1 D = 3.33564 ×
10−30 C m.
determine the dipole moment of I. The cell was calibrated
using OCS whose dipole moment was taken to be 2.38568(66)
× 10−30 C m.62 The theoretical second-order Stark coefficients
were calculated using the Golden and Wilson formalism,63
which is implemented in the program MB04.56 The out-ofplane component of the dipole moment, μc, was preset at zero
in the least-squares fit with the results shown in Table 4. The
experimental dipole moment components of this table, μa =
9.96(8), μb = 2.22(3), and μtot = 10.20(8) × 10−30 C m, deviate
more than expected from the CCSD(T) predictions (10.56,
3.15, and 11.02 × 10−30 C m, respectively; Table 1). It is
somewhat surprising that the very advanced CCSD(T)
calculations are not able to reproduce the dipole moment
components more accurately.
Assignment of the MW Spectrum of Form III. This
conformer has a sizable μa component of about 6 × 10−30 C m,
according to the CCSD(T) calculations (Table 2) and a value
of Ray’s asymmetry parameter (κ = −0.95) that is not much
different from the corresponding asymmetry parameter of I. A
typical, quite strong aR-pile-up series was therefore expected for
this form and searches for this series were first undertaken with
immediate success. Several assignments were confirmed by
MWRFDR experiments. The a-type lines assigned in this
manner were used to predict bQ-branch lines, which were soon
identified because most of them are among the strongest lines
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of the spectrum caused by a relatively large μb of approximately
11 × 10−30 C m (CCSD(T) value; Table 2). The assignments
of the strongest bR-branch lines were now straightforward.
Additional P- and R-branch lines were gradually assigned and
included in the least-squares fit. In this manner, 349 transitions
with maximum value of J = 71 and K−1 = 14 were assigned. All
quartic and one sextic (ΦKJ) centrifugal distortion constants
were fitted using Rotfit,57 with the remaining sextic constants
preset at zero. The spectrum is listed in Table 7S (Supporting
Information), and the resulting spectroscopic constants are
shown in Table 5.
as stated above for I. A total of 141 transitions with Jmax = 30
and K−1max = 4 were used to derive the spectroscopic constants.
Relative intensity measurements yielded 101(20) cm−1 for this
vibration, compared to 131 cm−1 predicted by the B3LYP
method (Table 3S, Supporting Information).
The experimental vibration−rotation constants calculated
from the entries in Table 5 are in this case αA = +364.38 (2), αB
= −7.08(1), and αC = −9.77(1) MHz, compared to +403.4,
−6.26, and −8.59 MHz (Table 3S, Supporting Information).
The agreement between the two sets of vibration−rotation
constants is again satisfactory.
The second excited state whose spectroscopic constants are
found in Table 5 is presumably an out-of-plane vibration
because its inertial defect (−0.1996(1) × 10−20 u m2) is in
absolute terms larger than that of the ground state
(−0.087098(27) × 10−20 u m2). A total of 65 transitions of
this form with Jmax = 31 and K−1max = 2 were employed to
obtain the spectroscopic constants of this excited state. Relative
intensity measurement yielded ca. 300 cm−1, compared with the
B3LYP frequency of 296 cm−1 (Table 3S, Supporting
Information). The experimental vibration−rotation constants
are αA = +21.90(3), αB = −2.32(1), and αC = −2.87(1) MHz,
compared to +26.89, −1.49, and −2.58 MHz, respectively
(Table 3S, Supporting Information). The agreement between
the two sets of vibration−rotation constants is satisfactory in
this case too.
Planarity of III. The inertial defect of the ground vibrational
state (Table 5) yields 97 cm−1 for the torsion about the C2N6
bond using the Oka formalism.60 This value agrees with
101(20) cm−1 found by relative intensity measurements. The
Hanyu et al. equation61 yields a lower value of 83 cm−1,
compared to the result using Oka’s formula,60 as expected.. The
torsional fundamental is well separated from the other
vibrational modes according to the B3LYP calculations
(Table 3S, Supporting Information), just as in the case of I.
All this, combined with the CCSD(T) result, is taken as
evidence that III has a completely planar equilibrium structure.
Dipole Moment of III. The determination of the dipole
moment of III was made in the same manner as reported above
for I. The results are presented in Table 6. The experimental
dipole moment components of this table, μa = 7.64(16), μb =
9.24(10), and μtot = 12.0(2) × 10−30 C m, are again found to
Table 5. Spectroscopic Constantsa of Form III of trans-NVinylformamide
A (MHz)
B (MHz)
C (MHz)
Δd (10−20 u
m2)
ΔJ (kHz)
ΔJK (kHz)
ΔK (kHz)
δJ (kHz)
δK (kHz)
ΦKJe (Hz)
rmsg
Nh
ground
first ex tors stateb
in-plane bend.c
19723.2338(55)
2976.65651(78)
2587.47826(79)
−0.087098(27)
19358.851(19)
2983.7425(17)
2597.2452(18)
−0.900684(74)
19701.332(33)
2978.9758(58)
2590.3460(58)
−0.1996(1)
0.6874(10)
−8.866(15)
103.162(69)
0.13766(23)
2.754(36)
−0.1342(89)
1.336
349
0.7269(28)
−9.193(65)
91.6(11)
0.14229(80)
2.292(99)
−4.5(32)
1.702
141
0.633(26)
−9.17(20)
92.5(73)
0.1420(15)
2.96(25)
−f
1.773
65
a
A-reduction Ir-representation.64 bFirst excited state of the torsion
about the C2N6 bond. cIn-plane bending vibration. dInertial defect.
Conversion factor: 505379.01 × 10−20 MHz u m2. eFurther sextic
centrifugal distortion constants preset at zero in the least-squares fit.
f
Preset at zero in the least-squares fit. gRoot-mean-square deviation of
a weighted fit, as defined in the footnote of Table 3. hNumber of
transitions used in the least-squares fit.
Attempts to get the nuclear quadrupole coupling constants of
the nitrogen nucleus failed for III, just as for I, because no wellresolved quadrupole hyperfine structures suitable for exploitation were observed for the b-type transitions, which were
explored in an attempt to derive these constants. However, the
shapes of several of these transitions were unsymmetrical and
definitely perturbed by this effect.
The experimental (Table 5) and CCSD(T) rotational
constants (Table 2) all agree to within about 0.5%, which is
again taken as an indication that the CCSD(T) structure is
close to the equilibrium structure. The inertial defect of III is
−0.087098(27) × 10−20 u m2, even smaller, in absolute terms,
than that of I (−0.142(5) × 10−20 u m2). The experimental and
B3LYP quartic centrifugal distortion constants are in quite
good agreement (Tables 2 and 5). The sextic centrifugal
distortion constant ΦKJ = −0.1342(89) Hz (Table 4) should be
compared with the B3LYP value (−0.219 Hz), shown in the
Supporting Information, Table 3S.
Vibrationally Excited State of III. The spectra of two
vibrationally excited states were assigned for this rotamer in the
same manner as described for the ground state. The spectra
consisting of 141 and 65 transitions, respectively, are listed in
Tables 8S and 9S of the Supporting Information, and the
spectroscopic constants are displayed in Table 5.
The strongest of these spectra, which has an inertial defect of
−0.900684(74) × 10−20 u m2 (Table 5) is presumed to belong
to the first excited state of the C2N6 torsion for a similar reason
Table 6. Second-Order Stark Coefficientsa and Dipole
Moment of Form III of trans-N-Vinylformamide
ΔνE−2/10−4 MHz V−2 cm2
transition
11,1
11,0
21,1
31,2
41,3
41,3
51,4
51,4
51,4
←
←
←
←
←
←
←
←
←
00,0
10,1
20,2
30,3
40,4
40,4
50,5
50,5
50,5
μa = 7.64(16)
obs
M=0
1.24(2)
M = |1|
9.36(13)
M = |2|
1.95(8)
M = |3|
1.18(3)
M = |4|
0.918(15)
M = |3|
0.349(5)
M = |5|
0.758(10)
M = |4|
0.406(5)
M = |3|
0.149(2)
Dipole Moment (10−30 C m)
μb = 9.24(10)
μc = 0.0b
calc
1.31
9.22
2.00
1.10
0.855
0.353
0.732
0.408
0.156
μtot = 12.0(2)c
a
Uncertainties represent one standard deviation. bBy symmetry; see
text. cIn debye units: 3.59(5) D. Conversion factor 1 D = 3.33564 ×
10−30 C m.
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■
deviate significantly from the CCSD(T) predictions (6.11,
10.65, and 12.28 × 10−30 C m; Table 1).
Searches for the Hypothetical Forms II and IV. The
theoretical calculations predict that there should be an energy
difference of 7−13 kJ/mol between I and II, and the latter form
was therefore expected to be present in small concentration, if
it exists at all as a stable form of cis-N-vinylformamide. This
conformer was calculated to have μa as the major dipole
moment component (Table 2). Searches were therefore
undertaken for the a-type R-branch transitions of this
compound using the spectroscopic constants of Table 2 to
predict their approximate frequencies. However, no assignments could be made.
The B3LYP and CCSD(T) methods predict rather different
(9.7 and 2.4 kJ/mol) energy differences between III and IV, as
discussed above. IV is predicted to have a comparatively large
μb of about 13 × 10−30 C m. Extensive searches for the
spectrum of IV were undertaken by employing the spectroscopic constants of Table 2, but no assignments could be made.
It is very unlikely that the spectrum of IV would have been
overlooked if the energy difference were as small as 2.4 kJ/mol.
It is concluded that IV must have a higher energy relative to III
than 2.4 kJ/mol. A conservative estimate is that the
hypothetical form IV is at least 4 kJ/mol less stable than III.
■
CONCLUSIONS
■
ASSOCIATED CONTENT
Article
AUTHOR INFORMATION
Corresponding Author
*Tel: +47 2285 5674. Fax: +47 2285 5441. E-mail: harald.
mollendal@kjemi.uio.no.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank Anne Horn for her skillful assistance. The Research
Council of Norway (Program for Supercomputing) is thanked
for a grant of computer time.
■
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The MW spectra of a mixture of cis- and trans-HNCO
forms of N-vinylformamide, (H2CCHNHC(O)H) have
been measured and analyzed. Rotational isomerism is possible
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The CCSD(T) rotational constants are in excellent agreement
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constants are in fair agreement with experiments, whereas
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additional rotamers must be at least 4 kJ/mol less stable than
the identified forms, if they exist at all.
S Supporting Information
*
Results of the theoretical calculations, including molecular
structures; geometrical parameters; dipole moments; harmonic
frequencies, IR intensities, Raman scattering activities, depolarization ratios, reduced masses, and force constants; and
vibrational energies and rotational constants. Microwave
spectra. This material is available free of charge via the Internet
at http://pubs.acs.org.
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