Article
pubs.acs.org/JPCA
Microwave Spectrum and Intramolecular Hydrogen Bonding of
2‑Isocyanoethanol (HOCH2CH2NC)
Harald Møllendal,*,† Svein Samdal,† and Jean-Claude Guillemin‡
†
Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, Blindern, NO-0315
Oslo, Norway
‡
Institut des Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, 11 Allée de
Beaulieu, CS 50837, 35708 Rennes Cedex 7, France
S Supporting Information
*
ABSTRACT: The microwave spectrum of 2-isocyanoethanol (HOCH2CH2NC) has been investigated
in the 12−120 GHz spectral range. The assignment of this spectrum was severely complicated by the
rapid transformation of 2-isocyanoethanol into its isomer 2-oxazoline, which has a rich and strong
spectrum. This process appeared both in a gold-plated microwave cell and in a brass cell and is
presumed to be catalyzed by metals or traces of base. The spectrum of one conformer was ultimately
assigned. This form is stabilized by an intramolecular hydrogen bond between the hydroxyl group and
the isocyano group and is the first gas-phase study ever of this kind of hydrogen bonding. The distance
between the hydrogen atom of the hydroxyl group and the nitrogen and carbon atoms are as long as 256
and 298 pm, respectively, indicating that covalent contribution to the hydrogen bond is minimal.
Electrostatic forces are much more important because the O−H and NC bonds are almost parallel
and the corresponding bond moments are practically antiparallel. The microwave work has been augmented by quantum
chemical calculations at the CCSD(T)/cc-pVTZ and MP2/cc-pVTZ levels of theory. Results of these calculations are generally
in good agreement with experimental findings.
■
INTRODUCTION
The Oslo laboratory has for a long time been interested in
intramolecular hydrogen bonding and used microwave (MW)
spectroscopy often augmented with quantum chemical
calculations to investigate a wide variety of internal hydrogen
bonds.1−4 Recent studies have shown that the π-electrons of
the triple bonds of the alkynyl (R−CC−R′) and the nitrile
(R−CN) functional groups are weak proton acceptors.5
Examples of alkynes with intramolecular hydrogen bonds
investigated by MW spectroscopy include propargyl alcohol
(HOCH2CCH),6 3-butyn-2-ol (H3CCH(OH)CCH),7 3butyn-1-ol (HOCH 2 CH 2 CCH), 8 − 1 2 4-pentyn-1-ol
(HOCH2CH2CH2CCH),13 propargyl thiol (HSCH2C
CH),14 3-butyne-1-thiol (HSCH2CH2CCH),15 propargyl
selenol (HSeCH2CCH),16 3-butyne-1-selenol
(HSeCH2CH2CCH),17 propargyl amine (H2NCH2C
CH),18 N-methylpropargyl amine (H3C(NH)CH2CCH),19
and 1-amino-3-butyne (H2NCH2CH2CCH).20
Similarly, intramolecular hydrogen bonding has been
reported for a number of nitriles comprising hydroxyacetonitrile (HOCH2CN),21 lactonitrile (H3CCH(OH)CN),22 3hydroxypropanenitrile (HOCH2CH2CN),23 3-mercaptopropionitrile (HSCH2CH2CN),24 Z-3-mercapto-2-propenenitrile (HSCHCHCN),25 aminoacetonitrile (H2NCH2C
N),26,27 3-aminopropionitrile (H2NCH2CH2CN),28 (Nmethylamino)ethanenitrile (H3C−NH−CH2CN),29 2-aminopropionitrile (H3C(NH2)CHCN),30 Z-3-amino-2-propenenitrile (H2NCHCHCN),31 and 3-phosphinopropionitrile (H2PCH2CH2CN).32
© 2014 American Chemical Society
In contrast to these many investigations of internal hydrogen
bonding with the π-electrons of the triple bonds in alkynes and
nitriles, no gas-phase studies of this interaction have been
reported for isonitriles (R−NC), the third functional group
possessing a triple bond. The present study of 2-isocyanoethanol (HOCH2CH2NC) by MW spectroscopy is therefore the
first gas-phase study ever of the ability of the isocyanide group
to participate in intramolecular hydrogen bonding.
Isocyanides have an interesting but relatively little explored
chemistry.33−35 Our two laboratories have therefore started to
investigate the physical properties of several isocyanides and
already reported the MW spectra of allenyl isocyanide (H2C
CCHNC),36 2-fluoroethyl isocyanide (FCH2CH2N
C),37 2-chloroethyl isocyanide (ClCH2CH2NC),38 E- and
Z-1-propenylisocyanide (CH3CHCHNC),39 cyclopropylmethyl isocyanide (C3H5CH2NC),40 4-isocyano-1-butyne
(HCCCH2CH2NC),41 and 4-isocyano-1-butene (H2C
CHCH2CH2NC).42 The present study of 2-isocyanoethanol
is an extension of these investigations focusing on the possible
intramolecular hydrogen bonding of this compound.
A model of 2-isocyanoethanol is shown Figure 1. Rotational
isomerism is possible about the C−C and C−O bonds resulting
in five possible conformers, denoted Conformer I−V. Atom
numbering is indicated on Conformer I. The O5−C1−C2−N3
and C2−C1−O5−H10 chains of atoms can conveniently be
Received: March 4, 2014
Revised: April 2, 2014
Published: April 2, 2014
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Scheme 1. Synthesis of 2-Isocyanoethanola
H NMR (CDCl3, 400 MHz) δ 3.50 (tt, 3H, 3JHH = 5.5 Hz, 2JNHqd =
1.8 Hz, CH2N); 3.60 (s brd, 1H, OH); 3.75 (tt, 1H, 3JHH = 5.5 Hz,
3
JNHqd = 2.4 Hz, CH2−O). 13C NMR (CDCl3, 100 MHz) δ 44.6 (1JCH
= 144.5 Hz (t), 1JCNqd = 6.9 Hz (t), CH2N); 60.0 (1JCH = 145.2 Hz (t),
CH2−O); 155.6 (1JCNqd = 6.2 Hz (t), NC). qd = quadrupolar coupling.
a1
instead of gaseous phosgene. The compound was purified by
distillation in vacuo of about 1 g to obtain the product with a
purity around 90%. The impurities were mainly unreacted
HOCH2CH2NHCHO and a little 2-oxazoline (<2%). 2Isocyanoethanol is a colorless liquid with a vapor pressure of
about 30 Pa at room temperature. The samples of 2isocyanoethanol were kept at liquid-nitrogen temperature or
at −80 °C when not in use.
Spectroscopic Experiments. The MW spectrum was
studied using the Stark microwave spectrometer of the
University of Oslo. Details of the construction and operation
of this instrument have been given elsewhere.42 The spectrum
was recorded in the 20−120 GHz frequency interval at room
temperature and a pressure of 5−10 Pa. A 2 m long Hewlett−
Packard MW cell with gold-plated walls and a 2 m homemade
brass cell were employed in the present experiments. Radiofrequency microwave double-resonance experiments
(RFMWDR), similar to those of Wodarczyk and Wilson,45
were also conducted to unambiguously assign particular
transitions.
■
RESULTS
Quantum Chemical Methods and Results. The present
ab initio calculations were performed employing the Gaussian
0946 and Molpro47 programs, running on the Abel cluster in
Oslo. Møller−Plesset second order perturbation calculations48
(MP2) and coupled-cluster calculations with singlet and
doublet excitations including noniterative triplet excitations,
CCSD(T),49 were undertaken. Peterson and Dunning’s50
correlation-consistent cc-pVTZ basis set was used in all
computations. The MP2 calculations were carried out using
Gaussian09, whereas the CCSD(T) computations were
executed with Molpro.47
The MP2/cc-pVTZ optimized structures of the five conformers I−V, their dipole moments, nuclear quadrupole
coupling constants of the 14N atom, vibrational frequencies,
vibration−rotation constants, differences between the equilibrium and ground-state rotational constants, and Watson’s51
quartic and sextic centrifugal distortion constants were first
calculated. No imaginary vibrational frequencies were found in
these calculations, which indicate that the rotamers I−V indeed
represent minima on potential energy hypersurface. The
precautions of McKean et al.52 were observed when calculating
the centrifugal distortion constants and the vibration−rotation
constants. The results of these computations are shown in
Tables 1S−5S of the Supporting Information.
The MP2 structures of the five conformers were then used as
starting points in CCSD(T) calculations of optimized
structures, rotational constants, dipole moments, and electronic
energies. Computation of CCSD(T) vibrational frequencies
and vibration−rotation constants could not be undertaken due
Figure 1. Representative conformers of HOCH2CH2NC. The MW
spectrum of V was assigned. This rotamer has an intramolecular
hydrogen bond formed between the hydrogen atom of the hydroxyl
group and the isocyano group.
used to describe the conformational properties of the five
rotamers. The O5−C1−C2−N3 link is antiperiplanar (ap) in I
and II and +synclinal (+sc) in the remaining three conformers.
The C2−C1−O5−H10 atoms have an ap orientation in I and
III, a +sc alignment in II and IV, and a −sc direction in V.
Mirror image forms exist for all conformers but I. Conformer V
is the only rotamer that can form an intramolecular hydrogen
bond with the π-electrons of the isocyano group.
MW spectroscopy is an ideal experimental method of
investigation due to its superior accuracy and resolution. The
experimental work is augmented by quantum chemical
calculations at a high methodological level. These computations
are useful for obtaining information that has been employed to
assign the MW spectrum and to investigate properties of the
potential energy hypersurface.
■
EXPERIMENTAL SECTION
Synthesis. Phosgene (20% in toluene), methyl formate, and
ethanolamine were purchased from Aldrich and used without
further purification. N-(2-Hydroxyethyl)formamide
(HOCH2CH2NHCHO) has been synthesized as previously
reported.43 2-Isocyanoethanol has been prepared by dehydration of the amide with phosgene as reported (Scheme 1).44
However, we used a solution of phosgene (20% in toluene)
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Table 1. CCSD(T)/cc-pVTZ Structures of Five Conformers of HOCH2CH2NC
conformer
a
I
C1−C2
C1−O5
C1−H6
C1−H7
C2−N3
C2−H8
C2−H9
N3−C4
O5−H10
152.3
141.9
109.5
109.5
142.9
109.0
109.0
117.7
96.0
C2−C1−O5
C2−C1−H6
C2−C1−H7
O5−C1−H6
O5−C1−H7
H6−C1−H7
C1−C2−N3
C1−C2−H8
C1−C2−H9
N3−C2−H8
N3−C2−H9
H8−C2−H9
C1−O5−H10
C2−N3−C4
105.6
109.3
109.3
112.1
112.1
108.5
110.2
109.9
109.9
109.0
109.0
108.6
108.1
178.0b
O5−C1−C2−N3
O5−C1−C2−H8
O5−C1−C2−H9
H6−C1−C2−N3
H6−C1−C2−H8
H6−C1−C2−H9
H7−C1−C2−N3
H7−C1−C2−H8
H7−C1−C2−H9
C2−C1−O5−H10
H6−C1−O5−H10
H7−C1−O5−H10
180.0
59.7
−59.7
59.3
−61.0
179.5
−59.3
−179.5
61.0
180.0
−61.1
61.1
II
Bond Length (pm)
153.1
141.6
109.0
109.4
143.0
109.0
109.2
117.7
96.1
Angles (deg)
110.5
109.3
109.6
107.0
112.3
108.0
110.3
109.9
110.5
109.1
108.5
108.4
108.1
177.0b
Dihedral Angle (deg)
178.2
57.8
−61.8
60.7
−59.7
−179.2
−57.4
−177.8
62.6
77.7
−163.4
45.1
III
IV
Va
151.9
141.7
109.4
109.6
143.0
109.1
109.1
117.7
96.0
152.7
141.3
109.0
109.6
142.8
109.3
109.2
117.7
96.1
152.7
141.2
109.6
109.1
143.3
109.0
109.1
117.8
96.3
107.5
109.2
108.0
111.9
111.5
108.7
111.3
109.5
110.1
108.5
108.5
108.9
108.3
178.6b
113.1
109.3
107.9
106.2
112.0
108.3
111.6
110.4
109.8
108.2
108.6
108.1
108.2
178.5b
112.0
109.3
108.5
111.8
106.7
108.4
109.7
110.1
110.8
108.2
108.7
109.3
107.5
175.9b
68.6
−51.3
−171.0
−52.9
−172.9
67.5
−170.9
69.1
−50.5
−162.8
−42.9
79.0
61.4
−59.1
−178.2
−56.6
−177.1
63.8
−174.2
65.4
−53.7
66.8
−173.4
−55.4
62.4
−56.5
−177.5
−62.0
179.0
58.0
179.9
61.0
−60.0
−64.1
−59.0
177.4
The MW spectrum of this conformer was assigned. bBent toward C1.
and IV (+9.61 kJ/mol). These energy differences are somewhat
less than their zero-point corrected counterparts reported in the
previous paragraph.
Some of the structural features of the five forms (Table 1)
warrant comments. The CCSD(T) N3C4 triple bond length
is 117.7 pm in I−IV and 117.8 pm in V, compared to
corresponding equilibrium bond lengths of 116.83506(16) pm
in HNC53 and 116.9(1) pm in CH3NC.54 The C2−N3 bond
length varies between 142.8 and 143.3 pm. The H3C−NC
equilibrium value is 142.2(1) pm in CH3NC.54 The C1−C2
bond lengths are between 152.7 and 153.1 pm, similar to the
equilibrium C−C bond length in ethane (152.2 pm).55 The
O5−H10 bond length has its longest value in V (96.3 pm),
0.2−0.3 pm longer than in the other four conformers. A slight
lengthening of this hydroxyl bond in V is consistent with weak
internal hydrogen bonding.
The C1−C2−N3 and C2−N3−C4 bond angles of the five
rotamers have their smallest values in V. These comparatively
small angles bring the isocyano group into closer proximity
with the hydroxyl group with enhanced hydrogen-bonding as a
to limited computational resources. The CCSD(T) structures
are given in Table 1; further structural details are given in
Tables 6S−10S of the Supporting Information. The rotational
constants calculated from the CCSD(T) structures are shown
in Table 2 together with CCSD(T) dipole moments and
electronic energies. The MP2 quartic centrifugal distortion
constants in the A-reduction form51 are also listed in this table.
It is seen from Table 2 that conformer V, which is the only
conformer stabilized by an intramolecular hydrogen bond,
represents the global minimum electronic energy, followed by I
(+5.76 kJ/mol), II (+6.84 kJ/mol), III (+8.35 kJ/mol), and IV
(+10.31 kJ/mol). The relative MP2 values corrected for zeropoint harmonic energies obtained from entries in Tables 1S−5S
of the Supporting Information are V (0.0 kJ/mol), I (+5.25 kJ/
mol), II (+7.02 kJ/mol), III (+7.19 kJ/mol), and IV (+10.19
kJ/mol). The two computational methods are thus in good
agreement.
The MP2 Gibbs energy differences obtained from the entries
in the Supporting Information, Tables 1S−5S, are V (0.0 kJ/
mol), I (+4.42 kJ/mol), II (+6.44 kJ/mol), III (+6.18 kJ/mol),
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Table 2. Spectroscopic Parametersa of Five Conformers of
HOCH2CH2NC
conformer
A
B
C
ΔJ
ΔJK
ΔK
δJ
δK
μa
μb
μc
μtot
I
II
III
IV
Rotational Constants (MHz)
26654.1
25916.7
11575.4
11301.7
2484.0
2458.7
3445.5
3450.6
2338.9
2325.9
2917.3
2894.9
Quartic Centrifugal Distortion Constants (kHz)
0.503
0.528
4.38
4.43
−18.8
−19.8
−31.8
−30.9
455
460
106
98.2
0.0709
0.0736
1.31
1.35
2.44
1.86
10.0
9.40
Principal Axis Dipole Momentsc (10−30 C m)
10.77
8.09
11.70
9.52
1.79
3.91
8.10
11.72
0.0d
4.55
3.52
4.39
10.91
10.07
14.66
15.73
Relative Electronic Energies (kJ/mol)
5.76
6.84
8.35
10.31
Relative MP2 Gibbs Energies (kJ/mol)
4.42
6.44
6.18
9.61
CCSD(T) due to available computational resources. MP2/ccpVTZ calculations were chosen for this purpose. The MP2
structure of 2-oxazoline and its energy was first calculated. The
results are given in Table 12S of the Supporting Information.
Interestingly, MP2 predicts this compound to have a slightly
nonplanar arrangement of the non-hydrogen atoms in contrast
to the CCSD(T) and spectroscopic results (see above). An
energy difference corrected for zero-point vibrations of 69.42
kJ/mol between 2-oxazoline and 2-isocyanoethanol, very
similar to the CCSD(T) result above, was obtained from
values given in Tables 5S and 12S of the Supporting
Information.
The transition state for the transformation of 2-isocyanoethanol into 2-oxazoline in the gas phase was then computed in
order to get an idea about the barrier separating the two
isomers. The transition-state structure, characterized by one
imaginary frequency of 1194i cm−1, is listed in Table 13S of the
Supporting Information. The electronic energy of the transition
state is 218.42 kJ/mol higher than the electronic energy of 2isocyanoethanol (from values in Tables 5S and 13S of the
Supporting Information). A barrier to transformation of 2isocyanoethanol into 2-oxazoline of about 220 kJ/mol is so high
that practically no transformation should occur in the gas phase
at room temperature in an uncatalyzed reaction.
Microwave Spectrum and Assignment. The theoretical
calculations predict that V is the lowest-energy rotamer of 2isocyanoethanol, being 5−6 kJ/mol more stable than the
second-lowest energy rotamer, conformer I. Further forms are
calculated to be even higher in energy (Table 2). Conformer V
is not only computed to be the lowest-energy conformer. It is
also favored by a statistical weight of 2:1 relative to I. The
spectrum of V was therefore expected to be the predominating
spectral feature. Conformer V is predicted to have its major
dipole moment components along the a- and b-inertial axes
(Table 2). Ray’s asymmetry parameter, κ,57 is about −0.83 and
a rich spectrum dominated by bQ-branch and aR-branch
transitions was therefore expected for this conformer.
Surprisingly, the first experiments showed no sign of 2isocyanoethanol. Instead, a strong and very rich spectrum of a
different compound was observed. This spectrum was soon
identified to belong to 2-oxazoline by means of its reported
MW spectrum.56 2-Oxazoline is much more volatile than 2isocyanoethanol, whereas the other impurity, unreacted
HOCH2CH2NHCHO, has much lower volatility. It was
therefore pumped several minutes on the sample at room
temperature before filling the cell with fresh sample in order to
get rid of as much 2-oxazoline as possible. This procedure,
which was repeated many times, did not help. The spectrum of
2-oxazoline reappeared. It seems very likely that 2-isocyanoethanol, which is stable at room temperature for a long time in a
glass cell and has a high MP2 barrier to transformation of about
220 kJ/mol in the gas phase (see above), is rapidly transformed
into 2-oxazoline in metal cells. Our observations suggest that
the transformation process, which has not been described
previously and was not foreseen, is catalyzed. Tiny amounts of
compounds absorbed on gold or brass cell walls, notably water,
are unavoidable and could play a role in the catalytic process. A
base seems to be a very effective catalyst. Thus, in another
experiment, we have added a drop of a base, 1,8diazabicyclo[5.4.0]undec-7-ene, to 2-isocyanoethanol in deuterochloroform, and this led to a rapid transformation of 2isocyanoethanol to 2-oxazoline, as observed by 1H NMR
spectroscopy.
Vb
10860.0
3668.5
2985.2
5.28
−30.6
77.1
1.67
9.22
4.39
8.36
1.24
9.52
0.0e
0.0f,g
a
The rotational constants have been calculated from the CCSD(T)/
cc-pVTZ structures in Table 1. The centrifugal distortion constants in
the A-reduction form51 and the relative Gibbs energies were acquired
in the MP2/cc-pVTZ calculations, while the dipole moments and
electronic energies were found in the CCSD(T)/pVTZ computations.
b
The MW spectrum of this conformer was assigned. cConversion
factor: 1 debye = 3.33564 × 10−30 C m. dFor symmetry reasons.
e
Electronic energy: −648113.14 kJ/mol. fThe fact that the statistical
weight of conformer I is half that of the other conformers has not been
taken into account in the value reported here for this conformer.
g
Gibbs energy at 298.15 K and 1 atm: −647801.09 kJ/mol.
result. The O5−C1−C2−N3 dihedral angles are close to their
canonical values of 180 and 60° with the exception of III, where
this angle is 8.6° larger for no obvious reasons. Deviation up to
17.7° (II) predicted for the characteristic C2−C1−O5−H10
dihedral angle is also hard to explain.
In the course of this work, it was seen that 2-oxazoline was
readily formed by a ring closure of HOCH2CH2NC, as
illustrated in Scheme 2. The optimized CCSD(T)/cc-pVTZ
Scheme 2. Formation of 2-Oxazoline from 2Isocyanoethanol
structure of 2-oxazoline was calculated in order to compare the
electronic energies of conformer V and 2-oxazoline. The
CCSD(T) structure and electronic energy of 2-oxazoline is
given in Table 11S of the Supporting Information. This
compound was found to have a symmetry plane (Cs symmetry)
according to the CCSD(T) computations, in agreement with
the spectroscopic study.56 The CCSD(T) electronic energy of
2-oxazoline is lower than the electronic energy of conformer V
by 73.67 kJ/mol, as can be calculated from the entries of Tables
10S and 11S of the Supporting Information.
The barrier to conversion of 2-isocyanoethanol into 2oxazoline in the gas phase is of interest, but these calculations
have to be performed at a lower methodological level than
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The unavoidable presence of 2-oxazoline was most
unfortunate for the assignment of the spectrum of 2isocyanoethanol because the 2-oxazoline spectrum is very rich
with transitions occurring every few MHz throughout the entire
MW region. The richness of its spectrum is caused by several
low-frequency ring-puckering modes, which are well Boltzmann
populated at room temperature and the fact that both a-type
and b-type spectra occur. 2-Oxazoline is much more volatile
than 2-isocyanoethanol, and this is another factor favoring the
spectrum of the former compound.
μb of 2-isocyanoethanol is approximately twice as large as μa
(Table 2). Initially, the relatively strong b-type Q-branch
transitions were searched for using the spectroscopic constants
of Table 2 to predict the approximate frequencies of these
transitions, which are the strongest ones of its spectrum, but no
definite assignments were obtained in this manner. The
experimental conditions were varied in these experiments. In
some of them, fumes of 2-isocyanoethanol were admitted to the
MW cells at room temperature. In other experiments the
sample was heated by a heat gun before admitting its fumes to
the cells. The sample fumes were floated through the cells in
other experiments keeping the sample tubes at room
temperature or holding them in water baths at 50−70 °C.
Having had no success so far in finding the b-type spectrum,
attempts to find a-type R-branch transitions were made. The
a
R-high-K−1 members are modulated at very low Stark fields.
This is a great advantage since most of the transitions belonging
to the spectrum of 2-oxazoline is not modulated and will not
contribute to the spectrum at such low fields. Spectra were
therefore recorded at Stark field strengths between 50 and 120
V/cm. By heating the samples with a heat gun and at the same
time pumping on the sample for several minutes and then
admitting fumes to the cell and immediately thereafter
recording spectrum, success was finally achieved. A typical
example is shown in Figure 2 of coalescing high-K−1 pairs of the
J = 16 ← 15 transition taken at a Stark field strength of about
50 V/cm. The intensity of the spectrum, which was always
inferior to the intensity of that of 2-oxazoline, declined rapidly
but was sufficient to allow measurements to be made in a
period of about 10−15 min after the cell had been filled with
fresh sample in the manner just described. Confirmation of the
assignments were made by RFMWDR45 experiments in many
cases, as well as fit to Watson’s A-reduction Hamiltonian51
using Sørensen’s program Rotfit.58 A typical example of a
RFMWDR spectral confirmation of an assignment is shown in
Figure 3.
Conformer V is relatively asymmetric (κ ≈ −0.83), and this
allowed fairly accurate predictions of the frequencies of b-type
lines to be made using only the aR-lines. The strong b-type lines
generally have slow Stark effects so assignments have to be
made at comparatively high Stark fields. The entire 2-oxazoline
spectrum is modulated under these experimental conditions
resulting in many overlaps. However, a large number of b-type
lines were unambiguously assigned and included in the leastsquares fit, which ultimately comprised 397 transitions shown
in Table 14S of the Supporting Information. The maximum
value of the principal quantum number was Jmax = 37, and the
maximum value of K−1 was 14. The Watson spectroscopic
constants are listed in Table 3. It was possible to get accurate
values for the five quartic centrifugal distortion constants. Two
sextic centrifugal distortion constants, ΦKJ and ΦK, were also
determined. Further sextic constants were preset at zero in the
least-squares fit.
Figure 2. Portion of the MW spectrum taken at a field strength of
about 55 V/cm demonstrating the complexity of the spectrum. The
high-K−1 absorption lines associated with the J = 16 ← 15 a-type
transition occur in this region. Values of the K−1 pseudo quantum
number are listed above several peaks belonging to the ground
vibrational state. The transitions with K−1 quantum numbers 13 and
14 almost coalesce at 106 316 MHz (far left). Nearly all the remaining
unlabeled transitions belong to 2-oxazoline. The intensity is in
arbitrary units.
Figure 3. RFMWDR spectrum of the J = 115 ← 105 pair of transitions
using a radio frequency of 5.65 MHz. The frequency of the 115,7 ←
105,6 transition is 73308.62 MHz, and the frequency of the 115,6 ←
105,5 line is 73317.71 MHz. The intensity is in arbitrary units.
It is seen that the CCSD(T) rotational constants (Table 2,
last column) deviate from the ground-state constants (Table 3)
by +0.82, −0.61, and −0.34%, in the cases of A, B, and C,
respectively. A difference of this order of magnitude is expected
because the ground-state and the CCSD(T) rotational
constants are defined differently. The ground-state constants
are effective parameters, whereas the CCSD(T) rotational
constants are calculated from an approximate equilibrium
structure.
Differences between the ground-state and the approximate
equilibrium rotational constants were obtained in the MP2
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The Journal of Physical Chemistry A
Article
Table 3. Spectroscopic Parametersa of the Ground
Vibrational State of Conformer V of HOCH2CH2NC
A (MHz)
B (MHz)
C (MHz)
ΔJ (kHz)
ΔJK (kHz)
ΔK (kHz)
δJ (kHz)
δK (kHz)
ΦKJ (Hz)
ΦKb (Hz)
rmsc
Nd
significantly higher energy than V. Searches for the other forms
(II−IV) were also negative. It should be noted that the
experimental conditions were unfavorable due to the
unavoidable presence of 2-oxazoline.
10949.8714(73)
3646.1213(15)
2974.9662(15)
5.1109(29)
−31.4734(92)
85.66(19)
1.60889(18)
10.0084(64)
−0.203(85)
−5.2(17)
1.433
397
■
DISCUSSION
Several forces seem to determine the conformational properties
of 2-isocyanoethanol. Oxygen is the second and nitrogen is the
third most electronegative element with Pauling electronegativities of 3.44 and 3.04, respectively.59 The O−C−C−N
sc conformation found in the three rotamers III−V should
therefore be favored by the gauche effect,60 which predicts that
a conformer having a maximum of sc interactions between
adjacent electron pairs and/or polar bonds have the lowest
energy.
Intramolecular hydrogen bonding is possible only in V. The
CCSD(T) nonbonded distance between the hydrogen atom
(H10) of the hydroxyl group and the nitrogen atom (N3) of
the isocyanide group is 256 pm and the distance between the
same hydrogen atom and the carbon atom of this group (C4) is
298 pm (Table 10S of the Supporting Information). The three
atoms H10, C4, and N3 form an angle of 57.8°, and the angle
between the three atoms H10, N3, and C4 is 99.2°. The
relatively long nonbonded distances between H10 on the one
side and N3 and C4 on the other and the unfavorable angles
between these atoms indicate that covalent contribution to the
internal hydrogen bond is not of great importance.
An interesting structural feature is the fact that the very polar
H10−O5 and N3C4 bonds with bond moments of 5.04 and
10.0 × 10−30 C m,61 respectively, are 2.1° from being parallel, as
derived from the structure (Table 1). C4 is the negative end
(formal charge = −1) and N3 the positive end (formal charge =
+1) of the isocyano group. This means that the bond dipoles of
the hydroxyl group and the isocyano groups are almost
antiparallel, which is ideal for a stabilizing dipole−dipole
interaction. This is assumed to be a major contribution to the
internal hydrogen bond making V the global energy minimum.
A rough estimate of the strength of the internal hydrogen
bond can be derived from comparing conformers IV and V,
which differs by a rotation of about 120° about the C1−O5
bond followed by minor structural adjustments. The gauche
effect should be about the same in IV and V because both have
an O5−C1−C2−N3 sc orientation. The hydrogen bond
strength is therefore estimated to be roughly equal to the
CCSD(T) energy difference between these two forms, namely,
∼10 kJ/mol (Table 2).
a
A-reduction and Ir-representation. Uncertainties represent one
standard deviation. Spectrum is listed in Table 14S of the Supporting
Information. bRemaining sextic centrifugal distortion constants preset
at zero in the least-squares fit. cRoot-mean-square deviation defined as
rms2 = Σ[(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. dNumber of transitions used in the fit.
calculations. The MP2 equilibrium constants are larger than the
ground-state constants by 15.87, 43.83, and 31.46 MHz in the
cases of A, B, and C, respectively (Table 5S of the Supporting
Information). Adding these values to the ground-state rotational constants (Table 3) one gets the following values for the
approximate equilibrium rotational constants: Ae = 10965.76,
Be = 3689.97, and Ce = 3006.42 MHz. Ideally, these rotational
constants and the CCSD(T) constants (Table 2, last column)
should be very close because both represent an approximate
equilibrium structure. However, the equilibrium constants are
larger than the CCSD(T) constants by 0.96, 0.58, and 0.71%, in
the cases of A, B, and C, respectively. These deviations are
caused by the shortcomings of the CCSD(T) and MP2
methods. Obviously calculations at even higher methodological
levels than CCSD(T) and MP2 are needed to reproduce
rotational constants at MW accuracy. Unfortunately, this is not
possible given our computational resources.
A comparison of experimental and MP2 quartic centrifugal
distortion constants (Table 2) and their experimental counterparts (Table 3) are in order. Deviations between 2.9 (ΔJK) and
10.0% (δK) are found for these parameters. The experimental
sextic centrifugal distortion constants are too inaccurate to
warrant comparison with their theoretical equivalents.
Searches for Further Conformers. The spectrum of a
conformer similar to I, with an energy 2.7(4) kJ/mol higher
than that of the hydrogen-bonded form of HOCH2CH2CN,
was assigned.23 In the present case, the CCSD(T) method
predicts the hypothetical conformer I to be 5.76 kJ/mol higher
in energy than V (Table 2). The statistical weight of I is half
that of V. However, I is predicted to have a comparatively large
μa ≈ 10.8 × 10−30 C m (Table 2), about 2.5 times as large as μa
of V (4.4 × 10−30 C m (Table 2)). An aR-spectrum similar to
that of I was expected for V, which has an asymmetry
parameter κ ≈ −0.988. Using this all information, the aRspectrum of I was estimated to be very roughly 30% of the
corresponding spectrum of V. Extensive searches for the aRspectrum of the hypothetical conformer I were undertaken at
very low Stark field strengths, but no assignments were
obtained. This is taken as an indication that I indeed has a
■
ASSOCIATED CONTENT
S Supporting Information
*
Results of the theoretical calculations, including electronic
energies; molecular structures; dipole moments; harmonic and
anharmonic vibrational frequencies; rotational and centrifugal
distortion constants; and 14N nuclear quadrupole coupling
constants. Microwave spectrum of V. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*(H.M.) Tel: +47 2285 5674. Fax: +47 2285 5441. E-mail:
harald.mollendal@kjemi.uio.no.
Notes
The authors declare no competing financial interest.
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■
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ACKNOWLEDGMENTS
We thank Anne Horn for her skillful assistance. Elin Katinka
Dankel is thanked for taking NMR spectra for us. This work has
been supported by the Research Council of Norway through a
Centre of Excellence Grant (Grant No. 179568/V30). It has
also received support from the Norwegian Supercomputing
Program (NOTUR) through a grant of computer time (Grant
No. NN4654K).). J.-C.G. thanks the Centre National d’Etudes
Spatiales (CNES) for financial support.
■
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