Article
pubs.acs.org/JPCA
Microwave and Quantum-Chemical Study of Conformational
Properties and Intramolecular Hydrogen Bonding of 2‑Hydroxy-3Butynenitrile (HCCCH(OH)CN)
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
‡
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 spectra of 2-hydroxy-3-butynenitrile, HCCCH(OH)CN,
and a deuterated species, HCCCH(OD)CN, have been investigated in the 38−120
GHz spectral region. Three rotameric forms, each stabilized by intramolecular hydrogen
bonds, are possible for this compound. The hydrogen atom of the hydroxyl group is
hydrogen-bonded to the π electrons of the alkynyl group in one of these conformers, to the π
electrons of the cyano group in the second rotamer, and to both of these groups
simultaneously in the third conformer. The microwave spectra of the parent and deuterated
species of last-mentioned form have been assigned, and accurate values of the rotational and
quartic centrifugal distortion constants of these species have been determined. The spectra of two vibrational excited states of this
conformer have also been assigned, and their frequencies have been determined by relative intensity measurements. Quantumchemical calculations at the MP2/cc-pVTZ and CCSD/cc-pVQZ levels were performed to assist the microwave work. The
theoretical predictions were generally found to be in good agreement with observations.
■
CH3CH(OH)CN,23,24 and (CH3)2C(OH)CN25 as well as
the alkynes HOCH 2 CCH, 26 H 3 CCH(OH)CCH, 27
HOCH2CH2CCH,28,29 and HOCH2CH2CH2CCH.9 The
situation in HBN is more complex than in these compounds
because both the nitrile and alkynyl groups can be involved in
internal hydrogen bonding. Rotation about its C−O bond may
in fact lead to the three rotameric forms depicted in Figure 1,
with atom numbering indicated on conformer I.
The H7−C2−O8−H9 dihedral angle can conveniently be
used to describe the conformational isomerism. This angle is
about +60° in I, approximately 180° in II, and near −60° in III.
One intramolecular hydrogen bond between H9 and the π
electrons of the C1N6 triple bond is present in I. A similar
situation is found in III, where H9 is hydrogen-bonded to the π
electrons of the C3C4 triple bond. In conformer II, H9 is
hydrogen-bonded to both of these triple bonds at the same
time. It should be noted that HBN is chiral and exists in the
mirror-image R and S configurations, whose corresponding
conformers have identical MW spectra. Figure 1 shows the
molecule in the S configuration.
There is another important reason for undertaking a study of
HBN: Cyanohydrins are versatile building blocks in organic
synthesis,30−32 but only a few gas-phase conformational and
structural studies have been reported for them.22−24 Further
INTRODUCTION
Intramolecular hydrogen bonding has for a long time been a
favorite research theme of the Oslo laboratory, and a number of
hydrogen bonds with a wide variety of hydrogen donors and
acceptors have been investigated over the years. In the past
several years, we have reported microwave (MW) spectra of the
following molecules having internal hydrogen bonds: 2isocyanoethanol (HOCH2CH2NC),1 2-aminopropionitrile
(H2NCH(CH3)CN),2 (2-chloroethyl)amine
(ClCH2CH2NH2),3 (chloromethyl)phosphine (ClCH2PH2),4
propargylselenol (HCCCH2SeH),5 2-propene-1-selenol
(H2CCHCH2SeH),6 2,2,2-trifluoroethanethiol
(CF3CH2SH),7 3-butyne-1-selenol (HSeCH2CH2CCH),8
4-pentyn-1-ol (HOCH2CH2CH2CCH),9 (Z)-3-mercapto-2propenenitrile (HSCHCHCN),10 (Z)-3-amino-2-propenenitrile (H 2 NCHCHCN), 1 1 3-butyne-1-thiol
(HSCH2CH2CCH),12 (methylenecyclopropyl)methanol
(H2CC3H3CH2OH),13 2-chloroacetamide
(ClCH 2 CO NH 2 ), 1 4 an d cyclopr opy lm et hy lseleno l
(C3H5CH2SeH).15 References to earlier work by us and others
are found in these papers as well as in several reviews.16−20
The cyanohydrin 2-hydroxy-3-butynenitrile, HCCCH(OH)CN, henceforth denoted as HBN, was chosen for
study this time. It is well-established that the π electrons of
nitrile (R−CN) and alkynyl (R−CC−R′) groups can act
as acceptors in intramolecular hydrogen bonds where an
alcohol group is proton donor. Examples include the nitrile
HOCH2CH2CN21 and the cyanohydrins HOCH2CN,22
© 2015 American Chemical Society
Received: November 11, 2014
Revised: January 2, 2015
Published: January 5, 2015
634
DOI: 10.1021/jp5112923
J. Phys. Chem. A 2015, 119, 634−640
Article
The Journal of Physical Chemistry A
on one side and glacial acetic acid (8.0 mL) on the other side
were introduced simultaneously via the two dropping funnels
over the course of 20 min. The suspension, stirred at room
temperature for 5 h, turned brown. It was then filtered, and the
solid was washed with 50 mL of ether. The yellow solution was
washed with water (4 × 20 mL) and brine (1 × 20 mL) before
drying over magnesium sulfate and evaporation of the solvent
in vacuo. Purification was performed by slow distillation on a
vacuum line (0.1 mbar) with gentle heating of the yellowbrown liquid to 40 °C and selective trapping of the colorless
cyanohydrin in a trap immersed in a bath cooled at −15 °C.
Yield: 5.67 g (70 mmol, 70%). This cyanohydrin can be stored
for months at −20 °C. 1H NMR (CDCl3, 400 MHz) δ 2.80 (d,
1H, 4JHH = 2.6 Hz, CCH), 4.27 (d, 1H, 3JHH = 6.8 Hz, OH),
5.23 (dd, 1H, 3JHH = 6.8 Hz, 4JHH = 2.6 Hz, CHO). 13C NMR
(CDCl3, 100 MHz) δ 50.7 (1JCH = 157.5 Hz (d), OCH), 75.3
(2JCH = 51.3 Hz (d), CCH), 76.8 (1JCH = 257.5 Hz (d), CCH),
115.8 (CN).
The deuterated species HCCCH(OD)CN was produced by conditioning the MW cell with heavy water and then
introducing the parent species. This resulted in roughly 50%
exchange of the hydrogen atom of the hydroxyl group with
deuterium.
Spectroscopic Experiments. The vapor pressure of HBN
is roughly 25 Pa at 22 °C. The spectrum was recorded at a
pressure of 5−10 Pa. The samples of HBN were stored in a
freezer at −80 °C. They had to be warmed to room
temperature in order to fill the MW cell with fresh sample.
During this process, the compound decomposed partly to
propynal and hydrogen cyanide, both of which were identified
by their reported MW spectra.42−46 The intensity of the
spectrum of propynal increased by a factor of 3 in the cell over
the course of 20 min, which showed that the decomposition
continued in this environment, possibly catalyzed by the cell
walls. The cell was therefore filled frequently with fresh
portions of HBN.
The MW spectrum was recorded using the Stark MW
spectrometer of the University of Oslo described in detail
elsewhere.47 Only salient features are reported here, namely,
the accuracy of this spectrometer, which is 0.10 MHz for
isolated lines, and the resolution, which is about 0.5 MHz for
strong transitions. Measurements were made in the 38−120
GHz spectral region. Radiofrequency microwave doubleresonance (RFMWDR) spectra48 were recorded to obtain
unambiguous assignments of selected transitions.
Figure 1. Models of three conformers of 2-hydroxy-3-butynenitrile.
Atom numbering is given on conformer I. The MW spectrum of II was
assigned.
studies, such as the present, might help us understand better
the chemical behavior of this important functional group.
It should also be mentioned that cyanohydrins are of
astrochemical interest. It has already been shown that the
simplest cyanohydrin, hydroxyacetonitrile (HOCH2CN), is
formed under astrophysical-like conditions from formaldehyde
(H2CO) and hydrogen cyanide (HCN).33 Formally, HBN
can be considered to be a hydrogen cyanide adduct to propynal
(HCCCHO). Propynal is an interstellar compound,34−36
which is also the case for the hydrogen cyanide molecule, which
is ubiquitous in the universe.37 The present study of its
rotational spectrum should be very helpful for a potential future
identification of interstellar HBN.
The several interesting aspects of HBN motivated this first
MW investigation. MW spectroscopy was chosen as our
method of investigation because of its unsurpassed accuracy
and resolution, which is ideal for this type of studies. The MW
investigation was assisted by advanced quantum-chemical
modeling, which is very useful for the assignment of complex
MW spectra because rather accurate values of spectroscopic
constants can be predicted in this manner and can be of
considerable use in the assignment procedure. Information
about parameters that cannot be obtained experimentally can
also be derived from these calculations, allowing a more
profound analysis of the problem at hand.
■
■
RESULTS
Quantum-Chemical Calculations. Frozen-core MP249
and CCSD50−53 computations were executed using the Abel
cluster of the University of Oslo. The MP2 calculations were
performed with the Gaussian 09 suite of programs54 while the
CCSD computations were done employing the Molpro
program,55 observing the default convergence criteria of the
two programs. The correlation-consistent cc-pVTZ triple-ζ and
cc-pVQZ quadruple-ζ basis sets56 were employed in the MP2
and CCSD calculations, respectively.
An MP2/cc-pVTZ potential function for rotation of the
hydroxyl group (Figure 2) was obtained by stepping the H7−
C2−O8−H9 dihedral angle (see Figure 1) in 10° intervals with
all of the remaining structural parameters allowed to vary freely.
This function has three minima corresponding to conformers
I−III. The MP2 structure of each conformer was optimized,
and the results are given in Tables S1−S3 in the Supporting
EXPERIMENTAL SECTION
Synthesis. A racemic mixture of HBN was synthesized
(Scheme 1) as previously reported38,39 with some small
Scheme 1
modifications. Into a 250 mL three-neck flask equipped with
a magnetic stirring bar, a nitrogen inlet and two dropping
funnels were introduced dry powdered sodium cyanide (0.163
mol, 8.0 g) and dry diethyl ether (125 mL). The suspension
was cooled to 0 °C, and 1 mL of glacial acetic acid was added.
Propiolaldehyde40,41 (0.10 mol, 5.4 g) in 10 mL of diethyl ether
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The Journal of Physical Chemistry A
Table 1. CCSD/cc-pVQZ Structures of Conformers I, II, and
III of HCCCH(OH)CN
I
C1−C2
C1−N6
C2−C3
C2−H7
C2−O8
C3−C4
C4−H5
O8−H9
C1−C2−C3
C1−C2−H7
C1−C2−O8
C3−C2−H7
C3−C2−O8
H7−C2−O8
C2−O8−H9
C2−C1−N6
C2−C3−C4
C3−C4−H5
Figure 2. Relative MP2/cc-pVTZ electronic energy as a function of
the H7−C2−O8−H9 dihedral angle.
Information. The MP2 computations predict that the global
energy minimum occurs for II at an H7−C2−O8−H9 dihedral
angle of 185.7° (−174.3°), whereas this angle is 59.6° for I and
284.4° (−75.6°) for III. Conformer II has an MP2 electronic
energy that is 6.91 kJ/mol less than the energy of I and 6.65 kJ/
mol less that that of III. Upon correction for zero-point
vibrational effects, these differences become 6.30 and 5.79 kJ/
mol, respectively. The potential function has three maxima
(transition states). The characteristics of these transition states
were calculated at the MP2 level, and the results are displayed
in Tables S4−S6 in the Supporting Information. The transition
states are located at H7−C2−O8−H9 dihedral angles of 111.3,
256.2, and 355.4° with electronic energies that are 10.47, 7.16,
and 12.10 kJ/mol higher than the energy of the global
minimum, respectively.
MP2 calculations were also undertaken to obtain harmonic
and anharmonic vibrational frequencies, the vibration−rotation
constants (the α’s),57 the Watson quartic and sextic centrifugal
distortion constants,58 the rotational constants r0 and re, and
dipole moments (see Tables S1−S3 in the Supporting
Information). The procedure recommended by McKean et
al.59 was followed when calculating the α’s and the centrifugal
distortion constants.
The optimized CCSD/cc-pVQZ structures of I−III are listed
in Table 1. Further details of the CCSD calculations are listed
in Tables S7−S9 of the Supporting Information. Table 2
contains the rotational constants calculated from the CCSD
structures, the MP2 quartic centrifugal distortion constants,58
the CCSD dipole moments, and the CCSD electronic energy
differences.
A few remarks on the CCSD structures are in order. The
O8−H9 bond length is 95.8 pm in all forms of HBN (Table 1),
which can be compared with the equilibrium O−H bond length
in methanol (95.6 pm).60 The slight elongation of this bond
length from its value in methanol is expected for the title
compound because of the internal hydrogen bonds in the three
conformers. The CCSD bond lengths of the C1N6 triple
bond are 115.1−115.0 pm, while re = 115.54 pm has been
reported for the CN bond in CH3CN.61 The re value of the
C3C4 triple bond in acetylene is 120.289 pm,62 which
slightly longer than the bond lengths of 119.9−120.0 pm found
in the present case.
The CCSD method predicts II to be the global minimum
(Table 2). The electronic energies of I and III are 6.08 and 5.73
kJ/mol higher, respectively. These values are very similar to the
II
Bond Distances (pm)
148.5
148.5
115.1
115.1
146.6
147.1
109.3
108.9
140.6
140.3
119.9
120.0
106.2
106.2
95.8
95.8
Bond Angles (deg)
110.5
109.9
106.7
107.2
111.3
111.4
108.3
109.1
108.9
113.2
111.1
105.8
108.5
108.6
178.1
178.5
178.4
177.0
179.4
179.9
Dihedral Angles (deg)
−59.7
70.0
178.2
−54.5
59.0
−173.8
−58.5
70.0
177.9
−54.3
177.9
−54.3
C1−C2−O8−H9
C3−C2−O8−H9
H7−C2−O8−H9
N6−C2−O8−H9
C4−C2−O8−H9
H5−C2−O8−H9
III
147.7
115.0
147.1
109.3
140.8
120.0
106.2
95.8
110.7
106.7
107.3
108.3
112.9
110.9
107.8
179.4
176.7
179.0
168.8
46.7
−75.1
169.4
45.6
45.5
Table 2. Theoretical Parametersa of Spectroscopic Interest
of Conformers I, II, and III of HCCCH(OH)CN
I
A
B
C
DJ
DJK
DK
d1
d2
μa
μb
μc
μtot
ΔE
II
Rotational Constants (MHz)
5909.8
5773.6
2856.6
2881.4
2026.2
2038.4
1.39
1.42
−8.03
−7.31
27.2
23.8
−0.597
−0.600
−0.0369
−0.0406
Dipole Moments (Db)
1.83
3.23
1.42
0.51
1.69
0.08
2.87
3.27
Relative Electronic Energiesc (kJ/mol)
6.08
0.0
III
5938.5
2852.7
2027.7
1.36
−7.64
26.6
−0.582
−0.0393
4.38
1.31
1.43
4.78
5.73
a
CCSD/cc-pVQZ rotational constants, dipole moments, and relative
electronic energies and MP2/cc-pVTZ centrifugal distortion constants
are shown. b1 D = 3.33564 × 10−30 C m. cRelative to conformer II.
Not corrected for zero-point vibrational effects.
MP2 results above. The lower energy of II relative to I and III
may reflect the fact that H9 is simultaneously bonded to two πelectron systems in II, whereas only one hydrogen bond is
present in the two other conformers. The prediction that I and
III have nearly the same energy indicates that the hydrogen
bonds in these two conformers have similar strengths.
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The Journal of Physical Chemistry A
Assignment of the Ground-State Spectrum of II. The
decomposition of HBN to propynal and hydrogen cyanide was
a severe complication because propynal has a very strong MW
a-type R-branch spectrum as well as a weaker b-type spectrum,
resulting in a dense spectrum over the entire investigated
spectral interval (38−120 GHz). Propynal also has several low
vibrational frequencies, and the MW spectra of excited states of
these vibrations added to the spectral richness. All of this
resulted in numerous overlaps with transitions of HBN. It was
also unfortunate that most of the transitions belonging to
propynal are much stronger than those of HBN. Another
negative factor was that the decomposition of HBN resulted in
a relatively rapid increase in the intensity of the propynal lines
accompanied by a simultaneous reduction in the intensities of
the HBN transitions.
It can be seen from Table 2 that the lowest-energy conformer
II has a comparatively large μa component of about 3.2 D, while
μb and μc are much smaller. We therefore concentrated on
finding a-type R-branch lines of this conformer using the
rotational and quartic centrifugal distortion constants of Table
2 to predict their approximate frequencies. RFMWDR searches
for selected transitions in the frequency region above 75 GHz
soon met with success. A typical example of an RFMWDR
identification is exemplified by the J = 2311 ← 2211 pair of
transitions shown in Figure 3.
CCSD dipole moment components are rather small (0.51 and
0.08 D, respectively), resulting in very weak transitions.
Accurate values of the rotational and quartic centrifugal
distortion constants have been obtained (Table 2). Attempts
were made to include sextic centrifugal distortion constants in
the least-squares fits, but the resulting constants had such large
standard deviations that it was decided to limit the fits to
include only quartic centrifugal distortion constants. The
spectroscopic constants shown in Table 3 should predict the
frequencies of rotational transitions that occur outside the
investigated spectral interval (38−120 GHz) with a high degree
of precision.
The CCSD rotational constants of the three conformers
(Table 2) have similar values with one exception, namely, the A
rotational constant of II, which differs from the two other A
constants by more than 100 MHz. The experimental effective
(r0) A constant (Table 3) is much closer to the CCSD A
constant of II than to the corresponding constants of I and III.
It is therefore certain that the spectrum in Table S10 in the
Supporting Information indeed belongs to II. The rotational
constants of the deuterated species HCCCH(OD)CN
discussed below confirm this assignment.
The effective (r0) ground-state rotational constants (Table 3)
are smaller than the CCSD constants (Table 2) by 16.7, 7.2,
and 6.5 MHz in the cases of A, B, and C, respectively. The
CCSD rotational constants are approximations of the
equilibrium counterparts, which are usually found to be smaller
than the effective constants since r0 values are normally longer
than re values, resulting in larger principal moments of inertia
and smaller values for the effective rotational constants. The
MP2 method predicts these differences to be 14.0, 5.9, and 7.6
MHz (Table S2 in the Supporting Information), in fair
agreement with the present findings.
There is very good agreement between the experimental
(Table 3) quartic centrifugal distortion constants and the MP2
equivalents (Table 2) with one exception, namely, d2, but this
experimental parameter is the least accurate centrifugal
distortion constant.
Vibrationally Excited States. The RFMWDR spectrum
revealed transitions belonging to several vibrationally excited
states. The spectra of two of these were assigned in the same
manner as discussed above for the ground-state spectrum. The
spectroscopic constants are included in Table 3, and the spectra
are found in Tables S11 and S12 in the Supporting
Information. The vibration−rotation constants α57 found by
subtraction of the excited-state rotational constants from their
ground-state equivalents are αA = −9.302(88) MHz, αB =
−9.5343(88) MHz, and αC = −0.401 MHz, which can be
compared with the MP2 values −11.2, −8.7, and −0.2 MHz,
respectively, calculated for the lowest bending vibration (Table
S2 in the Supporting Information). Rough relative intensity
measurements yielded 110(25) cm−1 for this vibration,
compared with the anharmonic MP2 frequency of 127 cm−1
(Table S2).
The values αA = −10.70(14) MHz, αB = 6.5467(98) MHz,
and αC = −2.670 MHz were calculated in a similar manner
from the entries in Table 3 for the other excited state. The
corresponding MP2 parameters of the second-lowest bending
fundamental are −16.0, 7.6, and −3.1 MHz, respectively.
Relative intensity measurements yielded ca. 180 cm−1 for this
vibration, in accord with an anharmonic fundamental of 195
cm−1 (Table S2). It is concluded that the MP2 calculations in
Figure 3. RFMWDR spectrum of the J = 2311 ← 2211 pair of
transitions. The RF was 5.85 MHz. The frequencies are 116237.69 and
116247.76 MHz for the 2311,13 ← 2211,12 and 2311,12 ← 2211,11
transitions, respectively. The intensity is in arbitrary units.
The frequencies of transitions having values of the pseudo
quantum number K−1 larger than about 10, which occur in this
spectral region, could then be predicted rather precisely. These
transitions appear as coalescing pairs with very rapid Stark
effects, and this property was useful in obtaining a secure
assignment. Further assignments were then gradually made. A
total of 275 aR transitions (listed in Table S10 in the
Supporting Information) were ultimately assigned and leastsquares-fitted to Watson’s Hamiltonian in the S-reduction Irrepresentation form58 employing Sørensen’s program Rotfit.63
The resulting spectroscopic constants are shown in Table 3.
The maximum value of J is 24, and the maximum value of K−1
is 23. None of the transitions displayed a hyperfine structure
due to quadrupole coupling of the 14N nucleus. Searches for band c-type lines were made, but none could be unambiguously
assigned. This is not surprising because the corresponding
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The Journal of Physical Chemistry A
Table 3. Spectroscopic Constantsa of Vibrational States of Conformer II of HCCCH(OH)CN and HCCCH(OD)CN
parent
Av (MHz)
Bv (MHz)
Cv (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
d1 (kHz)
d2 (kHz)
rmsb
Nc
deuterated
ground
lowest bend
second-lowest bend
ground
5756.892(46)
2874.2429(40)
2031.8990(50)
1.4360(35)
−7.358(16)
20.52(61)
−0.5912(34)
−0.0275(23)
1.296
275
5766.194(76)
2883.7772(78)
2032.300(10)
1.378(12)
−6.957(56)
29.4(21)
−0.6211(97)
−0.0648(87)
1.338
142
5767.59(13)
2867.6962(98)
2034.569(12)
1.202(10)
−4.606(43)
20.8(16)
−0.708(13)
0.142(10)
1.570
138
5518.987(85)
2854.6171(83)
2011.548(11)
1.3811(81)
−6.137(34)
20.5(11)
−0.5563(77)
−0.0274(45)
1.462
155
a
S-reduction Ir representation.58 Uncertainties represent one standard deviation. The spectra are listed in Tables S10−S13 in the Supporting
Information. bRoot-mean-square deviation, defined as 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 least-squares fit, and P is the number of
spectroscopic constants used in the fit. cNumber of transitions used in the fit.
and the π-electron systems of the triple bonds. There are three
polar groups in HBN that may interact. The most polar of them
is the cyano group, which has a bond dipole moment as large as
3.6 D66 with nitrogen as the negative end. The bond dipole
moment of the hydroxyl group is 1.5 D,66 while propyne
(CH3CCH) has a dipole moment of 0.7804 D with CH as
the negative end.67
In conformer I, the CCSD angle between the C1N6 and
O8−H9 bonds is 67.8° (from Cartesian coordinates in Table
S7 in the Supporting Information). The two groups are
oriented in such a manner that dipole−dipole stabilization
should be significant. The O8−H9 and C3C4 bonds are 3°
from being parallel, while the associated bond dipole moments
are antiparallel, resulting in a minor repulsion. The nonbonded
distances between the H9 atom and the C1 and N6 atoms are
256 and 340 pm, respectively (Table S7), which can be
compared to the sum of the Pauling van der Waals radius68 of
hydrogen (120 pm) and the half-thickness of an aromatic
molecule (170 pm), which is 290 pm. This suggests that the
covalent stabilization between H9 and the π electrons of the
C1N6 bond is not a large effect.
Conformer III resembles I. The bond dipole moments of the
O8−H9 and C3C4 groups stabilize III in this case, while the
electrostatic interaction caused by the O8−H9 and C1N6
groups destabilizes III. The nonbonded distance between H9
and C3 is 240 pm and the distance between H9 and C4 is 334
pm, resulting in a weak covalent stabilization.
In conformer II, the electrostatic interactions between the
O8−H9 group and both the C1N6 and C3C4 groups are
similar to those found in I and III, respectively. The covalent
stabilization with the π electrons of the two groups is also
similar. The much better situation for both electrostatic and
covalent contributions to the hydrogen bonding in II makes
this conformer several kJ/mol lower in energy than I and III, in
agreement with the present experimental findings.
this case were able to predict the correct sign and order of
magnitude for the α constants.
Deuterated Species. The spectrum of the deuterated
species HCCCH(OD)CN, which was assigned in a
straightforward manner, is listed in Table S13 in the Supporting
Information, and the spectroscopic constants are displayed in
Table 3. The substitution coordinates64 of the hydrogen atom
of the hydroxyl group in the principal axis system were
calculated from the rotational constants of the parent and
deuterated species (Table 3) using Kraitchman’s equations65
and were found to be |b| = 158.651(51) pm and |c| =
113.542(51) pm, while the a coordinate has a small imaginary
value. The uncertainties (one standard deviation) were
calculated from the standard deviations of the rotational
constants. The CCSD values of these coordinates are |a| = 8.39
pm, |b| = 157.52 pm, and |c| = 113.89 pm (Table S8 in the
Supporting Information), in good agreement with the
substitution coordinates above. The corresponding CCSD
values for conformer I are |a| = 78.85 pm, |b| = 213.39 pm, and
|c| = 1.39 pm (Table S7 in the Supporting Information), while
the values for III are |a| = 77.17 pm, |b| = 208.89 pm, and |c| =
16.54 pm (Table S9 in the Supporting Information). The
substitution coordinates of the hydroxyl group again show
conclusively that the assigned spectra belong to II and that
confusion with I or III is out of the question.
Searches for the Spectra of I and III. Conformer III has a
dipole moment component along the a principal inertial axis as
large as 4.4 D according to the CCSD method (Table 2).
Searches were performed for selected aR transitions of this
rotamer using the RFMWDR technique, but no characteristic
double-resonance signals were observed. This is taken as an
indication that there is a substantial energy difference between
III and II, producing insufficient intensities for the spectrum of
III. This is in accord with the MP2 and CCSD calculations,
which predict an energy difference of about 6 kJ/mol. Searches
for I were also negative. This form is also significantly higher in
energy than II as discussed above.
■
■
ASSOCIATED CONTENT
S Supporting Information
*
DISCUSSION
Several intramolecular forces appear to determine the
conformational properties of HBN. Internal hydrogen bonding
is present in all three forms. The hydrogen bonds are composed
primarily of two types of interactions, namely, dipole−dipole
interactions and covalent interactions between the H9 atom
Results of the theoretical calculations, including electronic
energies, molecular structures, dipole moments, harmonic and
anharmonic vibrational frequencies, rotational and centrifugal
distortion constants, and rotation-vibration constants, and
microwave spectra of the ground and vibrationally excited
states of the parent species and the ground state of the
638
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Article
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■
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. This work was
supported by the Research Council of Norway through a
Centre of Excellence Grant (Grant 179568/V30) and by the
Norwegian Supercomputing Program (NOTUR) through a
grant of computer time (Grant NN4654K).). J.-C.G. thanks the
French National Program Physique et Chimie du Milieu
Interstellaire (PCMI (INSU-CNRS)) and the Centre National
d’Etudes Spatiales (CNES) for grants.
■
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