Article
pubs.acs.org/JPCA
Rotational Spectrum, Conformational Composition, and Quantum
Chemical Calculations of Cyanomethyl Formate (HC(O)OCH2CN), a
Compound of Potential Astrochemical Interest
Svein Samdal,† Harald Møllendal,*,† and Sophie Carles‡
†
Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, P.O. Box 1033,
Blindern, NO-0315 Oslo, Norway
‡
Institut de Physique de Rennes, Département de Physique Moléculaire, UMR 6251 UR1-CNRS, Université de Rennes 1, Bâtiment
11 C, Campus de Beaulieu, F-35042 Rennes Cedex, France
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Publication Date (Web): August 12, 2015 | doi: 10.1021/acs.jpca.5b05285
S Supporting Information
*
ABSTRACT: The rotational spectrum of cyanomethyl formate (HC(O)OCH2CN) has been recorded in the 12−123 GHz spectral range. The
spectra of two conformers were assigned. The rotamer denoted I has a
symmetry plane and two out-of plane hydrogen atoms belonging to the
cyanomethyl (CH2CN) moiety. In the conformer called II, the cyanomethyl
group is rotated 80.3° out of this plane. Conformer I has an energy that is
1.4(6) kJ/mol lower than the energy of II according to relative intensity measurements. A large number of rotational transitions
have been assigned for the ground and vibrationally excited states of the two conformers and accurate spectroscopic constants
have been obtained. These constants should predict frequencies of transitions outside the investigated spectral range with a very
high degree of precision. It is suggested that cyanomethyl formate is a potential interstellar compound. This suggestion is based
on the fact that its congener methyl formate (HC(O)OCH3) exists across a large variety of interstellar environments and the fact
that cyanides are very prevalent in the Universe. The experimental work has been augmented by high-level quantum chemical
calculations. The CCSD/cc-pVQZ calculations are found to predict structures of the two forms that are very close to the Born−
Oppenheimer equilibrium structures. MP2/cc-pVTZ predictions of several vibration−rotation interaction constants were
generally found to be rather inaccurate. A gas-phase reaction between methyl formate and the cyanomethyl radical CH2CN to
produce a hydrogen atom and cyanomethyl formate was mimicked using MP2/cc-pVTZ calculations. It was found that this
reaction is not favored thermodynamically. It is also conjectured that the possible formation of cyanomethyl formate might be
catalyzed and take place on interstellar particles.
■
INTRODUCTION
Methyl formate (HC(O)OCH3) is a typical interstellar hotcore molecule abundant in massive star-forming regions. It was
found already in 1975 in Sagittarius B2 by means of its
rotational spectrum.1 Methyl formate has later been detected
toward low-mass star-forming regions2 and in cold clouds in the
Galactic center.3 The spectra of vibrationally excited states,4,5
13
C-isotopologues,6 and 18O-isotopologues7 of this interstellar
compound have been identified. A tentative identification of the
deuterated species DC(O)OCH3 in Orion has also been
reported.8
There are two rotameric forms of methyl formate. The O
C−O−C chain of atoms is synperiplanar (sp) in one of these
conformers9 and antiperiplanar (ap) in the second rotamer.10
The discussion above refers to the lower-energy OC−O−C
sp form, which is about 25 kJ/mol lower in energy than the ap
rotamer. Interestingly, a tentative detection in the interstellar
medium (ISM) of the higher-energy ap conformer has been
reported.10
The fact that methyl formate exists across a large variety of
interstellar sources suggests that further esters of formic acid
might also be formed in the ISM. Indeed, one additional such
© 2015 American Chemical Society
ester, ethyl formate (HC(O)OCH2CH3), has recently been
detected in two interstellar sources. 11,12 This finding
encouraged us to undertake this first investigation of the
rotational spectrum of yet another substituted methyl formate
ester, namely, cyanomethyl formate HC(O)OCH2CN,
hoping that this spectrum could form the basis for a potential
future identification of this compound in the ISM.
In cyanomethyl formate, one of the hydrogen atoms of the
methyl group of HC(O)OCH3 is substituted by a cyano group.
The cyano group is very prevalent in interstellar and
circumstellar compounds and found in about 25 of the
approximately 180 known compounds of this category.13 This
functional group is found in neutral molecules, cations, anions,
and radicals.13 The widespread occurrence of compounds
containing this group in the ISM is one reason why
cyanomethyl formate is suggested as a potential interstellar
compound.
Received: June 3, 2015
Revised: July 24, 2015
Published: July 24, 2015
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Article
The Journal of Physical Chemistry A
compounds, formic acid (HC(O)OH),14 and the cyanomethyl
radical (CH2CN),15 which might lead to the formation of
cyanomethyl formate (HC(O)OCH2CN).
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Publication Date (Web): August 12, 2015 | doi: 10.1021/acs.jpca.5b05285
Another reason for carrying out this research is that new and
much more sensitive radio astronomy observatories such as, for
example, the Atacama Large Millimeter/Submillimeter Array
(ALMA) in Chile have created a demand for rotational spectra
of candidate interstellar compounds such as the title
compound.
Cyanomethyl formate not only is of potential astrochemical
interest but also has interesting conformational properties,
which is associated with the rotation about its two C−O single
bonds. The quantum chemical calculations discussed below
indicate that there are three conformers of this compound.
These three rotamers are shown in Figure 1 and given Roman
■
EXPERIMENTAL SECTION
Compound and Spectroscopic Experiments. Cyanomethyl formate specified to be 99% pure was purchased from
Aldrich and used as received. The rotational spectrum, which
was studied using the Stark-modulated spectrometer of the
University of Oslo, revealed no impurities. Cyanomethyl
formate has a vapor pressure of roughly 60 Pa at room
temperature. The spectrum was recorded in the 12−123 GHz
spectral interval at a pressure of 5−10 Pa at room temperature,
or with the MW cell cooled to about 0 °C. It was not possible
to study the spectrum at lower temperatures, which would have
enhanced intensities, due to the rather low vapor pressure of
this compound. Salient features of the Oslo MW spectrometer,
which has previously been described in detail,16 is its accuracy
of 0.10 MHz for strong and isolated MW transitions and its
resolution of about 0.5 MHz. Radio-frequency microwave
double-resonance (RFMWDR) spectroscopy17 was also
performed with this spectrometer to confirm certain assignments.
■
RESULTS AND DISCUSSION
Quantum Chemical Methods. The Gaussian 0918 suite of
programs was used for the MP219 calculations, whereas the
Molpro20 set of programs was employed for the CCSD
computations. The frozen-core approximation was assumed in
all calculations and the default convergence criteria of the two
programs were always used. Dunning’s21 correlation-consistent
triple-ζ (cc-pVTZ) and quadruple-ζ (cc-pVQZ) basis sets were
used in the MP2 calculations, whereas the cc-pVQZ basis set
was chosen for the CCSD computations. The calculations were
performed employing the Abel cluster of the University of
Oslo.
Computational Conformational Analysis. The conformational characteristics of cyanomethyl formate can perhaps
best be described by the values of the C2−O4−C5−O6 and
O1−C2−O4−C5 dihedral angles. Two MP2/cc-pVTZ scans
were performed. In the first of these, the rotation about the
O4−C5 bond was studied by stepping the C2−O4−C5−O6
dihedral angle in 10° intervals, while keeping the O1−C2−
O4−C5 chain of atoms in the sp conformation (about 0°). All
structural parameters but the C2−O4−C5−O6 dihedral angle
were optimized. The resulting potential function, which is
shown in Figure 2, has two minima corresponding to
conformers II and I, respectively. The geometries of I and II
were optimized and their dipole moments, 14N nuclear
quadrupole coupling constants, harmonic and anharmonic
vibrational frequencies, Watson’s quartic and sextic centrifugal
distortion constants,22 rotation−vibration constants (α’s),23
and differences between effective (r0) and equilibrium (re)
rotational constants were computed; the results are listed in
Tables S1 and S2 of the Supporting Information.
It is seen from these two tables that conformer II has a MP2/
cc-pVTZ electronic energy that is 0.77 kJ/mol lower than that
of I. This difference becomes 0.18 kJ/mol (II lower-energy
form) after correction for the harmonic zero-point vibrational
energies given in the same two tables. The C2−O4−C5−O6
and O1−C2−O4−C5 dihedral angles were 180.0 and 0.0° for I
and 77.7 and 0.2° for II. The transition state for conversion
Figure 1. Models of the three conformers of cyanomethyl formate.
Atom numbering is indicated on conformer I. Microwave spectra of I
and II have been assigned, and the energy difference is 1.4(6) kJ/mol
with I as the lower-energy conformer. III is 23.6 kJ/mol higher in
energy than I according to CCSD/cc-pVQZ calculations. Its rotational
spectrum must consequently be extremely weak and it was not
assigned.
numerals for reference. Atom numbering is given on conformer
I. The O1−C2−O4−C5 and the C2−O4−C5−C6 dihedral
angles can conveniently be used to characterize the conformational properties of this compound. The O1−C2−O4−C5
dihedral angle is exactly 0° in I, 0.5° in II, and −179.6° in III,
whereas the C2−O4−C5−C6 dihedral angle is exactly 180° in
I, 80.3° in II, and 78.1° in III, according to the CCSD/ccpVQZ calculations discussed below.
The vast majority of the interstellar and circumstellar
compounds have been identified by their rotational spectra
and microwave (MW) spectroscopy was therefore chosen to
investigate the rotational spectrum of this compound in the
12−123 GHz spectral range. Moreover, rotational spectroscopy
is a superior conformational-analysis method due to its
unparalleled accuracy and resolution.
High-level quantum chemical calculations can now be used
to predict rather accurate rotational and centrifugal distortion
constants, which are useful in the assignment procedures of
complex rotational spectra. Reliable parameters not readily
available from experiment can sometimes be obtained from
these calculations as well. One example is reaction mechanisms
and we have also used quantum chemistry to investigate a
possible gas-phase reaction between two known interstellar
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The Journal of Physical Chemistry A
Figure 3. Relative electronic energy as a function of the C2−O4−C5−
C6 dihedral angle. The O1−C2−O4−C5 chain of atom is held in the
ap conformation. The minimum corresponds to III; see text.
Figure 2. Relative electronic energy as a function of the C2−O4−C5−
C6 dihedral angle. The O1−C2−O4−C5 chain of atom is held in the
sp conformation. The minimum at 77.7° corresponds to conformer II,
and the other minimum at 180° corresponds to I. The maximum at
129.7° has an energy that is only 3.55 kJ/mol higher than the energy of
II. The relatively flat portion of the potential curve between about 65°
and 180° implies that both I and II will undergo large amplitude
vibrations about the O4−C5 bond. Indeed, the MP2 anharmonic
torsional vibrations are 47.7 cm−1 in I and 62.2 cm−1 in II; see text.
characteristic C2−O4−C5−O6 and O1−C2−O4−C5 dihedral
angles are +78.3 and −179.1°, respectively, for III.
Computations of optimized CCSD/cc-pVQZ structures,
principal inertial-axes dipole moment components, total dipole
moment, and energy differences of the three conformers I−III
were carried out. Calculations of vibrational frequencies were
not done because they would be too costly at this high level of
theory. The CCSD structures, which are assumed to be close to
the Born−Oppenheimer equilibrium structures, and dipole
moments of the three forms are listed in Table 1, and further
results of these calculations are listed in Tables S5 − S7 of the
Supporting Information. The rotational constants calculated
from the CCSD structures in Table 1 are given in Tables 2 and
3 below together with their experimental equivalents. These
tables also contain the MP2 quartic centrifugal distortion
constants taken from Tables S1 and S2, as well as their
experimental counterparts.
Some of the results of these quantum chemical calculations
warrant comments. The CCSD computations predict conformer I to be the rotamer with the lowest electronic energy,
being 0.55 kJ/mol lower in energy than II, and 23.60 kJ/mol
lower in energy than III. The C2−O4−C5−O6 and O1−C2−
O4−C5 dihedral angles are 180 and 0°, 80.3 and 0.5°, and
+78.1 and −179.6° in the three cases. These results are close to
what was found in the MP2/cc-pVTZ computations above, but
the energy order of I and II is opposite from what was found in
the MP2/cc-pVTZ calculations.
It would be of interest to see what role the basis set plays for
the energies of I and II. MP2 calculations of optimized
structures of I and II were repeated using the cc-pVQZ basis
set. Calculations of vibrational frequencies at this level of theory
were too costly to be performed. The results are given in Tables
S1 and S2, respectively. It was found II is lower in electronic
energy than I, just as in the case of the of the MP2/cc-pVTZ
calculations (see above). The MP2/cc-pVQZ electronic energy
difference is 0.56 kJ/mol compared to 0.77 kJ/mol found the
MP2/cc-pVTZ calculations.
The CCSD prediction that I and II are much lower (∼23 kJ/
mol) in energy than III is in accord with the propensity of
formic acid esters to prefer sp conformations for the C2−O4−
C5−O6 link of atom, as exemplified by HC(O)OCH3,10
HC(O)OCH2CH3,24,25 and HC(O)OCH2F.26 Conformer III
between conformers I and II was calculated (Table S3) to have
an electronic energy that is only 3.55 kJ/mol higher than the
energy of II. This maximum occurs at 129.7° for the C2−O4−
C5−O6 dihedral angle. The rather flat part of the potential
function between about 65° and 180° indicates that the torsion
about the O4−C5 bond will be a large amplitude vibration in
both I and II.
The other maximum of Figure 2 occurs when the C2−O4−
C5−O6 and the O1−C2−O4−C5 dihedral angles both are
0.0°. The energy of this maximum is 28.55 kJ/mol higher than
the electronic energy of II. This high energy is presumably due
to the fact that the carbonyl oxygen atom and the cyano group
are brought into close contact in this conformation resulting in
a comparatively large repulsive interaction.
In the second scan, the C2−O4−C5−O6 dihedral angle was
again changed in 10° portions, whereas the O1−C2−O4−C5
link of atoms was ap (approximately 180°). The ensuing
potential function is drawn in Figure 3. This function is very
different from the first function (Figure 2) and has only one
minimum corresponding to conformer III instead of the two
minima (conformers I and II) found in the first scan, were the
O1−C2−O4−C5 link of atoms was sp. The energy of the two
barrier maxima are 8.57 kJ/mol at 0.0° and 6.25 kJ/mol at 180°.
The much lower barrier at 0° (8.57 kJ/mol) than in the
previous case (28.55 kJ/mol) may reflect the absence of
repulsion between the carbonyl oxygen atom (O1) and the
cyano group in the O1−C2−O4−C5 ap conformation.
MP2/cc-pVTZ calculations of the same series of molecular
parameters as those performed above for I and II were also
undertaken for III with the results shown in Table S4 of the
Supporting Information. Interestingly, III is as much as 23.45
kJ/mol higher in electronic energy than II (from Tables S2 and
S4). This difference becomes 21.46 kJ/mol after correction for
the harmonic zero-point vibrational energies. The corresponding energy difference in methyl formate is 25 kJ/mol.10 The
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Table 3. Spectroscopic Constantsa of Conformer II of
Cyanomethyl Formate at Different Vibrational States
Table 1. CCSD/cc-pVQZ Parameters of the Conformers I,
II, and III of Cyanomethyl Formate
I
Bond Distance (pm)
119.3
119.1
109.1
109.1
134.0
134.5
142.4
142.1
146.5
147.1
108.8
108.7
108.8
108.5
115.0
115.0
Angle (deg)
O1−C2−H3
125.7
125.7
O1−C2−O4
124.9
125.1
H3−C2−O4
109.4
109.2
C2−O4−C5
113.6
114.9
O4−C5−C6
107.6
111.5
O4−C5−H8
110.5
110.8
O4−C5−H9
110.5
106.1
C6−C5−H8
109.9
109.1
C6−C5−H9
109.9
109.2
H8−C5−H9
108.3
110.2
C5−C6−N7
178.1
178.6
Dihedral Angle (deg)
O1−C2−H3−O4
180.0
179.7
O1−C2−O4−C5
0.0
0.5
H3−C2−O4−C5
180.0
−179.8
C2−O4−C5−C6
180.0
80.3
C2−O4−C5−H8
60.0
−41.5
C2−O4−C5−H9
−60.0
−161.0
Dipole Moment (debyea)
μa
2.26
2.60
μb
0.09
3.49
μc
0.00b
0.66
μtot
2.26
4.40
Electronic Energy Diffrencesc (kJ/mol)
ΔE
0.0
0.55
O1−C2
C2−H3
C2−O4
O4−C5
C5−C6
C5−H8
C5−H9
C6−N7
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II
III
Av (MHz)
Bv (MHz)
Cv (MHz)
ΔJ (kHz)
ΔJK (kHz)
ΔK (kHz)
δJ (kHz)
δK (kHz)
ΦJ (Hz)
ΦJK (Hz)
ΦKJ (Hz)
ΦK (Hz)
ϕJ (Hz)
ϕJKc (Hz)
rmsa
std (MHz)a
Na
118.4
109.9
135.4
141.3
147.3
109.0
108.5
115.1
124.6
122.0
113.4
115.9
112.0
111.6
106.5
108.8
109.3
108.5
179.3
ground
first excited torsion (v21)
equilibriumb
7608.5723(26)
2608.94813(71)
2198.40201(72)
6.44242(73)
−36.4432(42)
99.941(11)
2.13646(33)
17.343(13)
−0.09001(29)
1.0602(25)
−5.416(20)
13.169(49)
−0.03629(16)
−1.0158(98)
1.323
0.12
641
7852.3358(51)
2549.4614(15)
2170.8155(15)
7.4003(39)
−51.5050(99)
163.91(15)
2.44371(50)
22.079(20)
−0.1504(33)
2.1423(60)
−12.290(31)
24.2(12)
−0.06238(27)
−1.946(17)
1.523
0.14
655
7704.9
2608.1
2206.0
5.30
−24.6
60.8
1.74
13.0
−0.0461
0.371
−17.3
43.8
−0.0191
−0.452
a
Defined in the footnote of Table 2. The spectra are listed in Tables
S17 and S18 of the Supporting Information. bRotational constants
from CCSD/cc-pVQZ structure in Table 1. MP2/cc-pVTZ centrifugal
distortion constants from Table S2. cϕK preset at zero in the leastsquares fit; see text.
−179.6
−179.6
0.9
78.1
−44.2
−162.5
has a Boltzmann factor relative to I as low as about 8 × 10−5 at
room temperature, which means that III will have an extremely
weak MW spectrum that would be impossible to detect with
our spectrometer. This rotamer is therefore not considered
further.
The existence of two conformers such as I and II with similar
energies is typical for substituted formate esters and has already
been found for HC(O)OCH2CH324,25 and HC(O)OCH2F.26
The C2−O4−C5−C6 chain of atoms takes the expected 180°
angle in I, and the unusual value of 80.3° in II (Table 1). The
corresponding C−O−C−C dihedral angle is 180° and 81.7°
(rα0-structure) in the corresponding two conformers of
HC(O)OCH2CH3,25 whereas the C−O−C−F dihedral angle
is 180° and 84(1)° in the two conformers of HC(O)OCH2F.26
0.78
3.12
2.30
3.95
23.60
1 debye =3.33564 × 10−30 C m. bFor symmetry reasons. cRelative to
Conformer I.
a
Table 2. Spectroscopic Constantsa of Conformer I of Cyanomethyl Formate at Different Vibrational States
Av (MHz)
Bv (MHz)
Cv (MHz)
2Pc (10−20 u m2)c
ΔJ (kHz)
ΔJK (kHz)
δJ (kHz)d
ΦKJ (Hz)f
rmsg
std (MHz)h
Ni
ground
first excited tors (v21)
second excited tors
third excited tors
lowest bend (v20)
equilibriumb
19304.2(23)
1775.2182(43)
1644.0460(44)
3.466(4)
0.20014(32)
−2.010(16)
0.0254(16)
−0.58(12)
1.191
0.13
330
18710.7(47)
1777.9569(74)
1650.9341(77)
5.140(9)
0.21687(54)
−3.3927(52)
0.0241(29)
−e
1.271
0.14
238
18114.4(78)
1781.622(13)
1659.412(13)
7.01(2)
0.25357(56)
−6.1409(42)
−e
−e
1.388
0.15
214
17354.5(79)
1786.461(13)
1670.405(13)
9.47(2)
0.3166(15)
−11.074(83)
−e
−e
1.762
0.21
70
19752.9(76)
1777.827(14)
1644.608(14)
2.56(1)
0.20690(74)
−2.915(23)
−e
−e
1.511
0.17
158
19731.7
1784.1
1653.0
3.14
0.193
−1.34
0.0218
−0.280
a
A-reduction, Ir-representation.22 Spectra listed in Tables S12−S16. bApproximate equilibrium rotational constants from CCSD/cc-pVQZ structure
(Table 1) and approximate equilibrium MP2/cc-pVTZ centrifugal distortion constants (Table S1). cPlanar moment defined by Pc = (Ia + Ib − Ic)/2,
where Ia, Ib, and Ic, are the principal moments of inertia. Conversion factor: 505379.05 × 10−20 MHz u m2. dFurther quartic constants preset at zero;
see text. eFixed at zero; see text. fFurther sextic centrifugal distortion constants preset at zero; see text gRoot-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 least-squares fit, and P is the number of spectroscopic constants used in the fit. hStandard deviation of the fit.
i
Number of transitions used in the fit.
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almost exclusively oriented parallel with its a-inertial axis (Table
1). Only pile-ups of a-type R-branch transitions separated by
almost exactly the sum of the B and C rotational constants were
expected for this rotamer. The weakness of the spectrum is
presumably largely caused by the existence of numerous
vibrationally excited states. The MP2 calculations predict five
normal vibrations for each conformer (Tables S1 and S2) with
frequencies below 500 cm−1. The torsions about the O4−C5
bond, which are as low as 47.7 cm−1 in I (Table S1, anharmonic
frequency) and 62.2 cm−1 in II (Table S2), will contribute most
to the reduction of intensity, because the Boltzmann factors are
much larger for these two normal modes than for the other
fundamentals.
This simple nature of the spectrum of I greatly facilitated its
assignment and the spectrum of this conformer was first
assigned. The CCSD rotational and the MP2 quartic centrifugal
distortion constants shown in Table 2, last column, were used
to predict the approximate frequencies of the spectrum of I,
which was assigned in a straightforward manner. The
assignments of several of these transitions were confirmed
using RFMWDR spectroscopy. The higher-K−1 members of
these series of aR-transitions have rapid Stark effects, which
were also very useful for their assignments. No b-type lines
were observed, presumably because μb is as small as 0.09 D
resulting in very weak b-type transitions. c-type lines were
excluded because μc is zero for symmetry reasons.
A total of 330 aR-transitions with J-values between 4 and 35
with a maximum value of K−1 = 12 were assigned. These lines
are listed in Table S12 of the Supporting Information. No
splittings due to quadrupole coupling of the 14N nucleus with
rotation were observed. The transitions were least-squares fit to
Watson’s A-reduction Ir-representation Hamiltonian22 using
Sørensen’s program Rotfit.32 The spectroscopic constants
obtained in this manner are shown in Table 2, second column.
Significant values different from zero could only be obtained
from our collection of aR-branch lines for three of the quartic
centrifugal distortion constants, namely, ΔJ, ΔJK, and δJ, and
one sextic constant, ΦKJ. The remaining centrifugal distortion
constants were preset at zero in the least-squares fit.
It was not possible to determine the dipole moment of this
conformer as well as that of conformer II because no low-J lines
with a clearly resolved Stark pattern that could be used for
quantitative measurements were observed.
Comparison of experimental and theoretical parameters is in
order. The value of twice the planar moment, 2Pc, where Pc =
(Ia + Ib − Ic)/2 and Ia, Ib, and Ic are the principal moments of
inertia, is characteristic for compounds containing a symmetry
plane and two out-of-plane hydrogen atoms attached to an sp3hybridized carbon atom, which is the case for I. The CCSD
value of 2Pc (Table 2, last column) is 3.14 × 10−20 u m2,
whereas the corresponding value is 3.466 (same order of
magnitude and units) for the ground vibrational state (second
column, same table). The experimental value of 2Pc (3.466(4)
× 10−20 u m2; Table 2) is larger than the theoretical
counterpart. This increase is presumably caused largely by the
torsional vibration about the O4−C5 bond.33
The CCSD rotational constants (Table 2, last column),
which are close to the equilibrium values, are larger than the
ground-state rotational constants (same table, second column)
by 427.5, 8.9, and 9.0 MHz in the cases of A0, B0, and C0,
respectively. This is what one would expect because the
equilibrium bond lengths (re) are generally shorter than the
ground-state bond lengths (r0), resulting in smaller principal
A C−O−C−X, dihedral angle of about 80°, where X is a
substituent, thus seems to be normal for one of the conformers
of substituted formates.
Formation of Cyanomethyl Formate in the ISM. The
chemistry in the ISM is very complex involving both the gas
phase and compounds adsorbed on “dust” particles.27−30 One
way to generate cyanomethyl formate from compounds already
known to exist in the ISM could be a reaction between
interstellar formic acid (HC(O)OH) 14 and interstellar
cyanomethyl radical (H2CCN)15 in the gas phase.
HC(O)OH + H 2CCN → HC(O)OCH 2CN + H
(1)
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Unrestricted MP2/cc-pVTZ calculations for this reaction
listed in Tables S7−S10 yielded an electronic reaction energy
corrected for harmonic zero-point vibrational energy (ΔEreact)
of +73.5 kJ/mol. Reaction 1 is therefore likely to be
nonspontaneous.
The transition-state structure, which was also computed, is
shown in Figure 4. This state is +84.0 kJ/mol higher in energy
Figure 4. Model of the transition state of the reaction between formic
acid (HC(O)OH) and the cyanomethyl radical (CH2CN).
than the combined energies of formic acid and the cyanomethyl
radical. This is a significant activation energy, which, together
with the value of ΔEreact (73.5 kJ/mol) implies that this gas
phase reaction is rather unlikely. However, a surface reaction
could be catalyzed, leading to the formation of cyanomethyl
formate from HC(O)OH and H2CCN. Other surface reactions
might also lead to cyanomethyl formate. There exists a large
reservoir of a variety of cyanides in interstellar space,13 which
may provide the necessary reagents leading to cyanomethyl
formate.
MW Spectrum and Assignment of the Ground-State
Spectrum of Conformer I. Survey spectra revealed a very
dense and relatively weak rotational spectrum with absorption
lines occurring every few megahertz throughout the investigated spectral range, 12−123 GHz. This is not surprising
because the rotational partition function of I is 1.17 × 105
(from the ground-state rotational constants) and the vibrational
partition function is 2.06 (from the MP2/cc-pVTZ harmonic
vibrational frequencies) at a temperature of 300 K. The
corresponding values are 1.33 × 105 and 1.92 for II.
Most of the observed transitions are likely to be due to II,
which has sizable components of the dipole moment along
both the a- and the b-axis (Table 1). b-type transitions would
be especially numerous for a compound with the rotational
constants similar to those predicted for II (Table 3, last
column). A minor part of the spectral lines should belong to I,
because it is calculated to be a near prolate rotor (Ray’s
asymmetry parameter31 κ ≈ − 0.94) with a dipole moment
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The Journal of Physical Chemistry A
relative intensity measurements yielded 173(30) cm−1. The
experimental α-constants are −454.7(79), − 2.609(15), and
−0.562(15) MHz compared to their MP2 equivalents −380.43,
−2.11, and −0.316 MHz, respectively (Table 1S).
Assignment of the Spectrum of Conformer II. This
conformer was predicted to have its major dipole moment
component (3.49 D; Table 1) along the b-axis. Searches were
first performed for the b-type Q-branch transitions, which are
among the strongest transitions of the spectrum, using the
CCSD and MP2 spectroscopic constants shown in the last
column of Table 3 as starting points. These transitions were
soon found. Successful searches for R-branch transitions of the
a- and b-varieties were then undertaken. The assignment of
several aR-branch lines were confirmed by RFMWDR experiments. The Stark effects of high-K−1 lines is very rapid, and this
facilitated the assignments of these transitions. No quadrupol
splittings due to 14N were seen.
The assignments of the aR-, bQ- and bR-transitions were
gradually extended to include additional transitions involving
higher and higher values of the J-quantum number. All Rbranch transitions up to J = 56 could be least-squares fitted
within their experimental uncertainties. Attempts to include
transitions with even higher values of J failed presumably
because they are too weak due to very small Boltzmann factors.
However, bQ-branch transitions could be fitted within their
experimental uncertainties only up to J ∼ 40. For higher values
of J, deviations of up to a few megahertz, more than10 times
larger than the experimental uncertainties, were encountered in
most cases. Inclusion of octic centrifugal distortion constants
did not improve the fit. These transitions were consequently
omitted from the fit. The same problem with these transitions
was encountered both for the A- and the S-reduction of
Watson’s Hamiltonian,22 but the fit using the A-reduction
seemed to be slightly better than the fit with the S-reduction
Hamiltonian. The reason for the unusual behavior of the high-J
b
Q-branch lines is assumed to be comparatively small higherorder vibration−rotation interactions.
The c-component of the dipole moment is 0.66 D according
to the CCSD calculations (Table 1). Searches for c-type
transitions were undertaken, but none of them could be
unambiguously assigned due mainly to the high spectral density
with frequent overlapping transitions and the interference of
Stark lobes. A total of 641 transitions shown in Table S17 of the
Supporting Information were used to determine the Areduction spectroscopic constants displayed in Table 3, second
column. All quartic and all sextic constants but ϕK were
determined. A value of ϕK significantly different from zero
could not be obtained. The value of this constant was hence
preset at zero in the least-squares fit.
Comparison of the experimental spectroscopic constants
(Table 2, second column) with the CCSD rotational constants
and MP2 centrifugal distortion constants, show that the CCSD
A and C rotational constants are larger than the experimental
equivalents by 96.3 and 7.6 MHz, respectively, whereas B is
slightly smaller by 0.8 MHz. However, MP2 calculations (Table
2S) predicts that the equilibrium value of A is smaller than the
ground-state value by 8.9 MHz, whereas the equilibrium values
of B and C are 32.92 and 23.77 MHz, respectively, larger than
the r0 values.
Table 3 reveals that the experimental quartic centrifugal
distortion constants are all much larger than the MP2
counterparts. The MP2 sextic constants deviate very much
form experiment. It is concluded that the MP2 predictions are
axis moments of inertia and larger rotational constants for the
equilibrium rotational constants compared to the ground-state
rotational constants. The re- and r0-rotational constants have
been calculated at the MP2 level (Table S1). The differences
between them are 183.2, 10.0, and 8.3 MHz, in the cases of A,
B, and C, respectively. The MP2 differences compared to the
differences between the CCSD and r0 rotational constants just
given, are satisfactory in the cases of the B and C rotational
constants, whereas a much larger discrepancy is seen for A.
Comparison of the experimental and MP2 centrifugal
distortion constants (Table 2) reveals good agreement in the
case of ΔJ, whereas the absolute value of the experimental ΔJK is
33% larger than the MP2 equiv. There are also significant
differences in the cases of δJ and ΦKJ. The MP2/cc-pVTZ
procedure is obviously not sufficiently refined to predict
accurate values for the centrifugal distortion constants and the
differences between the equilibrium and ground-state rotational
constants. However, more elaborate methods could not be
employed due to lack of computational resources.
Vibrationally Excited States of I. The ground-state
spectrum was accompanied by several weaker satellite spectra
with Stark and RFMWDR behavior very similar to that of the
ground-state spectrum. Detailed assignments were obtained for
four such spectra, which are assumed to belong to vibrationally
excited states of I. These spectra are listed in the Supporting
Information, Tables S13−S16 and their spectroscopic constants
are displayed in Table 2. The maximum value of the principal
quantum number J was 31 for all these excited states. The
maximum value of K−1 was 17 found for the first excited state of
the torsion (v21). The number of transitions of each excited
state is given in Table 2.
Three of the excited states, whose spectroscopic constants
are listed in columns 3−5 of Table 2, are assumed to belong to
successively excited states of the torsion about the O4−C5
bond of I (v21) because 2Pc increases upon successive excitation
of this mode.33 The increase in 2Pc and the variation in the
rotational constants from one excited state to the next are not
constant, which would have been the case for a completely
harmonic vibration. It is therefore concluded that the torsion
has a significant contribution from anharmonicity.
The frequency of the first excited state of the torsion about
the O4−C5 bond (v21) was determined by relative intensity
measurement to be 43(15) cm−1, whereas its anharmonic MP2
value is 49.8 cm−1 (Table S1). The increase in 2Pc upon
excitation can be used to get a rough value of the torsional
vibration. The torsional frequency, ω, is given roughly by ω =
(67.5 × 10−20 u m2)/Δ cm−1,33 where Δ is the increase in 2Pc
upon excitation. Using 1.67 × 10−20 u m2 for Δ obtained from
the ground and first excited state (Table 2), one gets 40 cm−1
for the torsional vibration.
The vibration−rotation constants (the α’s)23 are obtained as
αA = 593.5(52), αB = −2.7387(86), and αC = −6.8881(89)
MHz by subtraction of the first-excited-torsional-state rotational constants in Table 2 from the ground-state rotational
constants in the same table. The corresponding MP2 values are
(Table 1S) +572.56, −2.45, and −6.15 MHz. The agreement
between the experimental and theoretical vibration−rotation
constants is good given the approximate nature of the MP2
procedure.
The value of 2Pc decreases for the last excited state (Table 2,
column 6). A decrease is expected for a bending vibration that
maintains the Cs symmetry.33 The MP2 anharmonic frequency
of this lowest bending fundamental (v20) is 159.6 cm−1, whereas
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The Journal of Physical Chemistry A
A possible route to interstellar cyanomethyl formate could be
a reaction between interstellar formic acid (HC(O)OH) and
the interstellar cyanomethyl radical (CH2CN). A gas-phase
reaction was modeled with the MP2 method, but it turned out
that this reaction is not thermodynamically favorable. A
catalyzed reaction on interstellar dust might be feasible.
Other reaction paths other than the reaction between formic
acid and the cyanomethyl radical are also conceivable given the
large inventory of cyanides in interstellar space.
rather unreliable in the present case. Accurate predictions of
quartic and especially sextic centrifugal distortion constants
definitely require amuch more elaborate procedure than the
MP2/cc-pVTZ method.
Vibrationally Excited State. The spectrum of one
vibrationally excited state was assigned in the same manner as
described above for the ground-vibrational-state spectrum. A
total of 655 transitions were assigned for this excited state
(Table S18). The result of the least-squares analysis is given in
Table 3.
Relative intensity measurements yielded 60(15) cm−1 for this
vibration, which is assumed to be the first excited state of the
torsion about the O4−C5 bond (v21). The MP2 anharmonic
value is 62.2 cm−1 (Table 2S). The rotational−vibrational
constants obtained from columns 2 and 3 of Table 3 are αA =
−243.7635(57), αB = 59.4867(17), and αC = 27.5865(17)
MHz, whereas the MP2 method predicts −158.65, +46.44, and
+21.65 MHz, respectively. Interestingly, the centrifugal
distortion constants are generally much larger for the excited
state than for the ground vibrational state.
Energy Difference. The intensities of several transitions of
the ground vibrational state of conformer I were compared with
the intensities of transitions of the ground state of II observing
the precaution of Esbitt and Wilson.34 The energy difference
was calculated as described by these workers.34 The statistical
weight of II was assumed to be 2, whereas I was assumed to
have a statistical weight of 1. It was found that I is lower in
energy than II by 1.4 kJ/mol. The uncertainty corresponding to
three standard deviations was estimated to be ±0.6 kJ/mol. The
MP2 method above corrected for zero-point vibrational
energies predicts that II is lower in energy than I by 0.18 kJ/
mol, whereas the CCSD method predicts I to be lower in
energy than II by 0.55 kJ/mol (Table 1). The CCSD electronic
energy difference (0.55 kJ/mol) and the experimental energy
difference of the ground vibrational states (1.4(6) kJ/mol) are
in satisfactory agreement.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpca.5b05285.
Microwave spectra of the ground and several vibrationally excited states of conformers I and II. Results of the
theoretical calculations, including electronic energies,
molecular structures, dipole moments, harmonic and
anharmonic vibrational frequencies, rotational and
centrifugal distortion constants, rotation−vibration constants, and 14N nuclear quadrupole coupling constants
(PDF)
■
AUTHOR INFORMATION
Corresponding Author
*H. Møllendal. Tel: +47 2285 5674. Fax: +47 2285 5441. Email: harald.mollendal@kjemi.uio.no.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank Anne Horn for her skillful assistance. 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).
■
CONCLUSIONS
The rotational spectra of two rotamers, denoted I and II, of
cyanomethyl formate (HC(O)OCH2CN) have been assigned.
Conformer I has a symmetry plane consisting of all atoms but
the two hydrogen atom attached to cyanomethyl group. The
cyanomethyl group is rotated 80.3° out of this plane in rotamer
II. Conformer I is lower in energy by 1.4(6) kJ/mol relative to
II.
The ground-vibrational-state spectrum as well as spectra of
four vibrationally excited states of I have been assigned,
whereas the spectra of the ground vibrationally state and one
excited state were assigned for II. A large number of transitions
have been used to determine accurate spectroscopic constants
for both rotamers. These constants should be able to predict
very accurate frequencies for transitions occurring outside the
investigated spectral interval (12−123 GHz) and thus facilitate
a potential identification of interstellar cyanomethyl formate.
The spectroscopic work has been augmented by advanced
quantum chemical calculations at the MP2/cc-pVTZ and
CCSD/cc-pVQZ levels of theory. The CCSD structures of the
two rotamers, which are close to the Born−Oppenheimer
equilibrium structures, were derived. The MP2 calculations
predict centrifugal distortion constants and vibration−rotation
constants that are only in fair agreement with their
experimental counterparts. More refined and costly methods
must be used to obtain reliable predictions of these parameters.
■
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