Article
pubs.acs.org/JPCA
Microwave Spectrum and Conformational Composition of
(Azidomethyl)cyclopropane (C3H5CH2N3)
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 (azidomethyl)cyclopropane, C3H5CH2N3, has been
investigated in the 26−90 GHz spectral range at a temperature of about −30 °C. Five rotameric
forms of this compound, whose spectra can be distinguished by microwave spectroscopy, may exist. The
spectra of three of them denoted III, IV, and V were assigned. The ground vibrational state spectra of III
and V were assigned, while the ground and six vibrationally excited states were assigned for IV. These
three rotamers all have a synclinal orientation of the H−C−C−N chain of atoms, while the C−C−N−N
link is either + or − synclinal or antiperiplanar. Conformer IV, having synclinal orientation of the two said
dihedral angles, was found to have the lowest energy by relative intensity measurements. Rotamer V has
an energy that is 1.6(6) kJ/mol higher than the energy of IV, while the energy of III is 2.1(6) kJ/mol higher than the energy of
IV. Quantum chemical calculations were performed at the MP2/cc-pVTZ and CCSD/cc-pVTZ levels of theory. The rotational
constants obtained in the CCSD calculations are in good agreement with the experimental rotational constants, while the MP2
centrifugal distortion constants are generally in poorer agreement with their experimental counterparts.
■
INTRODUCTION
The properties of small organic azides (R-N3) have received
relatively little attention in the past. The main reason for this is
presumably their toxicity and the fact that some of them are
explosive and should be treated with utmost care. However,
recent studies1−4 have shown that azides have a number of
useful synthetic properties. The structures and conformational
behavior of gaseous organic azides should therefore be of
interest to better understand the chemistry of this class of
compounds. Roughly 25 years ago, Klaeboe, Nielsen, Priebe,
Schei, and co-workers synthesized several small organic azides
and performed studies of their structural and conformational
properties by electron diffraction, infrared spectroscopy, and
theoretical calculations, as summarized in a review.5 It was
established in these investigations that gaseous azides indeed
have unique conformational and structural properties.5
Very few conformational studies of azides have been reported
in the intervening period. However, one year ago, we presented
a novel synthesis of 2-fluoroethyl azide, FCH2CH2N3, and
investigated its conformational properties by MW spectroscopy
augmented with high-level quantum chemical calculations and
found that one rotameric form predominates in this case.6 In
the present work, our azide studies are extended to include the
first microwave (MW) investigation of (azidomethyl)cyclopropane, C3H5CH2N3.
This compound has two internal axes, namely, the C3H5−
CH2N3 axis and the C3H5CH2−N3 axis, about which rotational
isomerism may arise. Nine conformers are possible for this
molecule, but only five conformers can be distinguished by
means of their MW spectra. These five forms are shown in
© 2014 American Chemical Society
Figure 1 and given Roman numeral for reference. Confomer I is
unique, whereas mirror-image forms exist for the remaining
four rotamers. The enantiomers of II−V, which have identical
moments of inertia and dipole moments and identical MW
spectra, can be obtained by proper sign changes of relevant
dihedral angles.
Atom numbering is shown in Figure 1 on rotamer I. The
H6−C2−C9−N10 and the C2−C9−N10−N13 dihedral angles
can conveniently be used to characterize these five forms. The
H6−C2−C9−N10 link of atoms is exactly antiperiplanar (ap)
in I. This is also the case for the C2−C9−N10−N13 chain.
This form has a symmetry plane bisecting the cyclopropyl ring.
The former atomic arrangement is ap in II, while the C2−C9−
N10−N13 atoms have a +synclinal (+sc) arrangement in this
rotamer. Conformers III−V all have a +sc conformation of the
H6−C2−C9−N10 chain, whereas C2−C9−N10−N13 atoms
are ap in III, +sc in IV, and −sc in V.
MW spectroscopy is an ideal experimental method to study
the complex conformational problems such as the one
associated with the title compound, due to its extreme accuracy
and resolution. The fact that accurate energy differences can be
obtained from relative intensity measurements is another
advantage of this method. The MW work has been augmented
with high-level quantum chemical calculations to investigate
properties of the conformational hypersurface and to obtain
information that is useful for the assignment of the spectrum.
Received: June 24, 2014
Revised: July 29, 2014
Published: August 1, 2014
6971
dx.doi.org/10.1021/jp506261a | J. Phys. Chem. A 2014, 118, 6971−6978
The Journal of Physical Chemistry A
Article
compounds were distilled at 0.1 mbar at room temperature
using a vacuum line. A first trap cooled at −20 °C removed
some impurities, and the product was condensed in a second
trap cooled at −70 °C. The (azidomethyl)cyclopropane was
thus obtained in pure form in a 77% yield (1.87 g, 19.2 mmol).
1
H NMR (CDCl3) δ 0.25 (m, 4H, 3JHH = 6.0 Hz, 2 CH2 of the
cycle); 1.09 (m, 1H, 3JHH = 6.0 Hz, 3JHH = 7.1 Hz, CH); 3.07
(d, 2H, 3JHH = 7.1 Hz, CH2N3). 13C NMR (CDCl3) δ 3.3 (1JCH
= 162.8 Hz (t), CH2 cycle); 10.0 (1JCH = 162.1 Hz (d), CH
cycle); 56.0 (1JCH = 142.3 Hz (t), CH2N3).
Spectroscopic Experiments. (Azidomethyl)cyclopropane
is a colorless liquid with a vapor pressure of roughly 110 Pa at
room temperature. Its MW spectrum was recorded with the cell
cooled to about −30 °C using small portions of dry ice to cool
the waveguide. The pressure was 5−10 Pa during the
measurements. The MW spectrum was studied with Starkmodulation spectroscopy using the microwave spectrometer of
the University of Oslo. Details of the construction and
operation of this device have been given elsewhere.8 This
spectrometer has a resolution of about 0.5 MHz and measures
the frequency of isolated transitions with an estimated accuracy
of ∼0.10 MHz. The spectrum was investigated in the whole
26−90 GHz frequency interval. Radio-frequency microwave
double-resonance experiments (RFMWDR), similar to those of
Wodarczyk and Wilson,9 were also conducted to unambiguously assign particular transitions, using the equipment
described elsewhere.8
■
RESULTS
Quantum Chemical Calculations. The present frozencore MP210 and CCSD calculations were performed employing
the Gaussian0911 and Molpro12 programs running on the Abel
cluster in Oslo using Dunning’s13 correlation-consistent ccpVTZ basis set, which is of triple-ζ quality. The default
convergence criteria of the two computer programs were used.
MP2/cc-pVTZ computations of the structures, dipole
moments, vibrational frequencies, and Watson’s A-reduction
quartic centrifugal distortion constants14 were performed for
the five conformers I−V. All structural parameters were varied
freely in these calculations with no symmetry restrictions. No
imaginary harmonic normal vibrations were obtained, which is
an indication that these rotamers are minima on the potential
energy hypersurface. The precautions of McKean et al.15 were
observed in the computations of the centrifugal distortion
constants. Selected results of these calculations are found in
Tables 1S−5S in the Supporting Information.
The comprehensive CCSD/cc-pVTZ calculations of optimized structures, dipole moments, and electronic energies of
forms I−V were performed using the MP2 structures in Tables
1S−5S, Supporting Information, as starting points. The
optimizations were done without symmetry restrictions. The
resulting structures are listed in Table 1, while additional
structural details are listed in Tables 6S−10S of the Supporting
Information. The rotational constants calculated from these
structures are shown in Table 2. The dipole moment
components of the Molpro calculations were transferred to
the principal-axes dipole moment components with the results
listed in Table 2. Calculation of the centrifugal distortion
constants by the CCSD method is beyond our computational
resources. The MP2/cc-pVTZ quartic centrifugal distortion
constants in the S-reduction form14 are included in Table 2.
Some of the results in Tables 1 and 2 warrant further
comments. The geometry of the azido group is interesting. The
Figure 1. Five rotameric forms of (azidomethyl)cyclopropane. Atom
numbering is indicated on conformer I.
■
EXPERIMENTAL SECTION
Synthesis. Caution! (Azidomethyl)cyclopropane is potentially toxic and explosive and all experiments should be
performed behind a safety screen and in a well-ventilated hood.
The synthesis of (azidomethyl)cyclopropane has already
been reported but only few details were given.7 Since small
azides are potentially explosive on heating, we applied the
procedure we have already reported for 2-fluoroethyl azide6
using a high boiling solvent. This procedure allows the
purification of the expected product by room temperature
distillation since it is the sole low boiling compound of the
mixture when the reaction is completed. (Scheme 1).
Scheme 1
A stirred suspension of sodium azide (3.25 g, 50 mmol) in
triethylene glycol (50 mL) was degassed under vacuum (0.1
mbar) for 10 min at room temperature. The (bromomethyl)cyclopropane (3.38 g, 25 mmol) was then added, and the
resulting mixture was stirred under nitrogen at 50 °C for 20 h.
After checking of the complete transformation of the precursor
by 1H NMR spectroscopy on a small sample, the low-boiling
6972
dx.doi.org/10.1021/jp506261a | J. Phys. Chem. A 2014, 118, 6971−6978
The Journal of Physical Chemistry A
Article
Table 1. CCSD/cc-pVTZ Structures of Five Conformers of C3H5CH2N3a
I
C1−C2
C1−C3
C1−H4
C1−H5
C2−C3
C2−H6
C2−C9
C3−H7
C3−H8
C9−N10
C9−H11
C9−H12
N10−N13
N13−N14
150.2
151.0
108.0
108.0
150.2
108.2
150.7
108.0
108.0
147.8
109.4
109.4
123.2
112.9
C2−C1−H4
C2−C1−H5
C3−C1−H4
C3−C1−H5
H4−C1−H5
C1−C2−H6
C1−C2−C9
C3−C2−H6
C3−C2−C9
H6−C2−C9
C1−C3−H7
C1−C3−H8
C2−C3−H7
C2−C3−H8
H7−C3−H8
C2−C9−N10
C2−C9−H11
C2−C9−H12
N10−C9−H11
N10−C9−H12
H11−C9−H12
C9−N10−N13
N10−N13−N14
117.9
117.3
118.2
116.9
115.4
116.2
121.1
116.2
121.1
112.6
118.2
116.9
117.9
117.3
115.4
109.6
109.8
109.8
109.7
109.7
108.1
113.0
175.3b
H4−C1−C2−H6
H4−C1−C2−C9
H5−C1−C2−H6
H5−C1−C2−C9
H4−C1−C3−H7
H4−C1−C3−H8
H5−C1−C3−H7
H5−C1−C3−H8
H6−C2−C3−H7
H6−C2−C3−H8
C9−C2−C3−H7
C9−C2−C3−H8
C1−C2−C9−N10
C1−C2−C9−H11
C1−C2−C9−H12
C3−C2−C9−N10
C3−C2−C9−H11
C3−C2−C9−H12
H6−C2−C9−N10
H6−C2−C9−H11
H6−C2−C9−H12
−1.5
141.3
−146.6
−3.8
0.0
−145.0
145.0
0.0
1.5
146.6
−141.3
3.8
35.9
−84.7
156.5
−35.9
−156.5
84.7
180.0
59.4
−59.4
II
Bond Length (pm)
150.2
150.9
108.0
108.0
150.3
108.3
151.2
108.0
108.0
147.8
108.9
109.3
123.4
112.9
Angles (deg)
118.0
117.2
118.2
116.9
115.4
116.1
120.9
115.9
121.6
112.7
118.3
117.1
117.8
118.3
114.7
112.9
109.7
110.2
105.7
110.3
107.8
113.5
176.4b
Dihedral Angle (deg)
−1.8
140.8
−146.9
−4.3
0.4
−143.6
145.2
1.3
1.6
146.8
−141.6
3.5
35.1
−82.5
158.9
−36.9
−154.5
87.0
178.8
61.2
−57.4
6973
III
IV
V
150.1
150.8
107.9
108.1
150.4
108.1
149.9
107.9
108.1
148.1
109.4
109.4
123.3
112.9
150.7
150.7
108.0
108.1
150.4
108.2
150.5
107.9
108.1
147.9
108.7
109.5
123.5
112.9
150.2
150.7
108.0
108.1
150.5
108.3
150.5
108.0
108.1
148.1
109.4
108.8
123.4
112.9
118.1
117.2
118.1
117.8
114.8
116.7
118.9
116.9
118.6
114.7
118.2
117.7
118.0
117.7
114.7
108.8
110.6
109.6
109.9
109.9
108.0
112.9
175.3b
118.1
117.7
118.2
117.5
114.7
116.5
119.5
116.7
118.9
114.6
118.2
117.6
118.0
117.6
114.7
113.4
110.8
109.8
104.7
110.2
107.7
113.3
174.5b
118.2
117.1
118.0
117.9
114.8
116.6
118.9
116.3
118.7
115.2
118.2
117.8
118.1
117.5
114.7
113.4
110.8
109.8
109.8
105.0
107.7
112.9
175.3b
−0.7
143.7
−144.8
−0.3
0.2
−144.5
145.3
0.5
1.2
145.5
−143.1
1.2
−82.3
156.8
37.9
−152.1
87.0
−31.9
62.8
−58.1
−177.0
−1.1
143.6
−145.7
−1.0
0.1
−144.5
144.5
−0.1
1.4
145.7
−142.5
1.8
−88.3
154.3
35.5
−158.2
84.4
−34.4
57.1
−60.3
−179.1
−1.3
143.8
−145.4
−0.2
0.3
−144.6
145.3
0.4
1.1
145.3
−143.2
1.0
−78.7
157.3
38.4
−148.5
87.5
−31.4
66.8
−57.1
−176.1
dx.doi.org/10.1021/jp506261a | J. Phys. Chem. A 2014, 118, 6971−6978
The Journal of Physical Chemistry A
Article
Table 1. continued
I
C2−C9−N10−N13
H11−C9−N10−N13
H12−C9−N10−N13
a
II
Dihedral Angle (deg)
95.0
−145.0
−28.7
180.0
−59.3
59.3
III
IV
V
−174.3
−53.0
65.7
64.0
−175.1
−59.5
−65.5
59.0
174.6
The structures of the three conformers whose MW spectra were assigned are given in boldface. bN14 bent away from C9.
unusual value of this angle is presumably caused by steric
repulsion between the azido group and the cyclopropyl ring.
The nonbonded distance between H5 and N10 is 263.5 pm, the
nonbonded length between H8 and N10 is 270.1 pm (Table
7S, Supporting Information), while the distance between H8
and N14 is 262.4 pm. The sum of the Pauling van der Waals
radii17 of hydrogen (120 pm) and nitrogen (150 pm) is 270
pm. A smaller value than 95.0° would have brought the said
atoms into even closer proximity with steric repulsion as a
consequence.
The CCSD electronic energy differences between the various
conformers that are listed in Table 2 indicates that conformer
IV is the most stable form of the molecule, being more stable
than I, II, III, and V by 3.26, 3.89, 2.86, and 1.98 kJ/mol,
respectively. The corresponding MP2/cc-pVTZ energy differences corrected for harmonic zero-point vibrational energies
can be obtained from entries in Tables 1S−5S of the
Supporting Information as 3.81, 4.36, 3.98, and 2.49 kJ/mol,
respectively.
Microwave Spectrum and Assignment of Conformer
IV. The CCSD and MP2 calculations indicate that the energy
differences between the five forms of (azidomethyl)cyclopropane are a few kJ/mol, with rotamer IV as the
lowest-energy conformer. The MP2 computations of the
harmonic vibrational fundamental frequencies of rotamer IV
(Supporting Information; Table 4S) indicate that there are six
normal vibrations with frequencies below 500 cm−1. Similar
results were found for the four other rotamers (Table 1S−3S
and 5S, Supporting Information). The corresponding Boltzmann factors of these low-frequency vibrationally excited states
of each rotamer are therefore relatively large and the ground
vibrational state of each conformer is predicted to be
accompanied by a number of prominent satellite lines
belonging to the vibrationally excited state. All this should
result in a very rich and comparatively weak spectrum at −30
°C.
The observed spectrum is relatively weak with absorption
lines occurring every few MHz throughout the entire MW
region (26−90 GHz) in accord with expectations. The a-type
R-branch transitions of conformer IV were first searched for
because this conformer has its major dipole moment
component along the a-inertial axis (Table 2). The CCSD
rotational and the MP2 centrifugal distortion constants (Table
2) were used to predict the transition frequencies of this
spectrum. Ray’s asymmetry parameter18 κ is −0.834, and the
high-K−1 aR-transitions will therefore have rapid Stark effects
due to their near-degeneracy. Searches for these lines were
conducted employing a Stark field strength of about 110 V/cm.
These transitions turned out to be the strongest ones in the
spectrum under these experimental conditions, and they were
readily assigned. The assignments were confirmed by
RFMWDR experiments9 and fit to Watson’s S-reduction Irrepresentation Hamiltonian14 using Sørensen’s least-squares
program Rotfit.19 The assignments were gradually extended to
Table 2. CCSD/cc-pVTZ and MP2/cc-pVTZ Parameters of
Spectroscopic Interesta of Five Conformers of C3H5CH2N3
I
A
B
C
DJ
DJK
DK
d1
d2
μa
μb
μc
μtot
II
III
IV
Rotational Constants (MHz)
9013.7
5715.5
11838.5
5776.8
1362.3
1815.8
1184.4
1828.0
1316.7
1610.1
1142.7
1485.1
Quartic Centrifugal Distortion Constantsb (kHz)
0.150
1.57
0.168
1.28
2.62
−11.0
−4.76
−6.74
9.35
30.7
163
14.9
−0.0123
−0.436
0.00891
−0.402
0.00252
−0.0172
−0.00126
−0.0212
Dipole Moment (Debye)
1.29
1.55
1.83
2.14
1.41
0.75
1.02
0.26
0.0c
1.09
0.22
0.39
1.91
2.04
2.10
2.19
Energy Differenced (kJ/mol)
3.26
3.89
2.86
0.0
V
7240.5
1461.7
1392.7
1.66
−20.9
102
−0.436
−0.0214
2.00
0.27
0.85
2.19
1.98
a
CCSD rotational constants, dipole moments, and energy differences;
MP2 centrifugal distortion constants. The theoretical parameters of
the three conformers whose MW spectra were assigned are given in
boldface. bS-reduction Ir-representation.14 cBy symmetry. dRelative to
conformer I.
N10−N13 and N13−N14 bond lengths vary little from one
conformer to the next. The N10−N13 bond is longer than the
N13−N14 bond by more than 10 pm (Table 1), which is
consistent with a resonance hybrid description of the azido
group where the C3H5CH2−NN+N− and C3H5CH2−N−−
N+N resonance structures are the main contributors. The rs
length of the longer N−N bond in CH3N3 is 123.1 pm, while
the terminal N−N bond length is 113.7 pm.16 The first of these
is slightly shorter by 0.1−0.4 pm than the N10−N13 bond
lengths of (azidomethyl)cyclopropane, while the second is 0.8
pm longer.
The azido group of I−V is nonlinear with N14 bent away
from C9. The deviations from linearity vary from 3.6°
(conformer II) to 5.5° (IV), compared to 6.9° for this group
in CH3N3.16 The C2−C9−N10 angle are 3−5° smaller when
the C2−C9−N10−N13 link of atoms has a ap conformation
(rotamers I and III) than a ±sc conformation (II, IV, and V),
which may be due to repulsion between the azido group and
the ring or a slight rehybridization of the C9 atom in the sc
forms.
The H6−C2−C9−N10 dihedral angle is exactly 180° in I
and 178.8° in II and between 57.1° and 66.8° in the three other
conformers. The C2−C9−N10−N13 dihedral angle is exactly
ap (180°) in I and −174.3° in III. An interesting variation is
seen when this atomic arrangement has a +sc or a −sc
conformation. In IV this angle has a “normal” value of 64.0°.
The same is true for V (−65.5°). However, this dihedral angle
is as large as 95.0° instead of the expected ∼60° in II. The
6974
dx.doi.org/10.1021/jp506261a | J. Phys. Chem. A 2014, 118, 6971−6978
The Journal of Physical Chemistry A
Article
Table 3. Spectroscopic Constantsa of the Ground and Vibrationally Excited States of Conformer IV of C3H5CH2N3
vib. state
ground state
first ex. tors.
second ex. tors.
third ex. tors.
fourth ex. tors.
first ex. lowest bend.
combination mode
Av (MHz)
Bv (MHz)
Cv (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
d1 (kHz)
d2 (kHz)
HJK (Hz)
HKJ(Hz)
rmsc
Nd
5719.702(65)
1831.3867(21)
1481.6329(18)
1.44168(76)
−8.0232(72)
14.85b
−0.46243(81)
−0.01840(71)
0.0549(71)
−0.462(14)
1.201
450
5734.393(69)
1823.3073(21)
1480.4264(18)
1.49671(88)
−8.547(10)
14.85b
−0.47229(82)
−0.01677(63)
0.393(85)
−0.369(35)
1.314
427
5748.103(84)
1816.0608(25)
1479.2079(22)
1.55672(85)
−9.0968(89)
14.85b
−0.48728(88)
−0.01425(72)
0.0932(86)
−0.316(11)
1.230
372
5759.44(15)
1809.6902(29)
1478.1982(22)
1.6264(13)
−9.764(14)
14.85b
−0.5008(12)
−0.0160(11)
0.145(13)
−0.886(56)
1.419
300
5713.6(28)
1803.383(51)
1478.828(50)
1.6779(32)
−10.177(12)
14.85b
−0.496(13)
−0.145(25)
0.0b
0.0b
1.233
150
5769.135(71)
1819.0341(21)
1474.1330(20)
1.42599(61)
−8.3098(73)
14.85b
−0.46311(87)
−0.02096(72)
0.0b
−0.166(25)
1.293
366
5788.03(10)
1810.0239(25)
1473.2268(24)
1.51026(72)
−9.152(11)
14.85b
−0.4793(10)
−0.01986(88)
0.0
−0.304(47)
1.392
328
a
S-reduction Ir-representation.14 Uncertainties represent one standard deviation. The spectra are found in Tables 11S−17S of the Supporting
Information. bFixed at this value 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.
strongest of these satellite spectra is assumed to belong to the
first excited state of the torsional vibration. Relative intensity
measurements yielded 56(15) cm−1 for this mode compared to
the MP2 value of 62 cm−1 (Table 4S, Supporting Information).
The spectra of a total of four successively excited states (Table
12S−15S, Supporting Information) of this mode were assigned
with the resulting spectroscopic constants listed in Table 3. It is
seen from this table that the variation of the rotational
constants upon successive excitation is nearly constant for the
first three excited states of the torsion, whereas deviations from
this pattern are observed for the fourth excited state of this
mode. This deviation could be due to interactions with other
vibrational modes. The smooth variation of the rotational
constants of the first three excited states is typical for a
harmonic vibration.20
The 366 transitions of the spectrum of the lowest bending
vibration are listed in Table 16S, Supporting Information, while
the spectroscopic constants are given in Table 3. The MP2
frequency of this mode is 95 cm−1 (Table 4S, Supporting
Information). Relative intensity measurements yielded 89(20)
cm−1 for this vibration.
The last excited state of Table 3 is assumed to be a
combination mode consisting of the first excited state of the
torsion plus the first excited state of the lowest bending
vibration. The changes of the rotational constants from the first
excited state of the lowest bending vibration to this
combination state is almost, but not exactly, the same as the
changes from the rotational constants of the ground vibrational
state to the first excited state of the torsion. Its frequency was
determined to be 145(30) cm−1 from relative intensity
measurements.
Assignment of the Spectrum of V. This rotamer is
calculated to be 1.98 kJ/mol higher in energy than IV (Table 2)
and has its major dipole moment component of 2.00 D along
the a-axis. The Ray asymmetry parameter κ is −0.969, and
characteristic pile-up regions of aR-transitions separated by
approximately the sum of the B and C rotational constants are
predicted. The intermediate and high-K−1 members of these
series are modulated at low Stark fields. These properties were
exploited to assign the spectrum of this conformer. Typical
series of a-type transitions, such as the J = 17 ← 16 transitions
shown in Figure 2, were first identified. The assignments of the
a
R-lines were gradually extended to include values up to Jmax =
include higher and higher values of the principal quantum
number J and the lower K−1 transitions. Ultimately, 450 aRtransitions with Jmax = 29 and K−1,max = 24 shown in Table 11S
of the Supporting Information were assigned and used to
determine the spectroscopic constants shown in Table 3. The
quartic centrifugal distortion constant DK cannot be obtained
from this selection of transitions and were preset at MP2 value
(14.85 kHz). Two sextic centrifugal distortion constants, HJK
and HKJ, were fitted with the remaining sextic constants preset
at zero in the least-squares fit. Searches for b- and c-type lines
were made, but none were found. This is not surprising since μb
and μc are much smaller than μa (Table 2) resulting in
insufficient intensity of the b- and c-type lines.
The experimental rotational constants (Table 3) agree well
with the CCSD rotational constants (Table 2). The deviations
are −0.99% for A, 0.16% for B, and −0.23% for C. Differences
of this order of magnitude are to be expected because of the
different definitions of the rotational constants. The CCSD
constants are derived from an approximate equilibrium
structure, while the experimental rotational constants are
effective constants. The good agreement indicates that the
CCSD structure in Table 1 is indeed accurate, which was also
expected for computations at the high CCSD/cc-pVTZ level.
Much poorer agreement is seen for the experimental quartic
centrifugal distortion constants and their MP2 counterparts.
The best agreement is seen for DJ (+11.1%) and the worst for
DJK (−16.0%). Obviously, MP2/cc-pVTZ calculations are not
sufficient to predict accurate values for the quartic centrifugal
distortion constants in the present case.
Vibrationally Excited States of IV. The ground-state
transitions of conformer IV were accompanied by a series of
weaker transitions with very similar Stark effects and
RFMWDR patterns. These lines were considered to belong
to vibrationally excited states. The harmonic frequencies of the
lowest vibrational fundamentals are 62, 95, 213, 240, 356, and
451 cm−1 according to the MP2 calculations (Table 4S,
Supporting Information). The first of these fundamental
vibrations (62 cm−1) is the torsion about the C9−N10 bond,
while the other five fundamentals can best be described as
bending vibrations.
It is seen from Table 3 that the spectra of six vibrationally
excited states have been assigned. The assignment procedure
was similar to that of the ground vibrational state. The
6975
dx.doi.org/10.1021/jp506261a | J. Phys. Chem. A 2014, 118, 6971−6978
The Journal of Physical Chemistry A
Article
2 shows that the experimental A rotational constant is 2.83%
smaller than its theoretical counterpart, while the experimental
values of B and C are 1.66% and 0.33% larger than predicted by
theory. The experimental MP2 quartic centrifugal distortion
constants (Table 4) are all significantly larger than their
theoretical equivalents (Table 2).
Assignment of the Spectrum of III. This rotamer is
calculated to be 2.86 kJ/mol higher in energy than the energy
of the global-minimum form IV (Table 2). Its asymmetry
parameter κ is −0.992, and μa is the predominating dipole
moment component (Table 2). A pile-up spectrum, whose
features are very similar to those described for conformer V,
was predicted and soon identified. The J = 28 ← 27 a-type
spectrum is shown in Figure 3. The spectrum of III is even
Figure 2. Portion of the J = 17 ← 16 a-type transitions of conformer
V. The spectrum was taken at a field strength of about 110 V/cm.
Values of the K−1 pseudo quantum number are listed above several
peaks belonging to V. The transitions with K−1 quantum numbers 5,6,
3,4, and 3,12 are not resolved. The remaining unlabeled transitions are
presumed to belong to vibrationally excited states of V or to further
rotameric forms. The intensity is in arbitrary units.
31 and K−1max = 22. The c-dipole moment component is the
second largest component according to Table 2. Searches for ctype lines were made, but none could be unambiguously
assigned presumably due to their weakness and the spectral
richness with very frequent overlapping lines. The 407 aRtransitions used to determine the spectroscopic constants in
Table 4 are listed in Table 18S, Supporting Information. The
Figure 3. J = 28 ← 27 aR-transitions of conformer III. The spectrum
was recorded at a Stark field strength of about 110 V/cm. Values of the
K−1 pseudo quantum number are listed above several peaks belonging
to III. The transitions with K−1 quantum numbers 5, 6, and 7 are not
resolved. The remaining unlabeled transitions are presumed to belong
to vibrationally excited states of III or to other conformers. The
intensity is in arbitrary units.
Table 4. Spectroscopic Constantsa of the Ground
Vibrational State of Conformer V of C3H5CH2N3
Av (MHz)
Bv (MHz)
Cv (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
d1 (kHz)
d2 (kHz)
HJ (Hz)
HJK (Hz)
HKJ(Hz)
rmsa
Na
7035.0(17)
1485.9842(78)
1397.6745(81)
2.3199(20)
−31.1213(67)
102a
−0.6590(44)
−0.0257(19)
0.0274(12)
−0.2202(51)
−1.841(16)
1.238
407
weaker than the spectrum of conformer V. The assignments of
320 transitions (Table 19S, Supporting Information) were
made for values of J and K−1 up to 38 and 24, respectively, in
the same way as described in the previous paragraph. The K−1 =
1 or 0 are weak and difficult to modulate and frequently
overlapped, and definite assignments of these transitions could
not be made. The rotational constants, DJ, DJK, d1, HJK, and HKJ,
were fitted, while DK and d2 could not be determined from
these aR-lines. DK and d2 were fixed at the MP2 value (Table 2)
in the least-squares fit, whereas further sextic constants were
preset at zero. No vibrationally excited-state spectrum could be
assigned for intensity reasons. The experimental spectroscopic
constants are displayed in Table 5. The experimental rotational
constants of this table deviate by less than 0.6% from their
CCSD counterparts. Much larger differences are seen for the
quartic centrifugal distortion constants.
Searches for Further Conformers. Rotamers I and II are
predicted to be the highest-energy forms of (azidomethyl)cyclopropane with an energy difference of 3−4 kJ/mol relative
to the global-minimum conformer IV (Table 2). Form I has a
statistical weight of 1 vis-à-vis the four other conformers (II−
V) whose statistical weight is 2 relative to I. The assignment of
a
Footnotes are the same as those given for Table 3. The spectrum is
given in Table 18S of the Supporting Information.
rotational constants, four quartic centrifugal distortion
constants, DJ, DJK, d1, and d2, as well as three sextic centrifugal
distortion constants, HJ, HJK, and HKJ, were fitted. DK could not
be determined from these aR-lines and was fixed at the MP2
value (102 kHz; Table 2). Further sextic constants were preset
at zero in the least-squares fit. No excited-state spectra of this
conformer could be unambiguously assigned due to their
weakness.
Comparison of the experimental rotational constants listed in
Table 4 and the CCSD rotational constants displayed in Table
6976
dx.doi.org/10.1021/jp506261a | J. Phys. Chem. A 2014, 118, 6971−6978
The Journal of Physical Chemistry A
Article
Table 5. Spectroscopic Constantsa of the Ground
Vibrational State of Conformer III of C3H5CH2N3
Av (MHz)
Bv (MHz)
Cv (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
d1 (kHz)
d2 (kHz)
HJK (Hz)
HKJ(Hz)
rmsa
Na
possible that this results in steric repulsion and may therefore
be part of the explanation why IV is preferred over V.
The C2−C9−N10−N13 chain is antiperiplanar in III, whose
energy is +2.1(6) kJ/mol relative to IV. Steric repulsion is
absent in this rotamer, but this is not sufficient to make this
conformer the global minimum-energy form. One reason for
this could be the highly electronegative nature of the nitrogen
atom whose Pauling electronegativity value is 3.04.24 The socalled gauche effect25 predicts that sc forms are preferred for
highly electronegative atoms or groups of atoms, and this may
have played a role in the present case.
11897(44)
1177.057(38)
1136.174(38)
0.18116(46)
−5.5982(57)
163a
0.0103(44)
0.000126a
−0.0570(24)
1.5030(83)
1.331
320
■
ASSOCIATED CONTENT
S Supporting Information
*
Results of the theoretical calculations, including electronic
energies; molecular structures; dipole moments; harmonic
vibrational frequencies; and rotational and centrifugal distortion
constants. Microwave spectra of the ground and vibrationally
excited states of three conformers. This material is available free
of charge via the Internet at http://pubs.acs.org.
a
Footnotes are the same as those given for Table 3. The spectrum is
given in Table 19S in the Supporting Information.
the spectrum of I was therefore considered to be very
demanding in this weak spectrum. Searches were nevertheless
performed using Stark and RFMWDR spectroscopies, but with
negative outcome.
The chances to assign the spectrum of II looked a little
better, and considerable efforts were made toward this end.
This conformer was predicted (Table 2) to have an energy that
is about 1 kJ/mol higher than the energy of III, which has the
weakest of the three assigned spectra. In addition, II has dipole
moment components, which are significantly lower than μa of
III (Table 2). All this can explain why the very weak spectrum
of this form was not found despite extensive Stark and
RFMWDR searches.
Energy Differences. The energy differences between the
three identified conformers were determined by relative
intensity measurements21 performed on relatively strong, fully
modulated absorption lines observing the precautions of Esbitt
and Wilson.22 The intensity and thereby the energy differences
depend on the dipole moment component of the transitions in
question, and the theoretical values given in Table 2 were
employed in the energy-difference computations. Conformer
IV was found to be the global minimum being 1.6(6) kJ/mol
lower in energy than V, and 2.1(6) kJ/mol lower in energy than
III. The uncertainties are estimated to one standard deviation.
The corresponding CCSD values (Table 2) were 1.98 and 2.86
kJ/mol, respectively. The experimental values tend to be
slightly less than the CCSD predictions.
■
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.
■
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). J-C.G. thanks the Centre National d’Etudes
Spatiales (CNES) for financial support.
■
REFERENCES
(1) Meldal, M.; Tornøe, C. W. Cu-Catalyzed Azide-Alkyne
Cycloaddition. Chem. Rev. 2008, 108, 2952−3015.
(2) Brantley, J. N.; Wiggins, K. M.; Bielawski, C. W. Unclicking the
Click: Mechanically Facilitated 1,3-Dipolar Cycloreversions. Science
2011, 333, 1606−1609.
(3) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Click Chemistry:
Diverse Chemical Function From a Few Good Reactions. Angew.
Chem., Int. Ed. 2001, 40, 2004−2021.
(4) Bräse, S.; Gil, C.; Knepper, K.; Zimmermann, V. Organic Azides.
An Exploding Diversity of a Unique Class of Compounds. Angew.
Chem., Int. Ed. 2005, 44, 5188−5240.
(5) Klaeboe, P.; Nielsen, C. J.; Priebe, H.; Schei, S. H.; Sjøgren, C. E.
The Vibrational Spectra, Molecular Structure and Conformations of
Organic Azides. I. A Survey. J. Mol. Struct. 1986, 141, 161−172.
(6) Samdal, S.; Møllendal, H.; Guillemin, J.-C. Synthesis, Microwave
Spectrum, and Conformational Properties of 2-Fluoroethyl Azide
(FCH2CH2N3). J. Phys. Chem. A 2013, 117, 1935−1940.
(7) Sieh, D. H.; Michejda, C. J. Acid-Catalyzed Decomposition of
Trialkyltriazenes: Protected Alkyldiazonium Ions. J. Am. Chem. Soc.
1981, 103, 442−445.
(8) Samdal, S.; Grønås, T.; Møllendal, H.; Guillemin, J.-C.
Microwave Spectrum and Conformational Properties of 4-Isocyano1-butene (H2CCHCH2CH2NC). J. Phys. Chem. A 2014, 118,
1413−1419.
(9) Wodarczyk, F. J.; Wilson, E. B., Jr. Radio Frequency-Microwave
Double Resonance as a Tool in the Analysis of Microwave Spectra. J.
Mol. Spectrosc. 1971, 37, 445−463.
■
DISCUSSION
(Azidomethyl)cyclopropane displays a complex conformational
equilibrium consisting of at least three rotamers III, IV, and V.
There are probably several reasons why these three forms are
preferred over I and II. The identified forms III, IV, and V have
one thing in common, namely, a synclinal orientation of the
H6−C2−C9−N10 chain of atoms. This kind of sc preference is
typical for monosubstituted methylcyclopropanes (C3H5CH2X)
and displayed by the vast majority of these compounds.23 The
sc inclination seems to be almost independent of the nature of
the X substituent.
The lowest-energy conformer IV and the second lowestenergy rotamer V (+1.6(6) kJ/mol) both have sc orientation for
the C2−C9−N10−N13 link of atoms. There are no close
nonbonded contacts between N13 and atoms of the cyclopropyl ring in IV (Table 9S, Supporting Information), while a
close contact of 281 pm exists between N13 and H6 in V. It is
6977
dx.doi.org/10.1021/jp506261a | J. Phys. Chem. A 2014, 118, 6971−6978
The Journal of Physical Chemistry A
Article
(10) Møller, C.; Plesset, M. S. Note on the Approximation
Treatment for Many-Electron Systems. Phys. Rev. 1934, 46, 618−622.
(11) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; et al. Gaussian 09, revision B.01; Gaussian, Inc:
Wallingford, CT, 2010.
(12) Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz,
M.; et al. MOLPRO, version 2010.1, a Package of Ab Initio Programs;
Cardiff University: Cardiff, U.K., 2010. http://www.molpro.net/.
(13) Peterson, K. A.; Dunning, T. H., Jr. Accurate Correlation
Consistent Basis Sets for Molecular Core-Valence Correlation Effects:
The Second Row Atoms Al-Ar, and the First Row Atoms B-Ne
Revisited. J. Chem. Phys. 2002, 117, 10548−10560.
(14) Watson, J. K. G. Vibrational Spectra and Structure; Elsevier:
Amsterdam, The Netherlands, 1977; Vol. 6.
(15) McKean, D. C.; Craig, N. C.; Law, M. M. Scaled Quantum
Chemical Force Fields for 1,1-Difluorocyclopropane and the Influence
of Vibrational Anharmonicity. J. Phys. Chem. A 2008, 112, 6760−6771.
(16) Heineking, N.; Gerry, M. C. L. Nitrogen-14 Nuclear
Quadrupole Hyperfine Structure in the Microwave Spectrum of
Methyl Azide, CH3N3. Z. Naturforsch., A: Phys. Sci. 1989, 44, 669−674.
(17) Pauling, L. The Nature of the Chemical Bond; Cornell University
Press: Ithaca, NY, 1960.
(18) Ray, B. S. The Characteristic Values of an Asymmetric Top. Z.
Phys. 1932, 78, 74−91.
(19) Sørensen, G. O. Centrifugal Distortion Analysis of Microwave
Spectra of Asymmetric Top Molecules. The Microwave Spectrum of
Pyridine. J. Mol. Spectrosc. 1967, 22, 325−346.
(20) Herschbach, D. R.; Laurie, V. W. Influence of Vibrations on
Molecular Structure Determinations. I. General Formulation of
Vibration-Rotation Interactions. J. Chem. Phys. 1962, 37, 1668−1686.
(21) Townes, C. H.; Schawlow, A. L. Microwave Spectroscopy;
McGraw-Hill: New York, 1955.
(22) Esbitt, A. S.; Wilson, E. B. Relative Intensity. Rev. Sci. Instrum.
1963, 34, 901−907.
(23) Samdal, S.; Møllendal, H.; Guillemin, J.-C. Microwave
Spectrum, Conformational Properties, and Dipole Moment of
Cyclopropylmethyl Isocyanide (C3H5CH2NC). J. Phys. Chem. A
2013, 117, 5073−5081.
(24) Allred, A. L. Electronegativity Values from Thermochemical
Data. J. Inorg. Nucl. Chem. 1961, 17, 215−221.
(25) Wolfe, S. Gauche Effect. Stereochemical Consequences of
Adjacent Electron Pairs and Polar Bonds. Acc. Chem. Res. 1972, 5,
102−111.
6978
dx.doi.org/10.1021/jp506261a | J. Phys. Chem. A 2014, 118, 6971−6978