ARTICLE
pubs.acs.org/JPCA
Microwave Spectrum, Conformational Composition, and
Intramolecular Hydrogen Bonding of (2-Chloroethyl)amine
(ClCH2CH2NH2)
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
‡
Ecole Nationale Superieure de Chimie de Rennes, CNRS, UMR 6226, Avenue du General Leclerc, CS 50837, 35708 Rennes Cedex 7,
France
bS Supporting Information
ABSTRACT: The microwave spectrum of (2-chloroethyl)amine, ClCH2CH2NH2, has been
investigated in the 22120 GHz region. Five rotameric forms are possible for this compound.
In two of these conformers, denoted I and II, the ClCCN chain of atoms is
antiperiplanar, with different orientations of the amino group. The link of the said atoms is
synclinal in the three remaining forms, IIIV, which differ with respect to the orientation of
the amino group. The microwave spectra of four of these conformers, IIV, have been
assigned. In two of these rotamers, III and IV, the amino group is oriented in such a manner
that rare and weak five-membered NH 3 3 3 Cl intramolecular hydrogen bonds are formed.
The geometries of conformers I and II preclude a stabilization by this interaction. The energy
differences between the conformers were obtained from relative intensity measurements of
spectral lines. The hydrogen-bonded conformer IV represents the global energy minimum.
This rotamer is 0.3(7) kJ/mol more stable than the other hydrogen-bonded conformer III,
4.1(11) kJ/mol more stable than II, and 5.5(15) kJ/mol more stable than I. The spectroscopic work has been augmented by
quantum chemical calculations at the CCSD/cc-pVTZ and MP2/6-311þþG(3df,3pd) levels of theory. The CCSD rotational
constants and energy differences are in good agreement with their experimental counterparts.
’ INTRODUCTION
It is well-known that the amino group can be involved in
hydrogen (H) bonding. It acts as a proton acceptor (“base”) in
most cases. However, this group can also be a weak proton donor
(“acid”). One of our research interests has been to investigate the
ability to form intramolecular H bonds of the type NH 3 3 3 X,
where X is a proton acceptor. Microwave spectroscopy has
successfully been used to demonstrate that the amino group
indeed acts as a weak proton donor in many compounds
stabilized by internal H bonding. For example, in H2NCH2CH2F1 and H2NCH2CHF2,2 fluorine atoms are acceptors. In
H2NCH2CH2NH2,3,4 the amino group acts as both a proton
donor and a proton acceptor. A similar situation is found in
H2NCH2CH2NH(CH3).5 The oxygen atom accepts a H atom in
H2NCH2CH2OCH3.6,7 This is also the case for one of the
preferred conformers of amino acids exemplified by glycine810
and alanine.11,12 π-Electrons are involved in a number of cases
such as H2NCH2CHdCH2,1316 H2NCH2CH2CHdCH2,17
1-amino-1-ethenylcyclopropane,18 H2NCH2CtCH,19 H2NCH2CtN,20,21 H2NCH2CH2CtN,22 H2NCH2CH2CtCH,23
and 1-amino-1-ethynylcyclopropane.24 Pseudo π-electrons25 are
acceptors in (aminomethyl)cyclopropane,26 while the π-electrons of the phenyl group are active in intramolecular H bonding
in, for example, amphetamine.27
r 2011 American Chemical Society
An important finding made for the majority of the above
examples is that both H atoms of the amino group form internal
NH 3 3 3 X hydrogen bonds, which results in two conformers,
similar to rotamers III and IV of the title compound discussed
below. These two H-bonded conformers have been shown to
have almost the same energy.
Chlorine is a very electronegative element, with a Pauling
electronegativity28 of 3.16, which indicates that this element
could be an acceptor in a weak NH 3 3 3 Cl bond. However, no
gas-phase studies of compounds with intramolecular H bonds of
this type appear to have been reported. The potential ability of
amino groups to form H bonds with the chlorine atom and the
scant information available for this kind of interaction motivated
the present first MW study of the title compound . The choice of
(2-chloroethyl)amine, ClCH2CH2NH2, was made because this
compound represents the prototype for the formation of the fiveatom chain ClCCNH that might form a ring that is
stabilized by a NH 3 3 3 Cl hydrogen bond.
A total of five rotameric forms with all-staggered bonds can be
envisaged for this compound. Representatives of these five
Received: February 8, 2011
Revised:
March 16, 2011
Published: April 01, 2011
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The Journal of Physical Chemistry A
ARTICLE
Figure 1. Five conformers of ClCH2CH2NH2. Atom numbering is shown on conformer I. Microwave spectra of conformers IIV were assigned in this
work. Rotamers III and IV are stabilized by an intramolecular hydrogen bond between one of the hydrogen atoms of the amino group and the chlorine
atom and are the most stable forms of this compound.
rotamers are designated Roman numerals IV and depicted in
Figure 1. Conformers I and II have an antiperiplanar arrangement for the ClCCN chain of atoms, while this chain is
synclinal in the remaining three rotamers IIIV. Rotamers I and
II differ in the orientation of the amino group. Rotamer II has a
symmetry plane (Cs symmetry), whereas the amino group is
rotated roughly 120° from this conformation in I. No internal H
bonding is of course possible for I and II. Rotamers IIIV have
different orientations of the amino group, allowing five-membered intramolecular NH 3 3 3 Cl hydrogen bonds to be formed
in two rotamers, namely, III and IV. However, no such interaction is possible in V because both of the H atoms of the amino
group are oriented away from the chlorine atom. Mirror-image
forms exist for rotamers I and IIIV, which therefore have a
statistical weight of 2 for each of them. Rotamer II has a
symmetry plane and a statistical weight of 1.
Microwave spectroscopy is an ideal method for studying
complex conformational equilibria of the type represented by
(2-chloroethyl)amine because of its superior accuracy and resolution. The spectroscopic work has been augmented by highlevel quantum chemical calculations, which were conducted with
the purpose of obtaining information for use in assigning the
MW spectrum and investigating properties of the potentialenergy hypersurface.
’ EXPERIMENTAL SECTION
Synthesis.29 In a 100 mL two-necked flask equipped with a
rubber septum and a stirring bar was introduced 2-chloroethylamine hydrochloride (4.0 g, 34 mmol). The flask was adapted to
a vacuum line equipped with two traps. The first trap was
immersed in a cold bath cooled to 20 °C and the second
one, equipped with two stopcocks, was immersed in a bath
cooled to 100 °C. Pure 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU; 7.0 g, 46 mmol) was introduced by portions with a
syringe through the septum for 10 min. The formed free amine
was continuously extracted from the reaction mixture and
selectively condensed in the second trap. After 20 min of stirring
at room temperature, the reaction mixture was immersed in a
50 °C bath for 10 min. At the end of the reaction, the second trap
was disconnected from the vacuum line and attached to the
spectrometer. The compound can be kept for months in the
freezer (20 °C) under vacuum or nitrogen. It is stable enough
to be transferred quickly under air with a pipet. Yield: 2.0 g, 25
mmol (72%). 1H NMR (CDCl3, 400 MHz) δ 1.25 (s brd, 2H,
NH2); 2.89 (t, 2H, 3JHH = 5.6 Hz, CH2N); 3.47 (m, 2H, 3JHH =
5.6 Hz, CH2Cl). 13C NMR (CDCl3, 100 MHz) δ 43.4 (t, 1JCH =
136.5 Hz, CH2N); 47.9 (t, 1JCH = 149.7 Hz, CH2Cl).
Microwave Experiment. The spectrum of (2-chloroethyl)amine was studied extensively in the 2280 GHz frequency interval by Stark-modulation spectroscopy using the
microwave spectrometer of the University of Oslo. Some measurements were also performed in the 80120 GHz region.
Details of the construction and operation of this device have been
given elsewhere.3032 This spectrometer has a resolution of about
0.5 MHz and measures the frequency of isolated transitions with
an estimated accuracy of ≈0.10 MHz. Radio-frequency microwave double-resonance experiments (RFMWDR), similar to
those performed by Wodarczyk and Wilson,33 were also conducted to unambiguously assign particular transitions, using the
equipment described elsewhere.30 The experiments were performed at room temperature or at about 30 °C by cooling the
2 m Hewlett-Packard absorption cell with portions of dry ice.
The pressure of ClCH2CH2NH2 was roughly 10 Pa during the
measurements.
Quantum Chemical Methods. The present ab initio calculations were performed employing the Gaussian 03 suite of
programs,34 running on the Titan cluster in Oslo. Becke’s threeparameter hybrid functional35 employing the Lee, Yang, and Parr
correlation functional (B3LYP)36 was employed in the density
functional theory (DFT) calculations. MøllerPlesset secondorder perturbation calculations (MP2)37 and coupled-cluster
calculations with singlet and doublet excitations (CCSD)38,39
were also performed. The 6-311þþG(3df,3pd) basis set was
used in the MP2 calculations, whereas Peterson and Dunning’s40
correlation-consistent cc-pVTZ basis set was used in the CCSD
calculations. Both basis sets are of triple-ζ quality and diffuse
functions are included in the 6-311þþG(3df,3pd) basis set.
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Table 1. CCSD/cc-pVTZ Structures of Five Conformers of
ClCH2CH2NH2
conformer
I
II
III
IV
V
Bond Length (pm)
C1H2
109.5
109.0
109.6
109.0
109.0
C1H3
C1C4
109.0
151.8
109.0
152.2
109.3
151.4
109.3
152.2
109.9
151.6
C1N7
146.3
145.9
145.9
145.5
145.8
C4H5
108.5
108.7
108.6
108.7
108.6
C4H6
108.7
108.7
108.6
108.8
108.8
C4Cl10
179.3
179.5
180.0
179.8
179.2
N7H8
101.1
101.1
101.2
101.2
101.0
N7H9
101.1
101.1
101.2
101.2
101.1
H2C1H3
H2C1C4
107.4
109.2
106.9
109.3
107.5
108.8
107.0
109.0
107.3
108.9
H2C1N7
114.5
108.8
114.4
108.5
108.5
H3C1C4
108.8
109.3
107.1
107.4
106.8
H3C1N7
108.6
108.8
108.3
108.5
113.9
C4C1N7
108.4
113.7
110.4
116.2
111.3
C1C4H5
110.8
111.3
111.5
111.5
110.6
C1C4H6
111.3
111.3
110.7
111.5
111.1
C1C4Cl10
H5C4H6
110.7
109.2
111.0
109.1
111.0
110.3
111.2
109.4
112.4
109.0
H5C4Cl10
107.8
107.0
106.5
106.7
106.8
H6C4Cl10
106.9
107.0
106.6
106.5
106.8
C1N7H8
110.0
110.4
109.0
109.9
110.0
C1N7H9
110.9
110.4
109.7
109.3
109.9
H8N7H9
106.0
106.6
106.3
106.2
106.3
H2C1C4H5
H2C1C4H6
174.0
64.3
58.5
179.0
66.8
172.0
Angles (deg)
Dihedral Angle (deg)
177.4
60.8
59.3
177.5
H2C1C4Cl10
54.6
58.3
59.3
60.4
52.5
H3C1C4H5
57.2
60.8
56.7
57.0
48.8
H3C1C4H6
178.9
177.4
66.5
65.5
72.4
H3C1C4Cl10
62.3
58.3
175.3
176.0
168.0
N7C1C4H5
60.7
60.9
174.4
178.7
173.7
N7C1C4H6
61.0
60.9
51.2
56.2
52.6
N7C1C4Cl10
H2C1N7H8
179.9
45.2
180.0
179.2
67.1
57.8
62.4
178.8
67.0
52.0
H2C1N7H9
71.3
63.2
58.2
65.1
168.7
H3C1N7H8
165.0
63.2
177.7
62.9
67.5
H3C1N7H9
48.6
179.2
61.7
179.1
49.3
C4C1N7H8
77.0
58.8
65.3
58.1
171.7
C4C1N7H9
166.6
58.8
178.6
58.0
71.6
Calculations of the nuclear quadrupole coupling constants of the
chlorine nucleus in the principal-inertial axis system were performed using Bailey’s program,41 using the electric field gradient
obtained in the CCSD calculations.
’ RESULTS AND DISCUSSION
Quantum-Chemical Calculations. To assign a microwave
spectrum as complex as the one observed in the present case,
it is important to have as accurate rotational and centrifugal
distortion constants as possible. MP2 calculations with a large
basis set are known to produce accurate equilibrium structures42
and these MP2 structures have been used as starting points in the
even more accurate CCSD calculations. MP2/6-311þþG(3df,3pd) calculations of energies, structures, dipole moments,
vibrational frequencies, Watson’s quartic centrifugal distortion
constants43 were therefore first undertaken for IV. The geometry optimizations were performed observing the default
convergence criteria of Gaussian 03.44 No imaginary vibrational
frequencies were found for each of the five forms, which indicate
that they are indeed minima on the conformational-energy
hypersurface. The MP2 structures of the five forms are listed
in Table 1S in the Supporting Information, while additional
parameters of spectroscopic interest are displayed in Table 2S.
The energy differences in the latter table have been corrected
for zero-point vibrational energies. In Table 3S, the vibrational
frequencies of the five lowest fundamentals are listed.
The MP2 structures were then used as starting points in the
CCSD/cc-pVTZ calculations to get energies, structures, rotational constants, and dipole moments for conformers IV.
B3LYP/6-31G** force fields were employed as initial Hessians
in these calculations to speed up convergence. The CCSD
structures of these conformers are shown in Table 1, while
Table 2 contains additional CCSD parameters as well as the
Watson S-reduction quartic centrifugal distortion constants,43
which were obtained in the MP2 calculations. The energy
differences reported in Table 2 have been calculated from the
CCSD electronic energies.
The CCSD electronic field gradients were also computed,
and Bailey’s program41 was used to calculate the principal-axes
nuclear quadrupole coupling constants of the chlorine nucleus
from them. These parameters are also included in Table 2.
Calculations of quadrupole coupling constants of the 14N
nucleus were not undertaken, because this is a small and
unresolved effect in the observed MW spectra.
Comparison of the CCSD and MP2 structures in Tables 1 and
1S (Supporting Information) are in order. It is seen from these
two tables that there are relatively small differences between the
structures obtained with the two methods. Bond lengths generally agree to within about 0.5 pm, with one exception, namely,
the C4Cl10 bond length, whose CCSD lengths are about 1.5
pm longer than the MP2 lengths. Bond angles and dihedral
angles obtained in the two methods generally agree to within
approximately 1°.
Both the CCSD and MP2 methods predict the H-bonded
conformers III and IV to be the preferred forms of (2-chloroethyl)amine with practically the same energies (Tables 2 and
2S). There is also good agreement between the two methods
concerning the predicted energies of the less stable conformers I,
II, and V, which are roughly 4, 3, and 10 kJ/mol less stable than
III and IV according to these calculations.
It is noted that there are some interesting structural differences
between these two H-bonded rotamers III and IV, which are
perhaps best seen from a comparison of the C4C1N7 bond
angle and the N7C1C4C10 dihedral angle of the two
forms. The C4C1N7 bond angle is 110.4° in III and as large
as 116.2° in IV (CCSD values; Table 1). It is difficult to say
whether this almost 6° difference is a result of a slight rehybridization of the C1 atom or a 1,3-repulsion between H6 and H8
in IV. The nonbonded distance between these two atoms is
calculated to be 259.8 pm from the structure in Table 1,
compared to 240 pm, which is twice the Pauling van der Waals
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Table 2. CCSD/cc-pVTZ and MP2/6-311þþG(3df,3pd) Parameters of Spectroscopic Interest of Five Conformersa of
ClCH2CH2NH2
conformer
I
II
III
IV
V
Rotational Constantsb (MHz)
A
28062.5
27517.0
12598.4
12338.3
12925.9
B
C
2418.3
2307.8
2397.8
2294.8
3420.4
2929.0
3393.3
2907.0
3266.5
2868.1
16.04
17.61
Pseudo Inertial Defectc (1020 u m2)
Δ
8.00
8.91
15.32
d
Quartic Centrifugal Distortion Constants (kHz)
DJ
0.382
0.382
2.41
2.41
2.54
DJK
3.21
2.80
11.9
12.2
16.1
DK
106
94.4
44.1
45.1
66.0
d1
0.0266
0.0241
0.621
0.604
0.634
d2
0.000728
0.000161
0.0429
0.0395
0.0360
30
b
Dipole Moment (10
C m)
μa
7.89
4.78
5.61
1.84
9.49
μb
0.73
5.85
3.82
5.19
7.05
μc
2.91
0.00
3.13
3.53
1.16
35
b
Principal-Axes Nuclear Quadrupole Coupling Constants of the Cl Species (MHz)
χaa
51.77
52.08
19.41
18.36
22.02
χbb
17.90
17.88
9.70
11.30
6.92
|χab|
35.95
36.41
46.95
47.50
49.36
37
b
Principal-Axes Nuclear Quadrupole Coupling Constants of the Cl Species (MHz)
χaa
χbb
40.88
14.18
41.12
14.17
|χab|
28.26
28.62
15.77
7.22
14.96
8.46
17.84
5.02
36.98
37.42
38.85
0.0
10.00
bef
Energy Difference , , (kJ/mol)
ΔE
3.47
2.60
0.02
Minima on the potential energy hypersurface. b CCSD results. c Pseudo inertial defect defined by Δ = Ic Ia Ib, where Ia, Ib, and Ic are the principal
moments of inertia. Conversion factor: 505 379.05 1020 MHz μm2. d MP2 results. S-reduction.43 e Electronic energy. f Electronic energy of rotamer
IV: 1559 660.24 kJ/mol.
a
radius of hydrogen (120 pm).45 Moreover, there is a difference
of about 5° in the N7C1C4C10 dihedral angle, which
is 67.1° in III and 62.4° in IV. The latter conformer has
therefore a more compact structure. These structural differences
result in slightly different nonbonded distances between the
amino-group hydrogen atom involved in internal H bonding
with the chlorine atom. These distances are found to be 275.8 in
III and 280.2 pm in IV from the structures in Table 1 compared
to 300 pm, which is the sum of the van der Waals radii of chlorine
(180 pm) and hydrogen (120 pm).45
Microwave Spectrum and Assignment of the Spectrum of
Conformer III. The spectrum of (2-chloroethyl)amine was
expected to be relatively weak for several reasons. The prediction
that four of the five rotamers fall within a narrow range of only
about 4 kJ/mol (Table 2) means that the gas should contain
substantial amounts of each of these conformers. This results in
reduction of spectral intensities of each conformer compared to a
situation where there was only one rotamer present. The fact that
there are two isotopes of chlorine (35Cl 75.8% and 37Cl
24.2%) has a similar effect. The nuclear quadrupole coupling
associated with this element broadens and splits the spectral
transitions, which is another factor that has reduced intensities
as a consequence. Finally, there are four fundamental vibrations for each conformer with frequencies below 500 cm1
(Table 3S in the Supporting Information) and these states
are well populated at room temperature or at 30 °C according to Boltzmann statistics, resulting in further reduction of
intensities.
The observed spectrum was comparatively weak in accord
with these predictions and very dense with absorption lines
occurring every few MHz throughout the entire spectral range
(22 120 GHz). The frequencies of the strongest lines of the
spectrum were first checked against the NASA compilation of
microwave spectra.46 It turned out that there was a contamination of HOCH2CH2NH2, which has a relatively strong spectrum.
It was not possible to determine the concentration of this
contamination, but it was hardly more than a few percent.
Searches first concentrated on finding the spectra of III and IV
because these two conformers were predicted (Tables 2 and 2S)
to be the preferred forms of (2-chloroethyl)amine with practically the same energies. aR-spectra are generally easier to assign
than b- or c-type spectra primarily because high-K1 members
are modulated at low Stark voltages as a consequence of the neardegeneracy of pairs of K1-lines with the same value of K1.
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Table 3. Spectroscopic Constantsa of the Ground Vibrational
State of Conformer III of ClCH2CH2NH2
35
isotopologue
37
ClCH2CH2NH2
isotopologue
35
ClCH2CH2NH2
37
ClCH2CH2NH2
A (MHz)
12594.405(17)
12551.029(36)
A (MHz)
12360.280(44)
12315.805(45)
B (MHz)
C (MHz)
3414.1467(33)
2919.4401(33)
3336.1629(91)
2860.1327(89)
B (MHz)
C (MHz)
3381.197(39)
2895.280(39)
3302.371(39)
2834.589(39)
DJ (kHz)
2.477(14)
2.391(21)
DJ (kHz)
2.38(13)
2.48(12)
DJK (kHz)
13.164(38)
12.96(6)
DJK (kHz)
13.190(64)
12.694(58)
DK (kHz)
50.74(88)
50.74b
DK (kHz)
52.3(13)
53.1(13)
d1 (kHz)
0.6441(10)
0.529(13)
d1 (kHz)
0.6484(15)
0.6069(14)
d2 (kHz)
0.04666(47)
0.04666b
d2 (kHz)
0.04416(70)
0.04239(64)
1.611
2.013
rmsb
2.381
2.544
no. trans.c
76
60
rms
c
no. trans.d
a
ClCH2CH2NH2
Table 4. Spectroscopic Constantsa of the Ground Vibrational
State of Conformer IV of ClCH2CH2NH2
165
r
82
43
S-reduction, I -representation. Uncertainties represent one standard
deviation. Spectra in Tables 8S and 9S in the Supporting Information.
b
Fixed in the least-squares fit; see text. c Root-mean-square of a
weighted fit. d Number of transitions used in the fit.
Conformer III was predicted to have a much larger μa than IV
(5.61 and 1.84 1030 C m, respectively; Table 2) and we
therefore first focused on finding the aR-spectrum of the former
rotamer using the CCSD rotational constants and the MP2
quartic centrifugal distortion constants43 shown in Table 2 to
predict the approximate frequencies of the spectrum of III. An
a
R-spectrum having the predicted characteristics was indeed
found close to this prediction and confirmed by RFMWDR
experiments.33 We were also able to assign a number of b-type
Q-branch lines. The spectrum was fitted using a total of 165
transitions listed in Table 8S in the Supporting Information using
Sørensen’s program Rotfit.47 A weighted least-squares fit was
performed, where the spectral uncertainties of the transitions
were used as weights. Some of the b-type lines were split into two
components, each consisting of unresolved hyperfine quadrupole structure. Unfortunately, it was not possible to derive
accurate values for the nuclear quadrupole components from
these partly resolved splittings. The splittings were, however, in
accord with the predictions made using the CCSD values of the
principal-axes nuclear quadrupole coupling constants listed in
Table 2. The average frequencies obtained from the split lines
were used in the least-squares fit. The resulting spectroscopic
constants are listed in Table 3.
The CCSD structure was then used to predict the changes in
the rotational constants that occur when the 35Cl isotope is
substituted with the 37Cl isotope. These changes were added to
the experimental rotational constants of the 35Cl isotopologue
and used to predict the spectrum of the 37Cl variant, whose
spectrum was found within a few MHz from the predictions with
an intensity that was about 1/3 of that of the parent species, as
expected. This spectrum, consisting of 82 lines and listed in Table
9S in the Supporting Information, was fitted in the same manner
as that of the 35Cl species with the results displayed in Table 3.
Assignment of the Spectrum of IV. μb is the largest
component of the dipole moment in this case. We first focused
on finding the bQ-lines because they are the strongest ones in the
spectrum. Some of them should be split into two distinct
components according to the nuclear quadrupole constants in
Table 2, which was helpful in the assignment procedure. The
accuracy of the CCSD rotational constants was also very useful
because the spectrum was indeed found close to its prediction
using these constants. Several series of bQ-branch transitions
a
S-reduction, Ir-representation.43 Uncertainties represent one standard
deviation. Spectra in Tables 10S and 11S. b Root-mean-square of a
weighted fit. c Number of transitions used in the fit.
Table 5. Spectroscopic Constantsa of the Ground Vibrational
State of Conformer II of ClCH2CH2NH2
isotopologue
35
ClCH2CH2NH2
37
ClCH2CH2NH2
27120b
A (MHz)
27228.327(38)
B (MHz)
2399.2322(40)
2343.66(12)
C (MHz)
2295.2878(40)
2244.31(13)
9.090(24)
Δ (1020 u m2)c
9.02165(13)
DJ (kHz)
0.3752(91)
0.3819(64)
DJK (kHz)
DK (kHz)
2.753(43)
94.4b
3.01(19)
94.4b
d1 (kHz)
0.02468(16)
0.02468b
d2 (kHz)
0.000161
0.0000161b
rms
d
no. trans.e
b
1.428
1.260
82
38
a
S-reduction, Ir-representation.43 Uncertainties represent one standard
deviation. Spectra in Tables 6S and 7S. b Fixed; see text. c Defined by Δ =
Ic Ic Ib, where Ia, Ib, and Ic, are the principle moments of inertia.
Conversion factor: 505379.05 1020 MHz u m2. d Root-mean-square
of a weighted fit. e Number of transitions used in the fit.
were assigned first. The average frequencies were used in those
cases where partly resolved quadrupole splittings were observed.
The bR-branch lines were found after some searching. Attempts
to find a- and c-type lines were inconclusive and these candidates
were not employed in the final least-squares fit. A total of 76
transitions listed in Table 10S in the Supporting Information
were used to derive the spectroscopic constants displayed in
Table 4.
The spectrum of the 37Cl isotopologue (60 transitions) was
assigned in the same manner as described for its conformer III
counterpart. The spectroscopic constants are listed in Table 4
and the spectrum is shown in Table 11S.
Assignment of the Spectrum of II. Having assigned the
spectra of III and IV, the focus was shifted to II, which
was predicted to have a higher energy than III and IV by about
2.6 kJ/mol. This rotamer has rather similar magnitudes of μa and
μb (Table 2). We therefore first tried to find the aR-spectrum for
similar reasons as already mentioned for III. The spectroscopic
constants in Table 2 were so accurate that this was readily
achieved in spite of the fact that this spectrum was much weaker
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The Journal of Physical Chemistry A
ARTICLE
Table 6. Spectroscopic Constantsa of the Ground Vibrational
State of Conformer I of ClCH2CH2NH2
isotopologue
35
ClCH2CH2NH2
b
37
ClCH2CH2NH2
A (MHz)
28300
28280be
B (MHz)
C (MHz)
2412.319(72)
2304.867(75)
2356.82(37)
2253.25(39)
Δ (1020 u m2)c
8.091(13)
8.014(73)
DJ (kHz)
0.4056(54)
0.406be
DJK (kHz)
10.45(18)
10.45be
be
106be
DK (kHz)
106
d1 (kHz)
0.0266
d2 (kHz)
0.000728
d
rms
no. trans.e
be
be
1.518
37
0.0266be
0.000728be
1.100
8
a
S-reduction, Ir-representation.43 Uncertainties represent one standard
deviation. Spectra in Tables 4S and 5S. be Comments as for Table 5.
than those of III and IV, largely due to the energy difference.
The assignment of several of these transitions was confirmed
using the RFMWDR technique. However, an accurate value
of the A rotational constant could not be obtained from these
a-type transitions because the asymmetry parameter κ48 is
about 0.992. b-Type lines depend strongly on the A rotational
constants, which again depends strongly on the pseudo inertial
defect, Δ, defined by Δ = Ic Ia Ib. This means that Δ should
be close to 8.91 1020 u m2, as predicted in Table 2. The A
rotational constants was therefore given fixed values in the leastsquares fit and adjusted in a systematic manner so that Δ
approached 8.91 1020 u m2. The A rotational constants
obtained in this manner was then used with the other spectroscopic constants to locate the b-type transitions. A total of 82
a- and b-type transitions were assigned for conformer II of
35
ClCH2CH2NH2. These transitions are listed in Table 6S in
the Supporting Information and the spectroscopic constants
are displayed in Table 5. It is noted from this table that
Δ = 9.02165(13) 1020 u m2, its absolute value being
slightly larger than the theoretical counterpart Table 2 (8.91 1020 u m2).
The spectrum of the 37Cl species (Table 7S) of this conformer
was assigned in the same manner as described above for the
corresponding isotopologues of III and IV. RFMWDR experiments were again found to be very useful for this purpose. Only
a
R-type transitions were assigned in this case. The A rotational
constants was adjusted so that Δ got almost the same value as for
the 35Cl variant (Table 6S). The spectroscopic constants of this
conformer are also found in Table 5. The A rotational constant
was kept fixed in the least-squares fit.
Assignment of the Spectrum of I. This rotamer, which is
predicted to be approximately 3.5 kJ/mol less stable than III and
IV (Table 2), is predicted to have a comparatively large μa. The
high-K1 a-type R-branch pile-ups were found close to the
frequencies predicted using the spectroscopic constants shown
in Table 2. RFMWDR experiments were then performed to
assign several K1 = 3 pairs of transitions. However, it was found
that lines with K1 < 3 were difficult to assign unambiguously
owing to their relatively low intensities and the crowded nature of
the spectrum which results in frequent overlaps of rotational
transitions. No candidate lines with K1 < 3 were therefore used
in the final least-squares fit. The spectroscopic constants derived
from 37 transitions with K1 g 3 (Table 4S) are listed in Table 6.
The A rotational constant was adjusted in this case too to obtain
a value of Δ = 8.091(13) 1020 u m2 (Table 6), close to the
CCSD value of 8.00 1020 u m2 (Table 2).
RFMWDR experiments were performed to assign unambiguously K1 = 3 lines of the 37Cl isotopologue. Eight transitions
(Table 5S) were assigned in this manner. The A rotational
constants was obtained in the same manner as for its 35Cl
counterpart. The spectroscopic constants are listed in Table 6.
Structures. The CCSD rotational constants have been derived from an approximate equilibrium structure, whereas the
experimental rotational constants are effective constants. A
comparison of the two different sets of rotational constants is
informative but should be considered with some caution. It is
seen from Tables 2 and 3 that the experimental rotational
constants of III are very similar to their CCSD counterparts.
The largest deviation is found for the C rotational constants,
which deviates by about 10 MHz. The experimental constants of
IV (Table 4) also agree well with their counterparts of Table 2,
with a deviation of approximately 32 MHz for the A rotational
constant and smaller differences for B (12 MHz) and C (12
MHz). There is also a good agreement between the experimental
B and C rotational constants of both I and II (Tables 5 and 6) and
the corresponding CCSD constants. However, the A rotational
constants are very sensitive to small structural changes in these
two N7C1C2Cl10 antiperiplanar forms and this might
explain the larger differences seen for this particular constant
in the cases of conformers I and II. The indication obtained
from these observations is that the CCSD structures of IIV are
probably close to the equilibrium structures.
Energy Differences. The energy differences between the
rotamers were obtained by comparing the intensities of selected
rotational lines observing the precautions of Esbitt and Wilson.49
The energy differences were calculated as described by Townes
and Schawlow.50 Conformer II was assigned a statistical weight
of 1 due to its symmetry plane, while the other rotameric forms
were assumed to have a statistical weight of 2 because of the
existence of two mirror forms. Conformers III and IV were found
to have very similar energies, with IV found to represent the
global energy minimum, being 0.3(7) kJ/mol more stable than
III. Rotamer II is 4.0(10) kJ/mol less stable than IV, and I is
5.5(15) kJ/mol less stable than IV. The fact that III and IV have
the same energies within the measurement uncertainty is in
accord with the CCSD (Table 2) and MP2 (Table 2S) results.
The experimental energy differences between the two H-bonded
forms III and IV on the one hand and I and II on the other tend
to be slightly larger than obtained in the theoretical calculations.
Discussion. The experimental result that the two H-bonded
forms III and IV have practically the same energy is typical, as
already remarked in the Introduction. The fact that III and IV are
about 5.5 kJ/mol more stable than I and 4 kJ/mol more stable
than II is assumed to be largely a result of the intramolecular H
bonding that is present in III and IV, but not in I and II. These H
bonds must be quite weak because the nonbonded Cl 3 3 3 H
distances is 275.8 pm in III and 280.2 pm in IV (from the
structure in Table 1), only about 20 pm shorter than the sum
(300 pm) of the van der Waals radii of the H (120 pm) and Cl
(180 pm) atoms.45 The strength of the five-membered H bond
interactions can be taken to be roughly the same as the energy
differences between I and II on the one hand and III and IV on
the other, that is, 45 kJ/mol.
The nature of the H bonding in III and IV deserves comments.
Covalency is presumably of little significance because of the
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The Journal of Physical Chemistry A
comparatively long distance between the Cl atom and the H
atom involved in H bonding. However, the angle between the
CCl bond and the NH bond that is involved in H bonding is
7.4° in III and 6.1° in IV from being parallel. The bond moment
of CCl bond is 4.87 1020 C m with the chlorine atom as
the negative end51 and the bond moment of the NH bond is
4.37 1020 C m with nitrogen as the negative end.51 The two
bond dipoles involved in the H bonds are therefore practically
antiparallel and this favorable electrostatic interaction is assumed
to be a major contributor to the H-bond interaction.
A comparison of (2-chloroethyl)amine with its fluorine congener (2-fluoroethyl)amine, FCH2CH2NH2, is in order. Only
rotamers similar to III and IV were found for the latter
compound in a MW study.1 Typically, these two forms have
very similar energies.1 The three other conformers similar to I, II,
and V (Figure 1) have so high relative energies that they were not
observed in the MW spectrum.1 Moreover, the geometries of the
H bonds in two rotamers of (2-fluoroethyl)amine are very similar
to those observed for III and IV of the title compound. The
reason why the two H-bonded forms in (2-fluoroethyl)amine
were stabilized to a larger extent than their counterparts in
(2-chloroethyl)amine may not only be due to a larger electronegativity of fluorine (3.98)28 compared to chlorine (3.16).28
The so-called gauche effect,52 the tendency to maximize synclinal
(gauche) interactions when highly polar bonds are involved, is
presumably a more prevalent interaction in FCH2CH2NH2 than
in ClCH2CH2NH2 and may largely explain why the energy gap
between conformers, such as III and IV on the one hand and I
and II on the other, is larger in the fluorine variant than in
ClCH2CH2NH2.
’ ASSOCIATED CONTENT
bS
Supporting Information. Results of the MP2/6-311þþG(3df,3pd) calculations and the microwave spectra. This
material is available free of charge via the Internet at http://pubs.
acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Tel.: þ47 2285 5674. Fax: þ47 2285 5441. E-mail: harald.
mollendal@kjemi.uio.no.
’ ACKNOWLEDGMENT
We thank Anne Horn for her skillful assistance. The Research
Council of Norway (Program for Supercomputing) is thanked
for a grant of computer time. J.-C. G. thanks the Centre National
d’Etudes Spatiales (CNES) for financial support.
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