Article
pubs.acs.org/JPCA
Microwave and Quantum Chemical Study of the Hydrazino Group as
Proton Donor in Intramolecular Hydrogen Bonding of (2Fluoroethyl)hydrazine (FCH2CH2NHNH2)
Harald Møllendal,*,† Svein Samdal,† and Jean-Claude Guillemin‡
†
Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, Blindern, P.O. Box
1033, 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
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S Supporting Information
*
ABSTRACT: The microwave spectrum of (2-fluoroethyl)hydrazine (FCH2CH2NHNH2)
was studied in the 11−123 GHz spectral region to investigate the ability of the hydrazino
group to form intramolecular hydrogen bonds acting as a proton donor. This group can
participate both in five-member and in six-member internal hydrogen bonds with the
fluorine atom. The spectra of four conformers were assigned, and the rotational and
centrifugal distortion constants of these rotameric forms were determined. Two of these
conformers have five-member intramolecular hydrogen bonds, while the two other forms
are without this interaction. The internal hydrogen bonds in the two hydrogen-bonded forms are assumed to be mainly
electrostatic in origin because the N−H and C−F bonds are nearly parallel and the associated bond moments are antiparallel.
This is the first example of a gas-phase study of a hydrazine where the hydrazino functional group acts as a proton donor in weak
intramolecular hydrogen bonds. Extensive quantum chemical calculations at the B3LYP/cc-pVTZ, MP2/cc-pVTZ, and CCSD/
cc-pVQZ levels of theory accompanied and guided the experimental work. These calculations predict the existence of no less
than 18 conformers, spanning a CCSD internal energy range of 15.4 kJ/mol. Intramolecular hydrogen bonds are predicted to be
present in seven of these conformers. Three of these forms have six-member hydrogen bonds, while four have five-member
hydrogen bonds. The three lowest-energy conformers have five-member internal hydrogen bonds. The spectrum of the
conformer with the lowest energy was not assigned because it has a very small dipole moment. The CCSD relative energies of the
two hydrogen-bonded rotamers whose spectra were assigned are 1.04 and 1.62 kJ/mol, respectively, whereas the relative energies
of the two conformers with assigned spectra and no hydrogen bonds have relative energies of 6.46 and 4.89 kJ/mol.
■
INTRODUCTION
The Oslo lab has for a long time performed microwave (MW)
studies of a large number of compounds possessing intramolecular hydrogen (H) bonds. Much of our work has focused
on weak proton donor groups. Recently, we reported gas-phase
studies of alcohols,1−5 carboxylic acids,6 thiols,7−10 thiolcarboxylic acids,11 selenols,12−15 amines,16−18 amides,19,20 and
phosphines,21−24 where the said groups are proton donors in
internal H bonds with acceptors such as the fluorine
atom,1,6,9,11 the chlorine atom,17,19,23,24 π-electrons of triple
bonds,3−5,7,13,15,18 π-electrons of double bonds,2,8,10,14,21 and
Walsh25 pseudo-π electrons.12,20,22 References of many of our
earlier works are found in the cited literature as well as in
reviews.26−29
No gas-phase studies of hydrazines (R−NHNH2), where the
hydrazino group (−NHNH2) acts as a proton donor, appear to
have been reported. The hydrazino group is a fairly strong
proton acceptor and consequently a relatively weak proton
donor, just as in the case of its congener, the amino group.
In this work, we report the first MW study of (2fluoroethyl)hydrazine, henceforth referred to as FEH. This
compound is also the first studied example of a hydrazine that
© 2015 American Chemical Society
has conformers stabilized by intramolecular H bonding
between H atom(s) of the hydrazino group and the fluorine
atom.
Two different kinds of internal H bonds may exist for FEH.
One of these may form a five-member ring consisting of the F−
C−C−N−H chain of atoms, while the other type is composed
of the six-member F−C−C−N−N−H ring. Conformers
without H bonds are not expected to be very different in
energy than rotamers stabilized by this interaction because of
the weakness of the H bond interaction. A delicate conformational equilibrium was therefore foreseen to be present in
gaseous FEH. The “competition” that might exist between
conformers stabilized with five- or six-member H bonds on the
one side and rotamers with no H bond on the other was a
major motivation for undertaking this research.
The orientation of the substituent, R, of a substituted
hydrazine R−NHNH2 relative to the NHNH2-group has been
used to classify conformers as Inner or Outer,30 and this
Received: June 25, 2015
Revised: August 8, 2015
Published: August 10, 2015
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Article
The Journal of Physical Chemistry A
cooled with small portions of dry ice to ca. −30 °C during the
experiments to enhance the intensity of the spectrum. The
compound was kept at −80 °C when not in use. Traces of NH3
and N2H4 impurities were observed in the MW spectrum. The
spectrum was recorded at a pressure of 5−10 Pa using the
Stark-modulation spectrometer of the University of Oslo
described in detail elsewhere.33 Two salient features of the
spectrometer are the accuracy, which is ∼0.10 MHz for strong
and isolated transitions, and the resolution, which is ∼0.5 MHz
for such transitions. Measurements were performed in the 11−
123 GHz frequency interval. Radio frequency MW doubleresonance experiments (RFMWDR), similar to those of
Wodarczyk and Wilson, 34 were also undertaken. The
RFMWDR equipment is described elsewhere.33 This technique
makes it possible to assign unambiguously particular transitions.
nomenclature is also followed in the present case. Newman
projections of Inner and Outer conformers are shown in Figure
1. Rotational isomerism is possible about the C−C, C−N, and
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Figure 1. Newman projections of FCH2CH2NHNH2 viewed along the
N−N bond defining the Inner and Outer classification.
■
N−N bonds of FCH2CH2NHNH2, which results in a large
number of conformers. The MW work is guided by high-level
quantum chemical calculations, which indicates that 18
rotameric forms, nine Inner and nine Outer, exist for FEH
(see below). These theoretical predictions helped us assign
MW spectra of four conformers and allowed unambiguous
identification of the rotamers to which the assigned MW
spectra belonged.
RESULTS AND DISCUSSION
Quantum Chemical Calculations. Frozen-core MP2,35
CCSD,36−39 and B3LYP40,41 calculations were performed
employing the Gaussian 0942 and Molpro43 programs running
on the Abel cluster in Oslo. The MP2 and B3LYP
computations were undertaken using Gaussian 09, whereas
the CCSD calculations were performed with Molpro. The
default convergence criteria of the two computer programs
were used. Dunning’s44 correlation-consistent cc-pVTZ triple-ζ
basis set was employed in the MP2 and B3LYP calculations,
while the cc-pVQZ basis set, which is of quadruple-ζ quality,
was chosen for the CCSD computations.
The atom numbering F7−C1(H2,H3)−C4(H5,H6)−N8(H9)−N10(H11,H12) was chosen for FEH. The conformers
of this compound can conveniently be described by the values
of the F7−C1−C4−N8, C1−C4−N8−N10, C1−C4−N8−H9,
C4−N8−N10−H11, and C4−N8−N10−H12 dihedral angles.
These angles were varied in a systematic manner to locate
potential conformer candidates. The structures of these forms
were optimized at the MP2/cc-pVTZ level, and their
vibrational frequencies were calculated. A conformer is
characterized by having no imaginary frequencies. This was
found for 18 rotamers, which were given Roman numerals I−
XVIII for reference. The MP2 structures of these forms are
listed in Tables S1−S18 in the Supporting Information. Models
of the nine Inner rotamers are sketched in Figure 2, while the
nine Outer conformers are depicted in Figure 3.
The Watson quartic centrifugal distortion constants,45
nuclear quadrupole constants of the two 14N nuclei, and the
electronic energies corrected for zero-point vibrational energies
were also calculated for the 18 forms, and these parameters are
reported in Tables S1−S18. The centrifugal distortion
constants were calculated as described by McKean et al.46
It is shown below that eight conformers have relative
energies and dipole moments that make their spectra likely
candidates for assignments. B3LYP calculations of the
vibration−rotation constants (the α’s),47 the sextic centrifugal
distortion constants,45 and the differences between the
equilibrium rotational constants (re) and the ground-state
rotational constants (r0) were performed for these forms. The
B3LYP method was chosen because these calculations are
much less expensive than using the MP2 method with the same
basis set.
The MP2 structures were used as starting points in CCSD/
cc-pVQZ structure optimizations. These optimized CCSD
structures are assumed to be close to the Born−Oppenheimer
equilibrium structures, due to the very high theoretical level of
■
EXPERIMENTAL SECTION
Synthesis. The synthesis and NMR data of free FEH have
never before been reported in detail. FEH was prepared
(Scheme 1) starting from 2-fluoroethyl-4-methylbenzenesulfonate synthesized as previously reported.31
Scheme 1. Synthesis of FCH2CH2NHNH2
A modified form of the synthesis of Baklouti and Hedhli32
was used to produce 2-fluoroethylhydrazine: In a 100 mL threenecked flask equipped with a stirring bar, a nitrogen inlet, and a
dropping funnel was introduced hydrazine hydrate (30 mL,
0.62 mol). 2-Fluoroethyl-4-methylbenzenesulfonate (19.3 g, 88
mmol) was slowly added for 20 min, while the temperature was
maintained at 15 °C with an ice bath. At the end of the
addition, the solution was heated to 50 °C and maintained at
this temperature for 1 h. After it cooled at room temperature,
diethyl ether (20 mL) was added, and the organic phase
separated and was discarded (it contained a mixture of the
expected product and ∼10% of the disubstituted hydrazine
(FCH2CH2)2NNH2). The aqueous phase was then extracted
three times with chloroform (50 mL). After the organic phase
was dried on magnesium sulfate and filtered, the chloroform
was removed in vacuo, and the crude hydrazine was distilled in
a vacuum line (0.1 mbar) and selectively trapped in a cell
immersed in a bath cooled at −30 °C. Yield: 3.2 g, 47%. 1H
NMR (CDCl3, 400 MHz) δ 3.02 (td, 2H, 3JHH = 4.7 Hz, 3JHF =
28.2 Hz, CH2N); 3.19 (s brd, 3H, NHNH2); 4.60 (td, 2H, 3JHH
= 4.7 Hz, 2JHF = 47.5 Hz, CH2F). 13C NMR (CDCl3, 100
MHz) δ 55.2 (1JCH = 150.9 Hz (t), 2JCF = 18.9 Hz (d), NCH2);
81.9 (1JCH = 133.5 Hz (t), 1JCF = 164.9 Hz (d), FCH2).
Spectroscopic Experiments. FEH is a colorless liquid
whose vapor pressure is roughly 150 Pa at room temperature.
Fumes of this liquid were admitted to the MW cell, which was
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The Journal of Physical Chemistry A
Figure 2. Inner conformers of (2-fluoroethyl)hydrazine. The numbers in parentheses are the CCSD electronic energies in kJ/mol relative to the
energy of conformer XII (Figure 3), which is the global energy minimum conformer. Rotamers I and VI each have a six-member F−C−C−N−N−H
internal hydrogen bond, whereas III and V each have an intramolecular five-member F−C−C−N−H hydrogen bond, while II, IV, VII, VIII, and IX
all have no hydrogen bond. The MW spectrum of V has been assigned; see text.
Figure 3. Outer conformers of (2-fluoroethyl)hydrazine. Number in parentheses are the CCSD energies in kJ/mol relative to conformer XII.
Rotamer XV has a six-member H bond, while XII and XIV have five-member H bonds. The remaining conformers X, XI, XIII, XVI, XVII, and XVIII
have no hydrogen bond. The MW spectra of XIII, XIV, and XVI were assigned; see text.
rotational constants, and CCSD principal inertial axes dipole
moment components are listed in Table 2.
The results in Tables 1 and 2 warrant discussion. The C4−
N8−N10−H11 and C4−N8−N10−H12 dihedral angles
(Table 1) define the Inner and Outer conformations. It is
seen that both dihedral angles vary several degrees among the
conformers, between −82.3 and −88.2° for the C4−N8−N10−
H11 dihedral angle, while the C4−N8−N10−H12 angle varies
calculations. Vibrational frequencies could not be calculated at
this level of theory due to limited resources. The CCSD
structures, electronic energies, and dipole moments are listed
together with the MP2 results in Tables S1−S18 in the
Supporting Information. The characteristic CCSD dihedral
angles specifying the conformers are collected in Table 1, while
the MP2 energy differences corrected for zero-point vibrational
energies, the CCSD electronic energy differences, the CCSD
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Table 1. Characteristic CCSD Dihedral Anglesa of Conformers of (2-Fluoroethyl)hydrazineb
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dihedral angle
F7−C1−C4−N8
C1−C4−N8−N10
I
II
III
IV
V
VI
VII
VIII
IX
−70.2
−69.6
−60.2
67.4
65.1
66.9
−176.4
177.4
176.9
79.7
−176.6
−63.4
75.9
−173.2
−68.5
80.7
179.9
−68.1
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
−94.9
−68.1
−57.1
71.1
65.2
59.6
−177.3
176.0
−179.0
63.4
−179.8
−57.4
74.8
−177.4
−79.1
76.6
173.4
−62.4
C1−C4−N8−H9
C4−N8−N10−H11
Inner conformers
−160.6
−57.2
57.5
−163.9
−54.3
53.0
−159.4
−61.5
54.2
Outer conformers
−174.8
−58.0
63.8
−163.1
−56.4
43.9
−161.5
−65.2
61.3
C4−N8−N10−H12
−82.3
−86.0
−82.9
−88.4
−83.7
−87.9
−88.2
−85.1
−86.6
32.6
31.0
35.4
29.7
33.1
30.1
29.7
31.6
31.2
80.5
86.8
92.6
88.5
86.4
81.1
86.6
85.0
94.7
−160.7
−154.9
−150.0
−153.1
−156.0
−159.6
−155.2
−157.0
−147.5
In degrees. bAtom numbering: F7−C1(H2,H3)−C4(H5,H6)−N8(H9)−N10(H11,H12). Full CCSD structures are given in Tables S1 − S18 of
the Supporting Information.
a
Table 2. Theoretical Energy Differences, CCSD Rotational Constants and Principal-Axes Dipole Moment Components of
Conformers of (2-Fluoroethyl)hydrazine
energy difference (kJ/mol)
a
CCSD
2.41
9.39
5.35
7.66
0.73
5.98
6.00
8.10
12.47
14.31
11.79
0.0
6.14
1.62
8.60
4.98
10.73
10.35
3.50
9.67
5.90
8.15
1.04
6.62
5.94
7.76
12.58
15.43
11.98
0.0
6.46
1.63
9.34
4.89
10.33
10.38
MP2
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
a
b
rotational constants (MHz)
dipole moments (debye)
A
B
C
μa
μb
μc
μtot
8185.0
14462.9
9540.3
10248.3
13657.8
8118.1
14118.0
22287.7
14225.3
8625.4
14477.6
8899.5
10609.3
13771.3
8168.6
14202.2
22917.9
14195.5
3757.0
2420.7
3028.9
2839.8
2551.2
3740.0
2397.1
2053.3
2380.0
3460.4
2432.4
3286.2
2795.2
2542.6
3802.4
2411.3
2046.0
2430.7
2833.9
2267.4
2835.5
2727.8
2335.9
2819.9
2251.0
1953.0
2234.4
2749.0
2265.7
2962.6
2702.1
2329.3
2861.4
2261.9
1950.1
2246.2
0.765
0.416
0.500
0.486
0.071
1.428
0.602
0.632
0.180
2.140
1.539
0.130
2.785
0.574
0.533
3.173
2.116
2.009
2.093
3.014
1.550
1.919
2.579
1.730
0.036
0.469
0.618
1.568
2.188
0.243
1.817
0.007
0.980
0.981
1.258
0.734
1.089
0.257
2.221
2.258
0.907
0.373
0.484
0.332
0.207
0.878
2.045
0.349
0.648
0.476
1.674
0.372
1.703
2.121
2.480
3.054
2.754
3.003
2.735
2.274
0.773
0.854
0.676
2.795
3.367
0.444
3.387
0.745
2.011
3.342
2.993
3.012
MP2/cc-pVTZ corrected for zero-point energies. bCCSD/cc-pVQZ electronic energies.
between 29.7 and 35.5° for the Inner forms. The corresponding
variations are somewhat larger for the Outer conformers,
between 80.5 and 94.7° for the former dihedral angle, and
between −147.5 and −160.7° for the C4−N8−N10−H12
dihedral angle. It is also seen from Table 1 that several of the
F7−C1−C4−N8, C1−C4−N8−N10, and C1−C4−N8−H9
dihedral angles differ substantially from the “canonical” values
(±60 and 180°). These deviations indicate that nonbonded
forces influence the geometries of several of the conformers to
a considerable extent.
The Outer conformer XII (Figure 2) was found to be the
global energy minimum in both the MP2 and in the CCSD
calculations (Table 2), and this rotamer was assigned a relative
energy of 0.0 kJ/mol. The MP2 and CCSD energy differences
of all 18 conformers are in very good agreement (better than
∼1 kJ/mol), as shown in Table 2. The CCSD energy
differences of all 18 conformers span an energy interval of
15.43 kJ/mol (same table). It is impossible to say exactly how
accurate the CCSD energy differences are, but these energy
differences are likely to be accurate at least to within 2−3 kJ/
mol.
The nonbonded distances between the H atom involved in H
bonding with the F7 atom should be near or less than 260 pm,
which is the sum of the Pauling van der Waals radii48 of H (120
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Table 3. CCSD Geometries and Energies of Hydrogen-Bonded Conformers
conformer
Na
H bondb
distancec (pm)
angled
anglee(deg)
anglef
angleg (deg)
ΔEh (kJ/mol)
I
III
V
VI
XII
XIV
XV
6
5
5
6
5
5
6
F7···H11
F7···H9
F7···H9
F7···H12
F7···H9
F7···H9
F7···H11
255.2
260.4
248.1
218.9
258.1
247.5
242.9
F7···H11−N10
F7···H9−N8
F7···H9−N8
F7···H12−N10
F7···H9−N8
F7···H9−N8
F7···H11−N10
106.6
97.7
101.8
132.9
96.2
101.4
107.9
C1−F7, H11−N10
C1−F7, N8−H9
C1−F7, N8−H9
C1−F7, H12−N10
C1−F7, N8−H9
C1−F7, N8−H9
C1−F7, H11−N10
54.9
3.7
8.8
50.7
6.5
6.7
46.5
3.50
5.90
1.04
6.62
0.0
1.63
9.34
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a
The F7 atom can be involved in five- or six-member H bonds; see text. The numbers given in this column indicate how many atoms that are
involved in intramolecular H bonding. bAtoms involved in the intramolecular H bond. Dots indicate nonbonded interaction. cNonbonded distance
between F7 and the H atom involved in the H bond. dAtoms forming an angle characteristic of the H bond. eAngle in degrees of the characteristic H
bond. fAngle between the C1−F7 and the N−H bond involved in H bonding. gAngle in degrees. hElectronic energy relative to conformer XII.
dense with absorption lines occurring every few megahertz
throughout the entire spectral range (11−123 GHz) investigated. This was expected for the following reasons: A
thermodynamic equilibrium exists for the conformers of FEH,
and the concentration of each of them is dictated by
Boltzmann’s distribution law. Another factor that results in
reduced absolute intensity of the MW spectrum is the fact that
each conformer has approximately five normal vibrational
modes with frequencies below 500 cm−1 (Tables S1−S18). The
population in each vibrational state is determined by
Boltzmann statistics. Molecules of each state have their own
resolved MW spectrum.
It is seen from Table 2 that 10 rotamers (I, III, V, VI, VII,
VIII, XII, XIII, XIV, XVI) span a CCSD relative energy interval
of less than 8 kJ/mol. The Boltzmann factors of conformers
with relative energies larger than 8 kJ/mol are less than 0.02 at
−30 °C. Not only concentration, but the intensity of the
spectrum is also of great importance for the assignment of a
particular conformer. The intensity depends on the square of
the principal-axis component of the dipole moment. None of
the eight remaining high-energy (>8 kJ/mol) conformers II, IV,
IX, X, XI, XV, XVII, and XVIII have very large dipole moments
(Table 2) that could outweigh their low concentration, and no
searches for their spectra were therefore performed.
The relative energies of VII and VIII are 5.94 and 7.76 kJ/
mol (Table 2). Their largest dipole moment components are
∼0.6 D (same table). These two factors and the absolute
weakness of the observed spectrum make it very unlikely that it
would be possible to assign their spectra, and no efforts were
made in this direction. The spectra of the eight remaining lowenergy conformers I, III, V, VI, XII, XIII, XIV, XVI were
searched for, and we were able to assign the spectra of four of
them, namely, V, XIII, XIV, and XVI, as described below.
The following strategy was used to assign the spectra: The
B3LYP calculations for the eight conformers in question
yielded re- and r0-rotational constants (Tables S1, S3, S5, S6,
S12, S13, S14, and S16). The differences (A0 − Ae, etc.)
between these B3LYP constants were added to the CCSD
rotational constants to obtain the best possible approximation
of the experimental r0 rotational constants. These theoretical r0
rotational constants were used together with the MP2 quartic
centrifugal distortion constants to predict the approximate
frequencies of strong transitions for the conformer under
consideration. Searches were then undertaken for these
transitions using Stark and RFMWDR spectroscopies. The
spectra of conformers with low CCSD relative energies and
large dipole moments were first searched for.
pm) and F (140 pm). Inspection of the CCSD structures of the
18 conformers (Tables S1−S18) suggests that seven of them (I,
III, V, VI, XII, XIV, and XV) meet this criterion. The
nonbonded F···H distances of the seven H-bonded conformers
are collected in Table 3 together with several other parameters
of relevance for intramolecular H-bonding in these forms. It is
seen from this table that three of these conformers, namely, I,
VI, and XV, are stabilized by F7−C1−C4−N8−N10−H11 or
F7−C1−C4−N8−N10−H12 six-member H bonds, while four
(III, V, XII, and XIV) benefit from f ive-member H bonds
formed by the F7−C1−C4−N8−H9 link of atoms. The
conformers V, XII, and XIV, which all have five-member H
bonds, are the three lowest-energy forms of FEH, spanning an
energy interval of only 1.63 kJ/mol (Tables 2 and 3), followed
by I (3.50 kJ/mol), which has a six-member H bond. The
shortest nonbonded H···F7 distance is found in VI, which has a
six-member H bond. Interestingly, this conformer is 6.62 kJ/
mol higher in energy than XII, which is stabilized by a fivemember H bond. The conformer stability thus depends not
only on the shortness of the H bond. Other nonbonded effects
must have a say.
A preferred arrangement of many H bonds, especially
intermolecular, is the nearly linear arrangement of the, usually
three, atoms involved in H bonding. However, the strong
directive covalent forces in FEH will not permit the F7···H−N
angle to take a value near 180°. It is seen (Table 3) that this
angle is far from linear in five-member candidates (96.2−
101.8°). This angle is somewhat larger, ∼107° in the two sixmember conformers, I and XV, where the H11 atom is involved
in internal hydrogen bonding, and as large as 132.9° in VI,
where H12 takes part in this kind of interaction.
Another interesting feature emerges from Table 3: The
angles between the C−F and N−H bonds are a few degrees
from being parallel in all five-member H-bonded rotamers,
while these angles are roughly 50° in the six-member
counterparts. The bond moment of the C−F bond is 1.4
D,49 while the bond moment of N−H is 1.3 D.49 The bond
moments are nearly antiparallel in the five-member conformers,
which is ideal for electrostatic stabilization. It is concluded that
this electrostatic interaction is a major reason why the fivemember ring conformers V, XII, and XIV are the lowest-energy
forms of FEH. This characteristic H-bond geometry in the fivemember conformers of FEH closely resemble the two
corresponding H-bonded conformers of its amino analogue,
FCH2CH2NH2,50 as well as the three H-bonded rotamers of a
similar amine, namely, F2CHCH2NH2.51
Microwave Spectrum and Assignment Strategy. The
observed MW spectrum of FEH is relatively weak and very
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Table 4. Spectroscopic Constantsa of Conformer V of FCH2CH2NHNH2
ground
Av (MHz)
Bv (MHz)
Cv (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
d1 (kHz)
d2 (kHz)
HJ (Hz)
HJK (Hz)
HKJ (Hz)
HKc (Hz)
rmsd
Ne
first ex. lowest
second ex. lowest
first ex. second
state
torsion (v30)
torsion (2v30)
lowest torsion (v29)
13517.8137(25)
2521.48377(53)
2309.55877(54)
1.15304(28)
−11.6853(34)
90.9152(62)
−0.217667(28)
−0.010336(13)
0.000339(42)
0.0193(10)
−0.4973(30)
1.795(20)
1.067
611
13417.1942(71)
2521.3270(17)
2310.6401(17)
1.1963(16)
−11.9247(59)
79.03(34)
−0.226588(71)
−0.007486(30)
13321.338(14)
2522.0262(67)
2312.1600(66)
1.228(13)
−12.0168(95)
69.12(66)
−0.23607(11)
−0.005104(37)
13667.2030(93)
2513.1865(55)
2303.3259(54)
1.0887(89)
−11.9942(65)
108.36(44)
−0.207489(57)
−0.012650(26)
1.441
151
1.466
92
1.225
124
theoryb
13 637.0
2512.0
2304.4
1.18
−11.7
87.3
−0.219
−0.0101
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a
S-reduction Ir-representation.45 Uncertainties represent one standard deviation. bThe theoretical r0-rotational constants have been obtained from
the CCSD and B3LYP calculations; see text. The centrifugal distortion constants are from the MP2 calculations; see text. cFurther sextic centrifugal
distortion constants preset at zero. dRoot-mean-square deviation defined as rms2 = ∑[(νobs − νcalc)/u]2/(N − P), where νobs and νcalc are the
observed and calculated frequencies, u is the uncertainty of the observed frequency, N is the number of transitions used in the least-squares fit, and P
is the number of spectroscopic constants used in the fit. eNumber of transitions used in the fit. The spectra are listed in Tables S19−S22 of the
Supporting Information.
HKJ, and HK, were needed in the fit to get a root-mean-square
deviation comparable to the experimental uncertainties.
Attempts to include further sextic constants were made, but
these constants had large uncertainties; no really improved fit
was obtained in this way. These sextic constants were ultimately
preset at zero.
It is interesting to compare the experimental findings with
the theoretical predictions. Comparison of the experimental
rotational constants with the theoretical rotational constants
(Table 2) shows that the spectrum might belong either to V or
to XIV. However, the dipole moment components (Table 2)
exclude XIV. The assignment of the spectrum of Table S19 to
V is thus considered to be secure.
Comparison of experiment and theory is in order: The
experimental ground-state rotational constants (Table 4,
second column) and the theoretical counterparts obtained
from CCSD and B3LYP calculations are in good agreement in
the cases of B and C, while that of A deviates by 0.88%. It is
assumed that most of the deviation in the case of A is most
likely due to the B3LYP calculations of the third derivatives of
the potential energy hypersurface at the equilibrium structure.
This method is hardly sufficiently refined to yield highly
accurate values for the zero-point vibrational contribution to
the rotational constants.
Table 4 also reports the MP2 values of the quartic centrifugal
distortion constants, which are seen to agree very well with the
experimental equivalents. The same cannot be said about the
B3LYP sextic centrifugal distortion constants (Table S5), which
deviate so much from the experimental values in Table 4 that a
discussion is meaningless, and they have therefore not been
included in this table. The reason for this failure is again
assumed to be difficulties in obtaining accurate third derivatives.
Vibrationally Excited State of V. The spectrum of the
ground vibrational state of V was accompanied by several
satellite spectra that were assumed to belong to vibrationally
excited states of this rotamer. Three excited-state spectra
consisting of 151, 82, and 124 transitions listed in Tables S20 −
S22, respectively, were assigned. Their spectroscopic constants
Assignment of the Spectrum of V. This H-bonded
conformer has the second-lowest relative energy, which is 1.04
kJ/mol higher than the energy of XII (Table 2). Conformer V
has a small μa = 0.07 D and a large μb = 2.58 D, while μc = 0.90
D (Table 2). Searches were first made for b-type Q-branch
transitions, which were predicted to be the strongest transitions
of the spectrum of V. The theoretical r0-spectroscopic constants
in the last column of Table 4 were used to predict the
approximate frequencies of these transitions, which were readily
found. These bQ-transitions are also among the strongest
transitions of the entire spectrum. bR-branch transitions were
searched for next and found with ease. The assignments were
gradually extended to include higher and higher values of the
principal J quantum number up to Jmax = 88. Searches for
transitions involving even higher values of J were performed,
but no such lines could be unambiguously assigned because
they are too weak due to an unfavorable Boltzmann factor.
Finally, many c-type lines were assigned. They have generally
much lower intensities than the b-type transitions, because μc is
less than μb. The frequencies of μc lines could be very accurately
predicted using the spectroscopic constants obtained from the
b-type transitions. The frequencies of hypothetical a-type lines
could now be calculated with very high precision, but these
lines were not found because they are too weak caused by a
very small μa = 0.07 D (Table 2). No effects due to quadrupole
coupling of the two 14N nuclei with the molecular rotation were
observed in the spectrum of this rotamer as well as in the
spectra of the three other conformers assigned in this work.
Sørensen’s least-squares fitting program Rotfit52 was used in
the assignment procedure above. A total of 611 transitions
(Table S19 of the Supporting Information) could be fitted to
within their experimental uncertainties using Watson’s Sreduction Ir-representation Hamiltonian.45 Centrifugal distortion is prominent for the high J-transitions, with a maximum of
more than 4 GHz (Table S19). The experimental spectroscopic
constants are shown in Table 4, second column. Accurate
rotational and quartic centrifugal distortion constants were
obtained. Four sextic centrifugal distortion constants, HJ, HJK,
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are listed in Table 4, columns 3−5. No accurate sextic
centrifugal distortion constants were obtained from the leastsquares fits in these cases, and only quartic centrifugal
distortion constants were used in the final least-squares fits.
The two first spectra are assumed to belong to the first and
second excited states of the lowest torsional vibration (v30),
whereas the last spectrum is assumed to be the first excited
state of the second-lowest torsional vibration (v29). The MP2
calculations predict frequencies of 116 and 150 cm−1 for v30 and
v29, respectively, compared to 105(20) and 137(25) cm−1
obtained from rough relative intensity measurements.
The experimental vibration−rotation constants (the α’s)47
were computed by subtraction of the rotational constant of the
excited state under consideration from the corresponding
ground-state rotational constant. In this manner, αA =
100.6195(75), αB = 0.1568(18), and αC = −0.7682(18) were
found for the first excited state of v30 from the entries in Table
4. The B3LYP values are 98.47, − 1.24, and −1.54 MHz (Table
S5), which is considered to be good agreement given the
approximate nature of the theoretical calculations. The
experimental values of the corresponding rotation−vibration
constants of v29 are −149.3893(96), 8.2973(55), and
6.2329(55) MHz. The B3LYP equivalents are −148.63, 8.94,
and 6.56 MHz in good agreement with experiment.
Assignment of the Spectrum of XIII. The relative CCSD
energy of this form, which is without a H bond, is 6.47 kJ/mol,
and its Boltzmann factor is only 0.04 relative to XII. Its major
dipole moment component is μa = 2.79 D (Table 2). This
conformer is nearly a symmetrical top with Ray’s asymmetry
parameter53 κ = −0.976. The approximate frequencies of its aRspectrum were predicted using the theoretical ground-state
rotational constants (Table 5, last column) obtained as
described above, and the MP2 centrifugal distortion constants.
This spectrum has characteristic pile-ups associated with a-type
R-branch transition. Figure 4 shows the J = 14 ← 13 pile-up
region observed for this conformer.
Figure 4. J = 15 ← 14 pile-up region of conformer XIII taken at a
Stark field of ∼150 V/cm. The numbers above the peaks indicate the
value of the K−1 pseudo quantum numbers of the transitions. The K−1
= 8 and 11 transitions are overlapped by other transitions.
The first unambiguous assignment of this very weak
spectrum was obtained for aR lines using the RFMWDR
technique. Recording of the spectrum at relatively low Stark
fields eliminates transitions with slow Stark effects, while aRtransitions of XIII with K−1 > 2 are completely modulated at
low fields. The much-simplified spectrum obtained in this
manner was very helpful for the assignments of many additional
a
R-transitions.
This rotamer has a sizable μb = 1.82 D (Table 2). Searches
for b-type transitions were undertaken, but none were
unambiguously assigned, presumably due to low intensities as
well as the crowded nature of the spectrum with many
overlapping absorption lines. The spectroscopic constants
obtained from 181 aR-transitions (Table S23) are listed in
Table 5. The DK and d2 centrifugal distortion constants could
not be determined from the fit and were therefore preset at
zero. One sextic centrifugal distortion constant, HKJ, was
needed to get a satisfactory fit.
Comparison of the experimental rotational constants (Table
5) with the theoretical rotational constants (Table 2) reveals
that conformer XIII is the only suitable candidate. The
assignment of the spectrum is Table S23 conformer is thus
considered to be unambiguous. The differences between the
experimental and theoretical rotational constants are 1.54, 0.35,
and 0.17%, for A0, B0, and C0, respectively, which is considered
to be satisfactory given the approximations involved in the
theoretical calculations. The experimental and theoretical
quartic centrifugal distortion constants (Table 5) are in good
agreement in the cases of DJ and DJK, while a large difference is
seen for d1.
Assignment of the Spectrum of XIV. The energy of this
conformer, which has an internal five-member H bond, is only
1.63 kJ/mol higher than the energy of the global-minimum
form XII. The largest CCSD dipole moment component of
XIV, μa, is as low as 0.57 D, while μb is almost zero, and μc is
only 0.48 D (Table 2). Ray’s asymmetry parameter, κ, is
−0.963. A very weak a-type R-branch spectrum with features
very similar to that of conformer XIII (previous paragraph)
should exist for this rotamer as well. RFMWDR searches were
made for the spectrum of XIV using the theoretical
spectroscopic constants (Table 6) to predict suitable candidates
Table 5. Spectroscopic Constantsa of the Ground Vibrational
State of Conformer XIII of FCH2CH2NHNH2
A0 (MHz)
B0 (MHz)
C0 (MHz)
DJ (kHz)
DJKc (kHz)
d1 (kHz)
HKJd (Hz)
rmse
Nf
experiment
theoryb
10745.5(57)
2772.657(15)
2677.566(14)
2.9764(19)
−29.128(24)
−0.267(19)
−0.933(95)
1.222
181
10 579.8
2762.9
2672.8
2.78
−26.2
−0.488
a
S-reduction Ir-representation.45 Uncertainties represent one standard
deviation. bThe theoretical r0-rotational constants have been obtained
from the CCSD and B3LYP calculations; see text. The centrifugal
distortion constants are from the MP2 calculations; see text. cDK and
d2 kept at zero in the fit; see text. dFurther sextic centrifugal distortion
constants preset at zero. eRoot-mean-square deviation defined as rms2
= ∑[(νobs − νcalc)/u]2/(N − P), where νobs and νcalc are the observed
and calculated frequencies, u is the uncertainty of the observed
frequency, N is the number of transitions used in the least-squares fit,
and P is the number of spectroscopic constants used in the fit.
f
Number of transitions used in the fit. The spectrum is listed in Table
S23 of the Supporting Information.
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Table 6. Spectroscopic Constantsa of the Ground
Vibrational State of Conformer XIV of FCH2CH2NHNH2
experiment
A0 (MHz)
B0 (MHz)
C0 (MHz)
DJ (kHz)
DJKc (kHz)
d1 (kHz)
HKJd (Hz)
rmse
Nf
13723(11)
2514.552(55)
2303.728(56)
1.1535(14)
−12.656(11)
−0.238(13)
−0.588(33)
0.982
201
Table 7. Spectroscopic Constantsa of Conformer XVI of
FCH2CH2NHNH2
theoryb
Av (MHz)
Bv (MHz)
Cv (MHz)
DJ (kHz)
DJK (kHz)d
d1 (kHz)
HKJe (Hz)
rmsf
Ng
13781.5
2507.8
2300.8
1.18
−12.7
−0.217
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a−f
Comments as for Table 5. The spectrum is listed in Table S24 of
the Supporting Information.
ground state
lowest torsion (v30)
theoryb
14 017.1(22)
2391.4824(69)
2240.4157(71)
0.826 32(99)
−11.0917(88)
−0.0906(80)
−0.248(23)
1.101
187
c
14 167.5
2386.2
2238.2
0.795
−11.7
−0.117
13 927
2399.265(29)
2244.651(32)
0.8676(25)
−11.010(13)
0.0c
0.0c
1.658
82
a−b
Comments as for Table 5. cFixed. d−gComments as for c−f Table
5. The spectra are listed in Tables S25 and S26 of the Supporting
Information
for this kind of spectroscopy. These transitions were soon
identified, and the pile-ups of this conformer became evident
when the spectrum was recorded at relatively low Stark fields.
Ultimately, 201 transitions (Table S24) were assigned and used
to determine the spectroscopic constants displayed in Table 6.
Searches for c-type lines were unsuccessful, presumably because
of the small μc, as well as the complexity of the spectrum.
The rotational constants of XIV (Table 6) are similar to
those of V (Table 4), but the assignment of the spectrum of
Table S24 to the latter rotamer is assumed to be secure,
because no a-type spectrum was observed for V, while an a-type
spectrum is the only kind of spectrum observed for XIV.
The experimental and theoretical rotational constants agree
to within 0.43, 0.26, and 0.01% in the cases of A0, B0, and C0,
which is good agreement. The same can be said about the
quartic centrifugal distortion constants.
Assignment of the Spectrum of XVI. This conformer,
which is without a H bond, has a CCSD energy that is 4.89 kJ/
mol higher than the energy of XII (Table 2). The Boltzmann
factor of this form is thus 0.09 at −30 °C. XVI has a large μa =
3.17 D and κ = −0.974 resulting in a characteristic aR-pile-up
spectrum similar to those described in the two previous cases.
The spectrum is comparatively weak, but it is stronger than the
spectra of XIII and XIV. It was in fact so strong that it was
readily observed in the survey spectra. The spectrum of XVI
was assigned in the same manner as described for the spectra of
these two conformers. μb is 0.98 D (Table 2), but attempts to
find the strongest b-type lines failed presumably because of the
low concentration of XIV. Attempts to find c-type lines also
failed. This was expected because μc = 0.37 D (Table 2).
Ultimately, 185 aR-transitions with Jmax = 26 and K−1max = 20
listed in Table S25 of the Supporting Information were
assigned and used to determine the spectroscopic constants
listed in Table 7. The centrifugal distortion constants DK and d2
could not be obtained for similar reasons as described above for
XIII and XIV.
The rotational constants in Table 7 are similar to the
rotational constants of II, VII, and IX. Rotamer II is excluded
because μa is as small as 0.42 D and its CCSD relative energy is
as high as 9.67 kJ/mol. VII and IX are also excluded for similar
reasons. The assignment of the spectrum in Table S25 to
conformer XVI is therefore considered to be unambiguous.
The theoretical r0 rotational constants shown in the last
column of Table 7 are in good agreement with their
experimental counterparts in column 2. The largest difference
is seen for A0, where the theoretical constant is 1.1% larger than
the experimental equivalent. There is quite good agreement
between the experimental quartic centrifugal distortion
constants DJ, DJK, and d1 and the MP2 results (Table 7).
Vibrationally Excited State of XVI. A weak spectrum with
approximately half of the intensity of the ground-state spectrum
of XVI and with similar Stark modulation properties and
RFMWDR behavior was observed close to the ground-state
spectrum of XVI. This spectrum, which is listed in Table S26
and whose spectroscopic constants are displayed in Table 7, is
assigned as the first excited torsional state spectrum of the
lowest torsional vibration (v30) of XVI. This spectrum was so
weak that no unambiguous assignment could be made for K−1
lines less than 3. The A rotational constants will therefore be
very uncertain. This constant was fixed at 13 927 MHz, which
was derived by subtracting the vibration−rotation constant, αA
= 90.44 MHz obtained in the B3LYP calculations (Table S16),
from the A0 constant (Table 7). The MP2 vibrational frequency
is 100 cm−1 for this mode, while the B3LYP values of αB =
−5.32 and αC = −2.76 MHz (Table S16). The equivalent
experimental values obtained from the entries in Table 7 are αB
= −7.88(12) and αC = −4.13(12) MHz.
The Unsuccessful Searches. We searched for the spectra
of eight candidates, namely, I, III, V, VI, XII, XIII, XIV, and
XVI. The spectra of V, XIII, XIV, and XVI were assigned, while
no assignments were obtained for I, III, VI, and XII.
The relative CCSD energy of conformer I is 3.50 kJ/mol,
and its major dipole moment component is μb = 2.09 D (Table
2). Extensive searches have been made for its b-type Stark
spectrum, and the RFMWDR technique was also employed in
searches for its a-type spectrum, but to no avail. It is possible
that the energy difference is larger than 3.50 kJ/mol and that
this is the reason why success was not achieved.
Candidate III has a relative energy of 5.90 kJ/mol
(Boltzmann factor = 0.05 at −30 °C), and this may perhaps
explain why it was not found in extensive searches for its c-type
spectrum. Futile RFMWDR experiments were also performed.
Similar experiments were made for VI, whose CCSD energy is
6.62 kJ/mol higher than the energy of XII (Table 2) with the
same conclusion
The global energy minimum conformer XII has μa = 0.13, μb
= 0.24, and μc = 0.35 D, according to the CCSD calculations
(Table 2). The MW spectrum of this form will consequently be
extremely weak and would be almost impossible to assign in a
crowded Stark spectrum, such as the one at hand. Nevertheless,
futile attempts to assign the spectrum of XII were made, using
both Stark and RFMWDR spectroscopies.
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■
■
CONCLUSIONS
MP2/cc-pVTZ and CCSD/cc-pVQZ calculations indicate that
there are 18 different conformers of FCH2CH2NHNH2. Seven
of these conformers are stabilized by intramolecular hydrogen
bonds, where the hydrazino group acts as proton donor and the
fluorine atom is proton acceptor. Three of these rotamers have
six-member hydrogen bonds, whereas four have five-member
hydrogen bonds. The three lowest-energy conformers are all
stabilized by five-member hydrogen bonds demonstrating that
the hydrazino group indeed may act as a proton donor in
hydrogen bonds.
The reason why the majority of the conformers with fivemember H bonds are more stable than the rotamers with sixmember internal H bonds is presumed to be a favorable
orientation of the C−F and N−H bonds involved in H
bonding. These two bonds are nearly parallel, and their bond
moments are nearly antiparallel in the five-member species,
which is ideal for electrostatic interaction. A similar favorable
situation does not exist in the six-member forms.
The energies of the 18 rotamers span a CCSD energy
interval of 15.4 kJ/mol. The spectra of eight of the 18
conformers were assumed to be possible candidates for
assignment. The reasons for excluding 10 conformers were a
combination of high relative energies and/or small dipole
moment components. The absolute intensity of the MW
spectrum is small, as expected. Nevertheless, spectra belonging
to four different rotamers, two with five-member hydrogen
bonds and two without H bonds, were assigned, and rotational
and centrifugal distortion constants were determined.
Searches for the spectra of the remaining four conformers
were performed. The spectrum of the conformer with the
lowest energy of all conformers was not assigned, because its
dipole moment is very small. Two conformers have relative
energies larger than ∼6 kJ/mol, and this energy difference is
assumed to be the major reason why their spectra were not
found. The fourth conformer, whose spectrum was not assigned
despite a relative energy of 3.5 kJ/mol and a high dipole
moment, is more difficult to assess, but an underestimate of the
energy difference by the MP2 and CCSD methods could be the
reason.
■
ACKNOWLEDGMENTS
This work was supported by the Research Council of Norway
through a Centre of Excellence Grant (Grant No. 179568/
V30). It also received support from the Norwegian Supercomputing Program (NOTUR) through a grant of computer
time (Grant No. NN4654K). J.-C.G. thanks the Centre
National d’Etudes Spatiales (CNES) for financial support.
■
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ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpca.5b06095.
Results of the theoretical calculations, including MP2/ccpVTZ and CCSD/cc-pVQZ electronic energies, molecular structures, rotational constants, and dipole moments.
MP2 harmonic vibrational frequencies, rotational and
quartic centrifugal distortion constants. B3LYP/cc-pVTZ
vibration−rotational constants, differences between r0
and re rotational constants, and sextic centrifugal
distortion constants. Microwave spectra of four conformers. (PDF)
■
Article
AUTHOR INFORMATION
Corresponding Author
*Phone: +47 2285 5674. Fax: +47 2285 5441. E-mail: harald.
mollendal@kjemi.uio.no.
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.jpca.5b06095
J. Phys. Chem. A 2015, 119, 9252−9261
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