Article
pubs.acs.org/JPCA
Microwave and Quantum Chemical Study of Intramolecular
Hydrogen Bonding in 2‑Propenylhydrazine (H2CCHCH2NHNH2)
Harald Møllendal,*,† Svein Samdal,† and Jean-Claude Guillemin‡
†
Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, P.O. Box 1033
Blindern, NO-0315 Oslo, Norway
‡
Institut des Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, 11 Allée de
Beaulieu, CS 50837, 35708 Rennes Cedex 7, France
S Supporting Information
*
ABSTRACT: The microwave spectrum of 2-propenylhydrazine (H2CCHCH2NHNH2)
was studied in the 12−61 and 72−123 GHz spectral regions. A variety of intramolecular
hydrogen bonds between one or more of the hydrogen atoms of the hydrazino group and the
π-electrons are possible for this compound. Assignments of the spectra of four conformers, all
of which are stabilized with intramolecular hydrogen bonds are reported. One hydrogen bond
exists in two of these conformers, whereas the π-electrons are shared by two hydrogen atoms
in the two other rotamers. Vibrationally excited-state spectra were assigned for three of the
four conformers. The internal hydrogen bonds are weak, probably in the 3−6 kJ/mol range. A
total of about 4400 transitions were assigned for these four forms. The microwave work was
guided by quantum chemical calculations at the B3LYP/cc-pVTZ and CCSD/cc-pVTZ levels
of theory. These calculations indicated that as many as 18 conformers may exist for 2propenylhydrazine and 11 of these have either one or two intramolecular hydrogen bonds. The four conformers detected in this
work are among the rotamers with the lowest CCSD electronic energies. The CCSD method predicts rotational constants that
are very close to the experimental rotational constants. The B3LYP calculations yielded quartic centrifugal distortion constants
that deviated considerably from their experimental counterparts in most cases. The calculation of vibration−rotation constants
and sextic centrifugal distortion constants by the B3LYP method were generally found to be in poor agreement with the
corresponding experimental constants.
■
hydrazino group is capable of forming internal H bonds with πelectrons. However, an important difference exists between 2propenylhydrazine and the four other compounds just referred
to. In the title compound, the N−H and NH2 moieties of the
hydrazino group may both form internal H bonds, whereas only
one proton is available for H bonding in H2CCHCH2X,
where X = OH, NH2, SH, and SeH. The many interesting
aspects of H bonding in 2-propenylhydrazine motivated us to
carry out the first MW study of this compound, henceforth
referred to as 2PH. The present study is a continuation of our
extensive studies of intramolecular H bonding. References for
much of our work in this field are found in ref 1.
2PH has complex conformational properties because rotation
about the HC−CH2, CH2−NH, and NH−NH2 single bonds
may result in a large number of conformers. Substituted
hydrazines, RNHNH2, occur as what is traditionally referred to
as Inner or Outer conformers13 depending on the orientation of
the substituent, R, with regard to the NH2 group. In Inner
rotamers, the substituent R and the lone electron pair of the
NH2 group are antiperiplanar, whereas R and the lone pair are
INTRODUCTION
In a recent communication,1 we reported the microwave (MW)
spectrum of (2-fluoroethyl)hydrazine, FCH2CH2NHNH2. Our
reason for investigating this compound was to see whether the
hydrazino group would participate in intramolecular hydrogen
(H) bonding, acting as a weak proton donor, with the highly
electronegative fluorine atom being the acceptor. MW spectra
of four conformers of FCH2CH2NHNH2 were assigned and
two of these rotamers are indeed stabilized by internal H
bonding.
The electronegative fluorine atom is a unique and relatively
strong acceptor in H bonds and the question is whether the
hydrazino group would be capable of forming H bonds as a
proton donor with other atoms or groups than the exceptional
fluorine atom. It is well established that the π-electrons of
double bonds may act as a proton acceptor. Studies of 2propenylic (allylic) compounds have shown that moderately
strong intramolecular H bonding between the π-electrons and
the hydroxyl group exists in H2CCHCH2OH.2−5 It is more
surprising that one or more conformers of each of H2C
CHCH 2 NH 2 , 6 − 1 0 H 2 CCHCH 2 SH, 1 1 and H 2 C
CHCH2SeH12 are stabilized by this interaction because the
amino, thiol, and selenol groups are significantly weaker proton
donors than the hydroxyl group. A study of 2-propenylhydrazine (H2CCHCH2NHNH2) should reveal whether the
© 2015 American Chemical Society
Received: November 13, 2015
Revised: December 21, 2015
Published: December 23, 2015
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DOI: 10.1021/acs.jpca.5b11141
J. Phys. Chem. A 2016, 120, 407−416
Article
The Journal of Physical Chemistry A
synclinal in Outer conformers. An illustration of Inner and Outer
forms are shown in Figure 1.
Figure 1. Newman projections of 2-propenylhydrazine viewed along
the N−N bond.
2PH also contains the allyl group (H2CCHCH2−). Allylic
compounds, H2CCHCH2X, where X is a substituent,
generally prefer CCCH2X synperiplanar conformers
where the CCCX dihedral angle is close to 0°, or
anticlinal forms where this dihedral angle is about 120°. In
2PH, the conformational properties associated with the allyl
group come in addition to the Inner/Outer conformational
properties of the hydrazino group. The quantum chemical
calculations discussed below indicate that 18 conformers should
be taken into consideration. These rotamers are depicted in
Figure 2 (9 Inner forms) and in Figure 3 (9 Outer conformers).
Figure 3. Outer conformers. The CCSD energies in parentheses are
relative to conformer I. The spectra of none of these rotamers were
assigned.
quantum chemical methods was therefore chosen for this
investigation.
■
EXPERIMENTAL SECTION
Synthesis. The reported synthesis of 2PH14 was modified
to obtain a pure compound. Hydrazine hydrate (50 mL, 1 mol,
6 equiv) was introduced in a two-necked flask under nitrogen.
The reagent was cooled to 15 °C, and allyl bromide (20.2 g,
0.167 mol, 1 equiv) was added dropwise under stirring while
the temperature was maintained below 30 °C. At the end of the
addition, the mixture was heated to 70 °C and maintained at
this temperature for 1 h. After cooling to room temperature,
diethyl ether (30 mL) was added and the organic phase was
separated and discarded. The same procedure was applied with
a second addition of diethyl ether (30 mL). Both organic
fractions contained a mixture of mono- and diallylhydrazine.
Chloroform (50 mL) was then added to the aqueous phase,
and the organic phase was isolated. After the solvent was
removed, monoallylhydrazine was distilled in vacuo (0.1 mbar)
and selectively condensed in a trap cooled to −40 °C. A pure
product (>97%) was thus obtained in a modest 27% yield (3.25
g, 45 mmol). Unambiguous 1H and 13C NMR data of monoand diallylhydrazine are given below. This information
regarding the NMR spectra was not achieved in the previous
work.14 2-Propenylhydrazine: 1H NMR (CDCl3, 400 MHz) δ
3.05 (s brd, 3H, NHNH2), 3.35 (d, 2H, 3JHH = 6.4 Hz, CH2N),
5.20 (d, 1H, 3JHHcis = 13.2 Hz, CCH(H)), 5.21 (d, 1H,
3
JHHtrans = 17.8 Hz, CCH(H)), 5.78 (ddt, 1H, 3JHHtrans = 17.8
Hz, 3JHHcis = 13.2 Hz, 3JHH = 6.4 Hz, CCH); 13C NMR
(CDCl3, 100 MHz) δ 58.3 (t, 1JCH = 134.2 Hz, CH2N), 118.6
(t, 1JCH = 158.4 Hz, CH2), 134.5 (d, 1JCH = 151.9 Hz, CH). 1,1Di-2-propenylhydrazine: 1H NMR (CDCl3, 400 MHz) δ 2.85
(s brd, 2H, NH2), 3.13 (d, 4H, 3JHH = 6.4 Hz, CH2N), 5.15 (d,
2H, 3JHHcis = 13.2 Hz, CCH(H)), 5.21 (d, 2H, 3JHHtrans =
17.8 Hz, CCH(H)), 5.85 (ddt, 2H, 3JHHtrans = 17.8 Hz, 3JHHcis
= 13.2 Hz, 3JHH = 6.4 Hz, CCH); 13C NMR (CDCl3, 100
MHz) δ 63.6 (t, CH2N), 118.3 (t, CH2), 134.4 (d, CH).)
Spectroscopic Experiments. 2PH is a colorless liquid at
room temperature and has a vapor pressure of roughly 160 Pa.
Fumes of this liquid were admitted to the MW cell that was
Figure 2. Inner conformers of 2-propylenylhydrazine. CCSD
electronic energies in kJ/mol relative to the energy of conformer I
(global minimum) are given in parentheses. The MW spectra of I, VI,
VII, and VIII were assigned; see text.
Each form is given a Roman numeral (I−XVIII) for reference.
An analysis below indicates that as many as 11 of these forms
are stabilized by internal H bonds where either the −NH or the
−NH2 part are proton donor.
MW spectroscopy is ideal for studies of complex conformational equilibria such as the one presented by 2PH due to its
unparalleled accuracy and resolution. High-level quantum
chemical methods are now capable of predicting rotational
constants, energy differences between conformers as well as
dipole moment components along principal inertial axes that
are so accurate that the assignment of crowded MW spectra is
greatly facilitated. A combination of MW spectroscopy and
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The Journal of Physical Chemistry A
Table 1. Characteristic CCSD Dihedral Anglesa of Conformers of 2-Propenylhydrazineb
C1−C2−C3−N4
C2−C3−N4−N5
I
II
III
IV
V
VI
VII
VIII
IX
123.2
120.7
118.1
−14.3
10.5
8.4
112.3
121.3
125.0
−73.5
169.1
65.6
−78.8
176.5
63.8
−58.1
−176.8
74.5
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
129.8
108.8
122.3
7.1
−11.3
−9.9
131.5
120.3
116.0
72.8
−64.0
179.2
80.8
−69.5
−179.8
−73.5
172.1
61.9
C2−C3−N4−H11
C3−N4−N5−H12
C3−N4−N5−H13
83.0
84.9
83.3
81.7
84.8
88.1
−85.6
−84.4
−87.2
−31.7
−30.5
−33.3
−32.8
−30.8
−28.2
30.4
31.2
29.2
85.8
81.1
84.8
88.1
84.0
84.6
−86.9
−84.8
−90.7
−156.9
−161.3
−158.3
−155.0
−158.1
−158.6
155.8
158.2
152.8
Inner Conformers
168.0
51.1
−53.4
162.3
58.4
−56.2
60.7
−58.6
−167.9
Outer Conformers
−167.3
56.4
−60.2
−158.4
52.7
−59.4
165.7
52.0
−58.3
a
In degrees. bAtom numbering: C1(H6,H7)−C2(H8)−C3(H9,H10)−N4(H11)−N5(H12,H13). Figures with atom numbering of each conformer
and full CCSD structures are given in Tables S1−S18 of the Supporting Information.
Table 2. Theoretical Energy Differences, CCSD Rotational Constants, and Principal-Axes Dipole Moment Components of
Conformers of 2-Propenylhydrazine
energy difference (kJ/mol)
B3LYP
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
a
0.0
5.51
5.87
3.42
3.04
4.30
0.14
0.36
7.18
8.18
4.83
2.50
10.00
8.00
5.19
6.66
8.73
4.56
a
CCSD
b
0.0c
7.07
7.10
2.47c
3.32c
2.49c
0.33c
1.52c
8.06
8.91
5.04c
3.84c
10.14
7.35c
5.79c
8.05
10.28
5.00c
rotational constants (MHz)
dipole moments (Debye)
A
B
C
μa
μb
μc
μtot
10264.6
21698.2
13704.9
8878.4
15143.3
8588.6
9660.6
20605.9
15143.2
14525.8
9572.8
21154.3
9066.0
8763.3
15208.6
10905.6
22021.5
12997.1
3143.3
2239.5
2682.4
3664.8
2729.8
3827.4
3307.6
2262.5
2729.8
2660.5
3364.1
2255.7
3594.3
3732.4
2729.7
2979.7
2236.1
2779.5
2709.3
2194.6
2480.0
3024.8
2391.9
3039.0
2777.3
2232.3
2391.9
2434.0
2795.2
2222.5
2953.5
3006.9
2388.7
2632.2
2197.4
2537.0
1.204
0.726
1.421
0.568
0.242
1.417
1.814
0.783
0.242
1.808
0.198
0.224
1.992
0.691
0.501
1.714
0.612
0.198
0.216
1.060
0.570
1.321
1.857
0.566
0.020
1.473
1.857
1.218
0.283
0.348
1.194
0.977
0.233
0.681
1.347
0.851
1.180
1.264
0.975
1.226
0.221
1.003
0.229
0.527
0.221
0.737
1.875
1.780
0.124
0.905
1.793
1.167
1.327
1.485
1.700
1.803
1.816
1.890
1.886
1.826
1.829
1.749
1.886
2.301
1.907
1.828
2.326
1.500
1.876
2.183
1.987
1.723
B3LYP/cc-pVTZ energies are corrected for zero-point vibrational energies. bCCSD/cc-pVTZ electronic energies. cStabilized by H bonding.
calculations. The frozen-core approximation was utilized. The
default convergence criteria of Gaussian 09 were observed. The
atom numbering chosen for 2PH is C1(H6,H7)−C2(H8)−
C3(H9,H10)−N4(H11)−N5(H12,H13). Figures with atom
numbering of each of the 18 conformers are shown in the
Supporting Information together with their full structures.
It is convenient to describe the conformers of this compound
by the values of the C1−C2−C3−N4, C2−C3−N4−N5, C2−
C3−N4−H11, C3−N4−N5−H12, and C3−N4−N5−H13
dihedral angles. A systematic variation of these angles was
used to locate potential conformer candidates. The structures
of these candidates were optimized at the B3LYP/cc-pVTZ
cooled with small quantities of dry ice to a temperature of
approximately −40 °C to enhance intensity of the spectrum as
much as possible. Lower temperatures, which would have been
desirable because of increased spectral intensities, could not be
reached due to insufficient vapor pressure of 2PH. The MW
spectrum was recorded in the 12.4−61 and 72−123 GHz
spectral regions. The 50 kHz Stark modulated spectrometer
used in this study has been described in details elsewhere.15
Quantum Chemical Calculations. B3LYP16,17 and
CCSD18−21 computations were performed employing the
Gaussian 0922 program package. Dunning’s23 correlationconsistent cc-pVTZ triple-ζ basis set was used in all
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J. Phys. Chem. A 2016, 120, 407−416
Article
The Journal of Physical Chemistry A
atoms is close to the π-electrons of the C1C2 double bond.
Inspection of the interatomic distances of the CCSD structures
in Tables S1−S18 of the Supporting Information shows that
this occurs for as many as 11 of the 18 rotamers. One H bond is
found in most cases, but there are also examples where the πelectrons are involved in two H bonds at the same time. The
two H atoms in questions share the π-electrons in these cases.
H bond characteristics of these 11 rotamers are listed in Table
3.
level. Harmonic and anharmonic vibrational frequencies,
nuclear quadrupole coupling constants of the two 14N nuclei,
Watson’s quartic and sextic centrifugal distortion constants24
(observing McKean’s precautions25), vibration−rotational
constants (the α’s),26 and equilibrium (re) and effective (r0)
rotational constants were calculated using the optimized B3LYP
structures of the 18 conformers shown in Figures 2 and 3. No
imaginary harmonic vibrational frequencies were found for
these 18 forms and this was taken as evidence that a minimum
on the potential energy hypersurface (a conformer) had been
identified. Selected B3LYP results of the 18 rotamers are listed
in Tables S1−S18 of the Supporting Information.
The B3LYP structures were used as starting points in
CCSD/cc-pVTZ calculations of optimized structures and
dipole moments as well as its components along the principal
inertial axes. The CCSD structures should be close to the
Born−Oppenheimer equilibrium structures, due to the very
high theoretical level of calculations. Computation of CCSD
harmonic vibrational frequencies were not performed because
these calculations are costly. The full CCSD structures are
listed in Tables S1−S18 in the Supporting Information together
with the B3LYP results. The CCSD values of the five
characteristic CCSD dihedral angles are repeated in Table 1.
Rotamer I was found to have the lowest energy both in the
B3LYP and in the CCSD calculations. The B3LYP energies
were corrected for harmonic zero-point vibrational energy
contributions. Their energies relative to the energy of I are
listed in Table 2. The relative CCSD electronic energies,
rotational constants, principal-axes components of the dipole
moments, and the total dipole moments of the 18 forms are
displayed in the same table.
Comments are warranted for the results presented in these
two tables. The C3−N4−N5−H12 and C3−N4−N5−H13
dihedral angles (Table 1) define the Inner and Outer
conformations. Both these angles vary several degrees among
the conformers. In one series of Inner forms (conformers I−VI)
the C3−N4−N5−H12 dihedral angle varies between 81.7 and
88.1°, whereas the C3−N4−N5−H13 angle takes values
between −28.2 and −33.3°. In the other series of Inner
conformers (rotamers VII−IX), the former dihedral angle
varies from −84.4 to −87.2°, and the latter angle has a
minimum value of 29.2°, and a maximum value of 31.2°. There
are, of course, six different rotamers of the second series to
which VII−IX belong, but three of them are mirror images of
conformers belonging to the first series (I−VI). These
enantiomers have identical spectroscopic properties. Similar
variations are encountered for the Outer forms.
It is also seen from Table 1 that several of the C1−C2−C3−
N4, C2−C3−N4−N5, and C2−C3−N4−H11 dihedral angles
deviate in some cases by as much as ≈15° from their
“canonical” values (0, ±60, ±120, and 180°). These deviations
from normality can presumably be explained largely by the
influence of nonbonded forces on the geometries.
Table 2 reveals that the trends of the relative B3LYP and
CCSD energies are similar. The largest deviation is about 1.8
kJ/mol (conformer VI) between the relative B3LYP and CCSD
energies. The CCSD energies of all 18 conformers span a rather
narrow interval of 10.28 kJ/mol. It is not possible to say how
accurate the CCSD relative energies are, but it is assumed that
they are at least accurate to within 2−3 kJ/mol.
Hydrogen Bonding. The fact that both the −NH− and
−NH2 parts of the hydrazino group may be engaged in internal
H bonding requires that at least one of the H11, H12, H13
Table 3. Characteristics of Hydrogen Bonds in H2C
CHCH2NHNH2
conformer
I
IV
V
VIg
VIIg
VIII
XIh
XII
XIVh
XV
XVIII
H
atoma
H···C1b
(pm)
H···C2c
(pm)
H12
H12
H11
H11
H13
H11
H13
H11
H11
H12
H11
H11
H12
H11
H11
346.3
284.7
279.6
281.7
261.8
388.2
281.7
340.6
380.9
318.7
340.5
276.6
277.1
279.2
343.7
287.9
297.2
264.7
273.2
263.7
271.6
250.3
260.6
266.9
280.1
260.0
270.1
296.7
263.8
268.8
no. of ring
atomsd
5
6
4,
4,
5,
4
5,
4
4
5
4
4,
6
4,
4
5
5
6
no. of H
bondse
ΔECCSDf
(kJ/mol)
1
1
1
2
0.0
2.47
3.32
2.49
2
0.33
1
2
1.52
5.04
1
2
3.84
7.35
1
1
5.79
5.00
6
5
5
a
H atom involved in H bonding. bCCSD distance between H atom in
question and C1. Dots indicate nonbonded interaction. cCCSD
distance between H atom in question and C2. Dots indicate
nonbonded interaction. dNumber of atoms of the “rings”; see text.
e
Number of hydrogen bonds with the π-electrons of the double bond.
f
From Table 2. gBoth H11 and H13 involved in internal H bonding.
h
Both H11 and H12 involved in H bonding.
The criteria for selecting these conformers as candidates for
intramolecular H bonding have been that at least one of the
nonbonded distances between the H11, H12, and/or H13
atoms and the C1 and/or C2 carbon atoms is less than 290 pm.
This distance is the sum of the Pauling van der Waals radii27 of
the H atom (120 pm) and the half-thickness of an aromatic
molecule (170 pm). In the last IUPAC definition28 of the H
bond, it is stated that nonbonded distances between the H
atom and the acceptor is not alone a good measure of H
bonding. However, we have used the 290 pm limit just for
practical purposes.
H bonds are sometimes seen as a result of ring closure. Four,
five, or six atoms are often involved in these rings. Fourmembered rings include C2, C3, N4, and H11 in our case. The
five-membered rings are composed of C2, C3, N4, N5, and
H12, or H13, whereas the six-membered rings consist of C1,
C2, C3, N4, N5, and H12, or H13. Table 3 shows that all these
different types of internal H bonding are present in one or
more of the conformers of 2PH. Two H bonds are present in
VI, VII, XI, and XIV. Two H atoms share the π-electrons in
each of these cases. One H bond is present in each of the
remaining seven conformers.
Most interestingly, the relative CCSD energies of the H
bonded conformers (Table 3) demonstrate that there is no
simple relation between the nonbonded distance(s) and the
relative energies. Conformer I, which represents the global
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The Journal of Physical Chemistry A
Table 4. Spectroscopic Constantsa of Conformer I of CH2CHCH2NHNH2
ground state
Av (MHz)
Bv (MHz)
Cv (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
d1 (kHz)
d2 (kHz)
HJ (Hz)
HJK (Hz)
HKJ (Hz)
HK (Hz)
h1 (Hz)
h2 (Hz)
h3 (Hz)
rmse
Nf
αA (MHz)
αB (MHz)
αC (MHz)
10208.3162(22)
3129.53922(55)
2692.63232(53)
3.75457(58)
−28.5017(41)
98.999(18)
−1.14456(30)
−0.04992(21)
0.01754(17)
−0.1228(21)
−0.543(19)
3.457(73)
0.00868(22)
0.00085(21)
0.000361(63)
1.183
600
first excited C2−C3
torsion (v33)
second excited C2−C3
torsion (2v33)
first excited C3−N4
torsion (v32)
second excited C3−N4
torsion (2v32)
combination
mode (v33 + v32)
theoryb
10255.6344(50)
3135.1032(11)
2692.7863(11)
3.7026(15)
−28.2822(81)
99.941(52)
−1.13465(82)
−0.05310(46)
0.01754c
−0.0821(35)
−0.892(27)
3.457c
0.00734(23)
0.00085c
0.000196(85)
1.236
340
−47.3182(55)
−5.5640(12)
−0.1540(12)
10313.51(15)
3137.3874(35)
2691.4516(36)
3.6118(14)
−28.065(12)
98.999c
−1.1230(28)
−0.0426(18)
10256.56(17)
3120.3832(40)
2685.8862(38)
3.6915(15)
−28.224(10)
98.999c
−1.1241(28)
−0.0358(19)
10302.75(22)
3111.2270(54)
2679.1150(56)
3.6419(20)
−28.131(12)
98.999c
−1.1158(38)
−0.0434(23)
10310.94(33)
3125.1303(58)
2685.7502(58)
3.6518(23)
−28.006(14)
98.999c
−1.0957(54)
−0.0160(33)
10264.6
3143.3
2709.3
3.33
−26.8
99.7
−0.980
−0.0396
−0.988(56)d
−0.781(38)d
−0.741(45)d
−0.696(50)d
−0.127
0.977
192
−105.19(15)
−7.8482(35)
1.1807(36)
264
−48.24(17)
9.1560(40)
6.7461(38)
1.323
189
−95
18.31
13.52
1.209
175
−103.62(33)
4.4089(58)
6.8821(58)
a
S-reduction Ir-representation.24 Uncertainties represent one standard deviation. bCCSD rotational and B3LYP centrifugal distortion constants.
Fixed; see text. dFurther sextic constants preset at zero. eRoot-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. fNumber of transitions used in the fit. The spectra are listed in Tables
S19−S24 of the Supporting Information.
c
weak and extraordinary dense and that transitions of various
spectra frequently overlap.
The nine conformers II, III, IX, X, XIII, XIV, XVI, XVII, and
XVIII have CCSD energies relative to I that are 7 kJ/mol or
more higher (Table 2). Their Boltzmann factors relative to I
are hence less than 0.03 at −40 °C. No attempts to assign the
spectra of any of these conformers were made due to their
presumed low abundance. Searches have been performed for
the remaining nine forms using the CCSD rotational constants
and the B3LYP quartic centrifugal distortion constants to
predict their approximate spectra. The Stark modulation
patterns and radio frequency microwave double resonance29
(RFMWDR) experiments were used extensively to assign the
spectra. The least-squares fits of the spectra were done using
Watson’s Hamiltonian in the S-reduction form with the Irrepresentation24 employing Sørensen’s program Rotfit.30 No
resolved 14N nuclear quadrupole coupling was observed.
The ground vibrational-state spectra were accompanied by
two particularly strong spectra of vibrationally excited states of
the same conformer. These two spectra belong to the first
excited states of the torsions about the C2−C3 bond (v33) and
the C3−N4 bond (v32). The B3LYP frequencies of these
torsions are in the 80−160 cm−1 region (Tables S1−S18), and
their Boltzmann factors are consequently in the 0.6−0.4
interval. Once the spectrum of the ground vibrational state of a
rotamer had been assigned, searches were made for the spectra
of the excited states of these two torsional modes of this
rotamer. The B3LYP rotation−vibration constants (the α’s,
Tables S1−S18) were added to the rotational constants of the
ground vibrational state to get the best possible prediction of
the spectrum of each of the two excited states.
The third lowest vibrational state is the lowest bending
vibration (v31), whose B3LYP frequencies is roughly 250 cm−1
(Tables S1−S18) and the Boltzmann factor is about 0.2. The
energy minimum, has only one internal H bond characterized
by a nonbonded H12···C2 distance of 287.9 pm. This bond is
only 2.1 pm less than 290 pm, which was used above as a
practical criterion for internal H bonding. Rotamer I is, for
example, 2.49 kJ/mol lower in energy than VI, which has two H
bonds.
The nonbonded distance, 250.3 pm, between H13 and C2 in
VII (Table 3) is the shortest such contact that exists in the
conformers of 2PH. Moreover, this rotamer has two H bonds.
VII is nevertheless 0.33 kJ/mol higher in energy than I. The
conclusion is that there is no simple relationship between the
conformer energy and the shortness or number of H bonds.
The majority of the H-bonded conformers spans an energy
range of about 5 kJ/mol, whereas conformers without H bonds
span an energy interval of 7−10 kJ/mol relative to I (Table 2).
The fact that the hydrazino group form different H bonds
makes it difficult on this basis to roughly estimate the strengths
of the H bonds in each case, but 3−6 kJ/mol depending on the
type of interaction is a plausible estimate given these data.
Microwave Spectrum and Assignment Strategy. 2PH
has, as expected, a relatively weak and extremely dense MW
spectrum. This is due to several reasons. A thermodynamic
equilibrium exists in our experiments and each quantum state is
populated according to Boltzmann statistics. The 18 conformers span a relatively narrow energy interval of about 10 kJ/
mol and 8 rotamers have CCSD energies that differ by less than
5 kJ/mol (Table 2). Each conformer has six normal modes with
vibrational frequencies less than 500 cm−1 (Tables S1−S18 of
the Supporting Information). Each conformer will have its own
resolved spectrum not only for the ground vibrational state but
for vibrationally excited states as well. Obviously, all these
factors lead to a low Boltzmann population even for the ground
vibrational state of the most abundant conformer. The same
reasoning explains why the observed spectrum is comparatively
411
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The Journal of Physical Chemistry A
first excited-state spectra of this mode were only searched for
when the ground-state spectrum was comparatively strong. No
assignments of spectra belonging to this excited state could be
made.
Assignment of the Ground Vibrational-State Spectrum of Conformer I. This global-energy minimum rotamer
has its largest dipole moment components along the a- and cprincipal inertial axes, whereas μb is predicted to be small
(0.216 D). Survey spectra revealed a relatively strong spectrum
close to that predicted using the theoretical spectroscopic
constants shown in the last column of Table 4. The first
unambiguous assignments of transitions of this form were
achieved for a-type R-branch transitions using the RFMWDR
method. A typical example of a spectrum obtained by this
method is shown in Figure 4. These assignments were readily
rotational constants are expected in most cases because the
CCSD structures are close to the Born−Oppenheimer
equilibrium structures. The CCSD bond lengths are generally
smaller than the r0 bond lengths. The consequence of this is
that the CCSD principal moments of inertia are slightly smaller
and the rotational constants are larger than the r0 rotational
constants, just as observed in the present case.
The B3LYP calculations yielded the r0 and re rotational
constants (Table S1). However, differences between these two
sets of rotational constants, Ae − A0, etc., are very different from
their counterparts obtained from the experimental r0 and rCCSD
rotational constants. Similar results were obtained for the other
three conformers below. The B3LYP method is obviously not
capable of producing reliable values for these parameters in the
present case. This is unfortunate because accurate predictions
of these differences would have help tremendously for the
assignment of these complicated spectra.
All experimental quartic centrifugal distortion constants, but
DK, are larger than the B3LYP equivalents. The largest
deviation is observed for d1 (14.4%) and the smallest for DK
(0.7%). Much larger differences are seen for the sextic
centrifugal distortion constants, which is natural given the
simplifications involved in the B3LYP calculations.
Inspection of Table 2 shows that the CCSD rotational
constants of I differ so much from the other rotational
constants that it can be concluded that the spectrum shown in
Table S19 undoubtedly belong to rotamer I. The observation of
a- and c-type spectra corroborates this conclusion.
Vibrationally Excited States of I. Five spectra belonging
to vibrationally excited states of I were assigned. These spectra
are listed in Tables S20−S24 of the Supporting Information,
and the spectroscopic constants are collected in Table 4. The
assignments of several aR-lines of each excited state were
confirmed by RFMWDR experiments.
It was possible to assign c-type lines up to Jmax = 46 for the
strongest vibrationally excited state. The B3LYP calculations
(Table S1) predict that this is the first excited state of the
torsion about the C2−C3 bond (v33). Accurate values were
obtained for all quartic centrifugal distortion constants of this
excited state as shown in the second column of Table 4. Three
sextic constants were also obtained. The remaining four sextic
constants were kept constant at the ground-state values in the
least-squares fit.
Only aR-branch transitions were identified for the four other
excited states. The maximum value of J was typically about 20
and K−1max approximately 16 in these cases. It was not possible
to get accurate values for the quartic constant DK of these
states, and the value of the ground vibrational state was used
and kept constant in the fits of these spectra. One sextic
constant, HKJ, was also fitted keeping the remaining sextic
constants fixed at zero. The calculated values of the vibration−
rotation α-constants of the excited states are listed at the
bottom of Table 4 and used to assign the corresponding spectra
to individual excited states, as described below.
The torsion about the C2−C3 bond (v33) is the lowest
vibrational frequency according to the B3LYP calculations,
which predict 91.6 cm−1 for this vibration (Table S1). Relative
intensity measurements yielded 111(20) cm−1. The B3LYP
values of the vibration−rotation constants of the first excited
state of the torsion about the C2−C3 bond (v33) are αA =
−33.11, αB = −9.75, and αC = −2.64 MHz (Table S1)
compared to the experimental values −47.3182(55),
−5.5640(12), and −0.1540(12) MHz, respectively (Table 4,
Figure 4. RFMWDR spectrum of the J = 135 ← 125 pair of transitions
of conformer I. The RF frequency was 5.11 MHz. The pair at the left
belongs to the ground vibrational state. The pair with frequencies just
below 76.1 GHz is signals from the first excited state of the torsion
about the C2−C3 bond (v33), whereas the pair at the right belongs to
what has tentatively been assigned as the second excited state of v33;
see text. The lone line at about 76.080 GHz is spurious and does not
belong to the J = 135 ← 125 transition of conformer I.
extended to include additional a-type R-branch transitions up
to J = 21 and K−1 = 17. The preliminary rotational and quartic
centrifugal distortion constants obtained from the aR-spectrum
were next used to predict the frequencies of strong cQtransitions, which were found close to the predicted
frequencies. Additional c-type Q- and R-branch lines were
gradually included the least-squares fit. Ultimately, cQ-lines with
Jmax = 60 and cR-lines with Jmax = 52 were assigned. The
frequencies of b-type lines should be very precisely predicted
from this spectrum. However, no definite assignments of this
kind of transitions could be made, presumably because of a
small μb (about 0.2 D; Table 2) producing insufficient
intensities of the lines in question. The full spectrum consisting
of 600 transitions are listed in Table S19 and the spectroscopic
constants are repeated in the first column of Table 4.
Comparison of the experimental r0 rotational constants in
column 1 of Table 4 with the CCSD constants in the last
column reveals that the CCSD constants are larger than the r0
rotational constants by 0.55, 0.44, and 0.52% in the cases of A,
B, and C, respectively, which is very satisfactory. The fact that
CCSD rotational constants are a little larger than the effective
412
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The Journal of Physical Chemistry A
Table 5. Spectroscopic Constantsa of the Ground
Vibrational State of Conformer VI of CH2CHCH2NHNH2
column 3). There is only an order of magnitude agreement
between theory and experiment in this case. Obviously, B3LYP
calculations are not sufficiently refined to produce accurate
values in this case as well.
The spectroscopic constants listed in the third column of
Table 4 are assumed to belong to the second excited state of
the C2−C3 torsion. The α-constants of this state are not
exactly twice as large as the constants of the first excited state,
which would have been the case for a completely harmonic
vibration. However, their changes go in the expected direction
and have plausible magnitudes. It is therefore concluded that v33
is a very anharmonic vibration. An alternative assignment of this
spectrum as belonging to the first excited state of the lowest
bending vibration (v31) is ruled out because αB = 10.55 MHz
(Table S1), whereas −7.85 MHz is the experimental value
(Table 4).
The constants of column 4 of Table 4 are assumed to belong
to the first excited state of the torsion about the C3−N4 bond
(v32). Relative intensity measurements yielded 137(20) cm−1
for this vibration compared to the B3LYP harmonic frequency
of 149.7 cm−1. The B3LYP method predicts αA = −72.12, αB =
13.17, and αC = 8.21 MHz (Table S1). The absolute values of
these constants are all larger than their experimental counterparts (Table 4), but they have correct signs and relative
magnitudes.
Column 5 contains the spectroscopic constants of the second
excited state of v32. The α-values of this state are almost exactly
twice as large as the α -values of the first excited state of this
mode. The torsion about the C3−N4 bond, v32, thus appears to
be essentially harmonic and therefore behaves quite differently
from the torsion about C2−C3 bond (v33) discussed above.
The constants of a combination mode of the two torsional
vibrations are given in the sixth column. This assignment has
been made because the α-constants associated with this excited
state are almost equal to the sum of the corresponding
constants of the first excited states of v33 and v32.
Assignment of the Spectrum of the Ground Vibrational State of VI. The electronic CCSD energy of this
conformer is 2.49 kJ/mol higher than the energy of I (Table 2).
The Boltzmann factor of VI is 0.28 relative to I at the
temperature in question (−40 °C). The CCSD method
predicts that this conformer has μa = 1.417, μb = 0.566, and
μc = 1.003 D (Table 3). The aR-spectrum was assigned in the
same manner as described for the assignment of the
corresponding spectrum of I. Ultimately, 171 aR-lines with
Jmax = 18 and K−1max = 16 were assigned as shown in Table S25
of the Supporting Information. Many K−1-pairs of cQ-branch
lines with K−1 = 6 to K−1 = 12 coalesce and have rapid Stark
effects, and this was used to assign several members of them. A
few cR-branch lines were also assigned. b-type transitions were
not found, presumably because of their weakness. This is in
accord with a small μb (see above). Attempts to find spectra of
vibrationally excited states of this conformer were made, but no
further unambiguous assignments could be made due to the
weakness of the spectrum. Spectroscopic constants of VI are
listed in Table 5.
The CCSD rotational constants are all larger than the
experimental rotational constants by 0.55, 0.99, and 0.75% in
the cases of A, B, C, respectively, as expected (see above). All
B3LYP quartic centrifugal distortion constants have absolute
values that are larger than the experimental equivalents.
Deviations between 17% (DJ) and 42% (DJK) were found in
this case, which is not satisfactory.
A0 (MHz)
B0 (MHz)
C0 (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
d1 (kHz)
d2 (kHz)
rmsc
Nd
experiment
theoryb
8541.9120(47)
3789.8105(29)
3016.4362(27)
2.5438(28)
−6.0893(53)
12.879(24)
−0.7499(34)
−0.0708(18)
1.144
229
8588.6
3827.4
3039.0
2.98
−8.68
18.0
−0.937
−0.0836
a
S-reduction Ir-representation.24 Uncertainties represent one standard
deviation. bCCSD rotational and B3LYP centrifugal distortion
constants. 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 least-squares fit, and P is the number
of spectroscopic constants used in the fit. dNumber of transitions used
in the fit. The spectrum is listed in Table S25 of the Supporting
Information.
Inspection of Table 2 reveals that there are no other
conformers predicted to have rotational constants and a major
μa similar to VI. The assignment of the spectrum of Table S25
to VI is unequivocal.
Assignment of the Spectrum of the Ground Vibrational State of VII. This conformer is 0.33 kJ/mol less stable
than I according to the CCSD method. μa is much larger (1.814
D) than the other two dipole moment components, which are
0.020 and 0.229 D, respectively (Table 2). The assignments of
the spectra of the ground vibrational state and three
vibrationally excited states were made in a manner analogous
to those described for I and VI above. Searches for b-type and
c-type transitions resulted in no assignment, which is due to the
small values of the corresponding dipole moment components.
The spectrum of the ground state consisting exclusively of 311
a
R-transitions with Jmax = 21 and K−1max = 19 is listed in Table
S26 and the spectroscopic constants including quartic and one
sextic constant are shown in Table 6.
The CCSD rotational constants are larger than the
experimental counterparts in the case of A by 1.15%, and C
by 0.25%, whereas the CCSD value of B is smaller than the
effective equivalent by 0.22%. The B3LYP quartic centrifugal
distortion constants are all too large.
The rotational constants of VII are similar to the rotational
constants of XI. However, the major dipole moment
component of VII is along the a-inertial axis, whereas XI has
its major component along the c-axis. This is sufficient to
conclude that the spectrum of Table S26 undoubtedly belongs
to conformer VII.
Vibrationally Excited States of VII. The aR-spectra
(Tables S27−S29) of three such states were assigned. The
two lowest B3LYP vibrational frequencies are the torsions
about the C2−C3 (v33) and C3−N4 (v32) bonds occurring at
89.7 and 172.1 cm−1 (Table S7), compared to 98(20) and
162(25) cm−1 found in relative intensity measurements. The αconstants of these two excited states are predicted to be
−125.7, +23.6, and +12.7 MHz for v33 and −62.0, +20.8, and
+10.7 for v32. The experimental values in Table 6 are much less
than this but follow the same trend as the theoretical values.
This is the basis for the assignments to individual excited states
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The Journal of Physical Chemistry A
Table 6. Spectroscopic Constantsa of Conformer VII of CH2CHCH2NHNH2
Av (MHz)
Bv (MHz)
Cv (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
d1 (kHz)
d2 (kHz)
HKJd (Hz)
rmse
Nf
αA (MHz)
αB (MHz)
αC (MHz)
ground state
first excited C2−C3 torsion (v33)
second excited C2−C3 torsion (2v33)
first excited C3−N4 torsion (v32)
theoryb
9550.31(12)
3314.7538(28)
2770.2968(28)
4.2255(30)
−28.031(18)
75.9(65)
−1.3940(21)
−0.0560(20)
−1.274(28)
1.024
311
9657.01(16)
3297.1093(30)
2760.6801(28)
3.7912(35)
−25.559(22)
84.2(81)
−1.2326(22)
−0.0502(21)
−1.035(37)
1.124
264
−106.70(20)
17.6445(45)
9.6167(40)
9762.4(14)
3280.568(23)
2751.288(23)
3.4175(73)
−24.640(21)
75.9c
−1.230(29)
−0.0560c
−1.274c
1.363
159
−212.2(14)
34.186(23)
19.009(23)
9598.02(16)
3298.4308(35)
2761.1529(33)
4.2430(36)
−27.904(29)
93.9(79)
−1.3941(24)
−0.0540(22)
−2.02(15)
1.208
221
−47.71(20)
16.3230(45)
9.1439(43)
9660.6
3307.6
2777.3
4.41
−30.1
81.9
−1.45
−0.0729
−1.49
a
S-reduction Ir-representation.24 Uncertainties represent one standard deviation. bCCSD rotational and B3LYP centrifugal distortion constants.
Fixed; see text. dFurther sextic constants preset at zero. eRoot-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. fNumber of transitions used in the fit. The spectra are given in Tables
S26−S29 of the Supporting Information.
c
Table 7. Spectroscopic Constantsa of the Ground
Vibrational State and One Vibrationally Excited State of
Conformer VIII of CH2CHCH2NHNH2
in Table 6. Interestingly, the α’s of the second excited state of
v33 have almost exactly twice the values of those of the first
excited state indicating that this mode is essentially harmonic.
Assignment of the Ground Vibrational State of VIII.
The CCSD energy of this conformer is 1.52 kJ/mol higher than
the energy of I, and its Boltzmann factor is 0.46. μb is the
predominating dipole moment component calculated to be
1.437 D (Table 2). Searches for strong b-type Q-branch lines
led to the first assignments. bR-branch and aR-branch lines were
then found and included in the fit, which were gradually
extended to include aR-branch lines up to J = 26, bQ-branch
transitions with a maximum value of J = 77, bP-branch members
with Jmax = 77, and bR-branch lines up to J = 80. Attempts to
find c-type lines were also made, but these transitions are so
weak due to a small μc (Table 2) that unambiguous assignments
were not possible to obtain. The spectrum consisting of 947
transitions are listed in Table S30, whereas the spectroscopic
constants are shown in Table 7. The extensive spectrum made
it possible to determine all sextic centrifugal distortion
constants.
The CCSD rotational constants are again larger than the r0
rotational constants (Table 7) by 0.97, 0.41, and 0.32% in the
cases of A, B, and C, which is expected. The experimental
quartic centrifugal distortion constants are larger than the
B3LYP constants. The B3LYP sextic centrifugal distortion
constants generally deviate so much from their experimental
counterparts that a discussion is not warranted.
Inspection of Table 2 reveals that the rotational constants of
VIII differ so much from the rotational constants of the other
17 rotamers that an unambiguous conclusion can be drawn on
this basis alone.
Vibrationally Excited State. A total of 506 transitions
(Table S31) of one vibrationally excited state were assigned in
the same manner as described in the previous paragraph for the
ground vibrational state. The maximum value of J is 75 in this
case. All sextic centrifugal distortion constants were determined
for this excited state, as shown in Table 7. Interestingly, some of
the centrifugal distortion constants of the excited state vary
much from the ground-state values. This is especially true for
DK and HK.
ground state
A0 (MHz)
B0 (MHz)
C0 (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
d1 (kHz)
d2 (kHz)
HJ (Hz)
HJK (Hz)
HKJ (Hz)
HK (Hz)
h1 (Hz)
h2 (Hz)
h3 (Hz)
rmsc
Nd
αA
αB
αC
20407.7537(22)
2253.25296(26)
2225.03628(27)
0.73480(14)
−27.2957(22)
535.456(24)
0.128078(16)
−0.0115441(47)
0.002330(23)
−0.11488(72)
0.3847(59)
−0.33(16)
−0.0008693(25)
0.0001095(12)
−0.00000563(88)
0.906
947
first excited C2−C3
torsion
20127.7165(41)
2257.27550(55)
2232.78481(55)
0.76235(48)
−24.9439(56)
407.646(67)
0.138121(30)
−0.0123883(85)
0.00205(11)
−0.1205(21)
1.286(16)
−45.75(42)
−0.0009104(56)
0.0001330(21)
−0.0000027(18)
0.978
506
280.0372(47)
−4.02254(61)
−7.74853(61)
theoryb
20605.9
2262.5
2232.3
0.634
−24.6
527
0.0992
−0.00882
0.00149
−0.289
13.8
−259
−0.000487
0.0000590
−0.000000220
a
S-reduction Ir-representation.24 Uncertainties represent one standard
deviation. bCCSD rotational and B3LYP centrifugal distortion
constants. 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 least-squares fit, and P is the number
of spectroscopic constants used in the fit. dNumber of transitions used
in the fit. The spectra are listed in Tables S30−S31 of the Supporting
Information.
The B3LYP α-constants are+ 392.4, −2.95, and −5.39 MHz,
respectively, for v33, whose harmonic frequency is 95.3 cm−1
(Table S8), similar to the rough experimental value (103(20)
cm−1). These α-values have the correct order of magnitude and
correct signs with respect to the experimental values shown in
Table 7.
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The Journal of Physical Chemistry A
Searches for Further Conformers. About 4400 transitions have been assigned for the four rotamers. This includes
the vast majority of the strongest transitions. Many lines with
intermediate intensity have also been assigned. However, a
large number of mostly weak or very weak transitions have not
been accounted for. It is assumed that many of them belong to
unassigned spectra of vibrationally excited forms of the four
conformers, but they could also belong to conformers whose
spectra have not been assigned. Extensive searches for the most
likely candidates were performed.
Rotamer IV, which is one of these candidates, is predicted to
have a CCSD energy of 2.47 kJ/mol (Table 2) and hence a
Boltzmann factor of 0.28 relative to I. This form was not found
despite considerable efforts. It is of course a possibility that the
energy difference is larger than this value resulting in lower
intensity than expected for its spectrum. The fact that its largest
dipole moment component μb is not higher than 1.32 D (Table
2) is a disadvantage for the assignment of this species because
intensities are proportional to the square of the dipole moment
component.
The spectra of V (3.32 kJ/mol; Boltzmann factor = 0.20) and
XII (3.84 kJ/mol; Boltzmann factor = 0.14) could not be found
for reasons similar to those described in the previous paragraph.
It is not very surprising that spectra of XVIII (5.00 kJ/mol), XI
(5.04 kJ/mol), and XV (5.79 kJ/mol) have not been identified
because their Boltzmann factors are all less than 0.08.
involved are different. In VIII, H11 forms the H bond, whereas
H12 has this role in I. H11 of VIII is much closer to C2 (260.6
pm; Table 3) than H12 of I (287.9 pm; Table 3). The −NH2
part in VIII is rotated almost as far away from the π-electrons
(Table S8) as it can get, resulting in a minimal interaction
between this group and the H2CCH part. However, a short
distance between H11 and C2 and a favorable orientation of
the −NH2 part is obviously not enough to give VIII a lower
energy than I. Perhaps the π-electrons in I and H12 have a
better orientation for interaction with one another than what is
the case in VIII. H12 of I may also get help from H13, which is
not much further apart (about 2 pm; Table S1) from C2 than
H12 is. This presumed more favorable orientation may perhaps
explain why I is 1.52 kJ/mol lower in energy than VIII.
Conformer VI has a two H bonds with the π-electrons
involving H11 and H13. In spite of this, VI has a 2.49 kJ/mol
higher energy than I (Table 3). An essential difference between
VI and the three other forms discussed here is the C1−C2−
C3−N4 dihedral angle, which is 8.4° in VI (Table 1), whereas
this angle is roughly 120° in the three other conformers. The
C2−C3−N4−H11 dihedral angle of −58.6° brings the H11
atom into a favorable position. The orientation of the −NH2
moiety is also favorable for the interaction of H13 with the πelectrons, but the lone pair of the N5 atom is quite close to
these π-electrons with repulsion as a result. This could be one
reason why I is lower in energy than VI.
The theoretical calculations revealed strengths and weaknesses of the CCSD and B3LYP quantum chemical methods.
The CCSD calculations provided structures whose rotational
constants always were in good agreement with the observed
rotational constants of the four conformers. The same can be
said about the dipole moments and the relative energies. All
this was expected for this advanced method.
The B3LYP calculations provided structures (not given),
dipole moments (not given), and relative energies (Table 2)
that were of a good quality. This method predicts quartic
centrifugal distortion constants that on the average deviate by
more than 15% from the experimental constants. The B3LYP
vibration−rotation constants are only predicted on an order of
magnitude basis, whereas the sextic centrifugal distortion
constants are generally unreliable. Moreover, the differences
between the r0 and re rotational constants are not accurate. The
last three types of parameters depend on the calculation of the
third derivatives of the potential energy, which is very
demanding. B3LYP is obviously not refined enough to provide
very precise values of these parameters in the case of 2PH.
■
DISCUSSION
The previous study of FCH2CH2NHNH21 showed that the
fluorine atom acts as a proton acceptor in internal H bonds.
The present investigation shows that the hydrazino group is
capable of forming H bonds not only with the most
electronegative of all elements but also with π-electrons as
well as with strengths that are similar to those found in the
fluorine compound.
Conformers I, VI, VII, and VIII of 2PH, whose spectra were
assigned in this work, have one thing in common: They are all
stabilized by internal H bonds. I, VII, and VIII are also the
three most stable forms according to the CCSD calculations
(Table 2). The fourth species, VI, is only marginally less stable
than IV, whose spectrum was not assigned. The energy
difference is a mere 0.02 kJ/mol (Table 2).
Why I has a lower energy than V, VII, and VIII needs
explanation. In a complicated compound like 2PH, there will be
many nonbonded interactions and explaining subtle energy
differences is by no means a simple undertaking. Factors that
might be of importance in these cases are pointed out below.
The smallest energy difference (0.33 kJ/mol; Table 2) is
found between I and VII. Conformer I has only one H bond
(H12; Table 3), whereas VII has two, namely, H11 and H13,
which are sharing the π-electrons. The orientation of the heavy
atoms is similar in the two conformers (Table 1), but the
positions of the H atoms of the hydrazino group are different.
N5 is relatively close to C2 and the π-electrons of the double
bond in both cases (Tables S1 and S7, respectively). The lone
electron pair of N5 is favorably oriented with respect to the πelectrons in I pointing away from them. This is not the case in
VII where the lone pair and the π-electrons are in much closer
proximity. This is perhaps one reason why the two H bonds in
VII do not fully outweigh the destabilizing influence of the
lone-pair-π-electrons repulsion.
Conformer VIII is 1.52 kJ/mol higher in energy than I. Both
these rotamers have one H bond each, but the H atoms
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpca.5b11141.
Results of the theoretical calculations, including B3LYP/
cc-pVTZ harmonic and anharmonic vibrational frequencies, quartic and sextic centrifugal distortion constants,
vibration rotation constants, re and r0 rotational
constants, and nuclear quadrupole coupling constants
of the two 14N nuclei. CCSD electronic energies,
structures, rotational constants, and dipole moments.
Microwave spectra of the ground state of four conformers and of vibrationally excited states of three
rotamers. (PDF)
415
DOI: 10.1021/acs.jpca.5b11141
J. Phys. Chem. A 2016, 120, 407−416
Article
The Journal of Physical Chemistry A
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AUTHOR INFORMATION
Corresponding Author
*H. Møllendal. Phone: +47 2285 5674. Fax: +47 2285 5441. Email: harald.mollendal@kjemi.uio.no.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank Osamu Sekiguchi for performing a mass spectrometry investigation of 2PH. 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.
■
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