Microwave Spectrum and Molecular Conformation of (E)-Benzaldehyde Oxime

Article abstract of DOI:10.1006/jmsp.1999.7882

The microwave spectrum of (E)-benzaldehyde oxime, C₆H₅-CH=NOH and C₆H₅-CH=NOD, has been observed in the frequency range from 26.5 to 40.0 GHz. The spectrum of the ground vibrational state was assigned and fitted to the Watson's A-reduced Hamiltonian to obtain these rotational and centrifugal distortion constants: A = 5183.13(29) MHz, B = 895.367(3) MHz, C = 763.819(3) MHz, Δ_J = 0.019(3) kHz, and Δ_JK = 0.204(7) kHz for the normal species, and A = 5158.4(23) MHz, B = 869.44(2) MHz, C = 744.34(2) MHz, Δ_J = 0.023(2) kHz, and Δ_JK = 0.193(7) kHz for the deuterated species. The values of the ΔI (=I_c - I_a - I_b) obtained for the normal and deuterated species were -0.295(6) and -0.28(5) a.m.u. A², respectively. The molecular conformation of this molecule was a planar one in which the values of the dihedral angles, CCNO and CNOH, were almost 180° and 180°, respectively.

Full text of DOI:10.1006/jmsp.1999.7882

Journal of Molecular Spectroscopy 196, 283–289 (1999)
Article ID jmsp.1999.7882, available online at http://www.idealibrary.com on
Microwave Spectrum and Molecular Conformation
of (E)-Benzaldehyde Oxime
Nobuhiko Kuze,* Manabu Sato,* Keiichi Maue,* Tsuyoshi Usami,* Takeshi Sakaizumi,*
Osamu Ohashi,* and Kinya Iijima†
*Department of Chemistry, Faculty of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan;
and †Department of Chemistry, Faculty of Science, Shizuoka University, Oya, Shizuoka 422-8529, Japan
E-mail: n-kuze@hoffman.cc.sophia.ac.jp
Received December 14, 1998; in revised form April 19, 1999
The microwave spectrum of (E)-benzaldehyde oxime, C6H5—CH|NOH and C6H5—CH|NOD, has been ob-
served in the frequency range from 26.5 to 40.0 GHz. The spectrum of the ground vibrational state was assigned
and fitted to the Watson’s A-reduced Hamiltonian to obtain these rotational and centrifugal distortion constants:
A ϭ 5183.13(29) MHz, B ϭ 895.367(3) MHz, C ϭ 763.819(3) MHz, ⌬J ϭ 0.019(3) kHz, and ⌬JK ϭ 0.204(7)
kHz for the normal species, and A ϭ 5158.4(23) MHz, B ϭ 869.44(2) MHz, C ϭ 744.34(2) MHz, ⌬J ϭ 0.023(2)
kHz, and ⌬JK ϭ 0.193(7) kHz for the deuterated species. The values of the ⌬I (ϭIc Ϫ Ia Ϫ Ib) obtained for the normal
and deuterated species were Ϫ0.295(6) and Ϫ0.28(5) a.m.u. Å2, respectively. The molecular conformation of this
molecule was a planar one in which the values of the dihedral angles, CCNO and CNOH, were almost 180° and 180°,
respectively. © 1999 Academic Press
1. INTRODUCTION
vacuum distillation. The deuterated species (C6H5—
CH|NOD) was synthesized by refluxing a mixture of
C6H5—CH|NOH and D2O for 24 h. The product was
identified by its mass spectrum and the yield of the deuter-
ated species was ca. 50%. The microwave spectrum was
observed in the frequency range from 26.5 to 40.0 GHz with
Benzaldehyde oxime (C6H5—CH|NOH), which is the
prototype of the aromatic oximes, has two geometrical iso-
mers, syn form in which the C(|N)—H and N—O bonds
are in the eclipsed position ((E)-isomer) and anti form in
which the C(|N)—H and N—O bonds are in anti orienta-
tion ((Z)-isomer), as shown in Fig. 1. The (Z)-isomer
gradually inverts into (E)-isomer at ambient temperature.
The (E)- and (Z)-isomers of benzaldehyde oxime in solu-
tion were identified by NMR study (1). The molecular
structure of (E)-benzaldehyde oxime in the solid phase was
determined at 100 K by X-ray diffraction (2). The CCCN
dihedral angle, ␾, between the planes of the phenyl group
and the oxime moiety was reported to be about 20°. Sau-
vaitre has recorded the IR and Raman spectra of benzalde-
hyde oxime in the gas phase as well as in solution and solid
(3) and assigned the observed spectra by the normal coor-
dinate analysis, assuming the planar conformation. Our in-
terest was in the conformation of benzaldehyde oxime re-
lated to the dihedral angle, ␾, in the vapor phase. In this
work, we determined the molecular constants and the mo-
lecular conformation of (E)-benzaldehyde oxime in the vi-
brational ground state by the microwave spectroscopy.
2. EXPERIMENTAL SECTION
FIG. 1. Four possible molecular conformations with atom numbering of
(E)- and (Z)-isomers of benzaldehyde oxime. The CCCN dihedral angle ␾ is
(E)-Benzaldehyde oxime with 98% purity obtained from
Aldrich Chemical Company, Inc. was further purified by defined to be zero when the molecular conformation is planar.
283
0022-2852/99 $30.00
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284
KUZE ET AL.
TABLE 1
Principal Geometric Parameters and Relative Energies for Benzaldehyde Oxime Calculated
at MP2/6-31G(d, p) Level of Theorya
100 kHz square-wave Stark-modulated spectrometer at tion of each isomer in this molecule was the planar one in
room temperature. The sample was flowed in the waveguide
at the pressure of ca. 0.01 Torr. The microwave source
employed was a phase-locked YIG-tuned GaAs oscillator
(WJ 5610-302FD), using a signal generator (HP 8672A) as
a standard frequency oscillator.
which the dihedral angle CNOH was nearly 180° (see (E)-1
and (Z)-1 in Fig. 1) and the energy differences between the
planar conformer and the one with ␾ ϭ 90° were 4.6 and 3.0
kcal molϪ1 for the (E)- and (Z)-isomers, respectively (see
Fig. 2). Then the full optimizations were performed for both
isomers; the optimized structures were almost planar. We
found two more stable conformations; one was the (E)-
isomer in which CNOH ϭ 0° (see (E)-2 in Fig. 1) and the
other was the (Z)-isomer in which ␾ ϭ 50° and CNOH ϭ 9°
(see (Z)-2 in Fig. 1). The most stable conformation was
(E)-1. The optimized parameters, relative energy differ-
ences, rotational constants, and the components of the di-
pole moment in the principal axis system for four confor-
3. AB INITIO CALCULATIONS
The ab initio molecular orbital calculations have been
performed at the MP2/6-31G(d, p) level of theory (4, 5)
using the Gaussian 94 program package (6). The structural
optimizations of the (E)- and (Z)-isomers of benzaldehyde
oxime were carried out by varying the dihedral angle, ␾,
which was the torsional angle of the phenyl group, in steps
of 30° between ␾ ϭ 0° and 90°. The most stable conforma- mations of benzaldehyde oxime are summarized in Table 1.
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MICROWAVE SPECTRUM OF (E)-BENZALDEHYDE OXIME
285
TABLE 2
Observed Transition Frequencies (MHz) of (E)-Benzaldehyde Oxime for the Normal
and Deuterated Species
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KUZE ET AL.
TABLE 2—Continued
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MICROWAVE SPECTRUM OF (E)-BENZALDEHYDE OXIME
287
TABLE 3
Observed and Calculated Rotational Constants, ⌬I, and Centrifugal Distortion Constants
of Benzaldehyde Oxime
4. RESULTS AND DISCUSSION
In the spectral bunches for normal and deuterated species,
many absorption lines attributed to the vibrational satellites
were observed. In this study, however, the analysis of the
The microwave spectrum of C6H5—CH|NOH in the
26.5–40.0 GHz frequency region, observed with the Stark
field of 100 VcmϪ1, revealed K structures of a-type R
branch appearing with frequency intervals of 1660 MHz.
The assignment of the spectral lines ascribed to KϪ1 ϭ 0, 1,
and 2 were confirmed by observing their Stark behavior and
the shapes of the spectral lines at the Stark fields of 1600,
2000, and 2200 VcmϪ1, respectively. The intensities of these
lines were all weak. The spectral bunches for the deuterated
species were observed with intervals of 1620 MHz and were
not overlapped with the spectrum of normal species. The
intensities of the absorption lines for the deuterated species
were weak since the sample was a mixture of normal and
deuterated molecules. Therefore, the assignment of the
spectral lines for the deuterated species was limited to high
K lines. The total number of the spectral lines observed and
assigned to a-type R-branch transitions in the ground vibra-
tional states were 117 for normal and 103 for deuterated
species (see Table 2).
FIG. 2. The torsional potential curve of (E)- and (Z)-benzaldehyde oxime
as a function of the dihedral angle ␾ calculated from the MP2/6-31G(d, p) ab
initio MO calculations.
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288
KUZE ET AL.
TABLE 4
Structural Parameters of (E)-Benzaldehyde Oxime Derived by the Scaling of the
Whole Ab Initio Geometry for the Reproduction of the Observed Rotational Con-
stantsa
vibrational satellites could not be performed since the absorp- b-coordinate were determined to be 4.105(4) and 0.70(4) Å,
tion lines for low K transitions in the excited vibrational states respectively (errors in parentheses were 2.5 times the standard
were too weak to be measured and assigned.
deviation). The calculated coordinates of the hydrogen atom
The observed frequencies of the absorption line were fitted were a ϭ 4.113 Å for (E)-1 and a ϭ 3.416 Å for (E)-2
to obtain the rotational and centrifugal distortion constants, conformers on the basis of the ab initio calculation. The former
using Watson’s A-reduced Hamiltonian (7). Table 3 shows the value was consistent with the experimental one. Therefore, the
molecular constants in the ground vibrational state obtained observed molecular conformation was concluded to be (E)-1
along with those calculated by ab initio MO method.
as shown in Fig. 1.
The values of the ⌬I (ϭIc Ϫ Ia Ϫ Ib) obtained experimen-
In this study the observed absorption lines were only as-
tally were Ϫ0.295(6) and Ϫ0.278(2) u Å2 for the normal and signed to the a-type transitions. The calculated dipole moment
deuterated species, respectively. These two values meant that component (see Table 1) shows that the (E)-1 conformer has
the molecular conformation of this molecule was planar or only ␮ component and so the b-type lines of this conformer
a
almost planar. The result of the ab initio calculation of C6H5— may not be expected to be observed.
CH|NOH was consistent with this experimental result. In
The absolute value of ⌬I of (E)-benzaldehyde oxime
Table 3, the rotational constants for deuterated species esti- (Ϫ0.295(6) u Å2) was slightly larger than that of the related
mated by assuming the planar conformation of C6H5— compound, benzaldehyde (Ϫ0.1199 u Å2) (9). This suggests
CH|NOH optimized at the MP2/6-31G(d, p) level of theory that there may be some large-amplitude motion around the
also agreed with the experimental ones. Although it was con- C–C single bond, which is supported by the calculated vibra-
cluded that (E)-benzaldehyde oxime had the planar structure, tional frequency of the torsional motion around the C–C single
two rotational isomerisms of this molecule might be possible in bond, 78 cmϪ1 (HF/6-31G(d, p), no scaling).
respect to the rotation around the N–O bond, i.e., (E)-1 and
The molecular conformation of this compound in the solid
(E)-2 as shown in Fig. 1. To elucidate the orientation of the state is tetramer (2). Each molecule is slightly distorted with
hydroxyl group, the rs coordinates of the hydrogen atom the dihedral angle, ␾, of ca. 20°. This conformation may be due
bonded to the oxygen atom were calculated from the moments to an intermolecular hydrogen bond between the nitrogen and
of inertia, Ib and Ic, of normal and deuterated species by the the hydrogen atom of the hydroxyl group. However, in the gas
Kraitchman’s equations (8) on the condition of the planar phase, the molecular structure might become planar because of
molecular conformation (⌬I ϭ 0). The values of a- and no intermolecular interaction. This molecule may have the
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MICROWAVE SPECTRUM OF (E)-BENZALDEHYDE OXIME
289
TABLE 5
Comparison of r(C—C), r(C|N), and ЄCCN of (E)-Benzaldehyde Oxime
and Related Compoundsa
ACKNOWLEDGMENT
conjugation interaction between the phenyl ring and the oxime
moiety. The planar conformation of (E)-benzaldehyde oxime
We thank the Computer Center, Institute for Molecular Science, Okazaki
in the gas phase is consistent with that of N-benzylidenemeth- National Research Institutes, for the use of the NEC HSP and Gaussian 94
program (Revisions C.2 and E.2).
ylamine (C6H5—CH|NCH3) determined by gas-phase elec-
tron diffraction (10).
Table 4 shows the structural parameters of (E)-benzalde-
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