Generation, Microwave Spectrum, andAb InitioMO Calculation oftrans-1-Nitrosopropene, CH3CHCH - NO (synform)

Article abstract of DOI:10.1006/jmsp.1998.7774

trans-1-Nitrosopropene (synform) was generated in the gas phase by pyrolysis of 1-chloro-1-methyl-2-(hydroxyimino)ethane and identified by microwave spectroscopy. The microwave spectrum of the pyrolysate was observed in the frequency range from 8.0 to 40.

Full text of DOI:10.1006/jmsp.1998.7774

Journal of Molecular Spectroscopy 194, 79–86 (1999)
Article ID jmsp.1998.7774, available online at http://www.idealibrary.com on
Generation, Microwave Spectrum, and Ab Initio MO Calculation
of trans-1-Nitrosopropene, CH CH|CH—NO (syn form)
3
Takeshi Sakaizumi, Hideki Tanaka, Kouji Hirano, Nobuhiko Kuze, and Osamu Ohashi
Department of Chemistry, Faculty of Science and Technology, Sophia University, 7-1, Kioi-cho, Chiyoda-ku, Tokyo, Japan 102-8554
Received August 5, 1998; in revised form October 15, 1998
trans-1-Nitrosopropene (syn form) was generated in the gas phase by pyrolysis of 1-chloro-1-methyl-2-(hydroxyimino)eth-
ane and identified by microwave spectroscopy. The microwave spectrum of the pyrolysate was observed in the frequency range
from 8.0 to 40.0 GHz. The rotational constants (MHz) were determined as A ϭ 34 025(390), B ϭ 2315.62(2), and C ϭ
2
198.54(2) for CH CH|CH—NO (normal species) and A ϭ 34 012(530), B ϭ 2300.04(3), and C ϭ 2182.70(2) for
3
15 15
CH CH|CH— NO ( N isotopic species) in the ground vibrational state. The values of planar moment (P ϭ (I ϩ I
Ϫ I )/ 2) obtained for the normal and N isotopic species were found to be 1.62(10) and 1.52(13) uÅ , respectively. These
3
cc
a
b
1
5
2
c
values are almost the same within the limit of errors. This suggests that the nitrogen atom lies in or is close to the ab inertial
plane of the molecule and shows also that only two hydrogen atoms are located symmetrically out of the symmetry plane. The
pyrolysate was determined to be trans-1-nitrosopropene by comparing the observed and calculated rotational constants, ␬
(
Ray’s asymmetric parameter), and r coordinates of the nitrogen atom. One vibrationally excited state was observed and
s
Ϫ1
assigned to the C–N torsional mode (158(50) cm ). Interesting pyrolysates such as hydrogen cyanide and acetaldehyde were
also detected during the pyrolysis of the precursor. The lifetime of trans-1-nitrosopropene is found to be ca. 5 s in the
waveguide cell. © 1999 Academic Press
1
. INTRODUCTION
of XCHClCH|NOH 1 and its isotopic species, as shown in
Fig. 1 (X ϭ H (8, 9) and Cl (10)). The lifetime of
CH |CH—NO and ClCH|CH—NO are found to be ca. 30
Nitrosoalkenes possess a large synthetic potential since they
are comprised of both an alkene activated by an electron
withdrawing group of nitroso one and 1,3-diene system having
two double bonds of the C|C and N|O groups (1). Further-
more, the nitroso group itself reacts with a wide number of
reagents (2) and allows an easy and simultaneous incorporation
of the nitrogen and oxygen by the addition reactions (Diels–
Alder (3), ene-reaction (4), and (2 ϩ 2)-cycloaddition (5)).
However, in spite of the interesting nitrosoalkenes, isolated
nitrosocyclohexene was first reported only in 1976 (6).
2
and 60 s in the waveguide cell, respectively. The interesting
pyrolysates, such as XCH|O 5 and HCN 6 were detected
during its pyrolysis. The fact suggests that the trans–syn form
3
of XCH|CH—NO is first generated by isomerization of anti
form 2, while the by-products of XCH|O and HCN are
produced by cleavage of oxazete 4 generated by cyclization of
2
(8–10).
In this work, we attempted to generate and detect the unsta-
ble nitrosoalkenes by pyrolysis of 1-chloro-1-methyl-2-(hy-
droxyimino)ethane (X ϭ CH ). The observed pyrolysate was
Francotte et al. have prepared and characterized a series of
halonitrosoalkenes by treating the ␣-polyhalooximes with
NaHCO3 or K CO in dichloromethane (1). Only the ni-
3
determined to be trans-1-nitrosopropene (syn form) by com-
paring the observed and calculated rotational constants and rs
2
3
coordinates of the nitrogen atom. For the structural parameters,
trosoolefin of Cl C|C(CH )—NO could be isolated purely at
2
3
we discuss the C–N torsional mode of the pyrolysate and the
possible pyrolysis mechanism of the precursor by referring to
the results of ab initio MO calculation (MP2/6–31G**).
room temperature, whereas the others were stable in solution
for a few weeks and could be kept for months at Ϫ20°C.
Most of the transient nitrosoalkenes have been characterized
by in situ cycloadditions (vide infrared spectrum) (1). Their
instability can be explained by the ready decomposition of an
intermediate oxazete (four-membered ring molecule) produced
by intramolecular cyclization (1). This mechanism is supported
2. EXPERIMENTAL DETAILS
trans-1-Nitrosopropene was produced by pyrolysis of
by the fact that 4,4Ј-di-tert-butyl-4H-1,2-oxazete is isolated by 1-chloro-1-methyl-2-(hydroxyimino)ethane and also generated
thermolysis of the 2-tert-butyl-3,3-dimethyl-1-nitrosobutene by the chemical reaction of the precursor with potassium
(7).
hydroxide at room temperature using the vacuum solid–gas
Recently, we have succeeded in detecting by mass spectrom- reaction (VSGR) method (10).
etry and microwave spectroscopy the unstable nitrosoalkenes
Although the microwave spectral lines of the chemical re-
of trans-XCH|CH—NO (syn form) 3 generated by pyrolysis action product with KOH were stronger than those of the
7
9
0
022-2852/99 $30.00
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8
0
SAKAIZUMI ET AL.
electric furnace (max. 1500°C). The pyrolysis temperature was
monitored with a thermocouple fitted to the external wall of the
quartz tube and controlled by Digital Temperature Controller
(Asashi Rika Co.). The quartz tube was connected to an inlet
port of the ionic chamber of the mass filter or to an inlet port
on the side of Stark waveguide cell (3-m X-band waveguide).
To observe the microwave spectrum of 1-nitrosopropene, it
was necessary to use the high-speed rotary pump (350 l/min)
because its lifetime was very short.
The quadrupole mass spectrometer employed was Anelva
Ϫ6
AQA-360 type and the sample pressure (5 ϫ 10 Torr; 1
Torr ϭ 133.32 Pa) of the ionic chamber was adjusted with the
needle valve. A conventional microwave spectrometer was
employed with 100-kHz square-wave Stark modulation. The
microwave spectrum was observed in the frequency range from
8
.0 to 40.0 GHz. The three microwave sources employed were
a signal generator (HP-8672A) and two phase-locked YIG-
tuned GaAs oscillators (WJ 5600-301DF and 5610-302DF).
FIG. 1. Possible pyrolysis mechanism of XCHClCH|NOH (X ϭ H, Cl,
and CH ). The molecules of 3, 5, and 6 were identified by microwave
spectroscopy.
3
3
. AB INITIO MO CALCULATION
The GAUSSIAN 94 program (12) was employed in the ab
pyrolysate, their lines disappeared after ca. 10 min from the initio SCF calculations of the molecules. Molecular geometry
start of the reaction because the activation of KOH was lost. optimizations were performed by using second-order Møller–
Therefore, we used the pyrolysis method for observing the Plesset (MP2) perturbation theory (13) and 6-31G** basis set
microwave spectrum of this unstable molecule.
(14), and the vibrational frequencies were calculated at the
The precursor was prepared using hydroxylamine hydro- MP2/6-31G** level of the theory.
chloride and 2-chloropropionaldehyde according to the method
The transition states produced in the pyrolysis mechanism
used for the preparation of 1,1-dichloro-2-(hydroxyimino)eth- were confirmed by the identification of only one negative
ane (10). The aldehyde employed was generated by thermol- eigenvalue obtained at the MP2/6-31G** level of theory. It
ysis of its trimer, which was donated by Wakasugi (Kureha was performed to verify that two stable points connect to the
Chemical Industrial Co.). The precursor was purified under transition state in the intrinsic reaction coordinate (IRC) cal-
vacuum distillation at 78°C/11 mmHg. Methanol used in prep- culations (15).
aration was furthermore separated by repeating a trap-to-trap
distillation in vacuo. The normal precursor was identified by
observing bp (78–79°C/11 mmHg) (11) and H-NMR and
4. RESULTS AND DISCUSSION
1
mass spectra.
The microwave spectrum with the characteristic vibrational
1
5
The N isotopic precursor was also synthesized using the satellite of the pyrolysate observed at 485°C showed three
mixture of hydroxyl-amine hydrochloride (2 g) and its
isotopic compound (99 atom% N: 2 g, Matheson USA Co.), 26.5 and 40.0 GHz at 40 V/cm. They were readily assigned to
1
5
N
equidistantly spaced groups (B ϩ C Ӎ 4480 MHz) between
1
5
according to the method used for producing normal precursor. a-type R-branch transitions (K structure) with J ϭ 6 4 5 to
1
5
The mass spectrum of the N isotopic precursor showed the J ϭ 8 4 7. Furthermore, the observed microwave spectrum
new ionic peaks of 108 and 110 m/z, in addition to those of showed some interesting pyrolysates, such as acetaldehyde
3
5
37
1
07 ( Cl) and 109 ( Cl) m/z observed for normal precursor. (16) and hydrogen cyanide (17). The microwave spectrum of
The mass spectrum of the pyrolysates of the normal precur- the chemical reaction product of the precursor with KOH at
3
5
37
sor showed 27 (HCN), 36 (H Cl), 38 (H Cl), 44 (C H O), room temperature was just the same as that of the pyrolysate.
2
4
1
5
and 71 (C H NO) m/z. The ionic peak of 71 m/z showed a The microwave spectrum of the pyrolysate of the N isotopic
3
5
maximum intensity at 485°C, while those of 27 and 44 m/z precursor was observed lower at the frequency side than that of
increased as the pyrolysis temperature increases.
K structure of the normal species by ca. 250 MHz.
The mass spectrum of the pyrolysates (485°C) of the mix-
The spectral lines of KϪ1 ϭ 0 and 1 were assigned by
1
5
ture of the normal and N isotopic precursor showed the new observing their Stark behavior, while the KϪ1 ϭ 2 lines
1
5
15
ionic peaks of 28 (HC N) and 72 (C H NO) m/z.
showed broadening. The higher KϪ1 lines (KϪ1 Ͼ 2) were
3
5
The pyrolysis apparatus consisted of an unpacked horizontal buried in K structure and could not be observed. The a-type
1
5
quartz tube (300-mm length; 5-mm i.d.) heated in a handmade R-branch transitions of the normal and N isotopic species in
Copyright © 1999 by Academ ic Press

MICROWAVE SPECTRUM OF trans-1-NITROSOPROPENE
81
TABLE 1
Observed Rotational Transition Frequencies (MHz) of trans-CH CH|CH—NO
3
1
5
(
syn form) and Its N Isotopic Species in the Ground and Excited Vibrational
States
a
JЈ(KЈϪ1, KЈϩ1) 4 JЉ(KЉϪ1, KЉϩ1).
b
c
d
␯obsd Ϫ ␯calcd
.
Overlapped with the absorption line of acetaldehyde.
Not used in the fitting.
the ground and excited vibrational states were assigned as assigned because of the disturbance of the dense spectral lines
shown in Table 1. The observed spectral lines were fitted using of the A–E splitting due to methyl group and/or excited vibra-
Watson’s A-reduced Hamiltonian (18). Table 2 gives the mo- tional states of acetaldehyde (16).
lecular constants obtained from the least-squares fit of the lines
shown in Table 1.
The values of the planar moment (P ) were found to be
cc
.62(10) and 1.52(13) uÅ for the normal and N isotopic
2
15
1
The b-type Q-branch transitions of J1,JϪ1 4 J0,J series
were predicted in the frequency range of 31–40 GHz from the
obtained rotational constants, but these transitions could not be
species in the ground vibrational state, respectively. These
values are almost the same within the limit of errors. This fact
suggests that the nitrogen atom lies in or is close to the ab
inertial plane of the molecule. The value of ⌬I (ϭI Ϫ I Ϫ
c
a
2
I ϭ Ϫ3.23(18) uÅ ) of the normal species shows also that
TABLE 2
b
only two hydrogen atoms of the methyl group are located
stants, Planar Moments (u Å ), and C–N Torsional Frequency symmetrically out of the molecular symmetry plane. It was
Observed Rotational (MHz), Centrifugal Distortion (kHz) Con-
2
؊
1
(
␻/cm
)
of trans-CH CH|CHNO (syn form) and trans- found that this molecule has C symmetry and is a near-prolate
3
s
1
5
CH CH|CH NO (syn form) in the Ground and Excited Vibra-
3
symmetric top one (␬ ϭ Ϫ0.993).
tional States
To identify the pyrolysate observed, we assumed four pos-
sible pyrolysates (2, 3, 2؅, and 3؅ shown in Fig. 1), supported
by mass peak of 71 m/z (C H NO), having the methyl group
3
5
and discussion under Pyrolysis Mechanism. The candidates of
, 3, and 3؅, as shown in Table 3, were found to be the
2
near-prolate symmetric top molecules, but 2؅ was not because
of ␬ ϭ Ϫ0.247. Therefore, 2؅ was omitted from their candi-
dates. Table 3 gives the observed and calculated rotational
constants ( A, B, C), B ϩ C, ␬, the coordinates of the nitrogen
atom, and the relative energy differences. Assuming this mol-
a
Figures in parentheses are 2.5 times the standard deviation.
b
ecule to have C symmetry, the substituted coordinates of the
Pcc ϭ (Ia ϩ Ib Ϫ I )/ 2.
s
c
c
⌬
I ϭ Ic Ϫ Ia Ϫ Ib.
nitrogen atom were calculated from Kraitchman’s equation
Copyright © 1999 by Academ ic Press

8
2
SAKAIZUMI ET AL.
TABLE 3
Observed and Calculated Rotational Constants (MHz), r Coor-
5. EXCITED VIBRATIONAL STATE
s
dinates (Å) of Nitrogen Atom, and Relative Energies (kcal/mol) of
Some of the observed spectral lines were assigned to two
trans-CH CH|CHNO (syn form) and Its Isomers
excited vibrational states (V and V ). The vibrational satel-
3
a
b
lites of V were observed at the higher frequency side than the
a
line in the ground vibrational state by an interval of about
6
0–70 MHz for the J ϭ 7 4 6 to 8 4 7 transitions, while
the V were observed in the neighborhood of the line in the
b
ground vibrational state. Table 1 gives the spectral lines as-
signed for V , while those of V could not be completely
a
b
assigned. The vibrational frequencies of the V and V states
a
Ϫ1
b
were estimated to be 158(30) and 170(30) cm , respectively,
from the relative intensity measurements. The ⌬I value ob-
2
tained for Va was found to be Ϫ4.05(30) uÅ . This value
suggests that the vibration of the V state is of an out-of-plane
a
mode because the absolute value of ⌬I is larger than one of
2
Ϫ3.23(18) uÅ obtained for the ground vibrational state (21).
a
ab initio MO calculation at the MP2/6-31G** level.
Calculated by using the structural parameters of CH CH|CH and
b
The V state corresponding to the vibrational frequency of 158
cm can be ascribed to the excited state due to the C–N
a
3
Ϫ1
CH |CH–NO.
2
c
Figures in parentheses are 2.5 times the standard deviation.
c
␬
ϭ (2B–A–C)/(A–C).
d
Calculated from Kraitchman’s equation by using I and I (⌬I ϭ Ϫ3.12 u
TABLE 4
b
c
2
Å ).
a
Assumed and Fitted Structural Parameters of trans-
CH CH|CHNO (syn form) and Reproduced Rotational
3
Constants (MHz)
(
19) by using the four rotational constants of B and C of the
1
5
normal and N isotopic species and by assuming the value of
⌬
2
I (ϭI Ϫ I Ϫ I ) to be Ϫ3.12 uÅ . The observed values
c a b
were in good agreement with those of 3(trans-1-nitrosopropene
syn form)) predicted by ab initio MO calculation and were in
excellent agreement with those of 3 calculated by referring the
structural parameters of CH CH|CH (20) and
(
3
2
CH |CH—NO (9). The pyrolysate was determined to be
2
trans-1-nitrosopropene 3. This is consistent with the results of
ab initio MO calculation (MP2/6-31G**) that 3 is the most
stable among three candidates, as shown in Table 3. All of the
conformation for XCH|CH—NO generated by pyrolysis of
XCHClCH|NOH (X ϭ H, Cl and CH ) were found to be only
3
trans–syn form.
The barrier height (V ) due to internal rotation of the methyl
3
group of trans-1-nitrosopropene was obtained to be 1730 cal/
mol by ab initio MO calculation (MP2/6-31G**). The A–E
splitting was predicted to be Ϫ7.4 MHz for 212 4 111 transi-
tion, 7.4 MHz for 211 4 1 , Ϫ2.1 MHz for 3 4 2 , and 2.1
10
13
12
MHz for 312 4 2 . The two transitions of J ϭ 2 4 1 were
11
not observed because of the weak spectral lines. The spectral
lines of J ϭ 3 4 2 were observable; however, the splittings
of A- and E-levels were not observed because of very small
A–E splittings. The fact indicates that the V3 of trans-1-
nitrosopropene may be higher than 1730 cal/mol.
The microwave spectrum of 3؅, the isomer of 3, could not be
assigned because of the dense spectral lines due to the A–E
splitting caused by internal rotation of the methyl group and/or
excited vibrational states of acetaldehyde (16).
Copyright © 1999 by Academ ic Press

MICROWAVE SPECTRUM OF trans-1-NITROSOPROPENE
83
TABLE 5
a
Comparison of the Structural Parameters of trans-XCH|CHNO (syn form) and Those of XCH|CH
2
(
X ؍ CH , H, and Cl) in the Heavy Atoms Skeleton Studied by Ab Initio MO Calculation and Microwave
3
Spectroscopy
a
Bond lengths in Å and angles in degrees.
b
c
d
e
f
This work.
Used the structural parameters, improved those shown in Ref. (9).
Ref. (10).
Ref. (20).
Ref. (25).
Ref. (24).
Used MP2/6-31G** level.
The square brackets indicate the assumed parameters.
g
h
i
torsional mode because the variation of the rotational constants soethene (9). The four rotational constants calculated from
corresponding to the C–N torsional mode with the vibrational these parameters listed in Table 4 were in good agreement with
quantum number is much larger than that corresponding to the those observed within 0.01 MHz. The structural parameters of
C–C torsional mode. The result of ab initio MO calculation both Set I and Set II were consistent with the optimized
(
RHF/6-31G**) showed the C–N and C–C torsional frequen- structural parameters obtained by the ab initio MO calculation
Ϫ1
cies to be 143 and 216 cm , respectively. The vibrational (MP2/6-31G**) as shown in Table 5, while the assumed bond
frequency of 158(30) cm obtained from trans-1-nitrosopro- lengths of r(C—C) and r(N|O) were different from the
Ϫ1
pene (syn form) is very close to the C–N torsional frequencies theoretical values by ca. 0.01 and 0.03 Å, respectively.
Ϫ1
of trans-nitrosoethene (150(20) cm ) (8), trans-1-chloro-2-
The fitted structural parameters of Set I and Set II were
Ϫ1
nitrosoethene (120(30) cm ) (10), and trans-nitrosocyclopro- compared with those of the similar molecules, as shown in
pane (140 cm ) (22) within the errors. Therefore, we assigned Table 5. The obtained r(C|C) is longer than that of propylene
Ϫ1
the V state to the C–N torsional mode. On the other hand, the (20) because the nitroso group behaves as the electron with-
a
V state might be assigned tentatively to the C–C torsional drawing one. The obtained r(C—N) is slightly shorter than that
b
mode.
of trans-nitrosoethene (9) because the methyl and nitroso
groups take trans position, so-called trans effect.
The obtained ЄC–N–O is almost the same as that of trans-
nitrosoethene (9) and is consistent with the result calculated by ab
6
. MOLECULAR STRUCTURE
The partial r structural parameters of trans-1-nitrosopro- initio MO calculation. The obtained ЄC–C–C is almost the same
0
pene (syn form) were obtained by fitting three parameters (Set as that of propylene (20). The order of ЄX—C—C of trans-
I and Set II) to four rotational constants of B and C of the XCH|CH—NO (syn form) is found as follows: X ϭ CH Ͼ
3
1
5
normal and N isotopic species. Set I was made up of two Cl Ͼ H. Since the van der Waals radius of the methyl group (2.0
bond lengths of r(C|C) and r(C—N) and an angle of ЄC— Å) is larger than those of the chlorine atom (1.8 Å) and hydrogen
C—C, while Set II consisted of their two bond lengths and an atom (1.2 Å), and the bond length of r(C–CH ) is shorter than that
3
angle of ЄC—N—O. On the replacement of the hydrogen of r(C–Cl), the repulsion between the methyl group and the
atom of trans-nitrosoethene by the methyl group, their two hydrogen atom in cis position is, therefore, found to be the largest
bond lengths and the angle of ЄC—C—C may be expected to among the X groups. This trend is almost the same as that of
change by the effect of the substituted group, while the angle XCH|CH (X ϭ CH Ͼ Cl (24) Ͼ H (25)).
2
3
of ЄC—N—O may be slightly changed.
Table 4 gives the assumed and fitted structural parameters
and the observed and reproduced rotational constants, respec-
tively. The assumed structural parameters were quoted from
7. PYROLYSIS MECHANISM
The pyrolysates of 1-chloro-1-methyl-2-(hydroxyimino)eth-
1
5
those of propylene (20), nitrosomethane (23), and trans-nitro- ane and its N precursor were determined to be trans-1-
Copyright © 1999 by Academ ic Press

8
4
SAKAIZUMI ET AL.
FIG. 2. Relative energy differences between the two isomers of the pyrolysates of XCHClCH|NOH. Calculated for X ϭ H and CH by MP2/6-31G**
3
Ϫ1
level without consideration of the zero-point vibrational effect and for X ϭ Cl by MP2/6-311G** level (26). (1 cm ϭ 11.96 J/mol, 1 kcal/mol ϭ 4.184
kJ/mol).
1
5
15
nitrosopropene 3, trans-1-nitrosopropene N, acetaldehyde 5, microwave spectra of isotopic species (HC N) and CH |O
2
1
5
15
hydrogen cyanide 6, and hydrogen cyanide N. The spectral generated by pyrolysis of CH CHClCH| NOH were ob-
3
lines of 3 appeared at even lower pyrolysis temperature served. These facts suggest that they are generated by cleavage
(
350°C), while those of 5 and 6 appeared at higher pyrolysis of the four-membered ring produced by intramolecular cycliza-
1
5
temperature.
tion of CH CH|CH— N|O, 2. The pyrolysis mechanism
3
Although the two possible syn forms, 3 and 3؅, were, re- of X ϭ CH of XCHClCH|NOH was consistent with those of
3
spectively, predicted by isomerization of 2 and 2؅ generated by X ϭ H (8, 9) and Cl (10), as shown in Fig. 1.
the pyrolysis of CH CHClCH|NOH, as shown in Fig. 1, only
As shown in Fig. 2, the four-membered ring molecules of
3
3
was detected by microwave spectroscopy.
X ϭ H and Cl are slightly less stable than the corresponding
Figure 2 shows the result of their relative energy differences XCH|CH—NO (trans–syn form), while the two four-mem-
4: 0, 3: 1.17, 3؅: 2.08, 2: 5.07, 2؅: 6.22 kcal/mol [1 cal/mol ϭ bered ring isomers of X ϭ CH are slightly more stable than
(
3
Ϫ1
4
.184 J/mol ϭ 0.3498 cm ]) obtained from ab initio MO XCH|CH—NO (trans–syn form) and CH |CX—NO (syn
2
calculation (MP2/6-31G**) without consideration of the zero- form), respectively. Figure 2 shows the relative energy
point vibrational effect. The reason why 3 was generated by the differences between two isomers of the possible pyrolysates
pyrolysis of the precursor is that 3 may be the most stable of XCHClCH|NOH (X ϭ H, Cl, and CH ) calculated by
3
among possible pyrolysates 2, 2؅, and 3؅, except for 4. The 3 MP2/6-31G** level for X ϭ H and CH and by MP2/6-
3
may be generated by the isomerization of the unstable mole- 311G** level for X ϭ Cl (26). These indicate that the two
cule 2, while 3؅ could not be detected because it was less stable four-membered ring molecules substituted by the methyl
than 3 by 0.91 kcal/mol and 2؅ is also less stable than 2 by 1.15 group become slightly more stable than the nitroso mole-
kcal/mol.
On the other hand, 5 and 6 may be produced by another
cule.
This result of ab initio MO calculation suggests that it is
passway via intermediate, 4, from 2, as shown in Fig. 1. The possible to detect 4 (X ϭ CH ) by microwave spectroscopy.
3
Copyright © 1999 by Academ ic Press

MICROWAVE SPECTRUM OF trans-1-NITROSOPROPENE
85
FIG. 3. Potential energy surfaces for the pyrolysis of CH CHClCH|NOH from 2 to trans-CH CH|CHNO (syn form), CH |O, and HCN. Calculated by
3
3
3
MP2/6-31G** level of theory, taking account of the zero-point vibrational effect.
8
. REACTION ENERGY PROFILE
cyclization of 2 of X ϭ CH is smaller than that of 2 of X ϭ
3
H (49.96 kcal/mol (28)), but this is definitely large in compar-
ison with the usual thermal activation energies of ϳ12–24
kcal/mol of symmetry-allowed reactions, such as conrotatory
ring closure (29).
It is interesting to know how to cleave the four-membered
ring 4 of X ϭ CH3 in the comparison of the optimized
Figure 3 shows the potential energy surfaces for the pyro-
lysates of 1-chloro-1-methyl-2-(hydroxyimino)ethane such as
trans-1-nitrosopropene (anti form) 2, trans-1-nitrosopropene
(syn form) 3, 4-methyl-4H-1,2-oxazete 4, acetaldehyde 5, hy-
drogen cyanide 6, and the transition states (TS1, TS2, and
TS3), referring to the values for the total and zero-point ener-
gies for the products and reactants calculated at the MP2/6-
structures (27) of 4 and TS3. The four-membered ring (ϾC —
2
O—N|C —H) of 4 is almost planar because the two dihedral
1
3
1G** level of theory (27). The TS1 corresponds to the
angles of ЄC C NO and ЄOC C N are 0.3° and Ϫ0.3°, re-
2
1
2 1
isomerization of 2 to form 3, the TS2 to the ring closure of 2
to form 4, and the TS3 to the cleavage of 4 to form 5 and 6.
The TS1 is found to lie 6.44 kcal/mol above 2, as shown in
Fig. 3. On the other hand, the TS2 is found to lie 30.14
kcal/mol above 2, and TS3 to lie 59.76 kcal/mol above 4. The
spectively, and the two bond lengths of r(C —C ) and
r(O—N) are 1.485 and 1.452 Å, respectively. On the other
hand, the ring structure of TS3 is distorted and twisted
2
1
(
ЄC C NO: 19.9° and ЄOC C N: Ϫ30.4°, r(C –C ): 1.719
2 1 2 1 2 1
and r(O–N): 2.084 Å). The two dihedral angle and bond
lengths calculated for TS3 become much larger and longer than
those of 4. These results suggest that the four-membered ring
twists and the two bonds of C –C and O–N of TS3 lengthen
calculated energy difference of X ϭ CH (29.62 kcal/mol)
3
between the activation energy of the first reaction (from 2 to 4)
and that of the second reaction (from 4 to 5 and 6) is larger than
that of X ϭ H (23.91 kcal/mol) (28). The larger activation
energy is similar to the experimental results of Weiser and
Berndt (7). They reported that 4,4-di-tert-butyl-4H-1,2-oxazete
2
1
and are cleaved to form acetaldehyde and hydrogen cyanide.
(four-membered ring molecule) was generated by vacuum py-
ACKNOWLEDGMENTS
rolysis of 2-tert-butyl-3,3-dimethyl-1-nitrosobutene at 220°C,
while it was necessary to increase the pyrolysis temperature to
above 240°C to obtain di-tert-butylketone and hydrogen cya-
nide.
We thank Dr. Takashi Wakasugi, Kureha Chemical Industrial Co., Ltd., for
donating the sample of 2-chloropropionaldehyde (trimer). We also thank the
Computer Center of the Institute for Molecular Science for the use of the NEC
HSP Computer and the Library Program Gaussian 94, Revision E.2 (IMS
The activation energy of 30.14 kcal/mol calculated for the version).
Copyright © 1999 by Academ ic Press

8
6
SAKAIZUMI ET AL.
REFERENCES
Pople, GAUSSIAN 94, Revision B.2 and Revision E.2, Gaussian, Inc.,
Pittsburgh, PA, 1995.
. E. Francotte, R. Merenyi, B. Vandenbulcke, and H.-G. Viehe, Helv. Chim. 13. C. Møller, M. S. Plesset, Phys. Rev. 46, 618–622 (1934).
Acta 62, 1208–1218 (1981).
14. P. C. Hariharan, and J. A. Pople, Theor. Chim. Acta 28, 213–222 (1973).
. J. H. Boyer, in “The Chemistry of the Nitro and Nitroso Groups” (H. 15. C. Gonzalez and H. B. Schlegel, J. Chem. Phys. 90, 2154–2161 (1989); J.
Feuer, Ed.), Chap. 5, Inters. Publ., New York, 1969.
Phys. Chem. 94, 5523–5527 (1990).
. J. Hamer and M. Ahmad, in “1,4-Cycloaddition Reactions” Vol. 8, pp. 16. R. W. Kilb, C. C. Lin, and E. B. Wilson, Jr., J. Chem. Phys. 26, 1695–1703
1
2
3
4
19–452, Academic Press, New York, 1967.
(1957).
4
5
. G. T. Knight and B. Pepper, Tetrahedron 27, 6201–6208 (1971).
17. J. F. Westerkamp, Phys. Rew. 93, 716 (1954).
. L. L. Muller and J. Hamer, in “1,2-Cycloaddition Reactions” pp. 25–275, 18. J. K. G. Watson, in “Vibrational Spectra and Structure” (J. R. Durig, Ed.),
Inters. Publ., New York, 1967.
Vol. 6, pp. 33–35, Elsevier, Amsterdam, 1977.
6
. P. Ciattoni and L. Rivolta, Chim. Ind. (Milan) 49, 1186 (1967).
19. J. Kraitchman, Am. J. Phys. 21, 17–24 (1953).
7
. K. Weiser and A. Berndt, Angew. Chem. Int. Ed. Engl. 14, 70–71 20. D. R. Lide and D. Christensen, J. Chem. Phys. 35, 1374–1378 (1961); E.
1975).
Hirota and Y. Morino, J. Chem. Phys. 45, 2326–2327 (1966).
. T. Sakaizumi, A. Usami, H. Satoh, O. Ohashi, and I. Yamaguchi, J. Mol. 21. D. R. Herschbach and V. W. Laurie, J. Chem. Phys. 40, 3142–3153
Spectrosc. 164, 536–549 (1994).
(1964).
. T. Sakaizumi, M. Nishikawa, and O. Ohashi, J. Mol. Spectrosc. 171, 22. M. J. Corkill, A. P. Cox, and J. A. Norris, J. Chem. Soc. Chem. Commun.
18–524 (1995).
388–389 (1978).
0. T. Sakaizumi, M. Nishikawa, and O. Ohashi, J. Mol. Spectrosc. 178, 23. D. Coffey, C. O. Britt, and J. E. Boggs, J. Chem. Phys. 49, 591–600
13–124 (1996).
(1968).
1. A. Dornow, H. D. Jordan, and A. Muller, Chem. Ber. 94, 67–76 24. M. Hayashi and T. Inagusa, J. Mol. Struct. 220, 103–117 (1990).
(
8
9
5
1
1
1
1
(
1961).
25. E. Hirota, Y. Endo, S. Saito, K. Yoshida, I. Yamaguchi, and K. Machida,
J. Mol. Spectrosc. 89, 223–231 (1981).
2. M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson,
M. A. Robb, J. R. Cheeseman, T. A. Keith, G. A. Petersson, J. A. 26. J. M. Ugalde, private communication.
Montgomery, M. A. AlLahan, V. G. Zakrzewski, J. V. Oritz, J. B. 27. T. Sakaizumi, N. Kuze, H. Tanaka, and O. Ohashi, J. Mol. Struct. (Theo-
Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challa-
chem), to be published.
combe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, 28. J. M. Ugalde, J. Mol. Struct. (Theochem), 258, 167–174 (1992).
E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J.
Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez, J. A.
29. R. B. Woodward and R. Hoffmann, “The Conservation of Orbital Sym-
metry,” Academic Press, New York, 1970.
Copyright © 1999 by Academ ic Press