186
ZHANG, YU, AND BAUER
X
и
и
и
X
и
X
H2C"CH2 ϩ H2C"X
X
X
Scheme I
Since the first step in the pyrolysis of the ring gener-
ates H2C ϭ CH2 and H2C ϭ X, we focused on estab-
lishing the fate of the H2C ϭ X species, to determine
whether these products, at about 1000 K, remain intact
or fragment to produce reactive radicals that could at-
tack the parent compound or the ethylene. In Appendix
B we summarized published experimental magnitudes
and recently derived ab-initio estimates (via CBS-4).
It appears that the activation energies for fragmenta-
tion of H2C ϭ X range from ca. 80 to 90 kcal/mol; for
AZ only a rough estimate is available. The derived
much higher barrier. A recent calculation at the CBS-
4 level [8] for the enthalpy difference between the
ground state conformation of AZ and the planar tran-
sition structure {for the heavy atoms and H(N)} found
5.57 kcal/mol. The low-frequency IR and Raman
spectra were accounted for by an unsymmetrical po-
tential well for the flapping motion of the ring atoms,
with no second well for an axial H(N) [9]. The shoul-
der in the potential energy function appears between
the flapping vibrational levels 3 and 4. Indeed, the
CBS-4 calculation [8] for a structure initially con-
strained near the axial position, at an excitation of
1.65 kcal, showed no local minimum. However, later
microwave spectra suggest that there may be a very
shallow minimum at the location of the shulder [10].
The vacuum UV absorption spectrum of AZ was
recorded by Clark and Pickett [11]. Also, Clary and
Henshaw [12(a)] suggested, on the basis of a theoret-
ical analysis, that inversion at the N atom in AZ would
be induced by intense laser radiation. Electron impact
spectra of azetidine were recorded by Gallegor and
Kiser [12(b)].
1
kuni at 1000 K is ca. 4 ϫ 10Ϫ5 sϪ , so that the corre-
sponding half-times are several orders of magnitude
longer than the residence time in our reactor. Volkova
et al. [1(b)] reported that H2C ϭ NH did decompose
in
a
low pressure pyrolysis unit. (ca. 3 ϫ
10Ϫ3 torr, 400–700ЊC).
COMMENTS ON AZETIDINE
The gas-phase structure was determined by electron
diffraction and microwave spectroscopy [2]. The
equatorial position of H(N) was confirmed as the dom-
inate conformation. In the gas phase the interatomic
distances in the ring of AZ differ somewhat from those
in the crystalline solid and particularly from solid tri-
nitro-azetidine. So does the flap angle [3]. The vibra-
tional frequencies were remeasured, and with suitable
scaling were reproduced by ab-initio calculations at
the 6–31G* level [4]. The inversion rate at the N atom
was estimated from temperature-dependent NMR line-
SHOCK-TUBE EXPERIMENTS
The shock tube is of stainless steel with a 1 inch inner
diameter. The lengths of the driver and driven sections
are 120 cm and 170 cm, respectively. A damping tank
is attached to the driven section, next to the diaphragm
holder. Shock waves were generated by increasing the
pressure of the He driver until the mylar diaphragm
broke. Typical pressures on the driver and driven sides
are 80 psig and 300–400 torr, respectively. Two
piezo-electric pressure sensors are stationed 10 cm
apart at the end of the driven section. Their summed
signal was recorded and digitized through Biomation
8100 and Northern Tracor Signal Analyzer, and then
Þ
Þ
width measurement; ⌬H ϭ 5.0 kcal/mol and ⌬S ϭ
Ϫ10.8 eu [5]. Ab-initio calculations by these investi-
gators, at the MP3 level indicated a barrier height of
6.48 kcal. (At the same level, the derived inversion
barrier for ammonia was found to be 5.33 kcal, com-
pared with the microwave value of 5.93 [6]). An ear-
lier theoretical analysis [7] using STO-3G led to a