Ky. Song et al., COMPARISON OF QUANTUM AND SEMICLASSICAL VARIATIONAL TRANSITION-STATE MODELS FOR THE HO2-]H-2 MICROCANONICAL RATE-CONSTANT(O), The Journal of chemical physics, 103(20), 1995, pp. 8891-8900
Comparisons, for J=0, are made between HO2-->H+O-2 reaction path anhar
monic energy levels, variational transition states, and unimolecular r
ate constants determined by three different semiclassical models and a
quantum mechanical model. The semiclassical models are based on the r
eaction path Hamiltonian. However, to determine anharmonic energy leve
ls, the harmonic potential of this Hamiltonian is replaced by the actu
al anharmonic DMBE IV potential for the HO2 system. Two of the semicla
ssical models use Einstein-Brillouin-Keller (EBK) quantization to dete
rmine energy levels for motion orthogonal to the reaction path; i.e.,
one model neglects anharmonic coupling between modes, while the other
retains all the coupling. The third semiclassical model is based on a
quartic expansion of the potential and second-order perturbation theor
y to determine the energy levels. A comparison of the results of these
three semiclassical models shows that anharmonic coupling between mod
es orthogonal to the reaction path is unimportant for HO2 dissociation
. The separable EBK model gives a RRKM rate constant versus energy in
very good agreement with that obtained from a quantum mechanical calcu
lation which retains full coupling between modes in determining the re
action path energy levels. If anharmonicity is treated, the reaction p
ath Hamiltonian and its vibrator transition state give accurate RRKM r
ate constants for HO2 dissociation. Rate constants calculated with the
flexible transition states model are in very good agreement with thos
e of the semiclassical and quantum vibrator transition state models, i
f the O-2 stretch conserved mode is treated as an anharmonic oscillato
r in the flexible model. However, in contrast to the vibrator transiti
on state models, ''steps'' are not observed in the rate constants for
the flexible model, since the transitional mode is treated classically
. Harmonic and anharmonic rate constants are compared for both the vib
rator and flexible transition state models. (C) 1995 American Institut
e of Physics.