COMPARISON OF QUANTUM AND SEMICLASSICAL VARIATIONAL TRANSITION-STATE MODELS FOR THE HO2-]H-2 MICROCANONICAL RATE-CONSTANT(O)

Citation
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
Citations number
79
Categorie Soggetti
Physics, Atomic, Molecular & Chemical
ISSN journal
00219606
Volume
103
Issue
20
Year of publication
1995
Pages
8891 - 8900
Database
ISI
SICI code
0021-9606(1995)103:20<8891:COQASV>2.0.ZU;2-C
Abstract
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.