UNIMOLECULAR DISSOCIATIONS OF HCO, HNO AND HO2 - FROM REGULAR TO IRREGULAR DYNAMICS

We discuss the unimolecular dissociations of three triatomic molecules in their ground electronic states, HXO((X) over tilde) --> H + XO(n, j) with X = C, N and O, using quantum mechanical methods, classical tr ajectories and statistical theories. The calculations for HCO and HNO employ new nb initio potential-energy surfaces and the investigations for HO2 are performed on the DMBE IV potential. Our study focuses on t wo issues: the differences in the dissociation rates for the three sys tems and how they can be understood in terms of differences in the pot ential-energy surfaces and, secondly, on the reliability of statistica l theories (RRKM and SACM) in predicting the average rate. The interna l vibrational motion of HCO is mostly regular, even at energies much g reater than the threshold, which leads to a pronounced mode-specificit y in the dissociation rates; the RRKM rate is found to be an upper lim it for the quantum-mechanical rates with the average being overestimat ed by roughly a factor of five. At the other extreme, the dynamics of HO2 are essentially irregular, as confirmed by inspection of the wavef unctions and analyses of the energy spectrum of the bound states; as a consequence the fragmentation rates of HO2 are, on the average, well described by statistical theories. HNO is a mixed case showing both re gular and irregular motion; the RRKM rate is only a factor of two larg er than the quantum-mechanical average. HCO has a potential with a cle ar barrier at intermediate fragment separations and so exemplifies a s ystem with a tight transition state. The HNO potential, on the other h and, has a purely attractive exit channel and illustrates a system wit h a loose transition state. Although the potential for HO2 has no barr ier, the rapid change of the anisotropy between the inner and the oute r regions leads to pronounced dynamical barriers so that this system, too, belongs to the class of molecules with a tight transition state.