Unimolecular dissociations of HCO, HNO and HO2: from regular to irregular dynamics

Abstract

We discuss the unimolecular dissociations of three triatomic molecules in their ground electronic states, HXO(X)→ H + XO(n, j) with X = C, N and O, using quantum mechanical methods, classical trajectories and statistical theories. The calculations for HCO and HNO employ new ab initio potential-energy surfaces and the investigations for HO₂ are performed on the DMBE IV potential. Our study focuses on two issues: the differences in the dissociation rates for the three systems and how they can be understood in terms of differences in the potential-energy surfaces and, secondly, on the reliability of statistical theories (RRKM and SACM) in predicting the average rate. The internal vibrational motion of HCO is mostly regular, even at energies much greater than the threshold, which leads to a pronounced mode-specificity in the dissociation rates; the RRKM rate is found to be an upper limit for the quantum-mechanical rates with the average being overestimated by roughly a factor of five. At the other extreme, the dynamics of HO₂ are essentially irregular, as confirmed by inspection of the wavefunctions 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 regular and irregular motion; the RRKM rate is only a factor of two larger than the quantum-mechanical average. HCO has a potential with a clear barrier at intermediate fragment separations and so exemplifies a system with a tight transition state. The HNO potential, on the other hand, has a purely attractive exit channel and illustrates a system with a loose transition state. Although the potential for HO₂ has no barrier, the rapid change of the anisotropy between the inner and the outer regions leads to pronounced dynamical barriers so that this system, too, belongs to the class of molecules with a tight transition state.