THE RATES OF HCL LOSS FROM ENERGY-SELECTED ETHYLCHLORIDE IONS - A CASE OF TUNNELING THROUGH AN H-ATOM TRANSFER BARRIER

The dissociation rates of energy-selected ethylchloride and deuterated ethylchloride ions were measured as a function of the parent-ion internal energy by the method of photoelectron photoion coincidence. Previously performed ab initio calculations indicated that the rate-determining step for this reaction is an H-atom transfer from the beta carbon to the Cl atom via a substantial energy barrier of 92 kJ/mol (referenced to the zero-point energy). The ion internal energy range in which the experimental rates varied between 10(5) and 10(7) s-1 was found to lie well below the calculated barrier for H-atom transfer. The rates were modeled with the Rice-Ramsperger-Kassel-Marcus statistical theory which includes a tunneling step through an Eckart potential. The vibrational frequencies of both the normal and deuterated ethylchloride ions were determined by ab initio molecular-orbital methods. The theory accounted very well for the absolute rates including the strong deuterium isotope effect. The measured kinetic-energy release distribution appears nonstatistical. This indicates that the ion-dipole complex, which lies in between the transition state and the C2H4+ + HCl products, is ineffective in randomizing the excess energy among the product degrees of freedom.