The rates of HCl loss from energy-selected ethylchloride ions: A case of tunneling through an H-atom transfer barrier

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Abstract
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 β 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 105 and 107 s−1 was found to lie well below the calculated barrier for H‐atom transfer. The rates were modeled with the RRKM 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 C2H+4+HCl products, is ineffective in randomizing the potential energy of the reverse activation barrier.