The Determination of the Transition State Structure from the J Dependence of the Dissociation Energy E0 (J): The Methane and Ethane Ion Dissociation

Abstract

AbstractA threshold photoelectron photoion coincidence experiment has been setup which allows to measure breakdown curves of energy selected ions at different sample temperatures. These studies reveal the rotational energy of the transition state in unimolecular reactions, which in turn is used as a probe for the transition state structure. According to a model recently suggested this should allow the distinction of tight and loose transition states. The breakdown curves of methane and ethane ions in the region of the lowest reaction channel have been measured at 300 K and close to 0 K. In the case of the H loss from methane ions the shift between these two breakdown curves reveals that the dissociation energy Eo varies significantly with the rotational temperature as expected for a simple bond fission (loose transition state). In the case of H2 loss from ethane ions the shift between the breakdown curves at 300 and close to 0 K shows that E0 (J) does not vary significantly with the rotational temperature. This is to be expected for a concerted elimination reaction (tight transition state).