Transition state structures and angular momentum effects in the dissociation dynamics of energy-selected C4H+8 ions

Abstract

The photoionization and dissociation dynamics of energy-selected 1-butene ions have been investigated by the technique of threshold photoelectron photoion coincidence (TPEPICO) time of flight mass spectrometry. The absolute dissociation rates for the reactions leading to the loss of H, CH3, and CH4 have been measured for two samples prepared with very different internal energy and angular momentum distributions. First rotationally cold ions were prepared by photoionizing 1-butene molecules cooled in a seeded molecular beam. These rates were analyzed within the framework of RRKM theory with vibrator transition state structure for all three channels. Excellent agreement between theory and experiment was obtained when ab initio calculated transition state frequencies were used for the H loss and the CH3 loss transition states. A variational transition state theory (VTST) analysis shows that the CH3 loss transition state lies about 11 kJ/mol below the dissociation limit. Second, dissociation rates using an effusive source which contained a 298 K distribution of vibrational and rotational energy were measured. The vibrator-type transition state model, with inclusion only of the vibrational energy distribution, gives a good account of the total rates but significantly overestimates the H loss branching ratio. Excellent agreement is obtained, however, when the energies of the molecular ions and vibrator transition states are corrected for the rotational energy of each structure. K-rotor mixing with the vibrations does not change the calculated rates significantly. Finally, the analysis confirms a previous proposal [Faraday Discuss. Chem. Soc. 75, 57 (1983)] that an orbiting transition state (a la phase space theory) is not the rate limiting bottleneck at the energies used in this experiment.