ANGULAR-MOMENTUM IN ION-MOLECULE REACTIONS

The fragmentation of gaseous ions is described using a formalism based on RRKM and a joint distribution function of energy and angular momentum P(E,J); it is particularly suitable for the study of angular momentum effects, and is tested by evaluating the branching ratio for the competitive fragmentation pathways of an intermediate complex prepared in different rotational distributions with nearly identical total internal energy. The model is applied to the fragmentation of C4H8+ to C4H7+ (k1) and C3H5+ (k2) and compared with experiment. When C4H8+ is prepared by the condensation of ethylene ion and ethylene, k1/k2 ~ 0.11. When C4H8+ is produced by photoionization of neutral C4H8 olefins at a fixed internal energy corresponding to the heat of reaction of the condensation process, by using threshold photoelectron-coincident photoion mass spectrometry, k1/k2 ~ 0.39. Angular momentum clearly has a major effect on the relative contributions of competing reaction exit channels, particularly when one pathway leads to fragments with low reduced mass and polarizability. The possible neglect or inclusion of rotational energy and/or rotational angular momentum of the reactants in P(E,J) and k (E,J) leads to three different predictions, which increasingly emphasize the effect of angular momentum on the reaction outcome. Agreement between theory and experiment is satisfactory. © 1979, American Chemical Society. All rights reserved.