Why Do Losses of Polyatomic Fragments Strongly Dominate Losses of Atoms at High Internal Energies? Methyl versus Cl• Loss from the 2-Chloropropane Cation

Abstract

Previous work demonstrates that rates of H . losses generally increase much less rapidly with increasing internal energy than do those of losses of polyatomic fragments. To determine whether this is also the case for the losses of other atoms, the dependencies of the rates of Cl . and CH 3 . losses from CH 3 CHClCH 3 + . on ion internal energy are compared. These dependencies were established experimentally by photoionization mass spectrometry. The reactant ion and the transition states were characterized by B3LYP/6-31G(d), B3LYP/6-311G(d,p), and MP2/6-31G(d) theories. The transition states located are at very long bond lengths and close in energy to the corresponding dissociation products. Rate constants as a function of internal energy were obtained by RRKM theory. According to both experiment and theory, above its onset, methyl loss increases much more rapidly with internal energy than does loss of Cl . . This stems from three vibrational frequencies being substantially lowered in the transition state for the former but not in the latter reaction as they are being transformed into rotations of the separated products. This produces a much more rapid increase in the RRKM sum of states for the transition state to CH 3 CHCl + + CH 3 . , and thus also in the rate of that reaction. This, together with previous work, establishes that losses of polyatomic fragments from ions in the gas phase generally increase much faster with increasing internal energy than do losses of atoms.