Dissociation of energy selected Sn(CH3)4+, Sn(CH3)3Cl+, and Sn(CH3)3Br+ ions: evidence for isolated excited state dynamics

Abstract

The dissociation dynamics of Sn(CH₃)₄⁺, Sn(CH₃)₃Cl⁺, and Sn(CH₃)₃Br⁺ were investigated by threshold photoelectron photoion spectrometry using an electron imaging apparatus (iPEPICO) at the Swiss Light Source. The tetramethyltin ion was found to dissociate viaSn(CH₃)₄⁺ → Sn(CH₃)₃⁺ + CH₃ → Sn(CH₃)₂⁺ + 2 CH₃, while the trimethyltin halide ions dissociated viamethyl loss at low energies, and a competitive halogen loss at somewhat higher energies. The 0 K methyl loss onset for the three ions was found to be 9.410 ± 0.020 eV, 10.058 ± 0.020 eV, and 9.961 ± 0.020 eV, respectively. Statistical theory could not reproduce the observed onsets for the halogen loss steps in the halotrimethyltin ions. The halide loss signal as a function energy mimicked the excited state threshold photoelectron spectrum, from which we conclude that the halide loss from these ions takes place on an isolated excited state potential energy surface, which we describe by time dependent density functional calculations. The sequential loss of a second methyl group in the Sn(CH₃)₄⁺ ion, observed at about 3 eV higher energies than the first one, is also partially non-statistical. The derived product energy distribution resulting from the loss of the first methyl group is two-component with about 50% being statistical and the remainder associated with high translational energy products that peak at 2 eV. Time dependent DFT calculations show that a dissociative state lies in the vicinity of the experimental measurements. We thus propose that 50% of the Sn(CH₃)₄⁺ ions produced in this energy range internally convert to the state, on which they dissociate statistically, while the remainder dissociate directly from the repulsive state leading to high kinetic energy products.