J Phys Chem A. 2026 Jan 26. doi: 10.1021/acs.jpca.5c06933. Online ahead of print.
ABSTRACT
This study re-examines the bond cleavage mechanisms of fluoromethane cations (CH3F+) in their low-lying electronic states (X2E, A2A1, B2E) using a combination of threshold photoelectron photoion coincidence (TPEPICO) velocity map imaging and quantum chemical calculations. The C-H bond cleavage from the X2E state is confirmed to proceed via a statistical, thermodynamically controlled mechanism, as evidenced by a Boltzmann kinetic energy release distribution (KERD). In contrast, the C-F bond cleavage from the excited A2A1 and B2E states exhibits nonstatistical dynamics. For the A2A1 state, the dissociation is direct and rapid, characterized by a Gaussian-type KERD and a negative anisotropy parameter (β ≈ -0.5). Crucially, for the B2E state, our results contradict the previously proposed mechanism of internal conversion to the X2E state followed by statistical dissociation. Instead, we provide compelling evidence that CH3F+(B2E) undergoes internal conversion to the A2A1 state, which then dissociates directly. This revised pathway is supported by RRKM calculations, the near absence of CH2F+ fragments, and the similarities in KERD and β parameters between the B2E and A2A1 states.
PMID:41587419 | DOI:10.1021/acs.jpca.5c06933
