How Isomer and Conformer Structures Impact Dissociation Dynamics of Alkane Radical Cations

Ionization of alkanes to form radical cations activates their otherwise unreactive C-H bonds, facilitating important chemical processes such as hydrocarbon cracking. This work investigates the radical cation dissociation dynamics of hexane (C₆H₁₄) structural isomers by using femtosecond time-resolved mass spectrometry and quantum chemical calculations. All five isomers exhibit competition between the yields of fragment ions arising from direct C-C bond cleavage or dissociative rearrangement with hydrogen migration on dynamical time scales of ∼50-300 fs, suggesting that hydrogen migration in the metastable cations operates on such short time scales. Additional isomer- and conformer-specific dynamics are observed. Preferential dissociation pathways in the branched isomers are found to arise from geometric relaxation to cation structures with one elongated C-C bond. Coherent vibrational excitation along this elongated C-C bond in 3-methylpentane and 2,3-dimethylbutane results in ion yield oscillations in the first ∼300-400 fs after ionization. Enhanced depletion of the molecular ion signal in n-hexane compared to that in the branched isomers is attributed to a strongly coupled excited state in the most populated conformer that can be accessed by a two-photon transition. Collectively, these results provide a foundational understanding of dissociation dynamics in alkane radical cations and how these dynamics are affected by specific isomer and conformer structures.