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

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High Resolution Image Download MS PowerPoint Slide 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 (C 6 H 14 ) 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.

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