To roam or not to roam, that is the question for the methyl group in isopropanol cations

Isopropanol (2-propanol) dissociatively photoionizes primarily into the CH2CHOH+, CH3CHOH+, CH3CHCH3+ fragment ions, and, as a minor product, into (CH3)(2)COH+ in the 10.0-13.1 eV photon energy range as shown by Imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy. At internal energies of below 0.3 eV, the loss of CH4 dominates via a roaming pathway, in which the leaving CH3 abstracts a hydrogen atom from the other methyl group. At higher energies, the kinetically favored direct loss of a methyl radical quickly takes over as its transition state is looser. We use the measured CH3-loss appearance energy of 10.44 0.01 eV to confirm the heat of formation of protonated acetaldehyde, CH3CHOH+, as Delta(f)fH degrees 1/4 608 +/- 1 kJ mol(-1) at 0 K. The highest-energy dissociation observed leads to CH3CHCH3+ + OH, which corresponds to C-O bond scission. This process is the premier example of a nonstatistical dissociation which can be modeled using a statistical model, albeit with a physically meaningless appearance energy. This channel is shown to be non-statistical due to preferential OH loss from the first electronically excited ion state. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

Isopropanol dissociatively photoionizes primarily into CH2CHOH+, CH3CHOH+, CH3CHCH3+ fragment ions and as a minor product, into (CH3)(2)COH+ in the 10.0-13.1 eV photon energy range. The dissociation pathway changes at different internal energies, with the loss of CH4 dominating at lower energies via a roaming pathway and the direct loss of a methyl radical becoming more prominent at higher energies. The nonstatistical dissociation process is exemplified by the preferential OH loss from the first electronically excited ion state.