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

Abstract

Isopropanol (2-propanol) dissociatively photoionizes primarily into the CH2CHOH+, CH3CHOH+, CH3CHCH3+ fragment ions, and, as a minor product, into (CH3)2COH+ 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 ΔfH° = 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 non-statistical 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.