Dissociative photoionization mechanism of methanol isotopologues (CH3OH, CD3OH, CH3OD and CD3OD) by iPEPICO: energetics, statistical and non-statistical kinetics and isotope effects

The dissociative photoionization of energy selected methanol isotopologue (CH3OH, CD3OH, CH3OD and CD3OD) cations was investigated using imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy. The first dissociation is an H/D-atom loss from the carbon, also confirmed by partial deuteration. Somewhat above 12 eV, a parallel H2-loss channel weakly asserts itself. At photon energies above 15 eV, in a consecutive hydrogen molecule loss to the first H-atom loss, the formation of CHO+/CDO+ dominates as opposed to COH+/COD+ formation. We see little evidence for H-atom scrambling in these processes. In the photon energy range corresponding to the and ion states, a hydroxyl radical loss appears yielding CH3+/CD3+. Based on the branching ratios, statistical considerations and ab initio calculations, this process is confirmed to take place on the first electronically excited Ã2A′ ion state. Uncharacteristically, internal conversion is outcompeted by unimolecular dissociation due to the apparently weak Renner–Teller-like coupling between the and the à ion states. The experimental 0 K appearance energies of the ions CH2OH+, CD2OH+, CH2OD+ and CD2OD+ are measured to be 11.646 ± 0.003 eV, 11.739 ± 0.003 eV, 11.642 ± 0.003 eV and 11.737 ± 0.003 eV, respectively. The E0(CH2OH+) = 11.6454 ± 0.0017 eV was obtained based on the independently measured isotopologue results and calculated zero point effects. The 0 K heat of formation of CH2OH+, protonated formaldehyde, was determined to be 717.7 ± 0.7 kJ mol−1. This yields a 0 K heat of formation of CH2OH of −11.1 ± 0.9 kJ mol−1 and an experimental 298 K proton affinity of formaldehyde of 711.6 ± 0.8 kJ mol−1. The reverse barrier to homonuclear H2-loss from CH3OH+ is determined to be 36 kJ mol−1, whereas for heteronuclear H2-loss from CH2OH+ it is found to be 210 kJ mol−1.