Dissociative Photoionization of Dimethyl Carbonate: The More It Is Cut, the Bigger the Fragment Ion.

The dissociation of internal energy selected dimethyl carbonate (DMC) cations was studied by imaging photoelectron photoion coincidence spectroscopy (iPEPICO) in the 10.3-12.5 eV photon energy range. Vibrational fine structure is observed in the ground state band of the threshold photoelectron spectrum up to the dissociation threshold, and Franck-Condon simulations identify the O-C-O bend mode, in particular in combination with the C═O stretch mode, to be active. The DMC ionization energy was determined to be 10.47 ± 0.01 eV. The first dissociative photoionization product, CHOH, dominates the breakdown diagram from 11.0 to 11.8 eV and is also the lightest fragment ion at m/z = 31. Statistical modeling of the PEPICO data yields a 0 K appearance energy of E = 11.14 ± 0.01 eV. At higher photon energies, parallel dissociation channels are observed, leading to the daughter ions CHO, probably OHCHCHOH (m/z = 62, E = 11.16 eV), CHCHOH (m/z = 45, E = 11.46 eV), and CHOCO (m/z = 59, E = 11.47 eV). The m/z = 62 ethylene glycol ion may lose a CHO fragment and yield CHOH (m/z = 33) in a sequential dissociation at E = 11.54 ± 0.03 eV. Reaction path calculations explain the observed dissociation processes, except for the formation of the ethylene glycol cation. Composite method calculations of isodesmic and isomerization reaction energies are used to obtain the enthalpy of formation of dimethyl carbonate at ΔH(DMC(g)) = -548.3 ± 1.5 kJ mol. This puts the heat of formation of the cation at ΔH(DMC(g)) = 461.9 ± 1.8 kJ mol.