Competitive fragmentation pathways of acetic acid dimer explored by synchrotron VUV photoionization mass spectrometry and electronic structure calculations

In present study, photoionization and dissociation of acetic acid dimers have been studied with the synchrotron vacuum ultraviolet photoionization mass spectrometry and theoretical calculations. Besides the intense signal corresponding to protonated cluster ions (CH3COOH)(n) center dot H+, the feature related to the fragment ions (CH3COOH)H+ center dot COO (105 amu) via beta-carbon-carbon bond cleavage is observed. By scanning photoionization efficiency spectra, appearance energies of the fragments (CH3COOH) center dot H+ and (CH3COOH)H+ center dot COO are obtained. With the aid of theoretical calculations, seven fragmentation channels of acetic acid dimer cations were discussed, where five cation isomers of acetic acid dimer are involved. While four of them are found to generate the protonated species, only one of them can dissociate into a C-C bond cleavage product (CH3COOH)H+ center dot COO. After surmounting the methyl hydrogen-transfer barrier 10.84 +/- 0.05 eV, the opening of dissociative channel to produce ions (CH3COOH)(+) becomes the most competitive path. When photon energy increases to 12.4 eV, we also found dimer cations can be fragmented and generate new cations (CH3COOH) center dot CH3CO+. Kinetics, thermodynamics, and entropy factors for these competitive dissociation pathways are discussed. The present report provides a clear picture of the photoionization and dissociation processes of the acetic acid dimer in the range of the photon energy 9-15 eV. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754273]