Abstract
Mass spectra are measured for 2'-, 3'- and 4'-(ortho, meta and para) methyl substituted alkyl phenyl ketones excited at wavelengths ranging from 1200 to 1500 nm in the strong field regime. The selective loss of a methyl group from the acetyl group of the parent molecular ion upon excitation at similar to 1370 nm is attributed to an electronic resonance between ground D-0 and excited D-2 state of the radical cation. Depletion of the parent molecular ion is enhanced as the methyl substituent is moved from the 2' to 3' to 4' position on the phenyl ring with respect to the acetyl group. The phenyl-acetyl dihedral angle is the relevant coordinate enabling excitation to the dissociative D-2 state. Calculations on the radical cation of 2'-methylacetophenone show two stable geometries with dihedral angles of 7 degrees and 63 degrees between the phenyl and acetyl groups. The barrier to rotation for the 2' isomer limits population transfer to the D-2 state. In contrast, calculations on the radical cations of 3'- and 4'-methylacetophenone reveal no rotational barrier to prevent population transfer to the excited state, which is consistent with the enhanced dissociation yield in comparison with the 2' substitution. The enhanced dissociation of the 4' isomer as compared to the 3' isomer is attributed to its lower moment of inertia about the dihedral angle. (C) 2014 Elsevier B.V. All rights reserved.