Controlling dissociation of alkyl phenyl ketone radical cations in the strong-field regime through hydroxyl substitution position.

The hydroxy-substituted alkyl phenyl ketones 2'-, 3'- and 4'- (ortho, meta, and para) hydroxyacetophenone were excited in the strong-field regime with wavelengths ranging from 1200-1500 nm to produce the respective radical cations. For 2'- and 3'-hydroxyacetophenone, the parent molecular ion dominated the mass spectrum, and the intensity of the fragment ions remained unchanged as a function of excitation wavelength. In contrast, 4'-hydroxyacetophenone exhibited depletion of the parent molecular ion with corresponding enhanced formation of the benzoyl fragment ion upon excitation with 1370 nm as compared with other excitation wavelengths. Density functional (DFT) calculations suggest that dissociation occurs when the acetyl group in 4'-hydroxyacetophenone radical cation twists out-of-plane with respect to the phenyl ring, enabling a one-photon transition between the ground cation state D0 and the excited cation state D2 to occur. The DFT calculations also suggest that the lack of dissociation in the wavelength-resolved strong-field excitation measurements for 2'- and 3'-hydroxyacetophenone arises because both isomers have a barrier to rotation about the carbon-carbon bond connecting the phenyl and acetyl groups. These results help elucidate the effects of substituents on the torsional motion of radical cations and illustrate the potential for controlling molecular dissociation through the addition of substituents.