The proton affinity of a series of substituted acetophenones in their low-lying excited triplet state: A DFT study

Gas phase proton affinities (PA) and transition energies of a series of para-substituted acetophenones and their O-protonated counterparts(aromatic conjugated carbonyl systems) have been theoretically studied by DFT [B3LYP 6-311G (d,p)] calculations with complete geometry optimizations of the relevant excited state. The gas phase O-protonation turns out to be exothermic case and the local stereochemical disposition of the proton is found to be almost the same in each case. The presence of para substituent is seen to cause very little change of the proton affinities relative to unsubstituted acetophenones. Computed proton affinities are sought to be correlated with a number of computed system parameters such as the net charge on the carbonyl oxygen of the unprotonated and protonated species, charge on the proton of the protonated species and the computed hardness of the unprotonated species in the relevant excited state. The proton induced shifts are in general red shifts for the low-lying excited triplet state. The overall basicity is explained by distant atom contribution in addition to the contribution from the carbonyl group. (c) 2007 Elsevier B.V. All rights reserved.