The basicities of a series of substituted crotonaldehyde in their low-lying excited triplet state

Abstract

Gas phase proton affinities and transition energies of a series of substituted crotonaldehyde and their O-protonated counterparts have been theoretically studied by DFT (B3LYP 6-31G(d)) calculations with complete geometry optimizations of the relevant excited state. As in the case of ground states, 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 substituent is seen to cause very little change of the proton affinities (PA) relative to the unsubstituted crotonaldehyde. Electron releasing substituents increase PA by 0.005–0.05 hartree and the electron withdrawing substituents decrease it by 0.013–0.027 hartree with the exception of the nitro compound where it is increased by 0.1 hartree. Computed proton affinities are sought to be correlated with a number of computed system parameters such as the net charge on the carbonyl oxygen, charge on the proton of the unprotonated and the protonated species and the computed hardness of the unprotonated species in their relevant excited states. The proton induced shifts are in general red shifts for the lowest excited triplet state.