Theoretical study of photoacidity of HCN: the effect of complexation with water

Abstract

The character of the hydrogen bonding and the excited state proton transfer (ESPT) in the model system HCN⋯H₂O is investigated. The PES of the two lowest excited states of the H₂O⋯HCN complex was calculated using the CASPT2 method. The nonadiabatic coupling of the two states of the (π → π*) and (π → σ*) character is responsible for the excited state proton/hydrogen transfer. Compared to the ground state, the barrier for this process is significantly smaller. An increased number of water molecules in the complex with cyclic hydrogen-bonded network causes a large blue shift of the state of the (π → σ*) character. The question of the dissociation of the complex in its excited state is also addressed.