Spectroscopy of the cyano radical in an aqueous environment

The effect of bulk water on the B (2)Sigma(+) <-- X (2)Sigma(+) and A (2)Pi <-- X (2)Sigma(+) electronic transitions of the cyano radical is investigated. First, the cyano radical-water dimer is characterized to understand the nature of the interactions and parametrize molecular mechanics (MM) potentials. The carbon atom, which hosts the unpaired electron, is found to have a Lennard-Jones radius smaller than typical force fields values. Classical molecular dynamics (MD) is then used to sample water configurations around the radical, employing two sets of MM parameters for the cyano radical and water. Subsequently, vertical excitation energies are calculated using time-dependent density functional theory (TD-DFT) and equation-of-motion coupled-cluster with single and double substitutions (EOM-CCSD). The effect of water is modeled by point charges used in the MD simulations. It is found that both bands blue-shift with respect to their gas phase position; the magnitude of the shift is only weakly dependent on the method and the MM parameter set used. The calculated shifts are analyzed in terms of the solute-solvent interactions in the ground and excited states. Significant contributions come from valence repulsion and electrostatics. Consequences for experiments on ICN photodissociation in water are discussed.