Aqueous solvation of p-aminobenzonitrile in the excited states: a molecular level theory on density dependence.

Osawa et al. recently studied density dependence of electronic absorption spectra of p-aminobenzonitrile (ABN) in supercritical and subcritical water. They reported the peak position exhibits a minimum in a specific density. RISM-SCF-SEDD, which is a combination method of ab initio electronic structure theory and a statistical mechanics for molecular liquids, was applied to address the origin of this challenging phenomena. Highly accurate electronic structure theories (CASSCF and MCQDPT2) coupled with microscopic description of hydrogen bonding were employed over a wide range of density condition. We found that the solvation effects on the lower two excited states show different density dependence, suggesting that the turnover is attributed to the difference in sensitivity to solvent of the two states.