Hydrogen bond effects in the ground and excited singlet states of 4H-1-benzopyrane-4-thione in water-theory and experiment

The hydrogen bond energies for 4H-1-benzopyrane-4-thione (BPT) in its ground and two lowest excited singlet electronic states have been determined using ab initio methods. It was shown that the BPT molecule can form, as an acceptor, four relatively strong hydrogen bonds with water molecules, leading to a stable complex in the ground electronic state S-0. The hydrogen bonds involving the sulfur atom from the thiocarbonyl group were found to be stronger than those involving the oxygen atom from the benzopyran moiety. The former hydrogen bonds were predicted to become significantly weaker upon excitation to the S-1 state and, in contrast, stronger upon excitation to the S-2 state. Calculated changes in the hydrogen bond energy due to the S-0 -> S-1 and S-0 > S-2 excitation are in very good agreement with the experimental values obtained from the absorption solvatochromic study, according to a procedure proposed by us in [E. Krystkowiak, et al., J. Photochem. Photobiol. A: Chem., 2006, 184, 250]. The maxima of absorption spectra of the BPT-water hydrogen-bonded complex, calculated taking into consideration nonspecific solute-solvent interactions, are also in good agreement with the experimental results.