Structure and stability of salicylic acid-water complexes and the effect of molecular hydration on the spectral properties of salicylic acid

Density functional theory with B3LYP parametrization and 6-311++G(d,p) basis set has been used to investigate the structure and stability of salicylic acid-water complexes. The vertical excitation energies for these complexes have been computed using time-dependent density functional theory (with B3LYP parametrization and a 6-311++G(d,p) basis set). It is shown that the hydrogen bond between the carboxylic hydrogen and the oxygen of water is the strongest among all possible hydrogen bonds in the system. The hydrogen bond strength in salicylic acid-water complexes seems to be nearly additive. The change in absorption maximum (lambda(max)) corresponding to the vertical excitation energy for the first three excited singlet and triplet states of the complex with 1-3 water molecules is nominal (approximately 1-3 nm). But with the addition of the fourth water molecule, the lambda(max) for S(1) and T(1) decreases by approximately 17 nm and it increases for S(2) and S(3) by about the same amount. The decrease in lambda(max) for transition to the T(2) state on the addition of the fourth water molecule is only approximately 9 nm. There seems to be an intersystem crossing between the S(1) and T(3) states that could account for the observed fluorescence quenching of salicylic acid in water.