Substituent effects on proton transfer (PT) from the phenolic moiety to water in [p-XC6H4OH-(H2O)(n)](center dot+) (X = N(CH3)2, NH2, OH, H, CN, CF3, NO2, n = 1-3) clusters were investigated by density functional theory. Structural and electronic aspects related to substituent effects and hydrogen bonding were analysed. PT to water is inhibited (favored) by electron donating (withdrawing) substituents. The results indicate that para-substituents have a less important role in promoting proton transfer than the interactions with the water molecules. Anharmonic vibrational frequencies associated with the PT coordinate correlate with the sigma(+)(P) Hammett parameters. (c) 2007 Elsevier B.V. All rights reserved.