O-H...O versus O-H...S hydrogen bonding. 2. Alcohols and thiols as hydrogen bond acceptors.

In this paper, the effect of alkyl substitution at the hydrogen bond acceptor and its chain length on the strength and nature of hydrogen bonding is presented. In the present study we combine both experimental and computational methods to investigate the characteristics of O-H...O and O-H...S hydrogen bonding in the complexes of p-cresol (p-CR) with methanol (MeOH), ethanol (EtOH), methanethiol (MeSH), and ethanethiol (EtSH). The results indicate that, with an increase in the alkyl chain length, both O-H...O hydrogen bonding and O-H...S hydrogen bonding become stronger. Energy decomposition analysis emphasizes the dispersive nature of O-H...S hydrogen bonding. In addition, it revealed that the dispersion energy contribution in O-H...O hydrogen bonding increases with an increase in the alkyl chain length of the hydrogen bond acceptor. In the case of O-H...S hydrogen bonding, however, the dispersion energy contribution decreased from 68% for the H(2)S complex to 53% in the case of the MeSH complex; it remained unchanged with a further increase of the alkyl chain length. It was also observed that the red shifts in the OH stretching frequency did not correlate with the proton affinities of the O-centered acceptor vs the S-centered H-bond acceptor, in contrast with the known trend for the conventional H-bonded complexes. The IR/UV double resonance study enabled the assignments of the anti and gauche conformers of p-CR-EtOH and p-CR-EtSH.