Journal of Physical Chemistry A, Vol.120, No.35, 6902-6916, 2016
Acid-Base Formalism Extended to Excited State for O-H center dot center dot center dot S Hydrogen Bonding Interaction
Hydrogen bond can be regarded as an interaction between a base and a proton covalently bound to another base. In this context the strength of hydrogen bond scales with the proton affinity of the acceptor base and the pK(a) of the donor, i.e., it follows the acidbase formalism. This has been amply demonstrated in conventional hydrogen bonds. Is this also true for the unconventional hydrogen bonds involving lesser electronegative elements such as sulfur atom? In our previous work, we had established that the strength of OH center dot center dot center dot S hydrogen bonding (HB) interaction scales with the proton affinity (PA) of the acceptor. In this work, we have investigated the other counterpart, i.e., the H-bonding interaction between the photoacids with different pK(a) values with a common base such as the H2O and H2S. The 1:1 complexes of five para substituted phenols p-aminophenol, p-cresol, p-fluorophenol, p-chlorophenol, and p-cyanophenol with H2O and H2S were investigated experimentally and computationally. The investigations were also extended to the excited states. The experimental observations of the spectral shifts in the OH stretching frequency and the S1S0 band origins were correlated with the pK(a) of the donors. Ab initio calculations at the MP2 and various dispersion corrected density functional levels of theory were performed to compute the dissociation energy (D-0) of the complexes. The quantum theory of atoms in molecules (QTAIM), noncovalent interaction (NCI) method, natural bonding orbital (NBO) analysis, and natural decomposition analysis (NEDA) were carried out for further characterization of HB interaction. The OH stretching frequency red shifts and the dissociation energies were found to be lower for the OH center dot center dot center dot S hydrogen bonded systems compared to those for the OH center dot center dot center dot O H-bound systems. Despite being dominated by the dispersion interaction the OH center dot center dot center dot S interaction in the H2S complexes also conformed to the acidbase formalism, i.e., the D-0 and the OH red shift scaled with the pK(a) of the donor, similar to that observed in the OH center dot center dot center dot O interaction. However, the two classes of H-bonds follow different correlations. In addition we also discuss the nuances associated with the similarity and differences in the hydrogen bonding properties of the two classes in the ground electronic state as well as in the excited state.