O-H•••O versus O-H•••S Hydrogen Bonding. 3. IR-UV Double Resonance Study of Hydrogen Bonded Complexes of p-Cresol with Diethyl Ether and Its Sulfur Analog

In this work the hydrogen bonded complexes of diethyl ether (DEE) and diethyl sulfide (DES) with p-cresol (p-CR) were investigated. Only one conformer of the p-CR center dot DEE complex and three conformers of the p-CR center dot DES complex were found to be present under the supersonic jet expansion conditions. The conformational assignments were done with the help of IR-UV double resonance studies and ab initio calculations. The red shifts in the OH stretching frequency for the O-H center dot center dot center dot O and O-H center dot center dot center dot S hydrogen bonded complexes were quite close to each other. In fact, one of the p-CR center dot DES conformers showed a slightly larger red shift in the OH stretch than that in the p-CR center dot DEE conformer, which suggests that in this case sulfur is not a weak hydrogen bond acceptor as noted previously in case of the p-CR center dot H2O and p-CR center dot H2S complexes (Biswal et al. J. Phys. Chem. A 2009, 113, 5633). The natural bond orbital analysis also shows that the extent of overlap between sulfur lone pair orbitals (LP) and OH antibonding orbital (sigma*OH) was comparable to the oxygen (LP) and sigma*OH overlap, consistent with the similar magnitudes of the red shifts of OH stretch in the DES and DEE complexes. The computed binding energy of the p-CR center dot DES complex, however, was about 80% of the p-CR center dot DEE complex. The electron densities at the bond critical points indicated that the O-H center dot center dot center dot S interaction was weaker than the O-H center dot center dot center dot O interaction in this particular system also. The important finding of this study was that the IR red shifts in the H-bond donor X-H stretching frequency were not quite consistent with the computed binding energies and the atoms-in-molecules analysis contrary to the general understanding. Energy decomposition analysis suggests that O-H center dot center dot center dot S hydrogen bonding interaction is dispersive in nature and the dispersion contribution decreases with the increase in the length of the alkyl chain of the S hydrogen bond acceptor.