Exploring the nature and strength of S H···O hydrogen bond employing gas phase laser spectroscopy and quantum chemistry calculations

Abstract

Hydrogen bonding has been an important non-covalent interaction since its discovery in 1920 because of its inevitable role in the structures and functions of materials and biomolecules. This non-covalent interaction is represented by the symbol X-H···Y, where X and Y generally stand for very electronegative elements like O, N, F, etc. Recent research has revealed that hydrogen bonding is not limited to exclusively highly electronegative atoms; rather, less electronegative elements such as C, S, Se, P, and others in the periodic table may also engage in this type of non-covalent interaction, which is referred to as an unconventional hydrogen bonding. Sulfur-centered hydrogen bonding (SCHB) is one of the unconventional hydrogen bonding interactions that has drawn the most attention in the literature because of its significant importance in catalysis, drug discovery, material design, self-assembly, protein structure, and other areas. Sulfur can act as both hydrogen bond donor and acceptor. However, the majority of research on SCHB that has been published in the literature has been done on the X-H‧‧‧S system, in which X is an electronegative atom (O or N) and sulfur functions as a hydrogen bond acceptor. It has been reported in the literature that X-H‧‧‧S hydrogen bonds are as strong as conventional hydrogen bonds i.e., O-H‧‧‧O, N-H‧‧‧O, etc. There aren't many spectroscopic investigations of SCHB in the literature that use sulfur as a hydrogen bond donor.

In this thesis, we have explored the nature and strength of the S-H‧‧‧O hydrogen bond using a variety of gas-phase UV and IR laser-based spectroscopic techniques in combination with quantum chemistry calculations. We have studied 1:1 complexes of 2-Flurothiophenol (2-FTP) with different complexing partners such as water, alcohols, and ether. Our results demonstrate that the role of sulfur as a hydrogen bond donor is more significant than that as an acceptor due to an influence of the cumulative effect of the proton affinity (PA) ofthe acceptor and acidity (pKa) of the donor counterpart. We have also shown here a systematic modulation of the strength of the S-H‧‧‧O hydrogen bond by varying the hydrogen bond acceptor from water to ether. Additionally, a linear correlation between the strength of the S-H‧‧‧O hydrogen bond or IR red-shift in the S-H stretching frequency with the proton affinities of the hydrogen bond acceptors is established.