Nature and strength of sulfur-centred hydrogen bonds: laser spectroscopic investigations in the gas phase and quantum-chemical calculations

The importance of Sulfur centred hydrogen bonds (SCHBs) cannot be underestimated given the current day knowledge of its non-covalent interactions prevalent in many biopolymers as well as in organic systems. Based on the distance/angle constraints available from the structural database, these interactions have been interchangeably termed as van der Waals/hydrogen bonded complexes. There is a lack of sufficient spectroscopic evidence that can unequivocally term these interactions as hydrogen bonding interactions. In this review we present laser spectroscopic investigations of isolated binary complexes of H-bond donor-acceptor molecules containing Sulfur atom. The complexes were formed using supersonic jet expansion method and the IR/UV spectroscopic investigations were carried out on mass selected binary complexes. The pertinent questions regarding SCHBs addressed herein are (1) Is electronegativity the controlling factor to be a potent H-bond donor/acceptor? (2) How do SCHBs compare with their oxygen counterpart? (3) What is the nature of SCHBs, i.e. what are the dominating forces in stabilising these hydrogen bonds? (4) Do SCHBs follow classical H-bond acid-base formalism? (5) Are SCHBs found in peptides and proteins? If so, what are their strengths? Do they control the structure of the peptides? The experimental investigations were also supported by high level of ab initio computations.