S–H···O Hydrogen Bond Can Win over O–H···S Hydrogen Bond: Gas-Phase Spectroscopy of 2-Fluorothiophenol···H2O Complex

Abstract

Study of sulfur (S) centered hydrogen bonding (SCHB) interactions in the literature is mostly limited to the molecular systems where S acts as a hydrogen-bond acceptor. It has been found that this unconventional SCHB is similar in strength to any conventional hydrogen bonding interaction involving electronegative atoms. However, SCHB involving S as a hydrogen-bond donor is not explored much in the literature. Herein, we have studied the nature and strength of an unconventional S–H···O hydrogen bond in a 1:1 complex of 2-fluorothiophenol (2-FTP) and H2O using gas-phase electronic and IR spectroscopy in combination with quantum chemistry calculations. Both of the two conformers of 2-FTP···H2O observed in the experiment are found to be stabilized primarily by S–H···O hydrogen bonding interaction. O–H···S hydrogen-bonded conformers of the complex, which are higher in energy, are not observed in the experiment. There is a nice agreement between the theoretical and experimental IR spectra of the two observed conformers. The observed IR red-shift of 25–30 cm–1 in the S–H stretching frequency of both the conformers of the complex with respect to that of the 2-FTP monomer bespeaks that the S–H···O hydrogen bond present in 2-FTP···H2O is weak in nature. The present work demonstrates that the S–H···O hydrogen bond can have preference over the O–H···S hydrogen bond depending on the pKa values or proton affinities of the hydrogen bonding partners in a complex.