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

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 pK_a values or proton affinities of the hydrogen bonding partners in a complex.