Modulation of the strength of weak S–H⋯O hydrogen-bond: Spectroscopic study of the complexes of 2-flurothiophenol with methanol and ethanol

• Published in: The Journal of chemical physics 2024-06, Vol.160 (22)
• Main Authors: Metya, Surajit, Roy, Supravat, Mandal, Sourav, Huang, Qian-Rui, Kuo, Jer-Lai, Das, Aloke
• Format: Article
• Language: English
• Subjects: Ethanol; Hydrogen bonding; Hydrogen bonds; Infrared spectra; Infrared spectroscopy; Investigations; Methanol; Molecular orbitals; Physical properties; Protons; Quantum chemistry; Quantum theory; Vapor phases
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/5.0208086

Spectroscopic exploration of sulfur-centered hydrogen bonding involving a thiol group (S–H) as the hydrogen bond donor is scarce in the literature. Herein, we have investigated 1:1 complexes of 2-fluorothiophenol (2-FTP) with methanol (MeOH) and ethanol (EtOH) in the gas phase to examine the physical characteristics and strength of the S–H⋯O hydrogen bond. Structures, conformations, and the strength of the S–H⋯O interaction are investigated by measuring the electronic and Infrared (IR) spectra of the two complexes employing resonant two-photon ionization, UV–UV hole-burning, and IR–UV double resonance spectroscopic techniques combined with quantum chemical calculations. Three conformers of 2-FTP⋯MeOH and two conformers of 2-FTP⋯EtOH have been detected in the experiment. A comparison of the IR spectra obtained from the experiment with those of the low-energy conformers of 2-FTP⋯MeOH and 2-FTP⋯EtOH predicted from the theory confirms that all the observed conformers of the two complexes are primarily S–H⋯O hydrogen bonded. The IR red-shifts found in the S–H stretching frequencies in 2-FTP⋯MeOH and 2-FTP⋯EtOH concerning that in 2-FTP are ∼76 and ∼88 cm−1, respectively, which are much larger than that was reported earlier in the 2-FTP⋯H2O complex (30 cm−1). The strength and physical nature of different noncovalent interactions, including the S–H⋯O hydrogen bond existing in the complexes, are further analyzed using natural bond orbital analysis, quantum theory of atoms in molecules, and localized molecular orbital-energy decomposition analysis. The current investigation reveals that the S–H⋯O hydrogen bond can be strengthened by judicial choices of the hydrogen bond acceptors of higher proton affinities.