FTIR Spectra of Dialkyl Sulfide and Alkanethiol Complexes with Hydrogen Fluoride in Solid Argon

Hydrogen-bonded complexes of dialkyl sulfides and alkanethiols with HF have been prepared by condensing the argon-diluted reagents at 12 K. Infrared spectra of the (CH₃)₂S--HF complex revealed a v_s(HF) mode at 3382 cm^-1, which suggests that the latter has a weaker hydrogen bond than the (CH₃)₂O--HF complex. The two HF librational modes at 631 and 622 cm^-1 exhibit considerably less separation than the v₁ modes for the ether complex, which indicates less asymmetry in the sulfide librational potential. The v₁ modes for (CH₃)₂S- -HF are considerably lower in value than modes for (CH₃)₂O- -HF, which is characteristic of a less rigidly bound complex. The larger sulfide base has a higher proton affinity but is too diffuse in electron density to bind HF as strongly as the smaller oxide base. Similar results were obtained for the diethyl sulfide, methanethiol, and ethanethiol complexes with hydrogen and deuterium fluorides. Complexes involving one sulfide and two HF submolecules appear to be dominated by the chain structure.