Axial and equatorial hydrogen-bond conformers and ring-puckering motion in the trimethylene sulfide center dot hydrogen fluoride complex

Axial and equatorial hydrogen-bond conformers of the trimethylene sulfide...hydrogen fluoride complex have been generated and characterized in the supersonic jet of a molecular beam Fourier transform microwave experiment. It is shown that the ring-puckering large amplitude motion of trimethylene sulfide is responsible for the observed conformers. The axial conformer has been found to be the most stable and has been proved by the existence of relaxation of the high-energy equatorial form to it. This conformational preference has been explained in the context of a delicate balance between primary and secondary hydrogen bonds. The interconversion between both conformers takes place through the ring-puckering motion of the heterocycle, provided that the barrier to the ring inversion remains low after complexation, as all experimental findings indicate. The structural parameters of the trimethylene sulfide and the hydrogen bond have been derived from the analysis of the rotational spectra of the (C3H6S)-S-32...HF, (C3H6S)-S-34...HF, C-13(alpha) (C2H6S)-C-12-S-32...HF, and (CbetaC2H6)-C-13-C-12-S-32...HF isotopomers. Both conformers have C-s symmetry with the hydrogen fluoride located in the molecular symmetry plane of trimethylene sulfide, which is puckered at a similar angle to that found for the bare ring.