Nonbonding pairs in cyclic thioethers: Electrostatic modeling and ab initio calculations for complexes of 2,5-dihydrothiophene, thietane, and thiirane with hydrogen fluoride

Electrostatic potential energies V(ϕ) of a non-perturbing, protonic charge at fixed distances r from the S atom in three cyclic thioethers were examined as functions of the angles ϕ made by the r-vector with the C ₂ axis (thiirane and 2,5-dihydrothiophene) or the local C ₂ axis (thietane). The electrostatic PE V _HF (ϕ) of HF (HF modelled as an extended electric dipole) was also calculated and the results compared with geometries of the thioether⋯HF complexes calculated at the CCSD(T)-F12c/cc-pVTZ-F12 level. The latter reveal angular deviations θ ∼10-20° of the S⋯HF nuclei from collinearity in directions suggesting secondary interactions of F with H atom(s) of the rings. Angles ϕ made by the S⋯H hydrogen bond with the C ₂ (or local C ₂ ) axes in the complexes are systematically larger (∼4-9°) than indicated by the V _HF (ϕ) functions. Minima in the simple V(ϕ) versus ϕ functions occur at values smaller (∼5-10°) than those in the V _HF (ϕ) curves.