Molecular Structure of a Model <i>S</i> -Alkylthiolanium Ionic Liquid Based on NMR Measurements and DFT Predictions for Its Enantiomeric C–H···I ^– Hydrogen Bonded Units Achieved in an Unusual Use of QST2 and IRC Methods

Using atypical input geometries that led to asymmetric transition state (TS) structures, it was possible to construct the raw geometries of a majority of both units of the title ionic liquid <b>1a</b> of type A/B in the D₂O solution, mainly in a combination of QST2 and IRC methods. Their refinement in all-electron DFT-B3LYP calculations using a locally dense basis sets (LDBS) approach led to 34 complete pairs A/B as binary sets of enantiomeric forms of type A and B, which, according to time-averaged NMR spectra and calculated Gibbs free energy (Δ<i>G</i>) data, exist in a fast symmetrical two-site exchange A ⇌ B. The availability of these structurally diverse structures, in which iodine ion (I^-) occupies various spatial positions, stabilized by weak C-H···I^- hydrogen bonds confirmed by QTAIM analysis, made it possible to test selected DFT methods to calculate the δ_H, δ_C, and ^<i>n</i><i>J</i>_HH data against their experimental values. A modified joint two-nucleus approach (¹H/¹³C) with factor <i>m</i>_H = 6 was also used. The efficiency of these predictions for an ensemble of all pairs A/B, which is the multicomponent molecular structure of <b>1a</b> at the theoretical level used, was analyzed in light of their populations arising from consideration of Δ<i>G</i> data.