Conformational Analysis, Intramolecular Interactions, and Anharmonic Vibrational Spectra of the 2-Chloroethyl Chalcogenol Triad (X = O, S, Se)

Abstract

We present a systematic computational study of the conformational space, intramolecular bonding, and anharmonic vibrational spectra of 2-chloroethanol (ClCH₂CH₂OH), 2-chloroethanethiol (ClCH₂CH₂SH), and 2-chloroethaneselenol (ClCH₂CH₂SeH), together with their 1-chloro isomers. Conformational searches with the CREST/iMTD-GC algorithm followed by DFT optimization identify low-energy gauche conformers stabilized by intramolecular X–H•••Cl interactions; this gauche X–H•••Cl structure is the global minimum for the alcohol, whereas for the thiol and selenol, the global minimum is an anti-conformer and the gauche X–H•••Cl form lies only 0.1 and 0.8 kcal/mol higher. Composite coupled-cluster energetics (SVECV-f12 for O/S; DLPNO–CCSD(T)/CBS for Se) confirm the DFT conformational ordering, with mean absolute deviations of 0.10–0.14 kcal/mol and only small deviations for quasi-degenerate conformers. Natural population analysis reveals a progressive polarity reversal of the chalcogen from O to Se, while second-order NBO analysis shows that the gauche preference is hyperconjugative in origin, with negligible direct Cl lone-pair donation into σ*(X–H). Uniform double-hybrid (B2PLYP-D3) GVPT2 calculations yield well-behaved, monotonically decreasing anharmonic X–H shifts (−188, −104, and −90 cm^–1 for O–H, S–H, and Se–H) in both isomeric series and uniformly negative zero-point energy corrections. We also document a pronounced sensitivity of numerically differentiated quartic force fields to reference-geometry convergence and functional choice for the heavier chalcogens, with practical recommendations for anharmonic studies of Se-containing molecules.