Conformational stability, barriers to internal rotation of 2-aminothiophenol (d0 and d3): A combined vibrational and theoretical approach

The Raman (3700-100 cm−1) and infrared (4000-400 cm−1) spectra of liquid 2-aminothiophenol (2ATP) have been recorded and complemented with quantum mechanical calculations. Ten rotational isomers (Cs and/or C1 symmetry) have been considered due to the rotation of NH2 moiety around C-N bond, while the in-plane SH group is directed towards and/or away from NH2, respectively. Initial computational results are in favor of two conformers where the SH group is directed towards the NH2 moiety with C1 symmetry (7; gauche-1, 8; trans). An additional gauche-2 isomer (11) was revealed; with NH2 moiety perpendicular to the benzene ring and the out-of-plane SH group is directed towards NH2 while the lone pairs of electrons on nitrogen and sulfur atoms reside in opposite directions. From MP2/6-31G(d) calculations, the gauche-2 (11) is predicted to be more stable than trans (8) and gauche-1 (7) by 758 (2.17 kcal/mol) and 833 cm−1 (2.38 kcal/mol), respectively. Aided by theoretical and experimental outcomes, gauche-2 (11) conformer appears to dominate the liquid phase with minor spectroscopic features relating to gauche-1 (7) and/or trans (8) conformers. Aided by normal coordinate analysis, force constants and frequency calculations, a complete vibrational assignment has proposed for 2ATP (HSC6H4NH2) and its deuterated analogue (DSC6H4ND2). The structural parameters were optimized without any constrains with the addition of polarization and diffusion functions at 6-311+G(d) and 6-31++G(d,p) basis sets using RHF, MP2 and B3LYP, all in favor of gauche-2 (11) conformer. Additionally, the SH and NH2 barriers to internal rotations have been considered for 2-aminothiophenol and compared with 2-aminophenol (2AP).