Conformational stability, barriers to internal rotation of 2-aminothiophenol (d 0 and d 3): A combined vibrational and theoretical approach

Abstract

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 ( C s and/or C 1 symmetry) have been considered due to the rotation of NH 2 moiety around C–N bond, while the in-plane SH group is directed towards and/or away from NH 2, respectively. Initial computational results are in favor of two conformers where the SH group is directed towards the NH 2 moiety with C 1 symmetry ( 7; gauche-1, 8; trans). An additional gauche-2 isomer ( 11) was revealed; with NH 2 moiety perpendicular to the benzene ring and the out-of-plane SH group is directed towards NH 2 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 (HSC 6H 4NH 2) and its deuterated analogue (DSC 6H 4ND 2). 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 NH 2 barriers to internal rotations have been considered for 2-aminothiophenol and compared with 2-aminophenol (2AP).