Spectroscopic and theoretical studies of some 2-ethylsulfinyl-(4′-substituted)-phenylthioacetates

The analysis of the IR vCO bands of the 2-ethylsulfinyl-(4′-substituted)-phenylthioacetates 4′-Y-C6H4SC(O)CH2S(O)Et (Y = NO21, Cl 2, Br 3, H 4, Me 5, OMe 6) supported by B3LYP/6-31G(d,p) calculations along with the NBO analysis for 1, 4 and 6 and X-ray analysis for 3, indicated the existence of four gauche (q-g-syn, g3-syn, g1-anti and q-g2-syn) conformers for 1–6. The calculations reproduce quite well the experimental results, i.e. the computed q-g-syn and g3-syn conformers correspond in the IR spectrum (in solution), to the vCO doublet higher frequency component of larger intensity, while the computed g1-anti conformer correspond to the vCO doublet lower frequency component (in solution). NBO analysis showed that the ns → πC1O2∗, nO(CO) → σC1–S3∗, nO(CO) → σC1–C4∗ orbital interactions are the main factors which stabilize the q-g-syn, g3-syn, g1-anti and q-g2-syn conformers for 1, 4 and 6. The nO(CO) → σC1–S3∗ interaction which stabilizes the q-g-syn, g3-syn and q-g2-syn conformers into a larger extent than the g1-anti conformer, is responsible for the larger vCO frequencies of the former conformers relative to the latter one. The q-g-syn, g3-syn and q-g2-syn   conformers are further stabilized by σC4–S5σC4–S5 → πCO∗ (strong), πCO∗/σC4–S5∗, nO(CO) → σC6–H17[Et]∗ (weak) and πCO∗/σC4–S5∗ (strong) orbital interactions. The g1-anti conformer is also stabilized by σC4–S5 → πCO∗ (strong), πCO/σC4–S5∗, nO(CO) → σC6–H17[Et]∗, πC9C11[Ph] → σC4–H16[α-CH2]∗ (weak), nO(SO) → σC11–H23[o-Ph]∗ (medium) and πCO∗/σC4–S5∗ (strong) orbital interactions. The q-g-syn   conformer is further stabilized by O(CO)δ-⋯S(SO)δ+ attractive Coulombic interaction while the q-g2-syn conformer is destabilized by the nS5⋯O(CO)δ- repulsive Coulombic interaction. This analysis indicates the following conformer stabilization order: q-g-syn; g3-syn > g1-anti ≫ q-g2-syn. X-ray single crystal analysis of 3 indicates that it assumes in the solid a distorted q-g2-syn geometry which is stabilized through almost the same orbital and Coulombic interaction which takes place for the q-g2-syn conformer, in the gas, along with dipole moment coupling and a series intermolecular CH⋯O interactions.