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

Abstract

The analysis of the IR v CO bands of the 2-ethylsulfinyl-(4′-substituted)-phenylthioacetates 4′-Y-C 6H 4SC(O)CH 2S(O)Et (Y = NO 2 1, 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, g 3-syn , g 1-anti and q-g 2-syn ) conformers for 1– 6. The calculations reproduce quite well the experimental results, i.e. the computed q-g-syn and g 3-syn conformers correspond in the IR spectrum (in solution), to the v CO doublet higher frequency component of larger intensity, while the computed g 1-anti conformer correspond to the v CO doublet lower frequency component (in solution). NBO analysis showed that the n s → π C 1 O 2 ∗ , n O(CO) → σ C 1 – S 3 ∗ , n O(CO) → σ C 1 – C 4 ∗ orbital interactions are the main factors which stabilize the q-g-syn, g 3-syn , g 1-anti and q-g 2-syn conformers for 1, 4 and 6. The n O(CO) → σ C 1 – S 3 ∗ interaction which stabilizes the q-g-syn, g 3-syn and q-g 2-syn conformers into a larger extent than the g 1-anti conformer, is responsible for the larger v CO frequencies of the former conformers relative to the latter one. The q-g-syn, g 3-syn and q-g 2-syn conformers are further stabilized by σ C 4 – S 5 → π CO ∗ (strong), π CO ∗ / σ C 4 – S 5 ∗ , n O(CO) → σ C 6 – H 17 [ Et ] ∗ (weak) and π CO ∗ / σ C 4 – S 5 ∗ (strong) orbital interactions. The g 1-anti conformer is also stabilized by σ C4–S5 → π CO ∗ (strong), π CO/ σ C 4 – S 5 ∗ , n O(CO) → σ C 6 – H 17 [ Et ] ∗ , π C9 C11[Ph] → σ C 4 – H 16 [ α - CH 2 ] ∗ (weak), n O(SO) → σ C 11 – H 23 [ o-Ph ] ∗ (medium) and π CO ∗ / σ C 4 – S 5 ∗ (strong) orbital interactions. The q-g-syn conformer is further stabilized by O ( CO ) δ - ⋯ S ( SO ) δ + attractive Coulombic interaction while the q-g 2-syn conformer is destabilized by the n S5⋯ O ( CO ) δ - repulsive Coulombic interaction. This analysis indicates the following conformer stabilization order: q-g-syn; g 3-syn > g 1-anti ≫ q-g 2-syn . X-ray single crystal analysis of 3 indicates that it assumes in the solid a distorted q-g 2-syn geometry which is stabilized through almost the same orbital and Coulombic interaction which takes place for the q-g 2-syn conformer, in the gas, along with dipole moment coupling and a series intermolecular C H⋯O interactions.