Spectroscopic and theoretical studies of some N, N-diethyl-2-[(4-substituted)phenylsulfinyl] acetamides

Abstract

The analysis of the IR carbonyl bands of the N, N-diethyl-2-[(4-substituted)phenylsulfinyl] acetamides Et 2NC(O)CH 2S(O)Ph Y (Y = OMe 1, Me 2, H 3, Cl 4, Br 5 and NO 2 6) and the parent N, N-diethylacetamide 7, supported by HF/6-31G ∗∗ ab initio calculations of 3 and 7, indicated the existence of two pairs of cis ( anti, syn) and gauche ( anti, syn) conformers in gas phase, being the cis conformer pair more stable than the gauche pair. As in the case of the parent amide 7, the anti geometry is more stable than the syn one, for each pair of cis and gauche conformers. The higher stability of the cis conformer pair is in line with its geometry which allows a strong O ( CO ) δ - ⋯ S ( SO ) δ + charge transfer and Coulombic interactions along with the O ( CO ) δ - ⋯ ortho - H ( Ph ) δ + electrostatic interaction (intramolecular hydrogen bond). The gauche conformer pair is weakly stabilized through the O ( SO ) δ - ⋯ C ( CO ) δ + Coulombic interaction. The single carbonyl band found in solution (solvents of increasing relative permittivity) may be ascribed to the more stable and more polar cis conformer pair. The negative carbonyl frequency shifts of ca. −11 cm −1 (in n-hexane) for compounds 1– 5 give further support for the simultaneous occurrence of the O ( CO ) δ - ⋯ S ( SO ) δ + and O ( CO ) δ - ⋯ ortho - H ( Ph ) δ + interactions, which stabilize the cis conformer pair. X-ray single crystal analysis performed on 6 indicates that this compound assumes in the crystal a quasi- gauche ( q- g) conformation and the N, N-diethyl groups the anti geometry. The ( q- g) conformation is stabilized in the solid by crossed O ( CO ) δ - ⋯ S ( SO ) δ + and O ( SO ) δ - ⋯ C ( CO ) δ + orbital and Coulombic interactions. In order to obtain the large energy gain from the crystal packing, the q- g conformer forms centro-symmetric dimers which results in infinite chains stabilized by intermolecular hydrogen bonds.