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

The analysis of the IR carbonyl band of the N,N-diethyl-2-[(4′-substituted)phenylsulfonyl]acetamides Et2NC(O)CH2S(O)2C6H4Y (Y = OMe 1, Me 2, H 3, Cl 4, Br 5, NO26) supported by B3LYP/6-31G(d,p) calculations for 3, indicated the existence of three pairs (anti and syn) of cis (c) and gauche (g1 and g2) conformers in the gas phase, being the gauche conformers significantly more stable than the cis ones. The anti geometry is more stable than the syn one, for each pair of cis and gauche conformers. The summing up of the orbital (NBO analysis) and electrostatic interactions justifies quite well the populations and the νCO frequencies of the anti and syn pairs of c, g1 and g2 conformers. The IR higher carbonyl frequency component whose population is ca. 10%, in CCl4, may be ascribed to the least stable and most polar cis conformer pair (in the gas phase) and the lower frequency component whose population is ca. 90%, to the summing up of the populations of the two most stable and least polar gauche conformer pairs (g1 and g2) (in the gas phase). The reversal of the cis(c)/gauche (g1 + g2) population ratio observed in chloroform ca. 60% (cis)/40% (gauche) and the occurrence of the most polar cis(c) conformer only, in acetonitrile, strongly suggests the coalescence of the two gauche components in a unique carbonyl band in solution. A further support to this rationalization is given by the single point PCM solvation model performed by HF/6-31G(d,p) method, which showed a progressive increase of the c/(g1 + g2) ratio going from gas to CCl4, to CHCl3 and to CH3CN. X-ray single crystal analysis of 4 indicates that this compound assumes, in the solid state, the syn-clinal (gauche) conformation with respect to the [OCCH2S] moiety, and the most stable anti geometry relative to the [C(O)N(CH2CH3)2] fragment. In order to obtain larger energy gain from the crystal packing the molecules of 4 are linked in centrosymmetric dimers through two CH⋯O interactions (CH[O–Ph]⋯O[SO2]) forming a step ladder.

► The preferred conformations of 2-substituted-acetamides were determined.
► Theoretical data showed the existence of cis and gauche conformers.
► The IR higher carbonyl frequency in CCl4 was ascribed to the most polar cis conformer.
► The IR lower carbonyl frequency in CCl4 was ascribed to the gauche conformer.
► The NBO analysis and electrostatic interactions justifies the conformers found.