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An analysis of carbonyl bands in the infrared spectra of some 2-ethylsulfonyl 4′-substituted phenylacetates bearing substituents NO2 ( 1 ), H ( 2 ) and OMe ( 3 ), supported by B3LYP/6-31G(d,p) and SM5.42R at PM3 level calculations, along with natural bond orbital analysis (NBO) and X-ray diffraction (for  1 ) was performed. Theoretical data indicated the existence of two stable gauche conformations ( g  1  and  g  2 ). The  g  1  conformer is the most stable, least polar and has the lowest ν CO  frequency. The more intense, lower frequency carbonyl doublet component found in CCl 4  solution is assigned to the  g  1  conformer. As the solvent dielectric constant increases (going from CCl 4  to MeCN) the higher frequency ν CO  doublet increases in intensity. This behaviour is reproduced by the solvation free energy calculations, supporting the conformer assignments. NBO calculations indicate that the most important orbital interaction is LPO9 → π*C7=O8 for both conformers, which corresponds to [C=O ↔ C+–O-] conjugation. This stabilises the  g  1  conformer to a greater extent and is responsible for the lower ν CO  frequency. The sum of the selected NBO delocalisation energies for  1 – 3  indicates that the  g  1  conformer is more stable. It is concluded that the calculated greater stability of the  g  1  conformer is due to a balance of attractive electrostatic and orbital interactions along with relevant hydrogen bonds. The X-ray crystal structure analysis of  1  shows the presence of two crystallographic independent but almost superimposable molecules each which adopt a  cis  geometry. The molecules are consolidated into the three-dimensional crystal packing by C–H…O interactions as well as by nitro- N–O… π (phenyl) contacts.

Spectroscopic and theoretical studies of some 4′-substituted-phenyl 2-(ethanesulfonyl)acetates. Structure of 4′-nitrophenyl 2-(ethanesulfonyl)acetate