Journal of Mot!ecuhr Structure (Theochm), 184 (1989) 115-129 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 115 zyxwvuts AN AB INITIO COMPARATIVE STUDY OF THE’ELECTRONIC PROPERTIES OF SULFONAMIDES AND AMIDES JOSE ELGUERO*, PILAR GOYA and ISABEL ROZAS In&it&o de Quimica Mkdica, C.S.I.C., Juan de la Cierva, 3,28006 Madrid (Spain) JAVIER CATALAN and JOSR LUIS G. DE PAZ Departamento de Quimica, C-XZV, Facultad de Ciencias, Universidad Autdnoma de Madrid, Cantoblunco, 28049 Madrid (Spain) (Received 20 April 1988) ABSTRACT A comparative ab initio study of amides and sulfonamides has been carried out using minimal, extended and polarization basis sets. Rotational barriers and tautomerism are discussed compar- atively. The two main conclusions of this study are the absence in sulfonamides of a conjugation of the type present in planar amides and the relative insensitivity of the total energy to nitrogen hybridization in sulfonamides in contrast with amides. INTRODUCTION Continuing with our work on heterocycles containing the -N-SOS-N- moiety [ zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1 ] we decided to carry out a comparative ab initio study of the parent sulfon- amide, H-SOP-NH2, and formamide, H-CO-NH2, which would provide the basis for the understanding of the analogies and differences shown by the re- lated cyclic structures. Although formamide has been the subject of numerous experimental and theoretical studies [2,3], calculations on this molecule and its tautomer, for- mimidic acid, have been repeated and contrasted with those of the literature, in order to provide a reference for our results on sulfonamide. Three different tests can be used to compare formamide with sulfonamide. (i) Rotation about the N-C and N-S bonds. There is a direct relationship between the barrier to rotation and electronic delocalization. However, some complications arise from the difference between amides (an sp2 carbon and sp2 or sp3 nitrogen atoms) and sulfonamides (an sp3 sulfur and sp2 or sp3 nitrogen atoms). (ii) Hybridization of the nitrogen. There is a relationship between its hy- bridization and conjugation mediated through the conformation. 0166-1280/89/$03.50 0 1989 Elsevier Science Publishers B.V.