Crystallographic and theoretical investigations of the preferred conformations of three isomeric N-acetyl-dihydro-oxadiazoles

Structures of three isomeric 2- and 4- acetyl-5-ethyl-3-tolyl-dihydro-1,2,4 -oxadiazoles 1-3 have been determined by X-ray analysis of their single cry stals. 2-Acetyl-5-ethyl-3-(p-tolyl)- and 2-acetyl-5-ethyl-3-(m-tolyl)-2.5-d ihydro-1,2.3-oxaliazoles I and 2 are triclinic whereas 4-acetyl-5-ethyl-3-( p-tolyl)-4,5-dihydro-1.2,4-oxaliazole 3 is monoclinic. Molecular orbital ca lculations involving semi-empirical method (AM 1) and density functional me thod B3LYP/6-31G(d) were carried out with a view to compare these results w ith the crystallographic data. The bond lengths, bond angles and torsion an gles of all three compounds showed an excellent agreement between the calcu lated density functional method and crystallographic experimental values. ( C) 2001 Elsevier Science B.V. All rights reserved.