Ab initio molecular orbital study of the conformation of amide group: o-methylformanilide

HF/4-31G ab initio quantum mechanical calculations on o-methylformanilide a re used to analyse the conformational isomerism of the amide group in the m olecule. It is known from experimental literature sources that the compound has some remarkable features: (1) purl trans and cis rotameric forms assoc iated with the secondary amide grouping can be experimentally isolated and their properties studied; (2) for this molecule the cia -CO-NH- conformatio nal form predominates at ambient temperature in contrast to results for man y other secondary amides. The principal results of the ab initio calculatio ns carried out in the present study can be summarized as follows: (1) ab in itio calculations employing at least HF/4-31G basis set are needed to corre ctly describe the conformational state of the amide group in o-methyl-forma nilide; (2) the ab initio results show that the trans conformers have a pla nar structure while the cis isomer is nonplanar; (3) the N-H stretching mod e frequency for the trans form is estimated to be 49 cm(-1) higher than the respective N-H frequency in the cis conformer; (4) the experimental infra- red spectrum of the compound in CCl4 reveals that the cis -CO-NH- structure predominates at ambient temperature in accord with previous experimental r esults; (5) the theoretically estimated barrier of rotation around the amid e C-N bond (E-a = 19.2 kcal/mol) is in accord with the experimentally detec ted simultaneous presence of cis and trans forms at ambient temperature. (C ) 1999 Elsevier Science B.V. All rights reserved.