OUT-OF-PLANE DEFORMATION PATHWAYS OF THE R(X=)C-NR(2) FRAGMENT PRESENT IN AMIDES, THIOAMIDES, AMIDINES, ENAMINES, AND ANILINES - A CONCERTED STUDY MAKING USE OF STRUCTURAL DATA, MOLECULAR MECHANICS, AND AB-INITIO CALCULATIONS

The R(X=)C-NR(1)R(2) (R(1), R(2) = alkyl groups) fragment is present i n many classes of molecules and assumes, usually, a planar conformatio n owing to the C-N partial double-bond character. It can undergo,howev er, a cis-trans isomerization process by rotation around the C-N bond and concomitant nitrogen pyramidalization. In a previous paper (Gilli, G.; Bertolasi, V.; Bellucci, F.; Ferretti, V. J. Am. Chem. Sec. 1986, 108, 2420) the isomerization pathway was mapped by the use of some 90 crystal structures containing the fragment of interest, and a semiemp irical potential giving the total energy of the fragment during its de formation was proposed. In the present work the previous sample of cry stal structures is updated to the current state of the crystallographi c databases; the observed geometries are compared with the out-of-plan e deformation energy maps obtained by ab initio SCF calculations at th e 4-31G level for sample molecules (thioformamide, formamide, formamid ine, vinylamine, and aniline) representative of the five chemical clas ses investigated. It is shown that the originally proposed potential i s validated by this analysis and that the values of the energetic barr iers involved in the reaction, evaluated from the ab initio energy map s, are in good agreement with the available experimental data. It is f ound, moreover, that there is a specific class of compounds (o- and p- nitroanilines and polyconjugated enamines and amidines) which deviate from the general behavior. The crystal structures of two of these comp ounds are reported, and possible reasons for the discrepancies discuss ed.