The R(X=)C-NR(1)R(2) (R(1), R(2) = alkyl groups) fragment is present in many classes of molecules and assumes, usually, a planar conformation owing to the C-N partial double-bond character. It can undergo,however, 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 semiempirical potential giving the total energy of the fragment during its deformation was proposed. In the present work the previous sample of crystal structures is updated to the current state of the crystallographic databases; the observed geometries are compared with the out-of-plane deformation energy maps obtained by ab initio SCF calculations at the 4-31G level for sample molecules (thioformamide, formamide, formamidine, vinylamine, and aniline) representative of the five chemical classes investigated. It is shown that the originally proposed potential is validated by this analysis and that the values of the energetic barriers involved in the reaction, evaluated from the ab initio energy maps, are in good agreement with the available experimental data. It is found, 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 compounds are reported, and possible reasons for the discrepancies discussed.