N-DERIVATIVES OF FORMALDIMINES: THE REASON FOR THE HIGH NITROGEN INVERSION BARRIERS IN N-METHYL- AND N-CHLOROIMINES
The energy and electronic parameters of the nitrogen inversion in imines Н2С=NХНn (ХНn = СН3, NH2, OH, F, SiH3, PH2, SH, Cl) have been calculated with the DFT method (B3LYP 6-311+G(d,p)) in terms of natural bond orbital. It has been established that the interactions of the nitrogen lone pair (LP) with the bond orbitals at the imino carbon atom are practically independent of the X atom and contribute to the decrease of the inversion barriers (ΔЕі≠). While nN→σ*X–H, nN↔σX–H and nN↔nX interactions substantially depend on the heteroatom type and promote the increase in the ΔЕі≠ values with the rise in electronegativity of the X atom. The contribution of the interactions of the nitrogen LP with the Rydberg orbitals of the C=N–X group atoms is small and they cannot be the main reason of the decrease in the ΔЕі≠ values when X atoms of the second period are replaced by atoms of the third period of the same group. The interactions of the LP of the X atoms and the X–H bond orbitals with the C=N bond orbitals have the main influence on the inversion barriers. The contribution of nX→π*C=N interactions to the ΔЕі≠ values is dominant. The main reason of the “anomalous” inversion barriers of N-methyl- and N-chloroformaldimines is the destabilization of inversion transition states because of the reduction in the energies of σX–H →π*C=N and nX→π*C=N interactions and the rise in the energies of nN↔nСl interactions. The contributions of electronegativity of ХНn substituents and energies of intramolecular interactions to the ΔЕі≠ values have been determined.