The effect of C-substituents on the topomerization mechanism, inversion barriers and electronic character of C[dbnd]N bond in N-alkylimines: A theoretical study

Abstract

The mechanism of the degenerated Z,E-isomerization (topomerization) and the electronic structure of the imines R2CNR′ (R=NMe2, CF3, CO2Me, SMe, Me, OMe, F, CN, COMe; R′=Me, i-Pr, t-Bu) have been investigated with the DFT method (B3LYP/6-31+G(d,p)) in terms of NBO. It has been established that the topomerization of N-alkylimines passes through the inversion mechanism independently of C-substituent type (with the minor contribution of rotational component in individual cases). By means of the correlation analysis it was shown that interactions of the lone pair of the nitrogen atom with orbitals of C-substituents (nN→σ∗CR and nN↔σCR) have the main influence on the value of the inversion barrier (ΔEi≠) in the ground state. The increase of the energies of nN→σ∗CR interactions or the difference between energies of nN→σ∗CR and nN↔σCR interactions leads to reduction of the ΔEi≠ value, shortening of the CN bond, and promotes the inversion mechanism of the topomerization. The charge on the nitrogen atom mainly depends on conjugation effect of C-substituents, whereas the charge on the imino-carbon atom depends on their electronegativity (inductive effect). The increase of electronegativity of groups of the 2nd period elements at the imino-carbon atom leads to the raise of the positive charge on the imino-carbon atom whereas the increase of electronegativity of carbon-containing groups leads to the decrease of the positive charge. At the same time, the positive charges on the imino-carbon atoms for the imines with the mentioned C-substituents correlate with the values of χ-, σi-, σR-, σP-constants according to various linear dependences. These dependences differ in the sign and in the value of slope.