Substituent effects on electronic character of the CN group and trans/cis isomerization in the C-substituted imine derivatives: A computational study

Substituent effects (R = F, OH, OCF3, OCH3, CN, CHO, NH2, NO, CH3, CF3, PH2, SH, SiH3) on C-substituted methanimine were investigated using B3LYP and G3 calculations. The physical origin of substituent effects is explored by means of NBO (nature bond orbital) and AIM (atoms in molecules) as well as correlation analysis. The results reveal that the π-withdrawing/donating ability of substituents plays a dominant role in influencing the atomic charge on the CN nitrogen while the charge variation on the CN carbon atom attached by substituents can be well described by the substituent group electronegativity. AIM analysis indicates that the substituent changes the atomic charge adjacent to it through the modification of its interatomic surfaces. In terms of NBO analysis, the effect of substituents on the trans/cis inversion barriers can be attributed to the substitution induced variations on hyperconjugation of lone pair on the inverting nitrogen (nN) into the adjacent σ∗(CR) orbital in the transition state, and displays a strong dependence on the electronegativity of substituent groups. The isodesmic reaction suggests that the highly electronegative substituents along with strong π-donating character would obviously stabilize the C-substituted imines.