Kutsyk-Savchenko N. Electronic structure and inversion barriers of imines

The thesis is aimed at the investigation of the correlation between the imines chemical structure, their electronic structure and barriers of the nitrogen inversion by means of quantum-chemical methods. It was shown that the topomerization of N-alkyl-imines passes through the inversion of the nitrogen atom independently of electronic properties of substituents at the imino-carbon atom. It has been found that the replacement of the hydrogen atom at the nitrogen atom and/or at the imino-carbon atom by alkyl groups leads to the increase of inversion barriers because of the higher electronegativity of the carbon atom and/or the rise in the effective electronegativity of the imino-carbon atom. The intramolecular interactions have the subordinate importance. When NH- and N-alkylimines contain the substituents EHn (EMen) at imino-carbon atom, where E is an element of the 2nd or 3rd period (n = 3–0), their inversion barriers decrease moving from left to right over the period (with increasing electronegativity of substituents and the positive charge on the imino-carbon atom). The interactions of the lone pair of the nitrogen atom with the orbitals of C–E bond have the main influence on the change of the inversion barrier. The inversion barriers of N-alkylsulfenylimines are lower than the inversion barriers of isoelectronic N-alkylimines which is mainly caused by interactions of the lone pairs of the sulfur atom with orbitals of С=N bond. The interactions of the lone pair of the nitrogen atom with 3d-orbitals of the sulfur atom and antibonding orbital of substituent bond at the sulfur atom have the subordinate importance. The decrease of the inversion barriers in the rank N-sulfenyl-, N-sulfinyl- and N-sulfonylimines іs a result of the increase of the steric interactions between substituents at the C=N bond. The barriers of the nitrogen inversion must increase with the rise in oxidation state of the sulfur atom because of the all mentioned interactions weakening and the increase of the sulfur atom electronegativity. The negative charge on the nitrogen atom decreases when incorporating N-alkyl groups into the methanimine molecule as a result of the influence of the donor-acceptor interactions and higher electronegativity of the carbon atom comparing to the hydrogen atom. The rise in the negative charge on the nitrogen atom with the augmentation of the length and branching of N-alkyl groups and methylation degree of the imino-carbon atom is caused by the increase of the electron density transfer from C–H bonds of the methyl groups to the nitrogen atom at the expense of the interactions of bonding orbitals of C–H bonds with antibonding orbitals of N–C and C=N bonds.