The work is aimed at the development of chemical theory, namely the establishment of the relationship between the structure of amines and imines, their electronic structure, intramolecular interactions, and energy parameters of the inversion of the nitrogen atom based on the methods of quantum chemistry. At present, the mutual influence of substituents at the nitrogen atom, the geometry of molecules, the distribution of charges on atoms and groups, the hybridization of free electron pairs of nitrogen atoms, the contributions of s-orbitals to them, populations, energies of free electron pairs, donor-acceptor and repulsive electronic interactions on the inversion barriers of the nitrogen atom is practically unknown. A systematic study of the relationships between inversion parameters is relevant since only by considering related compounds it is possible to provide a reliable analysis of the factors affecting the inversion barriers, to determine the ways of obtaining nitrogen-containing compounds with certain configurational stability.
This study of the inversion in ammonia and formaldimine derivatives has revealed the relationship between the inversion barriers and the electronegativity as well as conformational energies of the substituents at the nitrogen atom. It has been shown for the first time that a decrease in the sums of bond angles at nitrogen atoms, negative charges on them, energies of free electron pairs of nitrogen atoms, and an increase in their energy difference between the ground and transition states, s-character and populations leads to an increase in the inversion barriers. The electronegativity of ХНn substituents is the dominant factor determining all other parameters of nitrogen-containing compounds. The increase in the electronegativity of X atoms within the period leads to the relative stabilization of the ground and transition states of molecules; the growth of the inversion barriers is due to the relative stabilization of the ground states, and not to the destabilization of the transition states of the inversion. The total effect of intramolecular interactions contributes to a decrease in the inversion barriers, and, with an increase in electronegativity of X atoms, decreases for the elements of the second period and increases for the elements of the third period. The charges on the imine carbon atoms increase in proportion to the energies of the donor-acceptor intramolecular interactions of the σ-bonds of the C-H, Si-H, and PEP heteroatoms with the antibonding orbitals of the C=N group. The dominant influence on the change in charges is exerted by the electronegativity of substituents (X atoms of the second period) or the energy of intramolecular interactions (X atoms of the third period). Abnormally high inversion barriers of N-methyl and N-chlorine derivatives are caused by the destabilization of inversion transition states due to the lack of nX→σ*N-H interaction in methylamine and the increase in the energies of nN↔nCl interactions in chlorine derivatives, absence or decrease of interaction energies of free electron pairs of X atoms with antibonding orbitals of C=N bonds in formaldimines. An analysis of the contributions from the interactions of free electron pairs of the nitrogen atom with the Rydberg orbitals of X atoms refutes the assumption about the significant contribution of nN→3dX-conjugation to the reduction of the inversion barriers.
