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The HN- and N-methylimines of formaldehyde, acetaldehyde and acetone: electronic structure and inversion barriers of nitrogen atom

Natalia V. Kutsik-Savchenko, Oleg S. Lebed, Aleksander V. Prosyanik


The transmission of electronic effects and factors which affect the inversion barriers of atom N (ΔEi¹) in unsubstituted (I) and N-methyl derivatives (II) of formaldehyde (a), acetaldehyde (b), and acetone (c) have been investigated using the DFT method within the NBO. It was defined that the negative charge on the N atom (qN) decreases with N-methylation. The energy of lone pare, qN and ΔEi¹ values (imines Ia, E-Ib; Z-Ib, Ic; IIa E-IIb; Z-IIb, IIc) increases and the negative charge on C= atom decreases with C-methylation. The reduction of qN values in N-methylimines is caused by the major influence of more electronegativity of C atom and minor influence of two-electronic interactions. The relative increase of qN values with increase of degree of C-methylation is caused by hyperconjugation of the methyl groups with the antibonding orbitals of C=N bond but not +I-effects of C-substituents. C-methylation increases the inversion barriers owing to the interactions of the orbitals of iminyl atom C and does not affect their changes due to the interactions involving orbitals of =C–R2 bonds. It denies the supposed dominant influence of nN®s*C–R interactions on ΔEi¹ values. In general, electronegativity of N-substituents and steric strain has the dominant influence on the inversion barriers but not the intramolecular interactions.


imines; inversion; electron density; intramolecular interactions


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