Extended planarity and π delocalization in triazine-based derivatives

Substituents bearing amino groups in positions 2, 4 and 6 of sym-triazines (1,3,5-triazines) share the planarity of the corresponding aromatic triazine moiety to different extent. In these systems, π-electron delocalization also ranges over adjacent N atoms of amino groups. The origin of both chemical features is attributed to the stabilization of the HOMO. Negligible inversion barriers were found, rotational barriers increasing as the exo amino groups become planar. Rigid rotor harmonic oscillator approximation for lower frequency vibrations makes Gibbs free energy inversion barriers unreliable. Natural population analysis shows a direct relationship between the planarity of the exo amino groups and the donor-acceptor interaction involving the exo nitrogen lone pair and the vicinal CN ring antibonding orbital.

Changes in geometry of amino groups as substituents in positions 2, 4 and 6 of 1,3,5-triazines are calculated by using DFT at the B3LYP/6-311++G(d,p) level. The planarity of the amino groups increases with the number of nitrogen atoms in the aromatic ring. π-electron delocalization spreads over adjacent N atoms of amino groups. Inversion and rotational barriers of the amino groups are also estimated.