Tautomeric equilibria and aromaticity of phosphodiazoles: An ab initio study

Geometric structures and thermodynamic parameters for all tautomeric forms of phosphodiazoles were computed at the MP2/cc-pVTZ level of theory. Results show a clear preference for the tautomeric forms where the tautomeric proton is connected to a nitrogen atom. Steric interactions between nitrogen and phosphorus lone pairs were investigated by Natural Steric Analysis. According to this methodology the destabilizing repulsion of lone pairs located on two vicinal nitrogen atoms is much larger than in the case where one of these atoms is a phosphorus atom. The maps of the Laplacian of the total electron density are presented to show shapes and delocalizations of nitrogen and phosphorus lone pairs. Aromaticity of phosphodiazoles was studied using NICS(1)ZZ and pEDA indices, which are mutually intercorrelated for theses compounds. The stability of a particular tautomeric form is linearly correlated with pEDA index of its five-membered ring.