Formation Mechanism of the Unsubstituted Chlorophosphazene ClP═NH: A Theoretical Study via Quantum Mechanical Calculations.

Although the synthesis of chlorophosphazene polymers has been explored for more than 100 years, the shortest yet most illusive monomer, ClP═NH, has never been isolated and fully characterized. Here we investigate the formation of ClP═NH from PCl and NH in chlorobenzene through quantum mechanical calculations. The potential energy surface was mapped using the MP2 Hamiltonian in conjunction with Dunning's correlation-consistent basis sets (aug-cc-pVXZ, where X = D and T). Along with HOMO/LUMO frontier molecular orbitals and natural bond orbital analyses, we found that instead of following the S1 path proposed in the literature, the reaction proceeds via an addition-elimination mechanism. Our results also indicate that due to the low-lying stable intermediates (IM), most steps are exothermic such that the production of ClP═NH·2HCl can be completed once the energy barrier for the formation of [PCl-NH]Cl is overcome. Therefore, our theoretical work might explain the challenges in isolating any of the IMs in a typical chlorophosphazene reaction in chlorobenzene.