Formation Mechanism of the Unsubstituted Chlorophosphazene Cl 3 P═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, Cl 3 P═NH, has never been isolated and fully characterized. Here we investigate the formation of Cl 3 P═NH from PCl 5 and NH 3 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 S N 1 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 Cl 3 P═NH·2HCl can be completed once the energy barrier for the formation of [PCl 4 -NH 3 ] + Cl - is overcome. Therefore, our theoretical work might explain the challenges in isolating any of the IMs in a typical chlorophosphazene reaction in chlorobenzene.