AN ABINITIO CHARACTERIZATION OF THE GASEOUS DIPHOSPHORUS OXIDES P2O1-5

The gaseous diphosphorus oxides P2Ox (x = 1-5) have been characterized by ab initio electronic structure calculations. A total of 20 stationary points have been located at the HF/3-21G* level, and 15 at the HF/6-31G* level, by using analytic gradients and the GAUSSIAN86 program. Vibrational frequencies were calculated from analytic second derivatives at these same levels for each stationary point. Ten of the points at each level were found to correspond to local minima. Also found is a transition state for the ring opening isomerization of cyclic P2O (C2v) to liner P2O. The energetically preferred structures are the following: P2O, linear (C∞v), analogous to N2O; P2O2, trans planar (C2h); P2O3, nonplanar oxo bridged (C2); P2O4, nonplanar oxo bridged (C1 but nearly Cs); and P2O5, nonplanar oxo bridged (C2). Isomerization energies were calculated at the MP2/6-31G*/ /HF/6-31G* and MP4SDTQ/6-31G*/ /HF/6-31G* levels; it is found that while correlation lowers the energies of P-P bonded isomers of P2O3 and P2O4 relative to the energies of their oxo-bridged isomers, the latter are nevertheless energetically preferred, by approximately 13 000 and 9500 cm-1 for these two molecules, respectively. It is noted that the oxo-bridged structures of P2O3, P2O4, and P2O5 closely resemble the local structures in the tetraphosphorus oxides P4O6 and P4O10, with the tetrahedral structures of P4O6 and P4O10 resolvable into a pair of C2-symmetry oxo-bridged P2O3 and P2O5 moieties, respectively. © 1990 American Chemical Society.