Vibrational Signatures of Unrealized Phosphorus Suboxide Intermediates in White Phosphorus Oxidation Reactions.

White phosphorus ignition notoriously produces the phosphoric acid anhydride P4O10, yet the intermediate oxidation steps remain undetermined. We report the first geometric and vibrational characterization of two P4O2 isomers, P3OPO and P3PO2, and substantiate a previously proposed cyclic P4O2 isomer. We formally assign the infrared bands observed by Andrews and Mielke at 898 and 891 cm-1 to the antisymmetric P-O-P vibrations of P3OPO species (Mielke and Andrews, 1990). Additional bands corresponding to terminal P═O and -PO2 stretches of P3OPO and P3PO2 discussed herein also went unrecognized due to the peculiar bonding of P4O2 species compared to oxo-bridged P4Ox (x = 3-6) species. Sequential addition of oxygen atoms to the P4 tetrahedron appears to form P3OPO and P3PO2, while cyclic P4O2 is formed from P2O dimerization. CCSD(T) geometries, CCSD(T) + MP2[δVPT2] fundamental frequencies, and enthalpies of formation extrapolated using focal-point analysis are reported. The predicted enthalpies of formation relative to tetrahedral P4 plus 3Σg- O2 for bent-P3OPO, extended-P3OPO, P3PO2, and cyclic-P4O2 are -93, -88, -85, and -63 kcal mol-1, respectively.