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

## TL;DR

This study identifies and characterizes new phosphorus suboxide intermediates formed during the oxidation of white phosphorus.

## Contribution

The first geometric and vibrational characterization of P4O2 isomers in white phosphorus oxidation is presented.

## Key findings

- Two P4O2 isomers, P3OPO and P3PO2, were identified with distinct vibrational signatures.
- A cyclic P4O2 isomer was confirmed through computational analysis.
- Enthalpies of formation for the isomers were calculated, showing varying stability.

## Abstract

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 P4O
x
(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 3Σ
g

– O2 for bent-P3OPO, extended-P3OPO, P3PO2,
and cyclic-P4O2 are −93,
−88, −85, and −63 kcal mol–1, respectively.