Resolving the trifecta of discrepancies regarding the phosphorus suboxide P4O

Abstract

Unsatisfied with vibrational and mechanistic discrepancies between three previous investigations, we characterized the P₄O potential energy surface with high-level coupled cluster methods for resolution, up to CCSD(T)/cc-pV(T+d)Z. Our computations substantiate the previous vibrational band assignments of Andrews and coworkers to cyclic and terminal P₄O. The antisymmetric P–O stretch of bridge-bonded P₄O was previously assigned to a band at 856 cm⁻¹, and the symmetric stretch was assigned to a lower frequency band at 553 cm⁻¹. In contrast, we find the symmetric P–O stretch of bridge-bonded P₄O to lie higher than the antisymmetric, and formally assign the symmetric stretch to a previously unassigned 770 cm⁻¹ band. Tentative assignments for the now-elusive antisymmetric P–O stretch of bridge-bonded P₄O are discussed herein. To resolve mechanistic discrepancies, we demonstrate that the isomerization pathway between terminal and bridge-bonded P₄O is a two-step process, requiring passage through a local-minimum intermediate on the potential energy surface. It was previously suggested that the antisymmetric P–O stretch band originally assigned to bridge-bonded P₄O (856 cm⁻¹) could instead belong to a D_2d-symmetry P₄O₂ species, but our computed fundamental vibrational frequency (654 cm⁻¹) does not support this suggestion. New insight has revealed our previous substantiation of the vibrational band assigned to cyclic P₄O₂ to be incorrect, and we now assign the positive combination antisymmetric P–O stretch to an unassigned 919 cm⁻¹ band. Focal point analysis with basis sets up to cc-pV(Q+d)Z and correlation treatment to CCSDT(Q) is performed with the addition of zero-point vibrational energies for cyclic (−95.5 kcal mol⁻¹), bridge-bonded (−92.5), and terminal P₄O (−81.7) relative to tetrahedral P₄ plus ³P oxygen as well as D_2d-symmetry P₄O₂ (−62.0) relative to tetrahedral P₄ plus O₂. We provide the energy of oxygen (+58.6 kcal mol⁻¹) using our methods for comparison to previously reported P₄O₂ species with optimal error cancellation.