Ab initio modeling of organophosphorus combustion chemistry

Abstract

An ab initio study has been conducted to calculate the thermochemistry of several organophosphorus intermediates, to assess the accuracy of estimated rate constants in current organophosphorus combustion mechanisms and to explore additional reaction pathways for methylphosphonic acid (MPA or PO(OH)₂CH₃). The organophosphorus reaction rates and thermochemistry were calculated for an elementary reaction rate model to predict MPA oxidation kinetics and reaction pathways in supercritical water. The thermochemistry for 17 phosphorus-containing species was calculated using the CBS-Q method, including species such as PO(OH)₂CH₂˙, PO(O˙)OHCH₃, and PO(OH)₂CH₂O˙, whose thermochemical values are unknown. Rate constants for fourteen reaction pathways involving MPA and its intermediates were calculated using conventional transition state theory. Unimolecular decomposition rates of MPA, bimolecular hydrogen abstraction rates between MPA and OH˙, and decomposition and bimolecular hydrogen abstraction rates of phosphoric acid and P˙O(OH)₂ were calculated and compared to estimated literature rate constants, when available. Agreement ranged from within one order of magnitude to more than six orders of magnitude difference. The most significant discrepancies occurred for reactions involving P–O bonds, indicating that estimates of these reaction rates based on analogous C–O bond chemistry are unreliable. Several types of free-radical organophosphorus reaction pathways were studied for the first time.