Quantum Chemical Study of the Thermochemical Properties of Organophosphorous Compounds

Organophosphorous compounds are involved
in many toxic compounds
such as fungicides, pesticides, or chemical warfare nerve agents.
The understanding of the decomposition chemistry of these compounds
in the environment is largely limited by the scarcity of thermochemical
data. Because of the high toxicity of many of these molecules, experimental
determination of their thermochemical properties is very difficult.
In this work, standard gas-phase thermodynamic data, i.e., enthalpies
of formation (ΔfH298°), standard entropies (S298°), and heat capacities (Cp°(T)), were determined using quantum chemical calculations and more
specifically the CBS-QB3 composite method, which was found to be the
best compromise between precision and calculation time among high
accuracy composite methods. A large number of molecules was theoretically
investigated, involving trivalent and pentavalent phosphorus atoms,
and C, H, O, N, S, and F atoms. These data were used to propose 83
original groups, used in the semiempirical group contribution method
proposed by Benson. Thanks to these latter group values, thermochemical
properties of several nerve agents, common pesticides and herbicides
have been evaluated. Bond dissociations energies (BDE), useful for
the analysis the thermal stability of the compounds, were also determined
in several molecules of interest.