Thermodynamic estimation of vapor pressure for organophosphorus pesticides

A method was developed to estimate vapor pressure of organophosphorus compounds having various functional groups or bond types. The method requires only information on chemical structure of the compounds. The thermodynamic approach using the modified Watson equation was successfully applied to estimate the vapor pressures of organophosphorus pesticides, including phosphorothionates, phosphorodithioates, phosphoramidothioates, phosphates, phosphorothiolates, and phosphorodithiolates. The optimization of the Watson method was found essential for correctly estimating the vapor pressures of organophosphorus compounds, as were the adequate molar refraction of phosphorus atoms in the Eisenlohr correlation and the introduction of an empirical constant for the nitro group in Meissner's equation. The number of possible hydrogen bond sites was found to improve the accuracy of estimated vapor pressures, especially for pesticides possessing the P=O moiety. Our modified Watson equation was used to successfully elucidate the vapor pressure of pesticides ranging from 10(-5) to 10(1) Pa at 298 K and could be used to correctly predict the vapor pressures of other organophosphorus pesticides not utilized in the analysis.