Surface binding of organophosphates on silica: Comparing experiment and theory

A consistent embedding hierarchy is applied to the calculation of binding enthalpies for organophosphate molecules to a silica surface and compared to experiment. The interaction of four probe molecules, dimethyl methylphosphonate (DMMP), diisopropyl methylphosphonate (DIMP), diisopropyl fluorophosphate (DFP), and sarin, with the silica surface is examined. Quantum chemical methods are employed to compute binding enthalpies and vibrational spectra for all interactions between probe molecules and silanol sites on the silica surface. Comparison with experimentally measured infrared shifts indicates that the theoretically modeled adsorption sites are similar to those found in experiment. The calculated binding enthalpies agree well with experiment for sarin, ΔH_ads,443K = -22.0 kcal/mol (calculated) vs -18.8 ± 5.5 kcal/mol (measured, 433 K < T_expt < 453 K), and DIMP, ΔH_ads,463K = -26.9 kcal/mol (calculated) vs -29.3 ± 0.9 kcal/mol (measured, 453 K < T_expt < 473 K). Agreement with experiment is less good for DMMP, ΔH_ads,463K = -19.7 kcal/mol (calculated) vs -26.1 ± 1.5 kcal/mol (measured, 453 K < T _expt < 473 K), and DFP, ΔH_ads,423K = -20.4 kcal/mol (calculated) vs -27.5 ± 3.1 kcal/mol (measured, 413 K < T_expt < 433 K).