Surface Binding of Organophosphates on Silica: Comparing Experiment and Theory

Abstract

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).

Infrared signature of micro-hydration in the organophosphate Sarin: an ab initio study

“…In Sarin the magnitude of the predicted IR P=O vibration red shift suggests that the adsorption free energy of water complexation would be comparable to the free energy of Sarin adsorption at a SiOH surface [37]. These P=O red shifts were an order of magnitude smaller than the experimental −147 to −650 cm −1 red shifts observed for the SiOH vibration following surface adsorption of OPs on silicon [35,37].…”

“…The predicted P=O vibration frequency using the B3LYP method would require a scaling factor of 0.99 to match experimental (Table 3), while the MP2 results would need to be scaled by 0.98. It should be noted there was some variation in the experimental P=O stretch frequencies reported (Table 3), ranging between 1279 and 1308 cm −1 , but we have elected to use the newer Sarin results of Taylor and co-workers [35] for our discussion. Both the B3LYP and MP2 methods underestimated the lower frequency vibrations and required scale factors of 1.04 (B3LYP) or 1.01 (MP2) for the P-F stretch, and 1.02 (B3LYP) or 0.99 (MP2) for the P-O-C vibrational mode.…”

“…[35]. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration.…”

Calculations on the unimolecular decomposition of the nerve agent VX

“…1,3 It is, therefore, important to study and mitigate against such synthetic toxic molecules. Numerous researches have been devoted to investigate different methods to degrade V-series agents including, among others, surface sorption, [4][5][6][7] hydrolysis, [8][9][10][11][12] perhydrolysis, 13 novel catalysts, [14][15][16][17][18] binding to metal ions, 19 and incineration. In recent studies, the thermally activated pericyclic H-transfer reaction in gas-phase via a 6-memberedring transition state (TS) has been investigated for the G-series OP nerve agent sarin (GB), 20,21 and for VX simulants.…”

Multifunctional ultra-high vacuum apparatus for studies of the interactions of chemical warfare agents on complex surfaces

“…39 More recently, the decomposition of sarin on Al 2 O 3 nanoparticles 40 was studied by attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy and the desorption of sarin from silica surfaces was investigated via reverse gas phase chromatography. 41 This body of work, while valuable, is limited in its ability to simultaneously track surfacebound and gas-phase reaction products in a way that provides overall reaction rates on surfaces that are well characterized with surface-sensitive methods such as photoelectron spectroscopy. Moreover, most of the published studies that employ actual CWAs have been conducted in solution or under atmospheric conditions, 36,37,[42][43][44] which preclude the application of surface analytical instrumentation.…”