The fundamental interactions of a series of chemical warfare agent (CWA) simulants on amorphous silica particulates have been investigated with transmission infrared spectroscopy and temperature-programmed desorption (TPD). The simulants methyl dichlorophosphate (MDCP), dimethyl cholorophosphate (DMCP), trimethyl phosphate (TMP), dimethyl methylphosphonate (DMMP), and diisopropyl methylphosphonate (DIMP) were chosen to help develop a comprehensive understanding for how the structure and functionality of CWA surrogate compounds affect uptake and hydrogen-bond strengths at the gas− surface interface. Each simulant was found to adsorb molecularly to silica through the formation of strong hydrogen bonds primarily between isolated surface silanol groups and the oxygen atom of the PO moiety in the adsorbate. The TPD data revealed that the activation energy for desorption of a single simulant molecule from amorphous silica varied slightly with coverage. In the limit of zero coverage and the absence of significant surface defects, the activation energies for desorption were found to follow the trend MDCP < DMCP < TMP < DMMP < DIMP. This trend demonstrates the critical role of electron-withdrawing substituents in determining the adsorption energies through hydrogen-bonding interactions. The infrared spectra for each adsorbed species, recorded during uptake, showed a significant shift in the frequency of the ν(SiO−H) mode as the hydrogen bonds formed. A clear linear relationship between the desorption energy and the shift of the surface ν(SiO−H) mode across this series of adsorbates demonstrates that the Badger−Bauer relationship, established origninally for solute−solvent interactions, effectively extends to gas−surface interactions. High-level electronic structure calculations, including extrapolation to the complete basis set limit, reproduce the experimental energies of all simulants with high levels of accuracy and have been employed to provide insight into the molecular-level details of adsorption geometries for the simulants and to predict the interaction energies for the CWA isopropyl methylphosphonofluoridate (sarin).

Sarin and soman are warfare nerve agents that represent some of the most toxic compounds ever synthesized. The extreme risk in handling such molecules has, until now, precluded detailed research into the surface chemistry of agents. We have developed a surface science approach to explore the fundamental nature of hydrogen bonding forces between these agents and a hydroxylated surface. Infrared spectroscopy revealed that both agents adsorb to amorphous silica through the formation of surprisingly strong hydrogen-bonding interactions with primarily isolated silanol groups (SiOH). Comparisons with previous theoretical results reveal that this bonding occurs almost exclusively through the phosphoryl oxygen (P═O) of the agent. Temperature-programmed desorption experiments determined that the activation energy for hydrogen bond rupture and desorption of sarin and soman was 50 ± 2 and 52 ± 2 kJ/mol, respectively. Together with results from previous studies involving other phosphoryl-containing molecules, we have constructed a detailed understanding of the structure-function relationship for nerve agent hydrogen bonding at the gas-surface interface.

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The kinetics and mechanism of reactions between gas-phase ozone and surface-bound C60 have been investigated by monitoring changes to reflection-absorption infrared spectra within a well-characterized film of C60 during exposure to a controlled flux of pure ozone. These ultrahigh vacuum studies provide direct infrared spectroscopic evidence for the formation and decomposition of a primary ozonide of C60. The spectral assignments of this highly unstable intermediate have been verified using electronic structure calculations. Theory and experiment revealed that C60 oxidized nearly exclusively via addition of ozone across the double bond that links two six-carbon-containing rings of the molecule. Following spectral characterization, the initial probability for ozone to react with the surface was found to be 5.8 ± 0.2 × 10(-4). Once formed, the ozonide quickly thermally decomposed to a variety of carbonyl-containing products.