Ab initio investigation of Sarin micro-hydration

The micro-hydration of the chemical warfare agent Sarin (GB) was investigated using ab initio calculations. The different structural conformers, water hydrogen bonding motifs, and adsorption energies for a series of gas phase optimized Sarin·nH2O (n = 1–4) clusters were evaluated. A rich and varied structural environment for these micro-hydrated clusters was observed, with hydrogen bonding between water and the PO oxygen being the dominating binding interaction. Only with the addition of at least four explicit waters does hydrogen bonding between water and either the isopropyl oxygen or the fluorine atom of Sarin become energetically favorable in these gas-phase optimized clusters. It is determined that the different isopropyl torsional configurations of Sarin give rise to changes in the water adsorption energies. The relative energies between the different Sarin conformers were also dependent on the number of explicit waters present within the optimized micro-hydrated cluster. Comparison of the energies for the Sarin·4H2O cluster did not reproduce the destabilization of the high energy Sarin conformer that was predicted from calculations employing an implicit water PCM solvent. The use of a mixed cluster/continuum model was shown to be unsuccessful for the computation of accurate free energies for these micro-hydrated Sarin clusters.