On the Mechanism of Carborane Diffusion on a Hydrated Silica Surface

Studies of motion of carborane wheel-like molecules on glassy surfaces provide an important contribution to the practical goal of designing molecular nanoscale transporters called nanocars. In these vehicles, carborane wheels are chemically coupled to chassis allowing these devices to participate in translations over the surface. As a preliminary step toward modeling dynamics of these species, we have investigated interactions of the p-carborane molecule with the hydrated (1010) surface of α-quartz. Quantum calculations with the CP2K package (cp2k.berlioz.de) have been performed for a series of model systems in order to explicitly estimate interaction energies between the carborane wheel and the surface. It was found that 8 kJ/mol for absorption of carborane on the surface provides a reasonable estimate for this interaction. Correspondingly, the four-wheeled vehicle would require at least 32 kJ/mol to activate its diffusion on the glassy surface. The latter value is consistent with experimental estimates of 42 kJ/mol for the activation energy as follows from single-molecule measurements of temperature dependence of the diffusion coefficient. It is argued that the translation of the four-wheeled carborane nanocars might be consistent with both rolling and hopping mechanisms of motion.