Molecular dynamics simulations of N,N,N,N-tetramethylammonium dicyanamide plastic crystal and liquid using a polarizable force field

A quantum chemistry based, dipole polarizable force field has been used to simulate the N,N,N,N-tetramethylammonium (TMA) dicyanamide (DCA) ionic salt, in both plastic crystalline and liquid phases. Simulations predicted the [TMA][DCA] crystal structure and dimensions in good agreement with experiment. Ion-counterion spatial distributions are used to understand the local environment and ion pairing of both ions in crystalline and liquid phases. The rotational dynamics of ions in the crystalline system are thoroughly explored. Arrest of the DCA rotational degrees of freedom was associated with the experimentally observed solid-solid phase transitions. The self-diffusion coefficient and conductivity were calculated for the liquid state; however no net ion diffusion is noted in the pristine crystalline state. Introduction of ion vacancy at 0.3% concentration is found to be sufficient to enable ion diffusive behavior and conduction at 425 K in the crystalline state, with good agreement found between the experimental and simulated conductivity.