Potential of mean force between two molecular ions in a polar molecular solvent: A study by the three-dimensional reference interaction site model

The orientationally dependent potential of mean force (PMF) between two cha rged polyatomic solutes immersed in a polar molecular solvent is obtained b y using the three-dimensional reference interaction site model (3D RISM) of the integral equation theory. We propose a partially linearized hypernette d chain (PLHNC) closure. The method is applied to the N,N-dimethylaniline c ation (DMA(+)) and the anthracene anion (AN(-)) in acetonitrile solvent (CH 3CN). We solve the 3D RISM integral equations by employing the modified dir ect inversion in the iterative subspace (MDIIS) method, The 3D site distrib utions of Solvent around each solute are obtained and discussed. The PMF be tween the solutes is calculated as a 3D profile dependent on the relative p osition of the solutes at six characteristic relative orientations. The PMF obtained is in qualitative agreement with the results of molecular dynamic s simulations. In the solvent, the AN(-) solute effectively attracts the DM A(+) dimethylamino group and repels its phenyl ring. We found that the most stable relative arrangement of the DMA(+) and AN(-) molecules in acetonitr ile solvent is different from that in gas phase.