On the nature of tetraalkylammonium ions in common electrochemical solvents: General and specific solvation – Quantitative aspects

The free energies of each of 80 tetraalkylammonium ion/solvent complexes [R4N+/(solv)n], with R ranging from methyl through butyl and n ranging from 1 through 4, were computed by density functional theory (DFT) in five common electrochemical solvents: dimethylformamide (DMF), dimethylsulfoxide (DMSO), acetonitrile (AN), dichloromethane (DCM), and methanol (MeOH). The energies of the complexes were computed both with and without their solvation energies. Additional computations of the energies of the individual components, both solvated and unsolvated, were also carried out. The resulting data permit construction of a thermodynamic cycle for each R4N+/solvent pair that in turn allows the determination of the extent of general and specific solvation energies for that pair. An additional series of computations for pentane as solvent were carried out. Since this solvent should not coordinate with tetraalkylammonium ions, these computations provide a test of the validity of the computational method. This work represents a useful new general protocol for assessing the relative importance of general and specific solvation in chemical systems.