DFT and related techniques help to unravel the dependence of proton transfer on solvent polarity in ion pairs of carbamates and thiocarbamates with amidinium- and imidazolium-based counterions.

Abstract

Small, but important, differences in the structure–property relationships between ionomers composed of amidinium or imidazolinium groups with alkylcarbamate or alkyldithiocarbamate counterions have been examined experimentally by us previously. To unravel the sources of these differences, DFT calculations are conducted here for ion-pair complexes (IPs) of these systems and their corresponding uncharged base and acid components (NPs). Calculations include IPs and NPs in which the amidine/amidinium and imidazoline/imidazolinium groups are anchored to a dimethylsiloxane pentamer. A surprising dependence of proton transfer on the dielectric constant (ε) of the medium is found for the systems: Whereas interconversion between the NPs and IPs is strongly dependent on medium dielectric in systems with an alkylcarbamate, none of the initial IP forms with an alkyldithiocarbamate transforms to an NP. Although the calculations do not include individual solvent–solute molecular interactions, they do probe how the components sense their bulk environments. The lack of detectable reversible proton transfer from the amidinium or imidazolinium cations to the dithiocarbamates is consistent with the “principle of proton affinity/pK_a equivalence”: Because the acidity of dithiocarbamates is higher than that of carbamates, the gap between their proton affinities (ΔPA) is decreased, favoring stronger electrostatic-based H-bonds in the ion-pair complexes of the dithiocarbamates. The differences can also be estimated from the Deuri–Phukan nucleophilicity index scale, which is based on DFT calculations and suggests a strong dependence of proton transfer on the nucleophilicity of the base and the dielectric constant of the solvent. Predictions from these calculations on some important experimental systems are mentioned.