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Basicity of neutral organic superbases with vinamidine structure in gas phase and acetonitrile : a density functional theory study

The gas-phase proton affinities (PAs) for a set of molecules with vinamidine structure are considered and their basicities in acetonitrile. It is shown that introducing double bonds to the imidazoline rings at the proton attachment site resulted in decrease in PA of the parent vinamidine. The increase in PA can be obtained by inducing modifications to the imidazoline ring at the junction of two diazepinylium rings. Placing methyl and dimethylamino substituents on the perimeter of the molecule further increased their gas-phase PAs. Studied vinamidine molecules are superbases, which possess PA values in the range between 261.0 and 284.2 kcal mol–1 in the gas phase and pKa values of 24.6–31.9 units in acetonitrile. Dismembering proton attachment site by opening the two diazepinylium and imidazoline rings resulted in the largest drop in PA values, indicating its importance in constraining the positions of imino nitrogen atoms in the neutral form of the molecule. Vinamidine molecules studied here present important pieces of the ladder of highly basic organic compounds for they possess accessible vinamidine molecular framework.

DFT calculations; vinamidine; superbasicity; gas phase basicity; pK_a

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