Engineering non-ionic carbon super- and hyperbases by a computational DFT approach: substituted allenes have unprecedented cation affinities

Abstract

The gas phase proton affinities (PAs) of substituted allenes in the gas phase were examined with the B3LYP/6-311+G(d,p) computational method. The calculations revealed that the designed allenes bearing a group having the potential for aromaticity offer scaffolds suitable for engineering robust non-ionic organic bases. Their protonation at the C(sp) site introduces super- and hyperbases as evidenced by their enthalpies of protonation ΔH (13) = 1243 and ΔH (24) = 1305 kJ mol⁻¹, which are close to the threshold of hyperbasicity of 1255 kJ mol⁻¹. It was conclusively shown that the basicity of allenes was dramatically amplified by substitution of electron releasing groups such as methyl and dimethylamino on the molecular framework. The allene disubstitued with two methylenecyclopropene groups gave the smallest superbase, possessing only hydrocarbon structure 23. Furthermore, interactions of some of the designed allene derivatives with the cations Li⁺, Na⁺, K⁺ and Cu⁺ were investigated using the mentioned computational method. The calculated gas-phase cation affinities and cation basicities for the molecules decrease as H⁺ > Cu⁺ > Li⁺ > Na⁺ > K⁺. The nucleus-independent chemical shift and harmonic oscillator model of aromaticity as two indices of aromaticity were used as a measure of the molecule/cation interaction.