Relationships between basicity, structure, chemical shift and the charge distribution in resonance-stabilized iminoamines

Abstract

Taking formamidine (gas phase proton affinity ≈950 kJ mol⁻¹) and guanidine (gas phase proton affintiy ≈990 kJ mol⁻¹) as the prototype iminoamines with resonance-stabilized cations, similar non-cyclic analogs based on a conjugated carbon atom backbone are studied computationally. A simple model of the charge delocalization is developed which predicts theoretical maximum gas-phase proton affinties (PAs) of ≈1114 kJ mol⁻¹ (unbranched), 1154 kJ mol⁻¹ (singly-branched) and 1209 kJ mol⁻¹ (doubly-branched) for such systems. It is also shown that the ¹H and ¹⁵N chemical shifts of the protons and nitrogens in the imine functions correlate both with basicity and with atoms-in-molecules multipole moments. The properties of these and related compounds are discussed in the context of proton sponges or superbases.