The intrinsic basicity of 4(5)-2′-aminoethylimidazole (histamine)

Article abstract of DOI:10.1021/ja00057a031

The gas-phase basicity (GB) of histamine (1) relative to ammonia (defined as the standard Gibbs energy change for reaction 1) has been measured by means of Fourier transform ion cyclotron resonance spectroscopy (FT-ICR): 1H⁺(g) + NH₃(g) ? 1(g) + NH₄⁺(g). The various tautomer/conformers of 1H⁺(g) were studied by means of ab initio SCF-LCAO-MO calculations at the 6-31G//6-31G level. The calculated proton affinity agrees well with that estimated from FT-ICR results. Ring protonation is slightly preferred over side-chain protonation. Chelation provides a major contribution to the stability of 1H⁺(g). Comparison of these results with aqueous solution data reveals dramatic differences due to solvation.