Gas phase equilibrium structure of histamine

The first gas electron diffraction (GED) experiment for histamine was carried out. The equilibrium structure of histamine in the gas phase was determined on the basis of the data obtained. The refinement was also supported by the rotational constants obtained in previous studies [B. Vogelsanger, et al. , J. Am. Chem. Soc. , 1991, 113 , 7864-7869; P. Godfrey, et al. , J. Am. Chem. Soc. , 1998, 120 , 10724-10732] and quantum chemical calculations. The proposed mechanism of tautomerization by simultaneous intermolecular transfer of hydrogens in a histamine dimer helps to explain the distribution of tautomers in different experiments. The estimations of the conformational interconversion times provided the explanation for the absence of some conformers in the rotational spectroscopy experiments. Equilibrium structure of histamine in the gas phase was determined, tautomeric and conformational conversion mechanisms were studied theoretically.