Variations of Tautomeric Preferences in Histamine Monocation-Ab initio Studies for "Essential" and "Scorpio" Conformations from the Gas Phase to Aqueous Solution

The polarizable continuum model (PCM) for geometries optimized at the RHF/6-31G* was applied to study the variations of the tautomeric preferences in the histamine monocation from the gas phase to aqueous solution. Seven solvents of different polarities (from cyclohexane to water) were chosen and calculations performed. A change of the tautomeric preference takes place already in apolar solvents containing heteroatoms. The ring N-aza protonated form (ImH+) is only favoured in the gas phase and cyclohexane,oe benzene, CCl4. The chain N-amino protonated form (AmH+-T1) predominates in other solvents: CHCl3, THF, acetone, water.