AB-INITIO DETERMINATION OF THE CRITICAL-POINTS OF THE POTENTIAL-ENERGY SURFACE OF THE N-(3)-H AND N-(1,N-3)-H TAUTOMERS OF HISTAMINE MONOCATION

Critical points of the potential energy surface (PES) of N-(3)-H and N -(1,N-3)-H tautomers of histamine monocation have been characterized b y means of RHF wavefunctions at the 6-31G*//6-31G** level. The PES ca n be divided into three areas: (1) trans area of the N-(3)-H tautomer; (2) proton transfer area; (3) area of N-(1,N-3)-H tautomer. In zone ( 2) critical points have been determined at the MP2/6-31G*//MP2/6-31G* level with frozen core. The potential energy obtained with MP2/6-31G C//6-31G** calculations is a good estimation of that obtained at the MP2/6-31G*//MP2/6-31G** level. In this region a gauche conformer of t he N-(1,N-3)-H tautomer with an internal hydrogen bond is localized as the most stable. This internal hydrogen bond can account in part for the high basicity of histamine. From this structure the proton is tran sferred to another hydrogen-bonded configuration of the N-(3)-H tautom er, the difference in energy and the proton transfer barrier being low . This phenomenon shows a small difference in the basicity between bot h nitrogens (N-(1) and N-amino) of the molecule, and consequently an e asy proton transfer between both moieties of the molecule should be pr oduced. The other two zones of the surface are very rich in critical p oints. The trans regions are not favoured at all in the gas phase. Sui table estimation of the potential energy can be obtained from the 6-31 G*//6-31G** calculations. These PES domains present low trans-gauche barriers towards the hydrogen-bonded conformers. The transition struct ures between different conformers are also characterized.