The Proton Transfer Properties of Imidazole

The imidazole molecule is paired with NH3 in order to examine the proton transfer properties of the former by ab initio methods. The primary minimum on the surface is ImH+···NH3 wherein the inter-nitrogen distance in the H bond is 2.89 Å. A second well appears in the surface, corresponding to Im···+HNH3, and the barrier between the two minima is rapidly enlarged, when the latter distance is elongated. When the NH3 is displaced from the N lone pair direction of the imidazole in the plane of the latter, the greater proton-attracting power of the imidazole relative to NH3 is enhanced; the opposite is observed when the NH3 is pulled out of the imidazole plane. This distinction is explained simply on the basis of the dipole and quadrupole moments of imidazole. The ability of imidazole to act as a proton shuttle from one molecule to another is examined by placing one NH3 molecule on either side. Taking H3NH+···ImH···NH3 as a starting point, the simultaneous transfer of two protons to form H3N···HIm···H+NH3 must overcome a large energy barrier. A stepwise process, passing through the H3N···HImH+···NH3 intermediate, is greatly favored energetically. If the central imidazole is permitted the freedom to translate between the two NH3 molecules, it is possible for the latter to be quite some distance apart. The imidazole will first approach within about 2.65 Å of the donor H3NH+ ion. The transfer to imidazole can then take place with little or no energy barrier. The protonated imidazole will then move close to the receptor NH3 before depositing the proton with it. In most cases, the largest energy barrier is associated not with the proton transfers, but with the motion of the imidazole cation. The barrier for this translation grows as the ultimate donor and acceptor NH3 molecules are moved further apart