EXPERIMENTAL AND COMPUTATIONAL STUDIES ON AMINOGUANIDINE FREE-BASE, MONOCATION AND DICATION .2. ACID-BASE PROPERTIES, GAS-PHASE PROTONATIONENERGIES AND TOTAL ENERGIES OF 2 TAUTOMERS OF THE FREE-BASE

The structures and protonation energies of aminoguanidine and aminogua nidinium ion were obtained from quantum chemical calculations by using the density functional method B3-LYP and the standard basis set 6-31G (d). The energy differences between the various forms of the two possi ble tautomers, the endiamine and the imidamide form, were investigated . The endiamine was found to be more stable than the most stable form of the imidamide by 5 kcal/mol in the gas phase. The proton affinity o f aminoguanidine was found to be 241.4 kcal/mol (B3-LYP/6-31G(d)) in t he gas phase. The liquidation pK(a2) for amininoguanidine and the guan idinium ion was determined to be 11.5 +/- 0.1. The reference values fo r guanidine free base and the guanidinium ion are 235.7 kcal/mol and 1 3.6, respectively. The difference in the order of basicity in the gas phase and in aqueous solution between aminoguanidine and guanidine is discussed in terms of symmetry, resonance, delocalisation, rehybridisa tion of nitrogen atoms, and electrostatic repulsion.