Protonation processes of unusual exothermicity

The protonation of guanidine [113-00-8] and biguanide [56-03-1] is more exothermic by 25 and 70%, resp., than the neutralization of OH-, the strongest base in aq. soln. The effect is caused by the symmetrization of the mesomeric mols. during the process (resonance energy). The protonated base mols. are also more rigid, which shows up in unually low and even neg. entropy changes.