Protonation of Pyrrole : AModel Hardness ( Softness ) Sensitivity Study "

The protonation mechanism of pyrrole is investigated in terms of various global and regional (fragment) hardness (softness) parameters and related quantities in order to identify the most sensitive criteria indicating the lmown o.-preference for electrophilic substitution in this molecule. Both rigid and relaxed hardness data are discussed. Numerical results for the H+... pyrrole system, obtained from a realistic semiempirical Atoms-in-aMolecule (AIM) hardness matrix, clearly show that the resultant AIM hardness (the inverse of the AIM softness) provides the most sensitive reactivity index. It measures the effective AIM hardness in a given molecular environment and predicts the ~-carbons to be effectively much harder than «-carbons, for all alternative approaches of proton. Relaxational contributions to the relevant hardnesses, representing the effect of moderating electron redistributions outside the response and displacement molecular fragments, are shown to be very small and to have practically no effect on the selectivity of protonation.