An ab initio quantum chemical study of the electronic structure and stability of the pyrrolyl radical: Comparison with the isoelectronic cyclopentadienyl radical

The electronic structure and stability of pyrrolyl are investigated using CASSCF, CASPT2 and G2(MP2) techniques. The ground state of pyrrolyl is found to be ²A₂, with five π-electrons, as in cyclopentadienyl. The computed N–H bond energy of pyrrole is 94.8 kcal mol⁻¹, while the heat of formation Δ_fH₂₉₈^o of pyrrolyl is deduced to be 70.5±1 kcal mol⁻¹. The Arrhenius parameters of N–H and C–H bond fission in pyrrole and cyclopentadiene and hydrogen abstraction reactions (by hydrogen) were also computed, indicating that pyrrolyl forms predominantly by C–H bond fission of pyrrolenine rather than by direct N–H bond fission.