INTERPRETATION OF PHOTOELECTRON-SPECTRA IN TERMS OF THE SEMIEMPIRICALAM1 METHOD .4. 3-SUBSTITUTED 2-PHENYLPYRROLES

Equilibrium torsional angles of 2-phenylpyrroles and their radical cat ions in the ground and ionized states have been estimated using UV pho toelectron and electron spectroscopies, respectively, in combination w ith AM1 quantum-chemical calculations. A theoretical study of the inte rnal rotation potential by means of the total energy partitioning appr oach has led to the following conclusions. a) In the case of molecules and radical cations of biphenyl and its heteroanalogs, the tendency t o adopt a planar conformation (V-2 barrier) is determined, in the firs t place, by the contribution of the two-center resonance component of the interannular bond energy. b) Non-valence interactions are vital fo r the stabilization of the gauche conformation. Their effect on the V- 4 barrier of biphenyls is stronger than that observed for 2-phenylpyrr oles.