Steric and electronic effects in methyl-substituted 2,2 '-bipyrroles and poly(2,2 '-bipyrrole)s: Part II. Theoretical investigation on monomers

The effects of N- and C-beta-methyl substitution in pyrrole and 2,2'-bipyrr ole were investigated through ab initio calculations and Atoms in Molecules analysis of the resulting wave functions. Replacement of a hydrogen atom w ith a methyl group in pyrroles lowers the ionization potential, with substi tution at C3 being more efficient than N-substitution because of the larger release of pi population to the ring in the former case. Full geometry opt imization at RHF/6-31G** level and as a function of the torsion angle tau b etween two adjacent rings demonstrates that the increasing loss of planarit y in the 2,2'-bipyrrole, N,N'-dimethyl-2,2'-bipyrrole, and 3,3'-dimethyl-2, 2'-bipyrrole series, adversely affects the positive contributions expected from methyl substitution. An intramolecular interaction energy model shows that the greater anti-planarization energy of N,N'-dimethyl-2,2'-bipyrrole, as compared to 3,3'dimethyl-2,2'-bipyrrole, is due to the larger decrease in the stabilizing electrostatic term and to the larger increase in the des tabilizing nonbonding contribution which occurs at t 0 degrees in the forme r. Calculations on the corresponding monocations and analysis of new conduc tivity measures on substituted poly(2,2'-bipyrrole)s suggest that the ease in achieving local chain planarity in doped polypyrroles should be more clo sely correlated to the anti-planarization energies of the charged monomers rather than to anti-planarization energies of the neutral monomers.