Structural studies of polypyrroles - I. An ab-initio evaluation of bondingthrough alpha and beta carbons

Detailed ab-initio calculations of a number of pyrrole oligomers are carrie d out in order to find out relative stability of structures bonded through alpha and beta carbons. Energetics of dimers, trimers and tetramers with al l possible linkage types and several pentamers are obtained from fully opti mized geometry. The linear as well as the branched forms of tetramers and p entamers are compared so that probabilities of branching can be estimated. The energy differences of different types of structures suggest that a grea t deal of branching is probable. It is shown that relative energies of 65 o ligomers can be fairly reproduced by a simple partitioning in terms of mono mer types. The approximate probability function generated in this manner is used in the following paper in a statistical mechanical approach to estima te the extent of bonding involving beta carbons as well as branching in pol ypyrroles. (C) 1999 Elsevier Science S.A. All rights reserved.