Quantum Chemical Conformational Analysis on Isotactic Polyacrylonitrile Using Molecular Model Compounds

Abstract

An attempt was made to decide the most stable structure for isotactic polyacrylonitrile using dimer and trimer model compounds with aids of semi-empirical quantum chemical methods (i.e., CNDO/2 and MINDO/3 methods) where possible conformations were considered by rotating main chain bond and bending nitrile triple bond from ordinary C-C-N straight bond at side chain. The results show that two adjacent nitrile groups have mutual attractive interactions and bend to get near to each other by assuming the prolonged CN bond length which reflects the tendency of double bond for CN groups in meso arrangement. This conclusion supports Kamide et al.’s proposal deduced from the systematic experimental results on chain stiffness and NMR peak assignment for stereoregular polyacrylonitrile. The bending nitrile bond structure of PAN was obtained for the first time from the view point of molecular orbital interactions in low molecular model compound.