A Theoretical Model of a Magnetic Polymer

Abstract

We report a theoretical model of a magnetic polymer consisting of benzenoid structures incorporated in a simple linear polyethylene-like polymer chain. The benzenoid structures are double ring annulenes having 12, 16, 20 and 24 outer ring π electrons and the benzene ring inside the main cyclic π system. Either six or three covalent bonds of benzene linked to the outer ring carbon atoms at regular intervals will bring planarity to the annulene systems. Hückel molecular orbital theory of quantum chemistry predicts that a 4n (n = 0, 1, 2,….) π alternating cyclic system has a triplet (2s + 1 = 3) stable ground state molecular orbital, provided the cyclic system is planar. Although cyclobutadiene is planar, it is quasi-stable. The 8, 12, 16, 20 and 24 annulenes are not planar but they are stable. If a benzene ring is introduced into the annulene ring and six or three carbon atoms of benzene are covalently linked to the carbon atoms of the outer ring annulene at regular intervals, the annulene systems might become planar. The presence of a benzene ring inside the annulene ring has little influence on the symmetry, electron distribution, energy level and molecular orbitals of the 4n system and as a result it is expected that these modified annulenes would have quite a stable triplet ground state. These predictions are here confirmed by DFT (density functional theory) geometrical optimisation of one of the systems at the B3LYP/6-31+G*level. Calculation of the nucleus independent chemical shift (NICS) at the centre of the ring produces strongly positive values for the triplet 4n annulene structure. Furthermore, the resulting systems do not undergo cyclo-addition (Diels Alder) reactions either at room temperature or in presence of UV radiation. If these planar star-like annulene molecules are incorporated into the polyethylene chain at regular intervals, the polymer is expected to be magneto- active.