Antiferromagnetic Linear Chains in the Crystalline Free Radical BDPA

The static magnetic susceptibility from 1.6 to 300°K and the heat capacity in fields of 0 and 5 kG from 0.9 to 10.2°K have been measured on polycrystalline samples of the complex of 1,3-bisdiphenylene-2-phenylallyl with benzene, commonly abbreviated BDPA. The data are shown to correspond to antiferro-magnetic Heisenberg linear chains characterized by a nearest-neighbor exchange integral of −4.6°K. Weak interchain coupling results in a paramagnetic—antiferromagnetic phase transition at 1.695°K in zero field, as evidenced by the sharp heat capacity anomaly at that temperature. Resolution of the heat capacity into a lattice T3 contribution and a magnetic contribution yields a Debye temperature of 52.5°K and a magnetic entropy per mole of 101% of the expected Rln2.