Density-functional study of intramolecular ferromagnetic interaction through m-phenylene coupling unit (II): Examination of functional dependence

Abstract

As a first step toward examination of ferromagnetic polymers and dendrimers by ab initio crystal orbital methods, we elucidated candidates for monomer units with the high-spin ground states in the previous study of Part I [J. Chem. Phys. 113, 4035 (2000)] by employing density-functional (DFT) methods using Becke’s and Becke’s three parameter exchanges with Lee–Yang–Parr correlation or Hartree–Fock (HF) molecular orbital and post HF approximations. However, it was found that further computations applying other DFT functionals should be carried out to clarify the level of approximations which appropriately describe the electronic structures of magnetic molecules. In this part II, we present details of numerical results concerning magnetic properties and electronic structures for m-phenylene molecules with three neutral and one cation radicals by spin-polarized density functional methods using variety of local and nonlocal functionals and unrestricted molecular orbital methods including Mo/ller–Plesset and coupled-cluster (CC) correlation corrections. The dependence of total, exchange and correlation energies, and spin densities on various approximated functionals is investigated thoroughly. The effective exchange integrals in the Heisenberg model are calculated by local and nonlocal DFT methods, and they are compared with those of complete active space (CAS) CI, CASSCF, and CASPT2. It is concluded that nonlocal DFT with density-gradient corrections can be used as a practical alternative to UCCSD(T) and CASPT2. The broken-symmetry Unrestricted Hartree–Fock (UHF) and DFT calculations of m-phenylene polyradicals with polar substituents are carried out to elucidate roles of superexchange interactions arising from the significant mixing of charge-transfer (CT) configurations. The resonance of covalent structures with CT or zwitterionic structures entails antiferromagnetic exchange interactions even in polyradicals with m-phenylene bridges; for example, substituted nitroxide polyradicals. Stable ferromagnetic polymers and dendrimers are designed on the basis of the theoretical grounds.