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.

Synthesis, Structure, and Unexpected Magnetic Properties of La3Re2O10

“…[26][27][28][29][30] In all of these calculations, the triplet states were consistently below the energy of the singlets. This would imply that the magnetic interaction between two dimers should be ferromagnetic.…”

Differential Polarization of Spin and Charge Density in Substituted Phenoxy Radicals

“…All other factors being equal, 28 polarization of unpaired spin density toward (or away) from the ligating atoms will increase (or decrease) the magnitude of electronic coupling. The notion of altering the magnetic properties of molecules through chemical means has been addressed both computationally [29][30][31][32] and experimentally. [33][34][35] Recent work by Shultz and co-workers, in particular, has clearly demonstrated that spin delocalization contributes to the attenuation of exchange interactions in bridged polynuclear systems.…”

Spin and Pseudo Spins in Theoretical Chemistry. A Unified View for Superposed and Entangled Quantum Systems

“…Judging from many other examples, [17][18][19][20][21] this tendency is reasonable. On the other hand, the potential energies for the HS state of 1 were shown in Figure 4B.…”

“…Here, we first examine ab initio spin Hamiltonian model for moleculebased magnets, where the effective exchange interactions are determined by the first-principle broken-symmetry (BS) density functional calculations. 17,18 As an example, the BS DFT calculations of a magnetic ring consisted of 34 hydrogen atoms is performed to determine the potential energy curves of the lowest -and highest-spin states of the species, and the effective exchange integrals are calculated by the energy difference between them. The natural orbital analysis of the BS DFT solutions is also carried out to obtain symmetry-adapted (SA) natural molecular mobitals (MO) and their occupation numbers.…”