Ab Initio Quantum Chemical Investigation of Intramolecular Magnetic Interaction in Some Diradical Derivatives of Imino Nitroxide and Nitronyl Nitroxide

Abstract

The magnetic properties of the monoradicals 2-(4-phenyl acetylene)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidozolyl-oxyl (1) and 2-(4-phenyl acetylene)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl-3-oxide (2) and the diradicals 2,2'-(1,2-ethynediyldi-4,1-phenylene)bis[4,4,5,5-tetramethyl-4,5-dihydro-1H-imidozolyl-oxyl] (3), 2,2'-(1,2-ethynediyldi-4,1 3,1-phenylene)bis[4,4,5,5-tetramethyl-4,5-dihydro-1H-imidozolyl-oxyl] (4), and 2,2'-(1,2-ethynediyldi-4,1 3,1-phenylene)bis[4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl-3-oxide] (5) are investigated by ab initio quantum chemical methods. The rule of spin alternation in the unrestricted Hartree-Fock (UHF) method clearly shows that the radical sites are antiferromagnetically coupled in 3 and ferromagnetically coupled in 4 and 5, which is consistent with a previous experiment. The molecular geometries are optimized at Hartree-Fock levels. This is followed by single-point calculations using the density functional (UB3LYP) treatment and the multiconfigurational complete active space self-consistent field (CASSCF) methodology. Magnetic exchange coupling constants are determined from the broken-symmetry approach. The calculated J values, -3.60 cm(-1) for 3, 0.16 cm(-1) for 4, and 0.67 cm(-1) for 5, are in excellent agreement with the observed values. Because of the very large size of the diradicals 3-5, the CASSCF (10,10) calculations cannot yield realistic J values. Nevertheless, the CASSCF calculations support the antiferromagnetic nature of the magnetic coupling in 3 and the ferromagnetic nature of the coupling in 4 and 5. The existence of an intramolecular magnetic coupling in 3-5 is also confirmed through computations of the isotropic hyperfine coupling constants for monoradicals 1 and 2 as well as diradicals 3-5.

Remarkable Differences in Spin Couplings for Various Self‐Paired Dimers of Ring‐Expansion‐Radicalized Uracil: A Basis for the Design of Magnetically Anisotropic Assemblies

“…We also calculated the magnetic coupling constants J of the rU‐rU diradicals using the formalism proposed by Yamaguchi et al . The exchange interaction between two magnetic centers is expressed by the Heisenberg spin Hamiltonian [Eq.…”

Tuning the Spin Coupling Interactions in the Nitroxide‐Based Bisphenol‐Like Diradicals

“…It should be noted that a number of calculational schemes have been attempted to obtain reliable J values . Despite its simplicity, an accurate estimate of J is still challenging.…”

“…It should be noted that an umber of calculational schemesh ave been attempted to obtain reliable J values. [40][41][42] Despite its simplicity,a na ccurate estimate of J is still challenging. For example, the use of the unrestricted Hartree-Fock method introduces spin contamination, which is an artificial mixing of different spin states,i nto singlet diradical calculations, resulting in ap oor description of the energy of the singlet spin state.…”

Quantum chemical approaches for controlling and evaluating intramolecular magnetic interactions in organic diradicals

“…The rule of spin alternation in the UHF or UDFT levels predicts the magnetic ground states without fail, although counter examples against the former regarding the SOMO interactions were reported. [23] In addition, it was known that intramolecular magnetic interactions between spin sources through a coupler can be influenced by several other factors: the geometrical connectivity, [23,[29][30][31][32][33][34] coupler chain length, [40][41][42][43] aromatic nature of a coupler, [32] dihedral angle, [44,45] and so on.…”