| 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 theunrestricted Hartree−Fock (UHF) method clearly shows that the radical sites are antiferromagnetically coupledin 3 and ferromagnetically coupled in 4 and 5, which is consistent with a previous experiment. The moleculargeometries are optimized at Hartree−Fock levels. This is followed by single-point calculations using thedensity functional (UB3LYP) treatment and the multiconfigurational complete active space self-consistentfield (CASSCF) methodology. Magnetic exchange coupling constants are determined from the broken-symmetryapproach. 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 excellentagreement 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 theantiferromagnetic nature of the magnetic coupling in 3 and the ferromagnetic nature of the coupling in 4 and5. The existence of an intramolecular magnetic coupling in 3−5 is also confirmed through computations ofthe isotropic hyperfine coupling constants for monoradicals 1 and 2 as well as diradicals 3−5.
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