Effects of excited-state spin-orbit coupling and ground-state hybridization upon the behavior of the in-plane g tensor in low-symmetry d1,9 systems

Models for Copper Dynamic Behavior in Doped Cadmium dl-Histidine Crystals: Electron Paramagnetic Resonance and Crystallographic Analysis

“…The in-plane g and A Cu tensor principal directions are slightly twisted (∼13°) away from the directed copper ground-state orbital lobes (Figure B). This could be due to deformation of the site geometry from a square-plane (the atom rms deviation is ∼0.12 Å and angular deviation from square is ∼12°) or from “apical” ligands which can introduce excited d xz and d yz states into the spin wave function . The postulated site has two such weakly interacting “apical” ligands, a carboxyl oxygen and an imidazole nitrogen that lie above the equatorial plane at 3.28 Å (3.25 Å at 100 K) and 2.84 Å (2.86 Å at 100 K) from the copper (Figure A) which then could theoretically account for the slight misalignment of the tensor.…”

“…This could be due to deformation of the site geometry from a square-plane 28 (the atom rms deviation is ∼0.12 Å and angular deviation from square is ∼12°) or from "apical" ligands which can introduce excited d xz and d yz states into the spin wave function. 29 The postulated site has two such weakly interacting "apical" ligands, a carboxyl oxygen and an imidazole nitrogen that lie above the equatorial plane at 3.28 Å (3.25 Å at 100 K) and 2.84 Å (2.86 Å at 100 K) from the copper (Figure 7A) which then could theoretically account for the slight misalignment of the tensor. However, there are undoubtedly local changes in the host structure upon doping that can also affect these correspondences.…”

Investigation by EPR and ENDOR Spectroscopy of the Nickel(I) Form of Cofactor F4301 of Methanobacterium thermoautotrophicum and of Nickel(I) Octaethylisobacteriochlorin

“…The relation between the g tensor and the molecular framework for square-planar d 9 complexes, chiefly those of Cu(II), was studied theoretically a number of years ago, − and is summarized by Pilbrow . Crystal-field terms of rhombic ( i.e.…”

“…However, Ni I Me 6 [14]4,11-dieneN 4 , prepared by electrochemical reduction in acetonitrile solution, exhibits an axial EPR spectrum, 56 although its crystal structure (as the perchlorate salt) shows two significantly different Ni(I)-N bond distances. 31 The relation between the g tensor and the molecular framework for square-planar d 9 complexes, chiefly those of Cu(II), was studied theoretically a number of years ago, [67][68][69] and is summarized by Pilbrow. 55 Crystal-field terms of rhombic (i.e., mixing of d x 2 -y 2 and d z 2 orbitals) and/or monoclinic (i.e., mixing of d xz and d yz orbitals) symmetry acting via spin-orbit coupling can lead to a rhombic g tensor, as well as to rotation of the in-plane g tensor directions (g 2,3 ) relative to the molecular coordinate system.…”

Studies by electron paramagnetic resonance spectroscopy of xanthine oxidase enriched with molybdenum-95 and with molybdenum-97

“…The orientations of the principal axes of the metal hyperfine interaction A(95,97Mo), however, are expected to depend upon the basic nature of the ground state and thus are more likely to be close to the molecular (or pseudosymmetry) axes than are those of g. The nature of the noncollinearity between g and A(95,97Mo) can give indications of the symmetry of the signal-giving species. For complexes of high symmetry, g and A(95,97Mo) are required to be collinear; for those possessing only a twofold rotation axis (C2) or a mirror plane (C,), one principal axis will coincide, and for complexes of lower symmetry (Ci, no symmetry, or C" with an inversion center), none of the principal axes are required to be coincident (Hitchman et al, 1969; Belford et al, 1977;Scullane et al, 1979). From the data reported in Table I it is clear that the Rapid signal-giving species possesses metal hyperfine couplings typical of a complex of Cs or C2 symmetry, while the hyperfine coupling of the Very Rapid species indicates that it possesses only an inversion center or, more probably, no symmetry.…”