Electron Spin Resonance of Met-Myoglobin: Field Dependence of g⊥eff

Abstract

The spin Hamiltonian for high‐spin Fe³⁺ in single crystals of met‐myoglobin has the form ${\mathcal{H}}_s{\mathrm{=D(S}}_z^2{\text{−25/4)+g}}_{\mathrm{|\; |}}{\mathrm{\beta H}}_z{S}_z{\mathrm{+g}}_{\perp }{\mathrm{\beta (H}}_x{S}_x{\mathrm{+H}}_y{S}_y)$ . In view of the large value of D the only ESR signal that can be observed is the transition from the m_s=—½ to m_s=+½ states. The resonance condition for the ground doublet is described by g^effβH=hν. When the magnetic field is applied along the z axis g∥^eff=g∥ and when the magnetic field is applied perpendicular to the z axis $g_{\perp }^{\mathrm{eff}}{\text{=3g}}_{\perp }{\text{[1–2(g}}_{\perp }{\mathrm{\beta H)}}^2{\text{(2D)}}^{\text{−2}}]$ . Through measurement of g⊥^eff and g∥^eff at 13 and 35 Gc/sec the spin‐Hamiltonian parameters were found to be g∥=2.002±0.001, g⊥=1.985±0.002, and 2D=8.76±1.2, cm⁻¹. These results are interpreted by replacing the pure 3d wavefunctions from which the (3d)⁵ electronic configuration of Fe³⁺ is usually formed by covalent molecular orbitals which are only part 3d. The values for g∥, g⊥, and D can be explained in terms of a single low‐lying pure antibonding state of ⁴A₂ symmetry 2000 cm⁻¹ above the ⁶A₁ ground state and coupled to it by the spin—orbit interaction. Numerical agreement between theory and experiment requires molecular orbits which have an average 3d character of only 68%. These highly covalent orbitals are in very good agreement with the theoretical results of Zerner and Gouterman.