Density functional theory of spin-coupled models for diiron-oxo proteins: Effects of oxo and hydroxo bridging on geometry, electronic structure, and magnetism

Rodriguez, Jorge H.; McCusker, James K.
April 2002
Journal of Chemical Physics;4/8/2002, Vol. 116 Issue 14, p6253
Academic Journal
We have performed a comprehensive study of the electronic structure and magnetic properties of structurally characterized models for diiron-oxo proteins. Results from Kohn-Sham density functional theory show that two complexes, with formula Fe[sub 2](μ-O)(μ-O[sub 2]CCH[sub 3])[sub 2](HBpz[sub 3])[sub 2] and [Fe[sub 2](μ-OH)(μ-O[sub 2]CCH[sub 3])[sub 2](HBpz[sub 3])[sub 2]][sup +], are strongly and weakly antiferromagnetically coupled, respectively, in agreement with experiment. The physical origin of the stronger and weaker exchange typically measured for oxo- and hydroxo-bridged diiron complexes, respectively, has been elucidated. The main superexchange pathways giving rise to molecular antiferromagnetism in both complexes have been identified. The dominant pathway in the oxo-bridged complex, Fe1(d[sub xz]):μ-O(p[sub x]):Fe2(d[sub xz]), was formed by π interactions whereas that of the hydroxo-bridged, Fe1(d[sub z²]):μ-O[sup H](p[sub ]):Fe2 (d[sub z²]), was formed by σ interactions. We also found a pathway mediated by the bridging acetates, Fe1(d[sub x²-y²]):bis(μ-acetato):Fe2(d[sub x²-y²]), which induces weak antiferromagnetism in the oxo-bridged complex but is significantly more important in the hydroxo-bridged complex. The antiferromagnetic exchange constants that parameterize the Heisenberg Hamiltonian H = JS[sub 1]·S[sub 2] have been predicted for both, strongly and weakly, coupled t complexes. Overall, the signs, trends, and magnitudes of the theoretical values (J[sup calc,sub μ-O] = + 152.7 cm[sup -1] J[sup calc,sub μ-OH] = + 23.3 cm[sup -1] were in excellent agreement with experiment. The geometries of the complete molecular structures have been optimized in C[sub 2ν] symmetry and used to calculate molecular properties such as atomic charges and spin densities. The electronic configurations (Fe:4s[sup 0.29]3d[sup 5.93],μ-O:2s[sup 1.92]2p[sup 4.99]; Fe:4s[sup 0.30]3d[sup 5.82]...


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