DOUBLE-EXCHANGE AND VIBRONIC COUPLING IN MIXED-VALENCE SYSTEMS - ELECTRONIC-STRUCTURE OF [FE4S4](3-POTENTIAL IRON PROTEIN AND RELATED MODELS() CLUSTERS IN HIGH)

The origin of the pair-delocalized ground state of spin S = 1/2, obser ved in the chemically symmetric, mixed-valence [Fe4S4](3+) cores in th e high-potential iron protein (HiPIP) and its synthetic analogues, is analyzed in the framework of an effective Hamiltonian model, comprisin g terms for excess-electron transfer (leading to double-exchange coupl ing of the paramagnetic Fe(III) cores), vibronic coupling (trapping th e excess electron), and Heisenberg-Dirac-Van Vleck exchange. The adiab atic potential surfaces of the system d(5)-d(5)-d(5)-d(6) are determin ed, and their extremal points, corresponding to definite electron dist ributions, are ascertained. The electron distributions depend essentia lly on the ratio of the transfer parameter and vibronic trapping energ y, beta/(lambda(2)/2 kappa). For small ratios, the excess electron is site-trapped; for ratios of larger magnitude (greater than or equal to 1), the delocalization behavior depends on the nature of the electron ic state considered. The transfer Hamiltonian has for beta < 0 an orbi tally nondegenerate ground state of high spin (S = 19/2), in which the excess electron is uniformly distributed over the four sites. However , for beta > 0, the transfer interaction stabilizes a highly orbital- and spin-degenerate electronic ground state, including spin levels ran ging from S = 1/2 to 17/2. The degeneracy is removed by vibronic inter action, leading to broken-symmetry, pair-delocalized states which appe ar in the energy order E(1/2) < E(3/2) < ... . Inhomogeneous HDVV exch ange, arising from differences in the coupling parameters for ferrous- ferric (J) and ferric-ferric (J(1)) interactions, has little effect on the composition of the broken-symmetry states but has a great impact on state energy. The spin structures of the two lowest broken-symmetry states of the total Hamiltonian are similar to those inferred from sp ectroscopic studies of HiPIP and synthetic analogues thereof.