A novel application of x-ray absorption spectroscopy has provided structural information for ions sorbed at oxide-water interfaces. As an example, in situ extended x-ray absorption fine structure (EXAFS) measurements of adsorbed selenate and selenite ions at ah alpha-FeOOH(goethite)-water interface have been performed; these measurements show that selenate forms a weakly bonded, outer-sphere complex and that selenite forms a strongly bonded, inner-sphere complex. The selenite ion is bonded directly to the goethite surface in a bidentate fashion with two iron atoms 3.38 angstroms from the selenium atom. Adsorbed selenate has no iron atom in the second coordination shell of selenium, which indicates retention of its hydration sphere upon sorption. This method provides direct structural information for adsorbed species at solid-liquid interfaces.

No abstract

No abstract

protoheme-(n-BuNC), 93985-12-7; chelated protoheme-(t-BuNC), 93985-1 3-8; chelated protoheme-(TMIC), 93985-14-9. 30-9; l-(t-BuNC),, 94089-29-9; l-(TMIC)2,93985-31-0; I -( C~H I~N C )~, BuNC), 93985-27-4; 2-(t-BuNC), 93985-28-5; 2-(TMIC), 93985-29-6; 38-7; C~H I~N C , 931-53-3; C~HIINH,, 108-91-8; C,HllCHO, 2043-61-0; Conclusion. Distal steric effects in anthracene heme cyclophanes and other cyclophanes can be large. They reside almost exclusively in the ligand association rates, suggesting a control by a conformational preequilibrium. Distal steric effects differentiate ligands by gross size and shape but do not significantly differentiate linear from bent binding diatomic molecules. Variations in p / € ? 2 are found to be the result of changes in polar rather than steric effects. support of the computer applications (Grant RR 00757) and to Prof. C. K. Chang for helpful discussions. Abstract:The phenolate-to-iron(II1) charge-transfer transition in a series of iron(II1) phenolate complexes has been investigated. NMR contact shifts for the phenolate protons are well correlated with the visible absorption maxima of the complexes and the Fe"'/Fe" redox potentials. The results indicate that the energy of the phenolate-to-iron(II1) charge-transfer band is sensitive to the crystal field strength of the other ligands coordinated to the ferric center. The stronger the other ligands are, the higher the energy of the phenolate charge-transfer band. The blue shift of the charge-transfer band is also reflected in smaller contact shifts for the phenolate protons and a more negative Fe"'/Fe" redox potential. On the basis of these studies, the probable identities of ligating species in transient dioxygenase intermediates are deduced. These studies also demonstrate that a square-pyramidal complex with an apical and a basal phenolate can give rise to phenolate charge-transfer bands of quite different energies. A dioxygenase active site approaching such a structure is proposed. Lastly, the axial ligand in Fe(sa~en)OC6H,-,4-CH, is shown to be an excellent model for tyrosine in resonance Raman studies of iron-tyrosinate proteins. Isotopic substitution studies on the model complex show that the ca. 570-cm-I feature found in these proteins cannot be assigned solely to an Fe-0 stretching vibration. 6868-6873. 4918-4924.

No abstract