Accurate Geometric Parameters Research Articles

X-ray and neutron diffraction studies of tricarbonyl(.eta.-pentamethylcyclopentadienyl)hydridomolybdenum at 163 K

The structure of [MoH([eta]-C[sub 5]Me[sub 5])(CO)[sub 3]] (1) has been determined by X-ray diffraction at 163(3) K [a = 6.969(2) [angstrom], b = 12.651(2) [angstrom], c = 7.954(2) [angstrom], [beta] = 103.36(2)[degrees], monoclinic, P2[sub 1]/m (No. 11), Z = 2] and neutron diffraction at 163.0(5) K [a = 6.968(2) [angstrom], b = 12.658(3) [angstrom], c = 7.953(2) [angstrom], [beta] = 103.34(2)[degrees]]. Refinements in space group P2[sub 1]/m against both data sets improved the description of the structure obtained by refinements in P2[sub 1] in this and an earlier study. The structure exhibits orientational disorder in the C[sub 5]Me[sub 5] group, yet relatively accurate geometric parameters have been obtained, with esd's from the neutron diffraction study of 0.007 [angstrom] for Mo-H, 0.010 [angstrom] (average) for C-H, and 0.003 [angstrom] (average) for all other bond lengths. The molecule adopts a [open quotes]four-legged piano stool[close quotes] geometry with the hydride ligand approximately trans to one of the Cp* ring carbons [Mo-H(1) = 1.789(7) [angstrom]]. Comparison of the title complex with related d[sup 4] CpMo(CO)[sub 3]L complexes (L = formally anionic, monodentate, [eta][sup 1] ligand) suggests that the conformation of these complexes is determined primarily b steric interactions between the ligand, L, and themore » Cp or Cp* ring, rather than by electronic effects of either Cp/Cp* or L.« less

Energy transfer between the flavin chromophores of electron-transferring flavoprotein fromMegasphaera elsdenii as inferred from time-resolved red-edge and blue-edge fluorescence spectroscopy

Both a mode-locked argon-ion laser and synchrotron radiation were used as excitation sources to obtain time-resolved polarized fluorescence of the two FAD cofactors in electron transferring flavoprotein fromMegasphaera elsdenii. Red-edge excited and blue-edge detected fluorescence anisotropy decay curves did not contain a fast relaxation process which was observed upon mainband excitation and detection. This relaxation was assigned to homo-energy transfer between the two FAD cofactors. Failure of energy transfer as observed with edge spectroscopy on this protein excludes restricted reorientational motion of the flavins as a possible mechanism of depolarization. From the global analysis of the fluorescence anisotropy decay surface obtained at multiple excitation and detection wavelengths, the distance between and the relative orientation of the flavins could be estimated. The methodology described has general applicability in other multichromophoric biopolymers and has the potential to acquire accurate geometrical parameters in these systems.

Stereochemistry of the isomers of 8-p-bromophenyl-6-cyano-2,9-dioxa-1-azabicyclo[4.3.0]nonane

1. We have determined the confirmations and accurate geometrical parameters for the two isomers of 8-p-bromophenyl-6-cyano-2,9-dioxa-1-azabicyclo[4.3.0]nonane. 2. Replacement of the nitro group by the cyano group proceeds mainly with inversion of configuration at the center of substitution.