The Mn cluster in the S(0) state of the oxygen-evolving complex of photosystem II studied by EXAFS spectroscopy: are there three Di-mu-oxo-bridged Mn(2) moieties in the tetranuclear Mn complex?

A key component required for an understanding of the mechanism of the evolution of molecular oxygen by the photosynthetic oxygen-evolving complex (OEC) in photosystem II (PS II) is the knowledge of the structures of the Mn cluster in the OEC in each of its intermediate redox states, or S-states. In this paper, we report the first detailed structural characterization using Mn extended X-ray absorption fine structure (EXAFS) spectroscopy of the Mn cluster of the OEC in the S(0) state, which exists immediately after the release of molecular oxygen. On the basis of the EXAFS spectroscopic results, the most likely interpretation is that one of the di-mu-oxo-bridged Mn-Mn moieties in the OEC has increased in distance from 2.7 A in the dark-stable S(1) state to 2.85 A in the S(0) state. Furthermore, curve fitting of the distance heterogeneity present in the EXAFS data from the S(0) state leads to the intriguing possibility that three di-mu-oxo-bridged Mn-Mn moieties may exist in the OEC instead of the two di-mu-oxo-bridged Mn-Mn moieties that are widely used in proposed structural models for the OEC. This possibility is developed using novel structural models for the Mn cluster in the OEC which are consistent with the structural information available from EXAFS and the recent X-ray crystallographic structure of PS II at 3.8 A resolution.     

Comparative performance of CF3I,CD3I and CH3I in an atomic iodine photodissociation laser

We have compared the performance of CF₃I, CD₃I, and CH₃I in an atomic iodine photodissociation laser over the pressure range 1–200 torr. At pressures below 5 torr, CD₃I produces larger energy outputs, while above 5 torr CF₃I gives superior performance. The crossing of the laser energy output versus pressure curves is explained on the basis of collisional quenching of I(²P_{$\text{1}\text{2}$})(≡I^*) by undissociated alkyl iodide.     

Photoreflectance of GaAs doping superlattices

We have performed room-temperature photoreflectance measurements on two GaAs doping superlattices having considerably different built-in potentials (1.2 eV and 85 meV). The first sample exhibits Franz-Keldysh oscillations, the period of the oscillations corresponding to the . A second dc pump beam has been used to change the electron-hole concentration and hence the built-in field. The spectrum of the second sample displays a number of features corresponding to quantized electron and hole states. There is qualitative agreement between experiment and theoretical calculation based on a two-band tight-binding model. In both samples the dependence of the amplitude of the photoreflectance signal on pump chopping frequency yields the minority carrier lifetime.     

Complexes of a dianionic bis(diphosphinomethanide) with lithium, sodium, potassium and zirconium. Effect of substitution on the Schlenk dimerisation of vinylidene phosphines

The vinylidene phosphine (Pr(n)(2)P)(2)C=CH(2) (1) undergoes Schlenk dimerisation on treatment with an excess of any of the alkali metals Li, Na or K to give the butane-1,4-diide complexes [(L)M{(Pr(n)(2)P)(2)CCH(2)}](2)[(L)M =(THF)(2)Li (6), (THF)(3)Na (7b), (DME)(2)K (8b)], after recrystallisation. Whereas the reaction between the analogous phenyl derivative (Ph(2)P)(2)C=CH(2) and K results in cleavage of a P-C bond, 1 reacts smoothly with K to give 8, with no evidence for P-C cleavage. Compound 6 is an excellent ligand transfer reagent: metathesis reactions between either 6 or its phenyl analogue [(THF)(2)Li{(Ph(2)P)(2)CCH(2)}](2) (2) and two equivalents of Cp(2)ZrCl(2) in THF give the corresponding dinuclear zirconocene derivatives [Cp(2)Zr(Cl){(R(2)P)(2)CCH(2)}](2) in good yields [R = Ph (11), Pr(n)(12)]. Compounds 6, 7b, 8b, 11 and 12 have been characterised by multi-element NMR spectroscopy and, where possible, by elemental analysis; compounds 6, 7b, 11 and 12 have additionally been characterised by X-ray crystallography.     

9,9‐Di‐n‐octyl‐9H‐fluorene

The title compound, C₂₉H₄₂, crystallizes from the melt as a triclinic crystal. The unit cell contains three crystallographically independent mol­ecules. The three fluorene ring systems are oriented such that two non‐equivalent ring systems related by a roto‐translation are separated by a third ring system oriented orthogonally to them. The octyl chains of all three mol­ecules are perpendicular to the fluorene ring system and force a 7.4 Å separation between the two coplanar mol­ecules, thereby preventing mol­ecular π‐stacking.     

Heteronuclear multiple magnetic resonance studies of quaternary ammonium salts derived from heterocyclic bases

¹⁴N tickling experiments performed with simultaneous decoupling of substituent protons are used to show that ²J(¹⁴N? H_ortho) and ³J(¹⁴N? H_meta) are both positive in the N-methyl pyridinium ion and related species. Long range coupling extending over as many as five bonds is observed between N-methyl protons and ring protons in ¹⁴N-decoupled spectra. Triple resonance decoupling is used to permit an analysis of the AA′MM′ spin system given by the ring proton of N-methyl pyrazinium iodide.