On the initial charge separation in bacterial reaction centers: Long-range electron transfer via an exciton-charge transfer (ECT) coupling mechanism

The low-lying electronically excited states for the reaction center of Rps. viridis are investigated using PPP/CI calculations. The six pigments are treated as three interacting pairs, the symmetric special pair dimer BC_MPBC_LP and the two loosely coupledasymmetric dimers BC_LABP_L and BC_MABP_M. It is shown that the charge transfer state BC_LA⁺BP_L⁻ can fall below the special pair excitation P* due to partial charge transfer from a histidine to BC_LA and due to stabilization of BP_L⁻ by a glutamic acid residue. As a result P* can decay in 2.8 ps into BC_LA⁺BP_L⁻ which goes over into the radical pair P⁺ BP_L⁻ in less than 1 ps. The first step can be described as an excitonic interaction between P* and BC_LA⁺ BP_L⁻.     

Vibrational spectra of M3PCr4O16 (M = K,NH4)

The infrared and Raman spectra of the two phosphochromates M₃PCr₄O₁₆ (M = K, NH₄) have been recorded and analysed. The spectra could be interpreted on the basis of the vibrations of Cr-O-P, O-P-O and CrO₃ groups. A large number of vibrational bands have been observed for each mode due to the existence of different Cr-O bond lengths. The anion is more distorted in potassium phosphochromate. The three ammonium groups are distinct.     

Catalysis of quinone-hydroguinone redox reactions at polypyrrole benzenesulphonate-coated platinum electrodes

The two-electron two-proton redox reaction of the benzoquinone-hydroquinone (Q/QH₂) couple in aqueous solution has been studied by cyclic voltammetry at platinum electrodes coated with polypyrrole benzensulphonate. In contrast to the behaviour on bare platinum, the voltammetric response of the Q/QH₂ couple on polypyrrole is close to the Nernstian behaviour within pH range from 1 to 7. The heterogeneous rate constant k_s determined from the peak separations is in the range of 10⁻³ cm s⁻¹. It is independent of the film thickness in a wide range and slightly pH-dependent, with a minimum at pH 4. Compared with related results based on polypyrrole perchlorate-coated gold electrodes, less aging of the catalytic efficiency is observed. Higher peak currents with increasing polypyrrole coverage suggest partial diffusion of the substrate into the bulk of the film. Preliminary studies on more complex quinones, ubiquinone-1, plastoquinone-1 and the bis-quinone of dibenzo [18] crown-6 demonstrate similar catalytic effects of polypyrrole layers. Potential applications of polypyrrole-coated electrodes in organic electrochemistry and bioelectroanalytical chemistry are discussed.     

Spectroscopic properties of alexandrite crystals II

Several aspects of the optical spectroscopic properties of alexandrite crystals not previously investigated are reported here. For Cr³⁺ ions occupying the Al³⁺ lattice sites with mirror symmetry in BeAl₂O₄, the positions of the zero-phonon lines for absorption transitions to the ²T_1g and ⁴T_2g levels are identified, and vibronic transition peaks in the fluorescence spectrum are compared to transitions appearing in the Raman spectrum and Stokes excitation spectrum. In addition, the effects of radiation trapping are shown to lengthen the fluorescence lifetime of the ²E_g - ⁴A_2g transition for ions in these sites at low temperatures. For Cr³⁺ ions occupying the Al³⁺ lattice with inversion symmetry, the ground state splitting of the ²E_g - ⁴A_2g transition is reported and the decrease of the fluorescence lifetime with temperature is shown to be due to the increase in vibronic emission probability as well as increased probability of direct radiationless decay.     

Fluorescence behaviour of highly concentrated rhodamine 6G solutions

The fluorescence quantum distributions E(λ) and fluorescence quantum efficiencies q_F of rhodamine 6G in methanol and in water are measured for various concentrations up to the solubility limit. The fluorescence spectra are separated in monomer and dimer (ground-state dimer and closely spaced pair) contributions. The stimulated emission cross sections for the monomers and the dimers are resolved.     

Is there a stable B2Π state for the CNO molecule?

We report MRD-CI calculations on the ground state X²Π and the excited states A²Σ⁺ and B²Π of the CNO molecule in linear geometry. The surfaces for oxygen and carbon extraction are calculated using a limited CI expansion of 47 configuration state functions; in the vicinity of the minima obtained with this procedure large-scale CI calculations are carried out including deter-mination of the spin-orbit splitting of the ²Π states of the minima. We find that the B²Π state will be difficult to detect spectroscopically due to an avoided crossing just at the equilibrium geometry of the ground state at R_CN = 2.25 a.u., R_NO = 2.30 a.u. Accordingly we find two shallow minima for B²Π at R_CN = 2.33 a.u., R_NO = 2.91 a.u. and R_CN = 2.78 a.u., R_NO = 2.28 a.u., respectively.     

FTIR detection of unstable molecules: Infrared spectrum of 2-azabutadiene

The infrared absorption spectrum of 2-azabutadiene in the gas phase was observed for the first time. The molecule was generated by a pyrolysis of 1-azetine. Most of the fundamental bands were identified and assigned by the help of an ab initio MO calculation of the force constants. The half-life of the molecule was about 7 min under our experimental conditions and no further isomerization was found.     

Utilisation de Me3SiI préparé in situ pour l'obtention de bis(trimethylsiloxy-1,3) diène-1,3

The in situ generation of iodotrimethylsilane, in the presence of triethylamine, is a convenient route to 1,3-bis(trimethylsiloxy)-1,3-diene.