sumably, mercaptoethanol selectively blocks Pd surface sites for activating bicarbonate, thus favoring the transfer of conductionband electrons to H2-evolving sites.A comparison of the selectivity factor can be made between the Pd-Ti02 systems and the previously examined p h o t~s y s t e m~~ consisting of the photosensitizer deazariboflavin, dRFI, the electron mediator MV2+, the electron donor oxalate, and the catalyst Pd(@-CD,60 "C) colloids. The best selectivity factor of the PdTi02 systems (SF = 2.3; see Table I) is larger than that of dRFI/MV2+/oxalate/Pd(P-CD,60 "C) (SF = 0.013), indicating that the former exhibits lower selectivity for HC0,-reduction with respect to H 2 evolution than the latter. The basis for this difference in selectivity remains to be established, however.In conclusion, electron transfer from Ti02 to properly prepared catalytic Pd surface sites promotes the reduction of C02/HC0,-to formate. The selectivity exhibited by the Pd-Ti02 particle systems is quite noteworthy in relation to other semiconductorbased systems, which generate a broad distribution of products. Specific sites on the Pd surface favor C02/HCO< reduction over H2 evolution. The experimental results reported here as well as those of others2" suggest that other metals might exhibit selectivity in catalyzing the reduction of CO, to other desirable organic photoproducts.We present a theoretical treatment for the Helmholtz free energy of an arbitrary discrete charge distribution in a central prolate or oblate spheroidal cavity surrounded by two and three concentric dielectric continua of the same symmetry. The electrostatic interaction is accounted for by a multipolar expansion. This model permits us to consider in a simple way nonlinear dielectric effects and the nonsphericity of solute molecules. The results are presented in a suitable form to discuss them in terms of a virtual polarization charge distribution. Also, we give an explicit expression for the contributions coming from the zeroth-order multipole moments for any number of shells.
A model for catecholamine storage in vesicles is analyzed within the SCRF-CNDO/~ approach including continuum solvent effects. The model considers the approach of cationic norepinephrine (NE) to a positively charged guanidinium moiety. Ion-pair formation is found for the whole range of dielectric constants. Even though stable states of H-bonded partners are found for large dielectric constants, this process is ruled out to occur because it involves too high energies. It appears that the medium's polarity is determinant in lowering the energy barrier between the ion-pair complex and the separated partners. Thus, as the medium dielectric constant increases, the equilibrium between the two states is enhanced.
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