Applying the method of density functional theory calculations, we examine the Raman and surface-enhanced Raman spectra (SERS) of crystal violet. The resulting optimized structure is of point symmetry D 3 , and the calculated Raman spectrum provides an excellent match with the observed normal Raman spectrum. This provides a reliable assignment of the symmetry and normal modes of the observed spectrum, which consists of bands assigned to modes of either a 1 or e symmetry. The e modes are not split, showing that D 3 symmetry remains, even on the surface. The SERS spectra, both normal and single-molecule, are dominated by the nontotally symmetric e vibrations, which are preferentially enhanced in accord with the Herzberg-Tellersurface selection rules. The mechanism involves intensity borrowing through vibronic coupling between a charge-transfer state and the lowest-lying π f π* transition. A quantitative measure of the degree of charge transfer is obtained by analyzing the potential dependence of SERS intensities. This indicates a considerable contribution of charge-transfer intensity to the overall SERS enhancement.

We explore the application of a previously suggested formula for determining the degree of charge transfer in surface-enhanced Raman scattering (SERS). SERS is often described as a phenomenon which obtains its enhancement from three major sources, namely the surface plasmon resonance, charge-transfer resonances as well as possible molecular resonances. At any chosen excitation wavelength, it is possible to obtain contributions from several sources and this has led to considerable confusion. The formula for the degree of charge transfer enables one to separate these effects, but it requires that spectra be obtained either at two or more different excitation wavelengths or as a function of applied potential. We apply this formula to several examples, which display rather large charge-transfer contributions to the spectrum. These are p-aminothiophenol (PATP), tetracyano-ethylene (TCNE) and piperidine. In PATP we can show that several lines of the same symmetry give the same degree of charge transfer. In TCNE we are able to identify the charge-transfer transition, which contributes to the effect, and are able to independently determine the degree of charge transfer by wavenumber shifts. This enables a comparison of the two techniques of measurement. In piperidine, we present an example of molecule to metal charge transfer and show that our definition of charge transfer is independent of direction.

The ubiquitously expressed CLC chloride transporters are involved in a great variety of physiological functions. The CLC protein fold is shared by Cl channels and 2Cl:1H antiporters. The antiporters pump three charges per cycle across the membrane with two Cl ions moving in the opposite direction of one proton. Multiconformational continuum electrostatics was used to calculate the coupled thermodynamics of the protonation of the extracellular-facing gating Glu (E) and Cl binding to the external (S) and central (S) sites in CLC-ec1, the Escherichia coli exchanger. S, S, and E are buried within the protein where the intersection of two helix N-termini creates a region with a strong, localized positive potential for anion binding. Our chemical potential titrations describe the thermodynamic linkage for binding the Cl to each site and protons to E. We find that the 2Cl:1H binding stoichiometry is a result of Cl binding to S requiring H binding to E, whereas Cl binding to S does not lead to proton uptake. When S binds a Cl, the protonated E moves upward, out of the positive helix cage. The increasing E proton affinity on binding the first Cl reduces the cost of binding the second Cl at either S or S. Despite the repulsion among the anions, the lowest energy states have two anions bound in the helix cage. The state with no Cl is not favored electrostatically, but relies on E blocking S and on the central residues Y445 and S107 blocking S.