We report the first observation of surface-enhanced Raman scattering (SERS) excited with ultraviolet (UV) light from transition metal electrodes. Adsorbed pyridine and SCN- on rough rhodium (Rh) and ruthenium (Ru) electrodes, respectively, have been studied using 325 nm laser excitation. In contrast, the best enhancers in the visible and near infrared, silver and gold, do not produce UV-SERS. The experimental data of UV-SERS are in agreement with our preliminary theoretical calculation based on the electromagnetic enhancement mechanism. The enhancement factor is about 2 orders of magnitude for the Rh and Ru electrodes when they are excited at 325 nm.

Density functional theory calculations were done to examine the potential energy surfaces of Ni(I)-catalyzed Negishi alkyl-alkyl cross-coupling reactions by using propyl iodide and isopropyl iodide as model alkyl electrophiles and CH 3ZnI as a model alkyl nucleophile. A four-step catalytic cycle involving iodine transfer, radical addition, reductive elimination, and transmetalation steps were characterized structurally and energetically. The reaction mechanism for this catalytic cycle appears feasible based on the calculated free energy profiles for the reactions. The iodine transfer step is the rate-determining step for the Ni(tpy)-CH 3 (tpy = 2,2'6',2''-terpyridine) reactions with alkyl iodides. For secondary alkyl electrophiles, the oxidative addition intermediate, Ni(III), prefers to undergo decomposition over reductive elimination, whereas for the primary alkyl electrophiles, Ni(III) prefers to undergo reductive elimination over decomposition based on comparison of the relative reaction rates for these two types of steps. In addition, thermodynamic data were employed to help explain why the yield of the coupled product is very low from the Ni(II)-alkyl halide reactions with organozinc reagents.

This discussion focuses on our recent approaches at aiming to optimize surface-enhanced Raman scattering (SERS) activity for transition metals (group VIII B elements), by intentionally fabricating desired surface nanostructures or synthesizing nanoparticles. The SERS activity of transition metals critically depends on the surface morphology of electrodes and on size, shape and aggregation form of nanoparticles. A correct surface roughening procedure for transition-metal electrodes is indispensable for fabricating cauliflower-like nanostructures that show a higher SERS activity. Two more methods have been explored to synthesize nanoparticles, i.e., cubic nanoparticles and gold-core palladium-shell nanostructures, respectively. Their SERS activities are considerably higher than those of normal spherical mono-metallic nanoparticles. To explain these observations, a preliminary theoretical calculation, using the three-dimensional finite difference time domain (3D-FDTD) method, was performed to evaluate the local electromagnetic field on transition metal surfaces. The result is in good agreement with the experimental data.

The potential-energy surfaces of the reactions of dirhodium tetracarboxylate (Rh2(II,II)) catalyzed nitrene (NR) insertion into C-H bonds were examined by a DFT computational study. A pure Becke exchange functional (B88) rather than a hybrid exchange functional (B3, BHandH) was found to be appropriate for the calculation of the energy difference between the singlet and triplet Rh2(II,II)-NH nitrene species. Rh2(II,II)-NR1 (R1 = (S)-2-methyl-1-butylformyl) is thermodynamically more favorable with a free energy lower than that of Rh2(II,II)-N(PhI)R1. The singlet and triplet states of Rh2(II,II)-NR1 have similar stability. Singlet Rh2(II,II)-NR1 undergoes a concerted NR insertion into the C-H bond with simultaneous formation of the N-H and N-C bonds during C-H bond cleavage; triplet Rh2(II,II)-NR1 undergoes H atom abstraction to produce a diradical, followed by subsequent bond formation by diradical recombination. The singlet pathway is favored over the triplet in the context of the free energy of activation and leads to the retention of the chirality of the C atom in the NR insertion product. The reactivities of the C-H bonds toward the nitrene-insertion reaction follow the order tertiary > secondary > primary. Relative reaction rates were calculated for the six reaction pathways examined in this work.

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Surface-enhanced Raman scattering (SERS) spectroscopy excited with a UV laser was successfully developed and the UV-SER spectra of various adsorbates, including pyridine and SCN-, on different transition metal electrodes were obtained. The experimental requirements for obtaining UV-SERS in an electrochemical system are given. The surface enhancement factor of a roughened Rh electrode covered with thiocyanate as a model molecule was estimated to be about two orders of magnitude in the UV region, consistent with our preliminary theoretical calculation based on the electromagnetic model. The investigation of SERS in the UV region will improve the understanding of the SERS enhancement mechanism and broaden the research field of SERS in areas such as surface science and the life sciences. Copyright (c) 2005 John Wiley & Sons, Ltd

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Some points on how to improve the detection sensitivity of confocal Raman microscopy for the study of surface-enhanced Raman scattering (SERS) of transition-metal electrodes are discussed, including the careful design of the spectroelectrochemical cell, proper selection of the thickness of the solution layer, the binning of charge-coupled device (CCD) pixels, and appropriate setting of the notch filter. Various roughening methods for the Pt, Rh, Fe, Co, and Ni electrode surfaces have been introduced in order to obtain SERS-active surfaces. It has been shown that the appropriate roughening procedure and the optimizing performance of the confocal Raman microscope are the two most important factors to directly generate and observe SERS on net transition-metal electrodes.