Donald E. Irish

Stable solvates in solution of lithium bis(trifluoromethylsulfone)imide in glymes and other aprotic solvents: Phase diagrams, crystallography and Raman spectroscopy

Lithium bis(trifluoromethylsulfone)imide (LiTFSI), a promising electrolyte for high energy lithium batteries, forms several stable solvates having low melting points in aprotic solvents. In a previous study (D. Brouillette, G. Perron and J. E. Desnoyers, J. Solution Chem., 1998, 27, 151), it was suggested, based on thermodynamic studies, that such stable solvates may persist in solution and influence their properties. To verify this hypothesis, phase diagrams and Raman spectra have been measured for solutions of LiTFSI in acetonitrile, propylene carbonate and glymes (n(ethyleneglycol) dimethyl ether or Gn), which have the chemical structure CH3–O–(CH2–CH2–O)n–CH3 for n = 1 to 4 and 10. The relative intensities of the LiTFSI and solvent Raman bands are proportional to the concentration for systems without solvates. The systems for which stable solvates were identified in the phase diagram show important changes in the relative intensities for both the LiTFSI and the solvent Raman bands at concentrations corresponding to particular stoichiometries and support the conclusion that stable solvates are present in the solutions. The structure of the crystalline G1:LiTFSI solvate was determined by X-ray crystallography. Structures for (G2)2:LiTFSI and (G1)3:LiTFSI solvates are proposed.

Adsorption of pyridine at the Au( 110)-solution interface

Chronocoulometry has been used in order to characterize quantitatively the energetics of pyridine adsorption onto a gold (100) single crystal electrode surface. Over the potential region investigated (−0.8 to+0.6 V), three orientations of the pyridine molecules on the gold surface have been observed. The pyridine orientation is influenced strongly by the electrode potential. At a positively charged surface, the pyridine assumes a vertical orientation with the nitrogen atom facing the gold surface. A limiting surface concentration corresponding to 6 × 10−10 mol cm−2 was determined for the vertical orientation. At a negatively charged surface and at low surface concentrations (Γ Γ > 1 × 10−10 mol cm−2) and for potentials close to zero charge, a third orientation, presumably intermediate between the flat and the vertical orientations, was observed. Reorientation from the intermediate to the vertical orientation takes the form of a phase transition. The potential of the phase transition coincides approximately with the potential of zero charge. The Gibbs energies of adsorption, electrosorption valencies, potentials and charges of the maximum adsorption were determined for the flat and the vertical orientation of pyridine. Adsorption of pyridine on the Au (100) surface is compared with the adsorption at polycrystalline Au and mercury electrodes.

Adsorption of pyridine at the polycrystalline gold—solution interface

Abstract Chronocoulometry has been employed in order to investigate quantitatively the adsorption of pyridine from aqueous solution onto a polycrystalline gold electrode. It was found that at a negatively charged surface, pyridine adsorbs in the flat orientation, reaching a limiting surface concentration of 3 × 10 −10 mol cm −2 . Under the influence of the electric field pyridine reorients so that on the positively-charged surface the pyridine molecules assume the vertical orientation, presumably with the nitrogen facing the gold surface. A limiting surface concentration of 7 × 10 −10 mol cm −2 is reached in the vertical orientation. The standard Gibbs energy of adsorption was found to reach fairly large values (−38 kJ mol −1 ). The electrosorption valency was also determined and found to be −0.6 indicating that a partial charge transfer is involved in the pyridine adsorption.

APPLICATIONS OF SURFACE ENHANCED RAMAN SCATTERING TO THE STUDY OF METAL-ADSORBATE INTERACTIONS

Surface enhanced Raman scattering (SERS) is a powerful technique for characterizing adsorbed species and processes at metallic surfaces. The giant signal enhancement (104–106 larger than normal Raman scattering) makes this technique sensitive to even sub-monolayer amounts of adsorbate on a surface. Consequently, the application of SERS to the in situ study of electrochemical processes provides useful mechanistic and structural information. In this review, advantages and limitations of electrochemical SERS techniques are presented along with experimental information about the nature of the metal-adsorbate interactions occurring in various aqueous and non-aqueous systems. Special emphasis is given to experimental results; however, the salient features of the enhancement theories are highlighted. Adsorbate orientation and SERS surface selection rules are discussed.

AN EXAMINATION OF THE RELATIONSHIP BETWEEN SURFACE ENHANCED RAMAN SCATTERING (SERS) INTENSITIES AND SURFACE CONCENTRATION FOR PYRIDINE ADSORBED AT THE POLYCRYSTALLINE GOLD/AQUEOUS SOLUTION INTERFACE

Surface enhanced Raman scattering (SERS) data are presented for pyridine adsorbed onto both smooth and roughened polycrystalline gold electrode surfaces. It was found that, for smooth surfaces, time independent (stable to within 10% of the initial value) intensities of the totally symmetric ring breathing mode of pyridine (1010 cm−1 band) could be measured by applying slow oxidation-reduction cycles between each intensity measurement. The potential and concentration dependence of the intensity of the 1010 cm−1 band was therefore determined. Independently, the isotherms for pyridine adsorption at smooth polycrystalline gold surfaces were obtained using chronocoulometry. The SERS data are compared to the surface concentration data. The SERS intensities are directly proportional to surface concentration at low coverages and deviate, becoming inverse, above half coverage concentrations. Lastly, the effect of surface roughness on the potential and concentration dependence of pyridine SERS was studied. No correlation between the SERS data from rough electrode surfaces and adsorption isotherms determined at a smooth surface was found.

A Raman spectroscopic study of lead and zinc acetate complexes in hydrothermal solutions

Principal Component Analysis (PCA) and bandfitting techniques were applied to Raman spectra of lead acetate and zinc acetate solutions measured at 25{degree}C. These results reveal the presence of strong, covalent Zn(Ac){sup +}, Zn(AC){sub 2}, Zn(Ac){sup {minus}}{sub 3} and Pb(Ac){sup +}, Pb(Ac){sub 2} and possibly Pb(Ac){sup {minus}}{sub 3} complexes in solution where (Ac) refers to the acetate ion, CH{sub 3}COO{sup {minus}}. Ligation numbers of the different complexes were obtained up to 250{degree}C and species of low-to-neutral charge were found to predominant at the higher temperatures. The spectroscopic evidence shows that the type of complex formed is a function of pH, ligand-to-metal ratio and temperature.

Surface enhanced raman spectroscopy of pyridine, pyridinium ions and chloride ions adsorbed on the silver electrode

Surface enhanced Raman spectra have been recorded from the Ag < KCl, pyridine electrode for solutions with a constant Cl− concentration and selected pH values in the range 1.9 to 8.5. Comparison of the profile of the 240 cm−1 band with spectra obtained from cells where the electrolyte was 1.0 M KC1, 1.0 M KBr and 0.1 M KBr/0.05 M Py substantiates the conclusion that the band arises from a Cl− to Ag vibration and not an N-bonded Py to Ag vibration. A local intensity maximum in the intensity vs. potential plot near the pzc for pH 8.5 and 5.8 is thus attributed to chloride stabilizing the physisorbed pyridine. The 240 cm−1 band does consist of two major components, however, suggesting two distinguishable distributions of sites. At low pH all of the pyridine is present as pyridinium ion, PyH+. The 1024 cm−1 band of this species has maximum intensity at about 0 V, and this intensity falls as the electrode is made more negative. It is suggested that this positive ion forms an ion pair with the chloride ion and that the two are desorbed together. The suggestion is consistent with a low SERS intensity for this band, relative to the intensity of physisorbed pyridine molecules (1008 cm−1) at pH 8.5, because the positive ion would be a chloride diameter removed from the metal surface.

RAMAN CHARACTERIZATION OF METAL-ALKANETHIOLATES

Abstract Raman spectroscopy has been used to characterize neat alkanethiol and various metal-alkanethiolate materials. Neat alkanethiol gives rise to two CS stretching peaks at 662 and 735 cm −1 , assigned to gauche and trans rotamers respectively. Only one CS stretching peak positioned at 725 cm −1 was found from CuC 12 and AgC 12 layered compounds, implying the absence of gauche rotamer near the thiolate group. An all- trans conformation of the chain is inferred from the peak position values of the CC stretching modes of CuC 12 and AgC 12 layered compounds. Gauche rotamers were observed in silver colloids capped with alkanethiolate.

Computer-Aided Visual Spectrum Analysis

A computer, operating in conversational mode, is being used for the analysis of overlapping Raman and infrared lines. Details of the procedure and the program are given. The fitting procedure is optical; operator intervention changes the parameters and thereby modifies the fit between synthesized curve and experimental curve. The goodness-of-fit is given by a difference measurement.

The Application of Raman Spectroscopy to Chemical Analysis

The development of the laser and its application as a source for the excitation of Raman spectra have resulted in the availability of Raman spectrophotometers at a cost competitive with ir spectroscopy. Quantitative analysis is a natural application. Procedures utilized with the conventional mercury lamp excitation and problems associated with laser excitation are reviewed. A bibliography of analyses is included.