Cation vibrational spectroscopic techniques

The direct evidence on which our view of cation solvation in polymer electrolytes is based comes mainly from spectroscopic techniques. IR and Raman studies have been carried out on a variety of systems (see Chapter 5, Torell and Schantz, 1989 and Freeh, Manning, Teeters and Black, 1988). Low frequency vibrational modes, around 860-870 cm associated with the cation-ether oxygen interactions in PEG based systems have been observed they are absent in PEO itself... 

For closer elaboration of the numerous radical cation states of molecules on energy and time scales (Figure 2a and c), photoelectron (PES)3,5,16 and electron spin resonance (ESR/ENDOR)10,25 spectroscopic techniques have complementary time ranges Vertical ionization energy patterns are measured with a time resolution of less than 10 15 s (Figure 2c) without any vibrational structural changes on electron ejection and can therefore be correlated to the eigenvalues calculated for the neutral molecule by... 

Various spectroscopic techniques have been applied to the characterization of carbocations as stable ions in solution and in the solid state. The techniques that have found most application in the study of hypercoordinate carbocations were developed primarily to be capable of distinguishing trivalent cations undergoing rapid degenerate rearrangements from bona fide hypercoordinate ions. In recent years, theoretical studies have become also very useful in the study of hypercoordinate carbocations. Furthermore, many properties such as chemical shifts ( H, etc.) and vibrational spectra can be accurately computed. 

The spectroscopic technique described above is applicable to most ionic states, but is particularly useful for nonfiuorescing or nonpredissociating molecular ion states, such as those of many radical cations. Because of the considerably lower energies of their first excited electronic states in comparison to their neutral parent molecules, internal conversion is enhanced, thus suppressing fluorescence. Typical examples are the cations of all mono- and of many di- and trihalogenated benzenes as well as of the benzene cation itself. For illustration, the UV/VIS spectrum of the monofluorobenzene cation shown in Fig. 4.41c was measured by the method described above and revealed for the first time vibrational resolution for this molecular cation. 

One important use of SFG vibrational spectroscopy is the orientational analysis of ionic liquids at gas-liquid interfaces. For example, the study of the structural orientation ofionic liquids using common cation types, that is, [BMIM], combined with different anions, gives information on the effects of both cation and anion types [3, 22, 26-28]. Additional surface analytical work includes SFG studies under vacuum conditions for probing the second-order susceptibility tensor that depends on the polar orientation of the molecule and can be correlated to the measured SFG signal intensities. Supporting information is frequently obtained by complementary bulk spectroscopic techniques, such as Raman and Fourier transform infrared (FTIR) analysis, for the analysis of the pure ionic liquids. 

When treating ion spectroscopy one should not forget anions. Similar spectroscopic techniques may be used as for cation spectroscopy. For instance dissociation spectroscopy is also possible for molecular anions. Since excited anionic electronic states mostly do not exist, one uses infrared multiphoton dissociation to study vibrational levels of the ground state. Another interesting technique is the photoelectron spectroscopy of anions (photodetachment photoelectron spectroscopy), which exhibit a very specific feature. This technique differs from cation <— neutral photoelectron spectroscopy in two respects (i) the final state is a neutral one thus anion photoelectron spectroscopy delivers information about neutrals rather than ionic systems, (ii) The initial state is anionic thus mass selection before spectroscopy is possible. As a result, mass selective spectroscopic information of neutral molecular systems is supplied which otherwise is not accessible. This is of particular interest for neutral systems which are only available in complex mixtures or are short-lived intermediate reaction products or radicals. 

Our studies also included IR spectroscopic investigation of the observed ions (Fig. 6.2). John Evans, who was at the time a spectroscopist at the Midland Dow laboratories, offered his cooperation and was able to obtain and analyze the vibrational spectra of our alkyl cations. It is rewarding that, some 30 years later, FT-IR spectra obtained by Denis Sunko and his colleagues in Zagreb with low-temperature matrix-deposition techniques and Schleyer s calculations of the spectra showed good agreement with our early work, considering that our work was... 

The third and fourth chapters deal with special classes of materials rather than measuring techniques as found in the first two chapters and the last one of this volume. In a brief, but powerful, review M.A. Subramanian and A.W. Sleight (chapter 107) discuss the chemistry, structure, electrical, magnetic and thermal behaviors spectroscopic (vibrational, ultraviolet-visible and Mossbauer) properties and luminescence of pyrochlores. Pyrochlores are ternary oxides of the general formula A2M2O7, where A can be a divalent or trivalent cation, and M is a pentavalent cation if A is divalent or a tetravalent cation if A is trivalent. Over several hundred rare earth pyrochlores are known, many are electrical insulators, some are semiconductors, and even a few are metallic in nature. 

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