Infrared spectroscopy with alkali metals

Solutions of alkali metals in liquid ammonia have been studied by many techniques. These include electrical conductivity, magnetic susceptibility, nuclear magnetic resonance (NMR), volume expansion, spectroscopy (visible and infrared), and other techniques. The data obtained indicate that the metals dissolve with ionization and that the metal ion and electron are solvated. Several simultaneous equilibria have been postulated to explain the unique properties of the solutions. These are generally represented as follows ... 

A significant rate enhancement for the C02 insertion process was noted in the presence of alkali metal counterions (Table I), even in the highly coordinating THF solvent. This rate acceleration was not, however, catalytic in alkali metal counterion, since the once formed carboxylate was observed to form a tight ion pair (76, 77) via its uncoordinated oxygen atom with the alkali metal ion, as evinced by infrared spectroscopy in the v(C02) region. That is, the counterion was consumed during the carbon dioxide insertion reaction. 

Prereactive behaviors were identified very early and an impressive series of examples was listed in a book by Klabunde in 1980 [266]. Electron spin resonance (ESR) studies reveal that in low-temperature matrices electron-transfer reactions are blocked as a rule and most, if not all, charge-transfer complexes involved in standard gas-phase harpoon reactions have been stabilized and observed in matrices. The ESR spectra of these systems revealed nearly complete electron transfer. Similar conclusions have also been drawn from infrared spectroscopy. For example, outside the field of alkali metal atoms, evidence of an AHNO complex has been obtained by this technique [267]. It should not be thought that every metal atom is able to make charge transfer with every molecule. For example, no indication exists of a charge transfer between Cu and NO in an argon matrix [268]. 

Infrared spectroscopy in the CO stretching region provides a sensitive probe for detecting contact ion pair formation between cations and carbonylates (12,13). Good examples are provided by the early work on the interaction of cations with tetracarbonylcobaltate (12) and by more recent work on a variety of metal carbonyl anions (13,14). In solvents such as THF, alkali metal carbonylates appear to be present in two or three different forms tight (contact) ion pairs, looser ion pairs, and ion triplets. For example, Fig. 5 shows the IR spectrum of Na[Mn(CO)s] in THF and its deconvolution into two different sets of bands, which are associated with two different species in solution (14). For one of these the evidence points to a contact ion pair of structure 6. 

The adsorption of molecular probes, followed by infrared or NMR spectroscopy and thermal desorption studies, is the most commonly adopted way to study basic sites. Carbon dioxide is frequently used in infrared studies, particularly of cationic zeolites with added alkali metals.Chloroform is also suitable, since the interaction with the chlorine atom and subsequently the C-Cl stretching frequency is a measure of the basic strength. NMR studies of basic zeolites have concentrated on the use of C containing probes such as methyl iodide and chloroform. Addition of methyl iodide results in its decomposition and the formation of methoxy groups at framework oxygens. The shift of the methyl carbon is expected to be related to the basicity of the framework, that is the tendency of the framework oxygens to donate electrons - C MAS NMR of methoxy groups prepared in this way shows a clear distinction between basic zeolites, such as Cs-X, and acidic zeolites, such as H-ZSM-5. 

In principle, all metallic elements can be determined by plasma emission spectrometry. A vacuum spectrometer is necessary for the determination of boron, phosphorus, nitrogen, sulfur, and carbon because the emission lines for these elements lie at wavelengths less than 180 nm, where components of the atmosphere absorb radiatjon. The usefulness for the alkali metals is limited by two difliculties (1) the compromise operating conditions that can be used to accommodate most other elements are unsuited for the alkalis, and (2) the most prominent lines of Li, K, Rb, and Cs are located at near-infrared wavelengths, which lead to detection problems with many plasma spectrometers that arc designed primarily for ultraviolet radiation. Because of problems of this sort, plasma emission spectroscopy is generally limited to the determination of about 60 elements. 

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