Spectroscopy, infrared cation interactions

Belton, P. S., Wilson, R. H., and Chenery, D. H. (1986). Interaction of group I cations with iota and kappa carrageenans studied by Fourier transform infrared spectroscopy. Int. J. Biol. Macromol. 8 247-251. 

The decomposition of Co2(CO)8 in faujasites has been studied in some detail. Low-temperature spin-echo ferromagnetic nuclear resonance spectroscopy shows that very small Co particles are formed in supercages of zeolite NaX by microwave plasma activation at low temperatures (86). In situ far-infrared spectroscopy revealed that adsorbed Co2(CO)s interacts with accessible supercage cations in NaY and CoY (239). Carbonyl complexes of different Co nuclearity, such as Co4(CO)i2 and Co(CO)4, are also formed (227,228). In HY the Co atoms are oxidized to Co ions by the zeolite protons. 

Up to now, infrared spectroscopy has been used mainly to determine the types of hydroxyl groups and the acidity of zeolites (39). The frequencies of the vertical and horizontal vibrations (with respect to the cavity wall) of H2O molecules adsorbed in zeolite A were determined by measurements in the far infrared ( 220 and —75 cm" ) (37). These values are in agreement with a simple theoretical model. A number of ultraviolet and ESR studies are reviewed (33). The difference has been established between the specific molecular interaction of aromatic molecules on zeolites cationized with alkali cations and the more complex interactions involving charge transfer in CaX and deca-tionized X and Y zeolites. These more complex interactions with CaX zeolites containing protonized vacancies and with decationized zeolites are similar. These phenomena are related to the interactions of molecules with acidic centers in zeolites which are stronger, as compared with the molecular adsorption. 

On the other hand, the spectroscopic techniques probe individual ionic species which build up the ionic aggregates. These techniques permit the investigation of the immediate chemical environments, the mobility of cations and water-ions Interactions. Metal nuclear magnetic resonance and Mossbauer spectroscopy are sensitive probes of counter cations and provide valuable information on the cations and their environment. Infrared spectroscopy is complementary to the above methods and addresses itself to the bound SO3" anions or water and the interaction of water molecules with the various species with which it is in contact. A common conclusion that is reached in the above mentioned studies is that four or five water molecules are needed to complete the hydration process. Reducing the level of moisture content (which surrounds the ionic species) below four water molecules per unit SOj site enhances the Coulombic interaction between the ionic species. This eventually leads to the formation of ion pairs in the dry membranes. These ion pairs do not necessarily disperse homogeneously in the fluorocarbon matrix but tend to form aggregates, phase separated from the matrix materials as demonstrated in the scattering studies. 

Carbon monoxide, a soft base, is expected to interact with a soft acidic surface site (19), The octahedral iron cations (+2.5 average oxidation state) are the softer of the acid sites on magnetite and may be expected to provide CO adsorption sites. The initial interaction should result in a carbonyl surface species, and such species have been observed by infrared spectroscopy (20-22)t... 

The work described in the present paper concerns the Influence of water and organic solvents on the ionic interactions in lightly sulfonated polystyrene (SFS) ionomers. The focus will be specifically directed towards the Influence of the solvent environment on the cation-anion and cation-cation interactions. Fourier transform Infrared spectroscopy (FTIR) was used to probe the former while electron spin resonance spectroscopy (ESR) was used to study the latter. Experiments were carried out with dissolved, swollen, and bulk ionomers. 

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. 

Infrared spectroscopy has been a valuable technique for exploring zeolite structures. It is useful for studying the nature of hydroxyl groups in zeolites, the interaction of cations with adsorbed molecules, and the fundamental framework structures of zeolites. 

Most acidity studies have been made using basic molecules such as ammonia, pyridine, and piperidine as probes. These molecules have the property that their interaction with Bronsted acid sites, Lewis acid sites, and cations and their hydrogen-bonding interactions give rise to different species detectable by infrared spectroscopy. Thus, adsorption on Bronsted acid sites gives rise to ammonium, pyridinium, and piperidinium ions with characteristic absorption frequencies of 1475, 1545, and 1610 cm"1, respectively. Adsorption on Lewis acid sites—tricoordinated aluminum... 

The adsorption behavior of diazines in X and Y zeolites has been studied by infrared spectroscopy (IR), temperature-programmed desorption (TPD), and simulation techniques. The studies showed that the interaction is determined by a donation of electron density from the nitrogen atoms of the probe molecules to the Lewis-acidic cations. The individual nature of the adsorption strongly depends on the Si/Al ratio of the zeolites, the kind of extraframework cation, and the positions of heteroatoms in the probe molecules. 

Infrared spectroscopy is a widely available technique and has been applied extensively in the study of microporous solids. Using Fourier Transform analysis, sensitive detectors and operating either in transmission or in diffuse reflectance (DRIFT) mode, powders can give spectra with high resolution and sensitivity. The method is most valuable when analysing the interaction of molecules with adsorption sites (acid or base) - this is described in Chapters 7 and 8. It does give some structural insights, however, for example on the environment of protons and on the presence of framework and non-framework cations. 

The interaction of benzene either with the cations or with the 12-R window can be quantified in an isotherm of adsorption, one for each type of sites. This has been done using infrared spectroscopy in NaY (75). 

XRD is useful to determine cation location in the faujasite structure. For some light cations such as Li or for structures with many possible sites, far infrared spectroscopy may give valuable information. It has been applied mainly to faujasites (figure 13) (80-82) and quite less to A or ZSM-5 (82). In each structure the cations in the various sites give rise to bands at a given wavenumber (80) which shift upon adsorption of H2O (81,82) or other adsorbates such as H2S, C6H6... (81). These results clearly indicate that the interaction cation - adsorbate described above implies also a change in the interaction of the cation with the zeolite framework. The deconvolution of the spectra may lead to the evaluation of the cation population in each site. 

Several mechanisms were offered to explain steric control in polymerizations of polar monomers. Furukawa and co-workers based their mechanism on infrared spectroscopy data of interactions between the cations and the growing polymeric chains in polymerizations of methyl methacrylate and methacrylonitrile. They observed a correlation between the tacticities of the growing molecules and the carbonyl stretching frequencies. The higher the frequency, the higher is the amount of isotactic placement in the resultant chains. The adducts, as in the initiation reactions, are resonance hybrids of two structures, A and B ... 

---