Alkali halide matrix

It can be seen from these considerations that the alkali halide matrix should be considered as a solvent and the interactions with the included solute will be a function of the chemistry of the system under discussion. This conclusion emphasizes the need for caution in correlating data obtained from various regions of the spectrum in which a common solute has been exposed to a variety of environments, whether of solvent-matrix nature or due to other solute molecules. 

Powders or fluff of polymers can be studied with the transmission technique by forming a KBr pellet. This approach disperses the sample in an infrared transparent matrix like KBr or NaCl. These inorganic salts have the property of cold flow so they revert to a glass-like consistency when sufficient pressure (10-15,000 Ib/in ) is applied to a finely divided powder. In this method, KBr is usually mixed with 1% of a finely ground sample in a special die. Pressure is applied with a press. The sample pellet formed is placed in the spectrometer and measured. The use of an alkali halide matrix to support and surround a solid sample for IR analysis was first presented by Stimson in 1952 [11]. This technique has been used widely because of its general applicability. The limitation of the KBr method (in addition... 

Hisatsune and co-workers [290—299] have made extensive kinetic studies of the decomposition of various ions in alkali halide discs. Widths and frequencies of IR absorption bands are an indication of the extent to which a reactant ion forms a solid solution with the matrix halide. Sodium acetate was much less soluble in KBr than in KI but the activation energy for acetate breakdown in the latter matrix was the larger [297]. Shifts in frequency, indicating changes in symmetry, have been reported for oxalate [294] and formate [300] ions dispersed in KBr. 

For the normal pellet technique where an alkali halide is used as pelleting material, reactions could occur between the matrix and the sample under study. Most of these reactions are found to be exchange reactions so that the sohd... 

Find the solubilities in water of the alkali halides. Calculate the molarity or molality (as convenient) of a saturated solution for each and plot them in matrix form with columns headed F, Cl, Br, ], and horizontal rows labeled Li, Nu, K, Rb. and Cs. Discuss any trends you notice. 

Rate Constants, Tunneling Matrix Elements, and Frank-Condon Factors for Orientational Relaxation of OH Dipoles in Alkali Halide Crystals... 

The radical anion of molecular oxygen (O ) has been prepared and trapped in a range of alcohols, water and benzene but not in aliphatic hydrocarbons (Bennett et al., 1968a). In contrast to COg the e.s.r. spectrum shows that 0 interacts strongly with its immediate environment. This interaction which alters the separation of the upper molecular orbitals of the anion is strongly dependent on the nature of the matrix. Previously, the Oj" radical ion has been stabilized only in ionic materials such as the alkali halides thus it is of particular interest to find that this anion can be trapped successfully in a non-polar matrix (benzene). There is some evidence (Evans, 1961), from optical spectroscopic studies that molecular oxygen can form a weak charge transfer complex with the 77-electron system in benzene and it seems probable that O2 is stabilized in benzene by the formation of a similar complex. 

Body Overlap Forces to the Dynamical Matrix of Alkali Halides. 

Poole, Liesegang, Leckey, and Jenkin (1975) have reviewed published band calculations for the alkali halides and tabulated the corresponding parameters obtained by various methods. Pantclidcs (1975c) has used an empirical LCAO method that is similar to that described for cesium chloride in Chapter 2 (see Fig. 2-2), to obtain a universal one-parameter form for the upper valence bands in the rocksalt structure. This study did not assume only one important interatomic matrix clement, as we did in Chapter 2, but assumed that all interatomic matrix elements scale as d with universal parameters. Thus it follows that all systems would have bands of exactly the same form but of varying scale. That form is shown in Fig. 14-2. Rocksalt and zincblende have the same Brillouin Zone and symmetry lines, which were shown in Fig. 3.6. The total band width was given by... 

Universal matrix elements were not available at the time of Pantelides s study, but now we may use them for a direct prediction of the susceptibility in the alkali halides. This is closely analogous to the calculation of susceptibility for the tetrahedral solids, done in terms of bond dipoles in Section 5-A by means of Eq. (5-7), and the corresponding calculation for the mixed tetrahedral solids. Since now we do not have independent two-electron bonds, however, the. susceptibility must be formulated differently the final result will be equivalent to the extreme polar limit of the tetrahedral solids. 

Before making application of this formula to tlie perovskites, let us make a brief application to ionic crystals- we summarized the results of this application in Chapter 13 -and to simple tetrahedral solids. In the alkali halides, we focus upon the occupied p states in the halogen ion and calculate the chemical grip associated with interaction of the halogen ion with the alkali, v stales. These arc the same couplings that were included in the calculation of ion softening in Section 14-C. The coupling W2 of Eq. (19-29) becomes the matrix element = 1.84 h /(md ), and 2W i, is to be identified with the = 9.1 h l(nul ) used in Table 14-2. Then (Eq. 19-29) becomes... 

In Section 19-E we found contributions to the effective charge 7 of the oxygen due to the matrix clement between an sp hybrid and a d slate and due to the matrix element For an oxygen displacement perpendicular to the Ti—O axis, the predicted transverse charge is simply this Z. However, for displacements along the Ti—O axis there are contributions from the transfer of charge between ions just as there were in the alkali halides, for which we obtained the transverse charge in Section 14-C. 

Matrix Isolation Studies of Alkali Halide Salt Molecules with Lewis Acids and Bases... 

The matrix isolation technique has been applied, in conjunction with the salt/molecule reaction technique, to model the high temperature gas phase reactions of alkali halide salt molecules. The reactions with Lewis acids such as SiFi, HF and CO2 yielded ion pair products which were quenched into inert matrices for spectroscopic study. Difficulties arising from lattice energy considerations in alkali halide salt reactions are minimized by the initial vaporization of the salt reactant. The reaction of such salt molecules with Lewis bases, including H2O and NH3, yielded complexes similar in nature to transition metal coordination complexes, with binding through the alkali metal cation to the base lone pair. 

These led to matrix studies of the structure of ion pairs and triple ions, such as the thorough studies by Devlin and coworkers on matrix isolated alkali nitrate (21), chlorate (22) and perchlorate ion pairs (23 ). For relatively simple salts, such as the alkali halides, investigations were conducted into the structure of the dimeric salt species (6, 7, ), which is present in a gas phase equilibrium with the monomeric salt species. These dimers have been found to be very strongly bound in a cyclic structure. 

Only occasionally have salt molecules been vaporized for use as a reactant toward another species in matrix isolation studies. Devlin (24,25,26) conducted extensive experiments in which salt molecules were vaporized and condensed into argon matrices containing from 1% to 90% H2O or NH3, to study the effects of stepwise solvation of the salt molecule, as a model for solution studies. Margrave (27 ) and Snelson (28) each have used salt molecules as reactants, but most commonly toward another salt molecule to form a mixed salt dimer. The work described below, which was initiated at the University of Virginia and has been continued at the University of Cincinnati, employs alkali halide salt molecules as reactants toward a variety of species, including both Lewis acids and Lewis bases. The initial intent was to react a salt molecule such as NaCl with HCl in an excess of argon to bring... 

The reactions of alkali halide and other salt molecules in the gas phase are of considerable interest to high temperature chemists reactions of CsF in either the gas phase or condensed phases are also of interest to catalytic chemists. The matrix isolation technique has proven itself valuable in the area of high temperature chemistry while inert matrices are condensed at 15 K, the technique allows a high temperature reaction to be initiated in front of the cold surface and then rapidly quenched to trap the initial products of the high temperature reaction. 

Solid solutions of BH4 in alkali halide single crystals were also investigated at room temperature in order to obtain information about changes in the symmetry caused by the matrix material (Memon et al., 1982). 

A number of experimental alternatives to traditional IR transmission spectroscopy are suitable for overcoming some of these complicating experimental factors. In the technique of diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (Hartauer et al. 1992 Neville et al. 1992) the sample is dispersed in a matrix of powdered alkali halide, a procedure which is less likely to lead to polymorphic transformations or loss of solvent than the more aggressive grinding necessary for mull preparation or pressure required to make a pellet (Roston et al. 1993). For these reasons, Threlfall (1995) suggests that DRIFTS should be the method of choice for the initial IR examination of polymorphs. He has also discussed the possible use of attenuated total reflection (ATR) methods in the examination of polymorphs and provided a comparison and discussion of the results obtained on sulphathiazole polymorphs from spectra run on KBr disks, Nujol mulls and ATR. 

Many other materials were studied as matrix materials for diffuse reflectance spectroscopy. Beside the previously noted alkali halides, other examples are silver halides, diamond, germanium, silicon, and chalco-genide glass, preferably of small particle diameters. For an extensive survey on different matrices, see Ref. °l... 

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