Separation energy alkali halides

The theory of Benedek35) also must be regarded as semi-empirical. The authors treat the ys of alkali halide crystals as a sum of three terms, namely y+, y, and yb. The first component represents the energy required to separate the positive ions, and the second the analogous work for the anions. Both are calculated more or less ab initio. On the other hand, the expression for yb, i.e., the thermal contribution, has no theoretical foundation. It is... 

Let us then turn to the ionic solids themselves. Kim and Ciordon (1974) recalculated the total energy for the alkali halides that form rocksalt structures and thereby computed from first principles the lattice spacing, the separation energy (the energy per ion pair required to separate the solid into isolated ions -this comes from the theory more naturally than does the cohesive energy, which is relative to isolated neutral atoms), and the bulk modulus. For KCI, the agreement of the values for the three properties with experimental values is typical of the calculations. The calculated (and in parentheses, the experimental) values for KCI are 3.05 (3.15) A, 175 (166) kcal/mole, and 2.3 (1.9) x 10 dync/cmA Again we may say that the interactions are quite well understood in terms of the microscopic theory. We shall return to the interpretation of these properties in terms of simple models in Section 13-D. 

If both L > To and I > tq, the Arrhenius law takes place with the effective activation energy E equal to that of the Coulomb attraction just after the creation of charged pair, similar to the case of elastic interaction. Note that since the actual value of the Onsager radius L in alkali halides could be as large sls L 100 A (T = 100 K, e Ri 5, ca = -cb = e), only a small fraction of pairs with a large enough initial separation, I L, has a chance to be separated [77]. 

In experimental fact, when a solid alkali halide is vaporized, the oppositely charged ions pair off into diatomic molecules, and the data obtained from the vapour can be used to verify the picture. For several such molecules, Table 3.1 shows the experimentally determined distances r0 (in Angstroms) between the centres of the ions, and the experimentally determined energy D (in electron volts) required to separate the ions by an infinite distance. Electrostatic theory says that each spherically symmetric distribution of charge should behave toward charges outside it as if its total charge were concentrated at its centre. Hence the picture predicts that D = e2/4ne0r0, and the last column of the table verifies the prediction. 

For MALDI-MS, van Kampen et al. [98], and their references therein] showed the importance of (1) the affinity of the analyte toward the Alkali+, whieh seems to be primarily governed by the charge density of the Alkali (2) the gas-phase availability of Alkali, whieh seems to be related to the lattice energies in the alkali-cation salt, and thus to the eounter anion used and (3) the matrix applied, which in turn may affect ion separation within the salt. For polyethylene glyeol (PEG) with an alkali-halide salt, the most abundant [M+Alkali]+-ion observed depends... 

Figure 3.4 Lowest two bound potential energy curves for an alkali halide molecule MX. The zeroth-order ionic and covalent curves, which intersect at the crossing radius R = R. . are shown at left. The adiabatic curves 6, (/ ) and s R) are shown at right the avoided crossing at R = Rc is caused by mixing of the degenerate ionic and covalent states, which have the same symmetry. The lower adiabatic state l/ (r R)> describes ionic M X" for R < R. but correlates with uncharged separated atoms M + X for large R. The higher adiabatic state R)> describes covalent MX (which is far more weakly bound that ionic M" X ) for R < R, and correlates with M" + X at large separations.

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