Halides electrostatic fields

This model of a complex or of a crystalline salt of a metal ion in a compound such as a halide or oxide is of an electrostatic, point charge, or point dipole type. The ligands or neighbors of the metal ion are treated as structureless, orbital-less point charges, which set up an electrostatic field. The effect of this field on electrons in the d orbitals of the metal ion is then investigated. 

When an ion is adsorbed on the surface of a dielectric, itself consisting of ions, we may expect Coulomb forces to act between the ions of the adsorbent and the adsorbed ion. A positive ion, adsorbed on top of a negative ion of an adsorbent, is attracted by this ion, but it is repelled by the ions surrounding the one with which it is in direct contact, attracted again by the then following ions, etc. The result is a rather weak attraction. Hiickel (S3) derived the following equation for the electrostatic field emanating from a cubic face of the surface of an alkali halide crystal ... 

Molecules that have dipoles, such as organic halides, ethers, ketones, nitrocompounds, etc., will be attracted by the electrostatic field emanating from the surface of an ionic crystal. The contribution toward the adsorption energy is given by... 

Nucleophilic photosubstitution reactions of benzylic chlorides have also been observed to occur with nucleophiles other than the alcohol solvents. n-Nucleophiles such as amine solvents147 and halide ions and acetate ions148, as well as 7r-nucleophiles such as toluene149 have been used. The latter, a photoalkylation reaction, was achieved by irradiation of benzyl chloride absorbed within zeolite micropores in a slurry in cyclohexane. In cyclohexane itself only products of PhCH2 are formed. This large medium effect is due to the strong electrostatic fields experienced in the zeolite cavities149. 

Most of the irregular SE s formed by irradiation interact with impurities that are the native irregular SE s of the crystal. Impurities interact with the irradiation products either by their stress field or, if heterovalent, by the electrostatic (Coulomb) field. Photolysis (radiolysis) is found in other than halide crystals as well. In oxides, the production of Frenkel pairs under photon irradiation is negligible. This has been ascribed to the fact that the reaction O2- +0" = 02 is endothermic, whereas the reaction X- +X = is exothermic. 

Of course, the traditional problem of the lack of precise knowledge of the heats of solvation for the passage of these ions into solution, makes the above criteria of stability less valuable to the condensed-phase chemist. A major breakthrough in this classical impasse has been achieved by Arnett and coworkers " who have recently carried out calorimetric measurements leading to reliable values of the enthalpy of ionisation of various alkyl, cycloalkyl and aiyl halides in solution. These determinations owe their validity to the use of superacid conditions and the NMR verification that the ions expected were in fact formed in those media without Ihe occurrence of secondary reactions. One of the most important conclusion of these studies is that on the whole the relative stabUities of carbenium ions are the same in the gas pha% and in the solvents used, i.e., electrostatic solvation effects do not alter the order of stability. The importance of this new experimental approach is quite obvious and one can except in the near firture considerable advances in the field of the thermodynamics of reactive carbenium ions in solution through the attmnment of a precise knowledge of AG° values for their formation in various media. 

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