Electron acceptor–electron donor interactions. Part XVIII. Charge-transfer interactions of some of the highest-valency halides, oxyhalides, and oxides with aromatic hydrocarbons and fluorocarbons. Ball–plane interactions. Group IV

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Abstract
We report a systematic examination of the interactions of the title. The disappearance of colour in the frozen mixtures of many weakly interacting materials is important to this study, and occurs when a binary system exhibiting an intermolecular charge-transfer transition produces a simple eutectic phase-diagram. The following generalisations are tested: (1) the covalent, highest-valency halides, oxyhalides, and oxides of the elements include high electron-affinity molecules that demonstrate charge-transfer spectra with the strictly π-donors such as the aromatic hydrocarbons and fluorocarbons; (2) as such donors produce stable complexes only in close ‘plane-to-plane’ combinations, and since the interactions are mostly of the ‘ball-to-plane’ type, the associations are very weak or just contact. Difficulties encountered in measuring and interpreting the measurement of extremely weak interactions are emphasized. Earlier reports of intermolecular transitions in TiCl4 and SnCl4 systems are confirmed and spectral changes for some GeCl4, PbCl4, and CCl4 systems are assigned similarly. On the other hand the interactions are extremely weak according to equilibrium and enthalpy studies, and many of the two-component mixtures are thermochromic, although X-ray diffraction patterns of the frozen materials in capillaries are diffuse. Positions of the absorptions are related to the order of vertical electron affinities. Silicon tetrachloride is effectively an inert, nonperturbing medium, and aromatic hydrocarbons dissolved in it produce spectra nearly identical to their cyclohexane solutions. Some anomalies of the methylbenzene spectra are described and a very low intensity absorption in hexamethylbenzene is identified tentatively as a Tâ†�S transition. The method of continuous variations is discussed and a previous equilibrium measurement based on this method is criticized.

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