Forces between Tetrahalide Molecules

Abstract

A review of the evidence furnished by melting and boiling points, entropy of fusion and evaporation, crystal structure, ionic‐bond character, radii, Raman frequencies, energy of evaporation and differential energy of expansion, polarizability, and solubility relations has led to the following conclusions: (1) Hindrance to rotation is small in liquid CCl₄ and CBr₄ but increases with increasing radius ratio of central atom to halide atom. This indicates that the van der Waals radii of the halide atoms in these liquids are considerably smaller than those assigned by Pauling. (2) In spite of the hindrance to rotation, the solubility relations can be calculated with considerable accuracy by means of equations derived for spherical molecules with radial force fields. (3) Attraction between molecules of different species is not appreciably influenced by the kind of bonding orbitals of the central atom, but is strongly influenced by the ionic character of the bond. (4) The apparent anomaly of the low boiling point and internal pressure of SiCl₄ is due to the large increase in molal volume between CCl₄ and SiCl₄; the attraction constant increases uniformly with M‐X distance in the series of chlorides of C, Si, Ge, Ti, Sn. (5) The attraction constant is closely related to the square of the polarizability, in accordance with the London theory. (6) The two isotropic Raman frequencies for the chlorides of C, Si, Ge, Ti, Sn, Pb decrease uniformly with increasing M‐X distance. (7) None of the properties of SiCl₄ requires for its explanation the assumption of ``double bond resonance.''