The Halogens and Hydrogen

The molecules in crystalline chlorine, bromine and iodine are packed in a different manner, as shown in Fig. 11.1. The rather different distances between atoms of adjacent molecules are remarkable. If we take the van der Waals distance, such as observed in organic and inorganic molecular compounds, as reference, then some of the intermolecular contacts in the b-c plane are shorter, whereas they are longer to the molecules of the next plane. We thus observe a certain degree of association of the halogen molecules within the b-c plane (dotted in Fig. 11.1, top left). This association increases from chlorine to iodine. The weaker attractive forces between the planes show up in the plate-like habit of the crystals and in their easy cleavage parallel to the layers. Similar association tendencies are also observed for the heavier elements of the fifth and sixth main groups. 

Inorganic Structural Chemistry, Second Edition Ulrich Muller 2006 John Wiley Sons, Ltd. 

The structure of iodine at four different pressures. The outlined face-centered unit cell in the 30-Gpa figure corresponds to that of a (distorted) cubic closest-packing of spheres. At 24.6 GPa four unit cells of the face-centered approximant structure are shown the structure is incommensurately modulated, the atomic positions follow a sine wave with a wave length of 3.89 x c. The amplitude of the wave is exaggerated by a factor of two. Lower left Dependence of the twelve interatomic contact distances on pressure 

The amplitude of the wave is parallel to b and amounts to 0.053 b. The interatomic distances differ from atom to atom within an interval of 286 to 311 pm. 

The structure of iodine at four different pressures. The outlined face-centered unit cell in the 30-Gpa figure corresponds to that of a 

Lower left Dependence of the twelve interatomic contact distances on pressure 

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No conformational dependence (NCD). Groups of this type include monatomic substituents such as hydrogen and the halogens cylindrical substituents such as the ethynyl and cyano groups, and tetracoordinate symmetric top substituents such as the methyl, trifluoromethyl and silyl groups. 

In these molecules both atoms have zero formal charge, but the physical properties indicate that there are nevertheless small charges of opposite sign on the hydrogen and the halogen atom, respectively. 

This dehydrohalogenation is called an alpha elimination (a elimination) because the hydrogen and the halogen are lost from the same carbon atom. The more common dehy-drohalogenations (to form alkenes) are called beta eliminations because the hydrogen and the halogen are lost from adjacent carbon atoms. 

COMPOUNDS OF IRON WITH HYDROGEN AND THE HALOGENS. 105 The following data have been obtained 1 for these compounds —... 

Chapter VI. Compounds of Iron with Hydrogen and the Halogens... 

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