Interhalogen compounds symmetry

CI2, Br2,12, and some interhalogen compounds XX form complexes with ammonia that are sufficiently stable to allow the structures to be determined in the gas phase. The structure of the complex H3NICI is shown in Fig. 18.6 [4]. The ICl molecule is situated on the threefold symmetry axis of the NH3 molecule, i.e. in the direction of the elecfron lone pair on the N atom. The N-I distance is 271 pm, 71 pm longer than the estimated single, covalent bond distance obtained from the MSS rule, but still 82 pm shorter than the sum of the van der Waals radii of N and 1. The experimental information needed for calculation of the dissociation energy of the complex is unfortunately not available, quantum chemical calculations have yielded dissociation energies ranging from 42 to 52 kJ mol [5,6]. 

Finally, the results for the chlorine fluorides CIF and CIF deserve special consideration. These two interhalogen compounds have unique structures. CIF has a T-like shape whereas CIF has a symmetry with one axial and four equatorial fluorine atoms (Fig. 4.3)... 

Interhalogen Compounds. Several of these compounds have been investigated by the infrared method and their structures elucidated. The compound IF5 illustrates how Raman and infrared spectra can together aid in structure determination. The Raman spectrum of the liquid and the infrared spectrum of the gas [ ] were measured. Lord et al. compared their observations with the predicted number of frequencies for seven possible symmetries for the molecule (Table I). 

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