Covalent halogenation

Although it is widely used as a solvent, CC14 is known to constitute health hazards, and its widespread use in dry cleaning has been largely halted. CC14 does not hydrolyze in water as do most covalent halogen compounds. 

It is possible to prepare the hydrogen halides by direct combination of the elements, although this method is difficult to control (especially in the case of HF and HC1) or does not give rapid, efficient reaction (as with HI). A third method, the preferred one for HI, is the hydrolysis of covalent halogen compounds containing iodine bonded to a nonmetal ... 

The group name thus emphasizes the facility with which these atoms form ionic compounds with a formal negative unit charge on the halo-gen (X ). Equally well known are covalent halogen compounds with bonds of the type R-X. In fact with the exception of helium, neon and argon all elements in the Periodic Table can form halogen compounds of some kind. 

The two other methods suggested by Deverell - the "T " and the "6 vs. T2" methods in the abbreviated terminology of Table 6.1 - have some attractive features. There are indications of a correlation between quadrupolar coupling constants and shielding values for halide nuclei in covalent halogen compounds (cf. Subsection 3.1.3) - corre-... 

For convenience the material has been divided up in different ways. On the one hand we have discussed separately the different para-meters that characterize the halogen NMR spectra and on the other we have treated separately halide ions and covalent halogen compounds, Such a sub-division, however, has its limitations - the need for a separate section for studies of the perchlorate ion exemplifies this. 

Introductory Aspects. Relaxation in Molecules or Ions with Covalently Bonded Halogens. Shielding Effects in Covalent Halogen Compounds. Scalar Spin Couplings. Relaxation of Chloride, Bromide and Iodide Ions. Shielding of Halide Ions. Quadru-pole Splittings in Liquid Crystals. Halide Ions in Biological Systems. Studies of the Perchlorate Ion. 

Quadrupole splittings of a covalent halogen compound in liquid crystals were first observed in Cl NMR spectra of CH2CI2 in polybenzyl-L-glutamate by Gill, Klein, and Kotowycs (14). Quadrupole splittings of halide ions in amphiphilic mesophases were first observed in 1971 by Lindblom, Winnerstrom, and Lindman (15). 

Biopolymer capsules have been prepared via non-covalent interactions between various biopol3mrers and bromoisobutyramide surface-functionalized templates [73]. These authors proposed that the adsorption of the biopolymer was achieved through non-covalent halogen bonding (Fig. 13.6). Enzymes, proteins, polypeptides, polynucleotides, and polysaccharides have been used for preparation of the capsules. Their mechanical stabihty has been improved by cross-linking with an amine reactive cross-linker [74] or via repeated refimctionalization of the deposited layer/biopolymer adsorption [75]. 

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