Chlorine, bromine and iodine nuclei

As already discussed above chlorine, bromine and iodine nuclei all have I > 1 and possess large electric quadrupole moments. It is therefore not surprising that in a majority of systems studied by NMR the dominant relaxation mechanism is due to quadrupolar interactions. The only exceptions encountered so far concern certain paramagnetic systems. 

All Stable chlorine, bromine and iodine nuclei possess an electric quadrupole moment and in non-symmetric environment their NMR spectra are usually broad and cannot be studied by ordinary high resolution techniques. Notable exceptions are halide ions in aqueous solution where the NMR signals become reasonably narrow (approximate line widths at half height are = 8 Hz for Cl and 5 Hz for Cl... 

Rapid magnetic exchange self-decouples chlorine, bromine, and iodine nuclei... 

Exercise 9-31 Sketch the proton nmr spectrum and integral expected at 60 MHz, with TMS as standard, for the following substances. Show the line positions in Hz neglect spin-spin couplings smaller than 1 to 2 Hz and all second-order effects. Remember that chlorine, bromine, and iodine (but not fluorine) act as nonmagnetic nuclei. 

A drawcard of NMR spectroscopy is that it can be used to obtain information on systems in both solid and solution phase. Arguably, this is an area in which NMR spectroscopy of chlorine, bromine and iodine has been underutilised because of the limitations associated with the quadrupolar nature of the nuclei. However, recently, it has shown wide potential application particularly as a tool for quantifying halide content in samples and for following reaction progress. 

Having discussed the recent work in the areas of chlorine, bromine and iodine NMR spectroscopy, it is also worthwhile to consider the advancements being made in NMR spectroscopy in general and how they might be specifically applied to overcome the inherent limitations present in examining NMR spectra of the quadrupolar nuclei. 

Protons are not coupled to chlorine, bromine, or iodine nuclei because of the strong electrical quadrupole moments of these halogen nuclei. For example, proton-proton coupling in CH3CH2C1 is unaffected by the presence of a chlorine nucleus the triplet and quartet are sharp. 

For covalent chlorine, bromine and iodine compounds the relaxation times of the halogen nuclei are extremely short and problems of sensitivity considerable even for pure liquids. Direct observations of NMR signals have therefore been reported only for chlorine compounds. Narrow signals are obtained when the nucleus is at a site of tetrahedral symmetry in covalent compounds (cf. Chapter 9). 

Absolute shielding data for the quadrupolar halogen nuclei are not yet available. Spin rotation interaction constants have however been determined for GIF [153 154 155], HCl [156], CICN [157], HBr [158] and HI [159]. In order to make use of these data to establish an "absolute" shielding scale for the magnetic chlorine, bromine and iodine isotopes it would be necessary to determine the halogen chemical shift of the gaseous compounds relative to a common reference such as an aqueous sodium halide solution. The experimental problems in such measurements are however by no means trivial. 

In the beginning of the chapters dealing with specific applications of chlorine, bromine and iodine NMR we have tried to lay out the basic spectroscopic principles involved. Much of this material is applicable to all quadrupolar nuclei. 

The chlorine, bromine, oxygen, etc., in excess completely change the properties of the fundamental nucleus and its derivatives, whilst the chlorine, bromine or iodine which have entered into the various nuclei do not alter the principal properties. Each nucleus, however little or much chlorinated, is capable of forming a hyperhalide by absorbing chlorine, an aldehyde or acid by absorbing oxygen. 

In the present monograph we have attempted to present a fairly comprehensive account of one set of related quadrupolar nuclei chlorine ( Cl and Cl), bromine ( Br and Br) and iodine While NMR studies of some of these nuclei were performed very early, many applications, especially in biological systems, are of recent origin and have still not reached a wide audience. 

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