Quadrupolar halogen nuclei

TABLE 1 Selected NMR properties of the NMR active quadrupolar halogen nuclei... 

It is useful to emphasise from the outset that alternative techniques for investigating solid samples, other than NMR spectroscopy of the quadrupolar halogen nuclei, tend to be considered first this is likely due to the (perceived) difficulty of the technique and may be related to the issues of sensitivity and line broadening mentioned above. However, the information gained is often complementary, and hence solid state nuclear magnetic resonance (SS NMR) spectroscopy is primarily used when it can provide unique insight unavailable with other techniques. 

Relaxation times of quadrupolar halogen nuclei may also be used to obtain sample information. One application of relaxation time measurements in the solution states is the ability to probe the level of ordering present in a solution, and it is of particular interest when compared directly to the solid state, an advantage of NMR spectroscopy highlighted earlier. 

Solution state standards for each of the quadrupolar halogen nuclei were also recommended (Table 1). As mentioned previously, one should carefully note both the concentration and solvent when preparing solution state standards, as the measured shift values have been shown to depend significantly upon these two variables (see also Section 4.5). Typical solvent isotope effects (expressed as the result of 5 X, H2O)—5(X, D2O), X = Cl, Br, I) are ca. 5 ppm for chlorine, 8-10 ppm for bromine, and 13 ppm for Na I, and are one (in the case of Cs) to three (Li/Na) orders of magnitude greater than the corresponding solvent isotope shifts for the alkali metals in the alkali metal halides. 

In practice, the SSNMR spectra of the quadrupolar halogen nuclei in non-cubic environments are expected to be dominated by the QI. This is due to the moderately large Q associated with each nuclide, coupled with typically modest CSA contributions (t)q5ical CSAs are observed to be on the order of 10 -10 ppm). However, recent work in particular vide infra) has shown that there are in fact many examples, particularly for chlorine-35/37, where contributions from CSA... 

Relaxation data on quadrupolar halogen nuclei may be obtained either from pulsed NMR studies or from line width measurements. The lower limit of T.j and T2 values that can be measured with a pulse spectrometer is set by the effective dead time of the receiver system after a pulse and by the width and amplitude of the pulse. At present... 

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. 

With a few exceptions, the listed spin couplings have been indirectly determined. As discussed in Section 1.5 there are essentially two methods available for determining scalar couplings to quadrupolar halogen nuclei. The first method is based upon the fact that the scalar contribution to the relaxation of a spin 1=1/2 nucleus (I) coupled to a quadrupolar halogen nucleus (S) is different for the two... 

Table 1 NMR properties of the half-integer spin quadrupolar halogen nuclei N.A.(%) / Q(mb) Ar crlRel.) S (%) y/10 (rad T- s h ...

R.P. Chapman, C.M. Widdifield, D.L. Bryce, Sohd-state NMR of quadrupolar halogen nuclei, Prog. Nucl. Magn. Reson. Spectrosc. 55 (2009) 215—237. 

---