Paramagnetic solutes

The color of the quinonoid compounds that may be obtained by disproportionation can be sufficiently like that of the radicals to cause confusion if visual observation or broad-band spectrophotometry is used.11 For example, Preckel and Selwood, using paramagnetism as a measure of the amount of radical, reported that solutions of triphenyl-methyl derivatives more or less rapidly lost their paramagnetism. The decomposed solutions were still highly colored, but the color was no longer dependent on the temperature as it is in the case of a radical-dimer equilibrium mixture. What is more striking, and an even more subtle and dirtier trick on the part of nature, is the fact that Preckel and Selwood s non-paramagnetic solutions were still rapidly bleached by exposure to the air. It is clear that radical-like reactivity is not a safe criterion for the presence of radicals. It is also clear that the ebullioscopic method is particularly unsatisfactory in view of the excellent chance for decomposition. 

The magenta appearance of the paramagnetic solution produced by dissolving thianthrene in c. sulfuric acid is due to the absorbance of the thianthrene radical ion(l-l-) (T " ") (4) formed by one-electron oxidation. Even before this was understood, the formation of such deep colors on disolution in c. sulfuric acid was used as a diagnostic test for a thianthrene ring system. 

MF5 was reported to combine with brown solutions of iodine in IF5 to give intense blue paramagnetic solutions, but only in the case of Ta was a stable solid of composition I[TaF6] isolated.62 KrF2, when allowed to react with NF3 in the presence of NbF5 in HF solution, gave (NF4)[NbF6].63... 

Fig. 4.6. Coaxial NMR tubes for the measurement of magnetic susceptibility of a paramagnetic solute. Solution A contains the paramagnetic solute and an inert probe substance. Solution B contains the probe substance in the same solvent. The measurement is then repeated substituting the paramagnetic solute in A with a diamagnetic analog of the same concentration.

The experimental setup consists of two coaxial tubes (Fig. 4.6) [18], one of which, e.g. the inner one, containing a solution of an inert probe substance and the paramagnetic solute, and the other containing a solution of only the inert probe substance in the same solvent. The shifts of the probe substance differ in the two solutions, and two different signals are observed. Their chemical shift separation is measured, and the experiment repeated with the same solution in the outer... 

The relaxation of solvent nuclei around a paramagnetic centre has been described by Solomon, Bloembergen and others [3-8]. The observed solvent relaxation rate, 1/T, obs, is the sum of a diamagnetic term l/Tid, corresponding to the relaxation rate of the solvent nuclei without the paramagnetic solute, and a paramagnetic term 1/T, p which is the relaxation rate enhancement caused by the paramagnetic substance ... 

Ps spin conversion reactions. Ps can suffer a spin conversion reaction with paramagnetic solutes without necessity of any change in the spin state of the latter [94]. Due to the spin statistics, after interacting with the odd electron of a solute molecule or ion, Ps (whether singlet or triplet) has normally 3 chances in 4 to become o-Ps, and 1 in 4 to be found as p-Ps due to the much shorter lifetime of p-Ps as compared to that of o-Ps, this corresponds in fine to transform a large portion of o-Ps into p-Ps ... 

Iodine has long been known to dissolve in highly acidic, oxidizing media, for example, oleum, to give bright blue paramagnetic solutions these solutions contain the I2 ion and salts with anions of extremely low basicity can be isolated. The Br2 ion may be similarly generated, but Cl2+ is still unknown except in the gas phase. These... 

If a proton of a diamagnetic molecule is present in a solution containing a paramagnetic solute, the induction ( local field ) B at the nucleus will be increased because of the alignment of the solute magnetic moments in the applied field. Evans has shown that this increase in the local field is given by... 

Here p is the density of the solution containing w kg of paramagnetic solute per cubic meter and q is th mass susceptibility of the solution without the paramagnetic material. The density of the latter is po so that Xo = Amass oPo and we obtain from Eqs. (10) and (11) the expression... 

To provide an internal reference, the solvent is sealed in a capillary, which is placed at the bottom of the NMR tube used for the sample solution. For this purpose, a melting-point capillary is closed at one end and a syringe is used to add the reference solution. For aqueous studies, a 2% solution of t-butyl alcohol in water can be used as reference and as solvent for the paramagnetic solute. The shift with respect to the methyl resonance of the r-butyl group is then monitored. With organic ligands such as acetylacetonate (acac) groups, complexes such as Cr(acac)3, Fe(acac)3, and Co(acac)2 are soluble in benzene and the proton resonance of the solvent is a convenient reference. The capillary is filled one-third full, the lower end of the capillary is cooled in ice, and the upper part is sealed off with a small hot flame. Alternatively, the reference solution can be placed in one compartment of a coaxial pair of cyhndrical NMR tubes, which are available commercially. In either case the spectral display should be expanded to permit an accurate measurement of the frequency shift. 

Neglecting the third term of Eq. (12), the mass susceptibility of a paramagnetic solute is readily determined from m and Ai. The diamagnetic mass susceptibilities 0 of the solvents, in SI units of m kg are -8.8 X 10 (benzene), —9.0 X 10 (toluene), and -9.0 X 10 (t-butyl alcohol-water solvent). For temperature-dependence studies, correct m according to Eq. (18), calculate A A/and plot versus HT. The slope of the best straight... 

Bloembergen, N. (1957) Proton relaxation times in paramagnetic solutions. Journal of Chemical Physics, 27, 572-573. 

A magnetic field has no appreciable influence on the viscosity of a paramagnetic solution. 5 The dependence of viscosity on temperature for potassium chloride solutions follows equation (13), 9.VIII E. 

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