Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

4 SPECTRAL LINE-BROADENING 149and methanolic solutions of transiton metal ions with unpaired electrons have beeninvestigated and much information has been obtained concerning the solvent inthe inner coordination shell of the metal. The nuclei in this solvation sheath interactstrongly with the unpaired electrons of the metal ion and thus experience adifferent magnetic field compared with those in molecules in the bulk of the solvent.Thus there is a chemical shift of, say, the water or methyl OH-protons of acoordinated molecule relative to the pure solvent. Most metals exchange solventwater rapidly, so only a single proton resonance line is observed for water resultingfrom an averaging of the resonances of the bonded and free molecules. Methanolbehaves similarly, and for Co2+ it has been possible, by lowering the temperature,to slow down the exchange to such an extent that no averaging of the NMR resonancesoccurred66. Another approach to this problem has been to look at the I7Oresonance line-broadening associated with the entering of 170H2 molecules intothe inner coordination shell of the paramagnetic cation67. From these NMR measurementsit has been possible to evaluate the rate coefficients of the reactionM(H20y++F0 + M(H,0Y++H20for many metals, among them Mn2+, Fez+, Co2+, Ni2+ and Cu2+. It is interestingto note that the rate coefficients measured in this way generally agree very wellwith those measured by relaxation studies on ion-pair formation of metal salts inwater (Section 3)68.4.2 ELECTRON SPIN RESONANCE AND OTHER SPECTRAL METHODSJust as certain nuclei possess spin and associated magnetic moments, so an unpairedelectron has a spin and a corresponding magnetic moment. As for the proton,the electron can have spin values of ++ and -4. When a strong magneticfield is applied the electron may take up only two positions and it may be inducedto pass between the two levels by the application of an oscillating electromagneticfield perpendicular to the steady fiekI6'. The experimental arrangement is rathersimilar to that used in NMR spectroscopy. The steady magnetic field correspondingto the resonance condition is determined for an oscillating field of fixed frequency.This time the resonance occurs in the microwave region when H,, is of the order oflo4 gauss. The frequency is higher than for NMR because of the much larger magneticmoment of the electron compared with, for example, the proton. It is traditionalin ESR spectroscopy to plot the variation of the absorption with field as thefirst derivative (i.e., the slope of the absorption DS. field plot is used as the ordinate,as in Fig. 17) so that the peak corresponds to the field at which the curve cuts theabscissa. This practice makes it especially easy to determine the distance betweenthe points of extreme slope, and this is the parameter which is usually measured inReferences pp. 176-1 79