Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

210 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSVoltages are induced, by the precessing nuclear magnetic moment, in a receivingcoil which is placed at right angles to the steady field (H,) and to the axis of thecoil providing the driving field. The induced voltages are then amplified anddetected on an.,oscilloscope in a resonant circuit. A maximum in the voltage willbe detected when the frequency of the rf current is equal to v, the angular frequencyof precession, i.e. when the rf field is in resonance with the precessing nuclei.G detailed description for an advanced design of apparatus is given by Gutowskyet al.'44. Commercial apparatus is now widely used.(c) Electron paramagnetic resonance (EPR)~ 40EPR can be used to observe the unpaired electrons in paramagnetic substancesand the technique is in many respects similar to NMR. The main differences are (i)that electron magnetic moments are 103-104 times larger than nuclear momentsand (ii) they generally possess a contribution to their magnetic moment fromorbital angular momentum, in addition to intrinsic or spin angular momentum.Because of their larger magnetic moment, resonance appears at rf frequencies ofseveral thousand megacycles. The rf frequencies at which resonance occurs aregiven bywhere u is the frequency, g is the Land6 splitting factor, u, is the electron spinmagnetic moment, Ho is the steady magnetic field and h is Planck's constant.IX-Axis sweepectioml couplerGraphick jGyratwFig. 17. Diagram of EPR spectrometer with automatic frequency control (AFC). From ref. 140.