Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 209TABLE 5NUCLEI POSSESSING A MAGNETIC MOMENT PAVOURABLE TO NMR STUDIEScillators to supply the rotating magnetic field and are known as the B10ch'~'(crossed-coil) and the P~und-Knight'~~ NMR Spectrometer respectively. A thirdmethod, due to Bloembergen et ~ 1 . utilises ' ~ ~ a radiofrequency bridge. The Blochspectrometer is most useful for the study of solids, but can only be used withsample temperatures up to 300" C, whereas the Pound-Knight spectrometer can beused up to 600" C.A block diagram of the crossed coil NMR spectrometer is shown in Fig. 16.The sample, contained in a tube, is placed in a uniform field of several thousandoersteds. This field is modulated, at 20 to 60 cycles per second, by coils the axesof which are parallel to each other and to the steady field. This modulation causesa recurrent scanning of the resonance condition. A high frequency radio transmittersupplies energy to the system through a coil placed at right angles to thesteady field. The effect of this alternating field is to change the tilt of the nucleialready oriented and precessing in the steady field. Choice of frequency is governedby the relationwhere v is the angular frequency of precession, pn is the nuclear magnetic moment,Ho is the uniform field, I is the nuclear spin quantum number and h is Planck'sconstant.AudioRE Transmitter R.CAmplRier Detector amplifierI 1Fig. 16. Block diagram of the crossed coil nuclear magnetic resonance spectrometer due to Bloch,Hansen and Packard. From ref. 134.References pp. 270-278