Field-tuning mechanism

Let us now turn our attention to the application of the sound wave to a liquid since this is the medium of importance to the practising chemist. The sound wave is usually introduced to the medium by either an ultrasonic bath or an ultrasonic horn (see Chapter 7). In either case, an alternating electrical field (generally in the range 20-50 kHz) produces a mechanical vibration in a transducer, which in turn causes vibration of the probe (or bottom of the bath) at the applied electric field frequency. The horn (or bath bottom) then acts in a similar manner to one prong of a tuning fork. As in the case of air, the molecules of the liquid, under the action of the applied acoustic field, will vibrate about their mean position and an acoustic pressure (P = P sin 2k ft) will be superimposed upon the already ambient pressure (usually hydrostatic, Pjj) present in the liquid. The total pressure, P, in the liquid at any time, t, is given by Eq. 2.4. 

Because of its piezoelectric properties, synthetic CC-quartz is used for frequency control in electrical oscillators and filters and in electromechanical transducers. When mechanically stressed in the correct direction, CC-quartz develops an electric polarization. The opposite is also tme an applied electric field gives rise to a mechanical distortion in the crystal. Thin sections of quartz are cut to dimensions that produce the desired resonance frequency when subjected to an alternating electric field the vibrating crystal then reacts with the driving circuit to produce an oscillation that can be narrowly controlled. Quartz is ideal for this application because it is hard, durable, readily synthesized, and can be tuned to high accuracy, for example, quartz crystal clocks can be made that are stable to one part in 109. 

The parameterization of a force field can be based on any type of experimental data that is directly related to the results available from molecular mechanics calculations, i. e., structures, nuclear vibrations or strain energies. Most of the force fields available, and this certainly is true for force fields used in coordination chemistry, are, at least partially, based on structural data. The Consistent Force Field (CFF)197,106,1071 is an example of a parameterization scheme where experimentally derived thermodynamic data (e. g., heats of formation) have been used to tune the force field. Such data is not readily available for large organic compounds or for coordination complexes. Also, spectroscopic data have only rarely been used for tuning of inorganic force field parameters113,74,1081. 

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