Definition of the Problem and Experimental Details

Solely from a conventionally measured impedance spectrum it is neither possible to draw conclusions on the existence of short-circuiting paths, nor to determine grain 

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The first problem is concerned with the definitions of the quantities calculated and measured. It has only gradually emerged over the last decades that there was considerable ambiguity about the definition of some of the reported parameters. In 1992 Willetts et al (referred to as WRBS) gave a detailed account of the various conventions that have been applied by different authors in defining the molecular response functions, but more recently Reiss has suggested that there are still inconsistencies in the way that the experimentally measured, macroscopic, response functions are reported. 

The important feature of the QSAR methods is that the most difficult part of the problem —the mechanism of interaction of the molecule with the bioreceptor, because in many cases the latter is, so far, unknown —remains a black box beyond detailed consideration. Only input to and outcome from this black box can be analyzed and compared in detail. In this process, the outcome is the biological effect obtained experimentally. The rigorous definition and determination of biological activity are not straightforward, but there are some conventional experimental methods. Also it is implied that biological activities provided for QSAR are well defined and determined by the same method for all the compounds in the series under consideration. 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

The following sections discuss each of these steps in detail and present a process that can be used to help insure successful problem definition. The two steps in the Solving the Problem phase of the project are not discussed in this chapter. Experimentation is dependent on the method that is used to probe the system and relies on the expertise of the person(s) collecting the data. Analysis of Experimental Results is the application of the chemomctric tools and is the topic of Chapters 3-5. 

We also hope that the need for more experimental details (for better analysis of certain conformational problems which have escaped more precise definition with the experimental methods hitherto employed) and the need to clear up the doubts that continue to surround previous conclusions, may stimulate interest in testing new techniques not yet applied to these problems. 

For adiabatic type calorimeters, the initial and the final temperatures are by definition different. To which temperature will a derived A//-value then refer In connection with microcalorimetric measurements, the problem may merely be academic as the temperature change may be very small. However, it is at this point instructive to analyze in some detail what we do when we calibrate an adiabatic calorimeter. Let the initial and the final state of the experimental process be represented by A and B and the corresponding temperatures by TA and TB, respectively. The experimental process is thus ... 

The problem of attempting to apply semiempirical calculations to catalytic and surface phenomena should not be minimized. The calculation is performed for a well-defined model which is a representation of an ill-defined experimental situation. The experimental system in the case of catalysis is seldom specified in detail such as surface structure, surface composition, ate of reaction, ratedetermining step, or a multitude of other factors. This lack of definition is an experimental and theoretical limitation. 

One of the hurdles in this field is the plethora of definitions and abbreviations in the next section I will attempt to tackle this problem. There then follows a review of calculations of non-linear-optical properties on small systems (He, H2, D2), where quantum chemistry has had a considerable success and to the degree that the results can be used to calibrate experimental equipment. The next section deals with the increasing number of papers on ab initio calculations of frequency-dependent first and second hyperpolarizabilities. This is followed by a sketch of the effect that electric fields have on the nuclear, as opposed to the electronic, motions in a molecule and which leads, in turn, to the vibrational hyperpolarizabilities (a detailed review of this subject has already been published [2]). Section 3.3. is a brief look at the dispersion formulas which aid in the comparison of hyperpolarizabilities obtained from different processes. 

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