Theory guiding experimental work

The interaction between experiment and theory is very important in the field of chemical transformations. In 1981 Kenichi Fukui and Roald Hoffmann received a Nobel Prize for their theoretical work on the electronic basis of reaction mechanisms for a number of important reaction types. Theory has also been influential in guiding experimental work toward demonstrating the mechanisms of one of the simplest classes of reactions, electron transfer (movement of an electron from one place to another). Henry Taube received a Nobel prize in 1983 for his studies of electron transfer in inorganic chemistry, and Rudolf Marcus received a Nobel Prize in 1992 for his theoretical work in this area. The state of development of chemical reaction theory is now sufficiently advanced that it can begin to guide the invention of new transformations by synthetic chemists. 

A few days later, Wurtz presented Kekule s benzene theory to the Societe Chimique in Paris.Kekule began by pointing out that no one, "as far as I am aware," had attempted to apply the theory of atomicity of the elements to aromatic compounds. He stated that he had had a "fully formed idea" on this question since 1858, having published hints in that direction in his major paper of that year, but he had not regarded it as appropriate to unveil it publicly and in detail until now. The theory that follows is very incomplete, he warned, and may not ultimately be verified, but it may also be very useful in stimulating and guiding experimental work, to confirm or refute it, hence his desire to put it forward now. 

From the above discussions, it is clear that the major contribution of theory in guiding future experimental work is the demand it places for high quality empirically determined data. Theory has rarely, if ever, contributed to a rise in Tc, and, without a clearly defined mechanism, the contributions of theory to the pursuit of high Tc remains low. 

Many branches of the kinetic theory of combustion processes have reached a stage where the theoretical concepts are fairly clear but where new experimental facts are essential before real progress can be made. Though valuable experimental work is proceeding, in many cases new experimental techniques seem to be required to provide novel information, without which theories cannot decide between conflicting views. This paper gives an analysis and interpretation of the literature, which may serve as a guide to workers in the combustion and allied fields. 

We wish to emphasize at the outset that we have not attempted to prove the correctness of the El mechanism for PVC dehydrochlorination. What we have tried to do, instead, is to use molecular orbital theory in order to (a) determine whether or not the El mechanism is a reasonable possibility, and to (b) elucidate theoretical principles which can serve as a guide for future experimental work in the PVC degrada-tion/stabilization field. Some preliminary results of this study were published in Part 1 of this series (5). 

This stability theory, which was also extended to amphoteric colloids — at first proved a useful guide in experimental work (e.g., also in the first stages of the investigations on coacervation, see p. 243, Chapter VIII, 3). But gradually facts were met with which are not compatible with it. Its usefulness became less and less and nowadays we may say it has only historical value. 

The above sketched theory of coacervation served as a useful guide in the further experimental work on the partial miscibility phenomena in hydrophilic sols and after its fundamental assumptions had later become more and more doubtful and another interpretation had already been put forward it still served at least as a serviceable scheme for the preliminary recording of the experimental facts... 

The other conclusion that comes from the above review is that there is still much work on these systems to be done. There is still only a rudimentary understanding of the size-dependent electronic structure and dynamics. There is a need for further synthetic, spectroscopic, dynamical, and theoretical work. Perhaps the area in which progress most needs to be made is theory. Further electronic structure calculations would be a tremendously valuable guide for further experimental studies. It is hoped that the work done to this point will stimulate further studies in all areas. 

Theory helps the experimentalists in many ways this volume is on chemical shift calculations, but the other ways in which theoretical chemistry guides NMR studies of catalysis should not be overlooked. Indeed, further theoretical work on two of the cations discussed above has helped us understand why some carbenium ions persist indefinitely in zeolite solid acids as stable species at 298 K, and others do not (25). The three classes of carbenium ions we were most concerned with, the indanyl cation, the dimethylcyclopentenyl cation, and the pentamethylbenzenium cation (Scheme 1), could all be formally generated by protonation of an olefin. We actually synthesized them in the zeolites by other routes, but we suspected that the simplest parent olefins" of these cations must be very basic hydrocarbons, otherwise the carbenium ions might just transfer protons back to the conjugate base site on the zeolite. Experimental values were not available for any of the parent olefins shown below, so we calculated the proton affinities (enthalpies) by first determining the... 

Using the van t Hoff-Le Bel theory as a guide, Fischer extended his cyanohydrin syntheses from the work with xylose and arabinose to researches on all the other aldoses that were available in quantity at that time the list consisted of glucose, mannose, galactose, rhamnose, lactose and maltose. The experimental results with the disaccharides... 

The Brunauer-Emmett-Teller (45) theory of physical adsorption was the first and is still probably the best and most useful theory of multilayer adsorption covering the complete range in p. This is not to say, however, that it is a really satisfactory theory. As will be made clear below, the assumptions of the theory are extremely crude, but they are still sufficiently good to contain a number of the important qualitative features actually observed experimentally. The role of the BET theory in physical adsorption is in fact rather analogous to that of the van der Waals equation in the theory of liquids it was the first important theory in the field it has stimulated virtually all the work that followed it it is still extremely useful as a qualitative guide but it is not quantitatively correct. 

In the field of nanoscale materials, SIESTA has probably made its largest impact in the study of carbon nanotubes. This is a field which has captivated the attention of researchers for their unusual electronic and mechanical properties. Simulation and theory have played a major role, often providing predictions that have guided the way for experimental studies. Work done with SIESTA has spanned many aspects of nanotube science vibrational properties [239-241], electronic states [242-246] (including the effect of lattice distortions on the electronic states [247-250]), elastic and plastic properties [251-254], and interaction with other atomic and molecular species [255-259]. Boron nitride nanotubes have also received some attention [260, 261]. 

A chemist s work, like that of any scientist, involves observation and experimentation. The view of matter and its changes that can be seen in the laboratory is considered the macroscopic component of chemistry (/). Chemists use symbols and mathematics to describe their observations and findings (the symbolic component). Finally, chemists relate what they have observed to die action of particles (atoms and molecules) in the submicroscopic or particulate component. Johnstone (7) explained that chemists consider these multiple components or representations simultaneously and can easily move between the representations as needed. The Particulate Nature of Matter (PNM) is the underlying theory for the particulate component of chemistry that Johnstone described, the idea diat matter is composed of particles. For more information concerning the PNM, see an earlier work by Johnstone (2) or Gabel s chapter in the Chemist s Guide to Effective Teaching (5). 

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