Computational-experimental chemistry

It should also be acknowledged that in recent years computational quantum chemistry has achieved a number of predictions that have since been experimentelly confirmed (45-47). On the other hand, since numerous anomalies remain even within attempts to explain the properties of atoms in terms of quantum mechanics, the field of molecular quantum mechanics can hardly be regarded as resting on a firm foundation (48). Also, as many authors have pointed out, the vast majority of ab initio research judges its methods merely by comparison with experimental date and does not seek to establish internal criteria to predict error bounds theoretically (49-51). The message to chemical education must, therefore, be not to emphasize the power of quantum mechanics in chemistry and not to imply that it necessarily holds the final answers to difficult chemical questions (52). 

Ab initio quantum chemistry has advanced so far in the last 40 years that it now allows the study of molecular systems containing any atom in the Periodic Table. Transition metal and actinide compounds can be treated routinely, provided that electron correlation1 and relativistic effects2 are properly taken into account. Computational quantum chemical methods can be employed in combination with experiment, to predict a priori, to confirm, or eventually, to refine experimental results. These methods can also predict the existence of new species, which may eventually be made by experimentalists. This latter use of computational quantum chemistry is especially important when one considers experiments that are not easy to handle in a laboratory, as, for example, explosive or radioactive species. It is clear that a good understanding of the chemistry of such species can be useful in several areas of scientific and technological exploration. Quantum chemistry can model molecular properties and transformations, and in... 

When the uncertainty associated with AHf is 5 kcal/mol, rate and equilibrium constants can be estimated within a factor of 10 at process temperatures, i.e., 500-1,500 K. This level of accuracy may be acceptable for preliminary mechanism development work and for the identification of important reactions in a DCKM. However, it would clearly be desirable to know AHf within 1 kcal/mol, which would lead to the determination of rate and equilibrium constants that are accurate within a factor of two. Since this level of accuracy is very close to the limits of accuracy of most experimental measurements, improvements in AHf are often difficult. Consequently, computational quantum chemistry holds a great promise for the accurate determination of AHf. 

Computational quantum chemistry has emerged in recent years as a viable tool for the elucidation of molecular structure and molecular properties, especially for the prediction of geometrical parameters, kinetics and thermodynamics of highly labile compounds such as nitrosomethanides. However, they are difficult objects for both experimental (high toxicity, redox lability, high reactivity, explosive character etc.) and computational studies, even with today s sophisticated techniques (e.g. NO compounds are often species with open-shell biradical character which requires the application of multi-configuration methods). 

The aforementioned progress in NMR spectroscopy (and other experimental methods as well) in combination with computational chemistry has reached a stage in which an understanding of the most general features of organic reactions on solid acids may reasonably be expected in several years. This does not yet quite exist this report is written in a time at which the sophisticated application of NMR and computational quantum chemistry to solid acids is becoming widespread, and specialists in various areas are suddenly having to evaluate evidence from other specialties. 

Hopkinson was hired by York to teach theoretical organic chemistry (the Woodward-Hoffmann rules were then a hot topic) and to carry out experimental chemistry. Despite the limited computing capacity at York at the time, he managed to complete some work on the electrophilic addition to alkenes. He is probably best known, however, for his work on proton affinities, destabilized carbocations,234 organosilicon compounds, silyl anions and cations, and more recently, on the calculation of potential energy surfaces and thermodynamic properties. He has had a particularly fruitful collaboration with Diethard Bohme.235... 

The best-known uses of computers in chemistry rely on floating point computation. Numerical quantum chemistry, chemometrics and the collection and evaluation of experimental data, e.g. in X-ray crystallography, modern spectroscopy (advanced NMR, MS, IR) and chemical dynamics are major areas where floating point computation is indispensable. 

Until recently the application of computers in chemistry was restricted to two fields. First, there were numerical calculations as for the analysis of experimental data and in quantum mechanical computations, and second, computers were used for documentation and information retrieval. 

This was all to change in 1970 when the first chink in the armor of the inherent superiority of experiment over computation appeared. Within a few years, computational results pointed out serious experimental errors and thereby secured a place for computational chemistry as an equal partner with experiment in exploring the chemical sciences. All of these centered on the simple molecule methylene, CHj, establishing what Schaefer has called the paradigm of computational quantum chemistry . 

In my junior year, I got a summer job with a company later to be called Cleveland Crystal Corp. I published my first paper in Nature in 1959 on the absolute configuration of the cadmium selenide crystal, based on X-ray intensities. I was also interested in computers, but I understood that with the computer I would be working on problems for others whereas I wanted to work on my own problems. Being able to carry out a successful research project with my own hands gave me the courage and the conviction to pursue a career in experimental chemistry. 

Probably even more important to computational quantum chemistry is the development in the analytical evaluation of first, second and higher derivatives of the potential energy with respect to nuclear coordinates . These analytical derivative methods are indispensable to the location and characterization of the stationary points (minima or transition states) on the potential energy surface and have greatly advanced the scope of applicability of ab initio calculations. Ab initio calculations are in a position to predict many new types of the heavier group 15 compounds and provide valuable information for the interpretation of complex experimental data. 

Thanks in part to the computational advances described in the preceding section, coupled cluster theory has developed into arguably the most accurate and computationally affordable method of modern computational quantum chemistry. The results of coupled cluster calculations are commonly found in the chemical physics literature, and, when the accuracy of experimental results is questioned, the CCSD(T) method is often used to settle the debate. In spite of this success, coupled cluster theory is far from applicable to all problems of chemical interest. The majority of the current research efforts may be divided into four overlapping categories ... 

Although there has been an exponential growth of computational organotin studies in the last 30 years, a substantial portion of the current publications combine both computational and experimental techniques. This chapter includes examples of this type along with those based on only computational methods. While the majority of the current literature is included, in depth discussion is reserved for the most intriguing studies to give an overview of the current state of computational organotin chemistry. 

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