Quantum structure data

This by no means exhaustive discussion may serve to indicate the value of the information provided by magnetic data relative to the nature of the chemical bond. The quantum-mechanical rules for electron-pair bonds are essential to the treatment. Much further information is provided when these methods of attack are combined with crystal structure data, a topic which has been almost completely neglected in this paper. It has been found that the rules for electron-pair bonds permit the formulation of a set of structural principles for non-ionic inorganic crystals similar to that for complex ionic crystals the statement of these principles and applications illustrating their use will be the subject of an article to be published in the Zeitschrift fur Kristallographie. 

The only structural data available for S7+ comes from quantum chemical calculations43 and suggests that S7+ is a C2 symmetric S7 homocycle in the chair conformation and exhibits a much less pronounced bond lengths alternation... 

The structural data for Sg2+ rely on good quality quantum chemistry calculations. 

Region C 1380-1600 A. The absorption spectrum is highly structured in this region138, so that significant participation of electronically excited N20 is a distinct possibility. Quantum yield data (Table 10), however, suggest that the... 

Representative examples are provided in Table 5-19. Only a single (intermolecular) distance is examined for each system, underlying the fact that the experimental structure data are incomplete. The usual quantum chemical models have been surveyed. Comparisons with molecular mechanics models have not been included even though force fields such as MMFF have been explicitly parameterized to reproduce known hydrogen-bond distances. 

Quantum chemical calculations need not be limited to the description of the structures and properties of stable molecules, that is, molecules which can actually be observed and characterized experimentally. They may as easily be applied to molecules which are highly reactive ( reactive intermediates ) and, even more interesting, to molecules which are not minima on the overall potential energy surface, but rather correspond to species which connect energy minima ( transition states or transition structures ). In the latter case, there are (and there can be) no experimental structure data. Transition states do not exist in the sense that they can be observed let alone characterized. However, the energies of transition states, relative to energies of reactants, may be inferred from experimental reaction rates, and qualitative information about transition-state geometries may be inferred from such quantities as activation entropies and activation volumes as well as kinetic isotope effects. 

Characterization of transition-state geometries and energetics and ultimately reaction mechanisms remains a challenge for quantum-chemical models. The complete absence of experimental structural data and the need to interpret experimental reaction rates in terms of transition-state theory greatly complicates assessment of the theory, but it also increases its value as an exploratory tool. Nowhere is the problem more acute than in dealing with reactions in solution. 

After finishing his first paper on the nature of the chemical bond, in 1931, Linus Pauling stopped basing his ideas on mathematical proofs. Chemists, he understood, were not trained to appreciate the difficult mathematics of quantum physics. To communicate with them, he developed his own theoretical style, made up in equal parts of a broad application of Erwin Schrodinger s wave mechanics, structural data from X-ray crystallography, other laboratory results from across the field of chemistry, and Pauling s own insights. 

The host BaGdBgOie exhibits a quantum efficiency = 0.65 when doped with the ion Eu +. Eu + emission from the same host is characterized by a broad band peaking in the blue at 460 run. Structural data for these compounds have not been reported. 

---