Based on electronic structure

Recently, Moskaleva et al. have proposed a new mechanism based on electronic structure calculations." Earlier experimental studies by Kasdan et al. determined that methyne (HC) has a doublet ground state and with a doublet-quartet energy splitting (AEdq) of 71.5 + O.SkJ/mol." Moskaleva et al. noted that the initially proposed mechanism (for HCN and N(" S) atom formation) is therefore spin-forbidden, and they also proposed a more favorable and spin-allowed reaction on the doublet surface. This new route on the doublet energy surface proceeds through the formation of an NCN intermediate, with concomitant formation of (doublet) hydrogen atom. 

Since the periodic table is based on electron structure, two elements in the same group (Pb and element 114) should have similar electron configurations. 

Properties Based on Electronic Structure and Molecular Geometry... 

During the past few decades, a wide variety of molecules with transition metal-carhon mulhple bonds have been studied. The chemistry of doubly bonded species - carbenes - is particularly interesting because it leads to several synthetically important transformations, and for this reason, metal carbenes are the main subject of this chapter. Our discussion begins with a classification of metal-carbene complexes based on electronic structure, which provides a way to understand their reactivity patterns. Next, we summarize the mechanistic highlights of three metal-carbene-mediated reactions carbonyl olefinafion, olefin cyclopropanafion, and olefin metathesis. Throughout the second half of the chapter, we focus mainly on ruthenium-carbene olefin metathesis catalysts, in part because of widespread interest in the applications of these catalysts, and in part because of our expertise in this area. We conclude with some perspectives on the chemistry of metal carbenes and on future developments in catalysis. 

The Styx formalism and the more successful Wade s rules for the electronic requirement helped provide a yes/no answer for the existence of polyhedral borane clusters. But one should not overemphasize the importance of electron-counting rales. Molecules satisfying the same rale need not have the same thermodynamic stability. Electron-counting rules can be used as a tool for predicting the existence of molecules, but not for assessing their relative stability. Criteria based on electronic structure theory are required for evaluating the stability of molecules. Qualitative ideas can help in this process as well. The ring-cap orbital overlap compatibility is one such idea that connects the deltahedral boranes to aromatic hydrocarbons. 

To add predictive power to the descriptor approach, researchers have attempted to develop activity descriptors based on material properties that can be intuitively determined prior to material synthesis. One of the popular approaches uses parameters based on electronic structure. For example, Bockiis and Otagawa studied the correlation between catal3riic activity and the electronic structure of perovskites [9]. They concluded that... 

A number of approximate VCD models are based on electronic structure calculations. The LMO modeH is a more accurate molecular orbital (MO) approach in evaluation of VCD intensities and has been implemented at the ab initio level. The nuclear and the electronic contributions to the dipole transition moments are treated completely separately in this model. The expressions of nuclear contributions are derived with full nuclear charges. For the expression of the electronic contributions, the BO approximation is invoked after the electronic part of the magnetic dipole operator is modified in such a way that the BO difficulty is avoided. Nonvanishing electronic contributions to the magnetic transition moments are thus obtained, which come from the displacements of the centroids of the localized MOs during vibrations. 

Accordingly, nanomaterials cannot be treated as small copies of macroscopic materials, but, instead, their properties depend critically on the arrangement of the atoms. Thus, for nanomaterials one can no longer apply the above-mentioned scaling laws, i.e., the systems of interest have sizes far from the thermodynamic limit and one has left the scaling regime so that every single atom counts. Therefore, theoretical studies of the properties of nanostructures have to be based on electronic-structure methods, like those described in, e.g., ref. 1. In most cases one imposes the Born-Oppenheimer approximation, i.e., for a system with M nuclei and N electrons one fixes the structure, R = assumes that... 

Approximate Models of VCD Based on Electronic Structure Methods... 

Paths 1 and 2 of Fig. 2 represent simplistic but descriptive manifestations of the SAR logic. However, the pyridine for phenyl substitution example (path 3) can optimize properties such as aqueous solubility due to the incorporation of a polar atom, in this case nitrogen. And finally, path 4 represents the important SAR concept of bioisosterism in which substitutions can be rationally made based on electronic structure and atomic size of functional groups and have literature precedent for activity enhancement for example, the thiophene for phenyl substitution in path 4 [35]. Another valuable example of bioisosterism is the fluorine for hydrogen substitution which has successfully aided numerous probe and drug development programs [36]. 

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