Hybridisation model

For other purposes, however, it is undoubtedly helpful to take advantage of the simple picture provided by the hybridisation model, even though hybridisation is an extra concept to leam. It immediately reveals, for example, that all four bonds are equal. It can be used whenever it offers a simplification to an argument as we shall find later in this book, but it is good practice to avoid it wherever possible. In particular, the common... 

M.o. theory and the transition state treatment In 1942 Wheland proposed a simple model for the transition state of electrophilic substitution in which a pair of electrons is localised at the site of substitution, and the carbon atom at that site has changed from the sp to the sp state of hybridisation. Such a structure, originally proposed as a model for the transition state is now known to describe the (T-complexes which are intermediates in electrophilic substitutions... 

Carbon has six electrons around the atomic core as shown in Fig. 2. Among them two electrons are in the K-shell being the closest position from the centre of atom, and the residual four electrons in the L-shell. TTie former is the Is state and the latter are divided into two states, 2s and 2p. The chemical bonding between neighbouring carbon atoms is undertaken by the L-shell electrons. Three types of chemical bonds in carbon are single bond contributed from one 2s electron and three 2p electrons to be cited as sp bonding, double bond as sp and triple bond as sp from the hybridised atomic-orbital model. 

Indeed, it may be possible to develop and hybridise this model with the successful features of other models such as that due to Alexandrov and Mott [8,32] or Jansen and Block [33]. The complexity [34] of cuprate superconductors makes it extremely difficult to describe all features of the cuprates in a single model. 

Possible explanations for this unusual behaviour have been offered by first-principles band structure calculations and synchrotron X-ray structural studies. The former revealed that, because of hybridisation between Ba and C orbitals, the rigid-band model is not appropriate for the description of the electronic properties and the calculated N(ev) for K3Ba3C60 and Rb3Ba3C60 are almost identical [69]. The structural analysis revealed positional disorder of the Ba2+ and K+ ions in the distorted tetrahedral sites of the bcc structure and the existence of short Ba-C and K-C contacts, consistent with strong hybridisation between the K, Ba and the C60 states [70]. It is important to notice that in K3Ba3C60 there is a perfect matching between the size of K+ and Ba2+ ions, while in both the Rb+ and Cs+ analogues, there is a considerable mismatch, which leads to fundamental structural... 

In conventional Rietveld refinements, a structural model based on the arrangement of atoms in the unit cell is generally used. However, the imaging ability of MEM in the MEM/Rietveld method enables one to visualise more detailed features like the bonding electron distribution associated with hybridised orbitals [86]. was found to adopt a pseudo-monoclinic cell with... 

This led to the concept of fragmentation of the total molecular surface area in combination with multivariate analysis (Stenberg et al. 2001) towards predictive models of drug permeability for more complex datasets. Permeability models were established based on so-called partitioned total surface area (PTSA) descriptors. Each of the PTSA descriptors corresponds to the surface of a certain atom type, differentiated by hybridisation, which results in individual descriptors for e.g. sp3, sp2, and sp carbon atoms. The resulting permeability model based on 19 descriptors finally consisted of oxygen, nitrogen and polar hydrogen surfaces, while the main contribution for prediction of Caco-2 permeability was attributed to PSA. In addition some more lipophilic contributions... 

Robinson and Whalley [195] interpreted the mechanism of conformational transmission in two ways. Inspection of a Dreiding model reveals that a change from to hybridisation at C(2) is accompanied by a slight flattening of the... 

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