Bond physical significance

From the results described above it is clear that a different QSPR model can be obtained depending on what data is used to train the model and on the method used to derive the model. This state of affairs is not so much a problem if, when using the model to predict the solubility of a compound, it is clear which model is appropriate to use. The large disparity between models also highlights the difficulty in extrapolating any physical significance from the models. Common to all models described above is the influence of H-bonding, a feature that does at least have a physical interpretation in the process of aqueous solvation. 

Kaznessis et al. [24] used Monte Carlo simulations on a data set of 85 molecules collected from various sources, to calculate physically significant descriptors such as solvent accessible surface area (SASA), solute dipole, number of hydrogen-bond acceptors (HBAC) and donors (HBDN), molecular volume (MVOL), and the hydrophilic, hydrophobic, and amphiphilic components of SASA and related them with BBB permeability using the MLR method. After removing nine strong outliers, the following relationship was developed (Eq. 37) ... 

The erbium ion is coordinated to eight carboxyhc oxygens [2.362—2.415 A)] from four oxalate (acid oxalate) moieties which forms a square antiprism around Er(III). A water molecule forms the cap (Er—OH2 =2.441 A) above the large square face of the antiprism. The acid oxalates (HOOCCOO) and oxalate ions occupy the crystallographic sites at random. The statistically averaged oxalate groups are centrosymmetric and planar. A very short H-bond (2.43 A) has been observed between two water molecules in two equivalent molecules, but the physical significance is difficult to assess because these waters are disordered in the molecule. 

Because of the disorder, the bond parameters have no physical significance as they represent only the average values of all the possibilities in the mixed crystal. Therefore it is impossible to deduce the nature of the bonds in the crystal from X-ray diffraction data. 

One problem with the network equations is that they can, on occasion, give rise to negative bond valences which have no physical significance (expect to indicate that, from a chemical point of view, the bond should not exist). Rutherford (1998) has explored the resonance bond model as an alternative to the use of the loop equation (Section 14.4) while Rao and Brown (1998) have suggested using the method of maximum entropy (Section 11.2.2.1). 

The physical significance of 2 in Equation (73) is somewhat harder to define. At first glance it appears to be the length of the repeating unit, about 0.25 nm for a vinyl polymer. We must remember, however, that the derivation of Equation (73) assumed that the coil was connected by completely flexible joints. Molecular segments are attached at definite bond angles, however, so an actual molecule has less flexibility than the model assumes. Any restriction on the flexibility of a joint will lead to an increase in the dimensions of the coil. The effect of fixed bond angles on the dimensions of the chain may be incorporated into the model as follows. 

Curved-arrow notation is also a very useful device with which to generate resonance structures. In this application it is truly a bookkeeping system. Since individual canonical forms do not exist but are only thought of as resonance contributors to the description of a real molecule, the use of curved-arrow notation to convert one canonical form to another is without physical significance. Nevertheless it provides a useful tool to keep track of electrons and bonds in canonical structures. For example, the structures of carboxylate resonance contributors can be interconverted as follows ... 

Fig. 4.10 The model of a C/C double bond as a a In bond is at bottom really equivalent to the sps/sps + sps/sps model both result in the same electron distribution, which is the physically significant thing. There are no gaps in electron density between the carbons as the contribution to density from the n bond (or one of the sp5/sp5 bonds) falls off, the contribution from the n bond (or the other sps/sps bond) increases. The electron density falls off smoothly with distance from the C/C axis. For some purposes one of the models,

The Fajans-Sidgwick distortion picture of bond-type transitions is not easily translated into quantitative language for one does not generally speak of an anion as being 35-percent distorted. The resonance concept, however, can be used to describe the character of a bond between two different atoms in language that is probably no more physically significant but that can be put into equation form. 

This generalization may be useful in a number of cases. However, the vocabulary taken from the physics of semiconductors is highly unsuited to oxides, because, in most cases, dangling bonds have no physical significance. The conservation of the number of electrons per bond between bulk and surface is also not a well founded assumption. 

The physical significance of the above evaluation of as a basis for estimation of ffads.H s the assumption that chemisorption of H involves formation of a quasi-diatomic M—H bond and that the polarity of this bond is characterized by the electronegativity difference, Xm Xh- Directly determined initial heats of adsorption of H, that is, for -> 0, are found (/4) to be related to d> for transition metals, and this effect originates on account of partial charge transfer in H chemisorption determined by the electron affinity of the metal, -O, and characterized inter alia by Xm - Xh-... 

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