Bond valence scheme

Within the computational scheme described in the course of this work, the available information about the atomic substructure (core+valence) can be taken into account explicitly. In the simplest possible calculation, a fragment of atomic cores is used, and a MaxEnt distribution for valence electrons is computed by modulation of a uniform prior prejudice. As we have shown in the noise-free calculations on l-alanine described in Section 3.1.1, the method will yield a better representation of bonding and non-bonding valence charge concentration regions, but bias will still be present because of Fourier truncation ripples and aliasing errors ... 

Several quantum-chemical studies have been performed on Hg(CN)2 and related species, applying different approaches with consideration of relativistic effects in order to get MO schemes and energies as a basis for discussion of bonding, valence XPS,105 UPS,106 XANES and EXAFS spectra.41 The latter study also showed Hg(CN)2 to be dissolved in H20 in molecular form (/-(Hg—C) 202, r(C—N) 114 pm), and obviously not to be hydrated, a remarkable finding insofar as solvates of Hg(CN)2 with various donor molecules are well known.2 However, in contrast to Cd(CN)2 (see above), Hg(CN)2 as such does not form clathrates. 

In the original (22) derivation of Eqn. II.2, Eqn. II.4 was, in effect, used as the starting point for the discussion the logic was accordingly obscured by the fact that Eqn. II.4 already contained all the essentials of the object of derivation. Although these deductions are improper in the classic sense, it is still possible that Eqn. II.4 may be admissible in an extended valence scheme. However, in this situation fundamental difficulties arise in correlating the number of bonds pi and q] (Def. II.9) with the total number of electrons (Defs. II.3 and 11.4). Hence the relations... 

A similar situation is observed in the case of the cation-radical derived from dimethylhexa-phenyl diphosphafnlveninm dication as a resnlt of redaction of the latter by the sodinm salt of the naphthalene anion-radical (Biaso et al. 2006). According to ESR, x-ray experimental data, and results of DFT calculations, the diphosphafnlveninm cation-radical detains the excess electron density within the exocyclic double bond (see Scheme 3.41). Stabilization is gained by conjngation with the two pentavalent phosphorus atoms. Biaso et al. (2006) rationalize the electronic strnctnre throngh two valence-isomeric fluidic forms of the distonic type. 

Oxazinyl anions obtained from 4//-l,3-oxazines are considered to exist in equilibrium with valence bond tautomers (Scheme 57). 

Although the discussions of the preceding molecules have been couched in valence bond terms (Lewis structures, hybridization, etc.), recall that the criterion for molecular shape (rule 2 above) was that the cr bonds of the central atom should be allowed to gel as far from each other as possible 2 at 180°. 3 at 120°, 4 at 109.5°, etc. This is (he heart of the VSEPR method of predicting molecular structures, and is, indeed, independent of valence bond hybridization schemes, although it is most readily applied in a VB context. 

However, there exists a vast literature of ab initio quantum chemical methods which are described in terms of either the molecular orbital (MO) or valence bond (VB) schemes for determining the electronic and geometric structure of molecules. The application of these methods to surface problems has advanced rapidly in recent years, as we shall discuss in this section. 

The aziridine derivative 117, a photochemical valence tautomerization product of berberinephenolbetaine, was also converted into the benzindan-oazepines by regioselective C-14—N bond cleavage (Scheme 24)." The treatment of 117 with p-toluenesulfonic acid in benzene under reflux gave rise to the benzindenoazepine 118 whose N-methylation with dimethyl sulfate in HMPA in the presence of sodium hydride afforded the n-methyl derivative 119." Since 119 has already been converted" to ( )-c -alpini-genine (120) and ( )-ds-alpinine (121), this synthesis constitutes a formal synthesis of these alkaloids. 

Interestingly, Michalak and coworkers [47] have compared the results of using point-charge polarized monomer electron densities with those obtained using unperturbed densities for halogen bonds. They find that polarizing the molecular components of the complex reduces the contribution assigned to donor-acceptor interactions in their ETS-NOCV (extended transition state - natural orbitals for chemical valence) scheme, but does not eliminate it entirely. This is consistent... 

A general electron-valence scheme of chemical bonds is proposed, based on an analysis of the crystal structure of monosilicides with an FeSi-type lattice. 

On the basis of the actual coordination of metal and silicon atoms in CrSi2, corresponding to a distorted tetrahedron, and taking into account the fact [17] that stable tetrahedral d s-hybrid bonds are theoretically more than twice as stable as sp -hybrid bonds, the electron-valence scheme of bonds in CrSi2 can be represented as depicted in Fig. 3. 

An electron-valence scheme of bonds in CrSi2 is proposed, based on the assumption that the tetravalent chromium atoms are bonded to the four nearest silicon atoms, lying in adjacent hexagonal layers, by somewhat distorted d s tetrahedral covalent bonds, and the silicon atoms are bonded by sp hybrid bonds to two chromium atoms and two silicon atoms. 

The bonding in the Fe2C20 core may be described by three valence bond contributors (Scheme 10), two of which require an Fe->Fe dative bond to satisfy the EAN requirements of both metal... 

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