Valence bonds partial

The resonating-valence-bond theory of the electronic structure of metals is based upon the idea that pairs of electrons, occupying bond positions between adjacent pairs of atoms, are able to carry out unsynchronized or partially unsynchronized resonance through the crystal.4 In the course of the development of the theory a wave function was formulated describing the crystal in terms of two-electron functions in the various bond positions, with use of Bloch factors corresponding to different values of the electron-pair momentum.5 The part of the wave function corresponding to the electron pair was given as... 

Cooper et al. [30] were successful in rationalizing the striking variation in the S-S equihbrium bond lengths of FSSF (189.0 pm), ClSSCl (195.0 pm) and HSSH (205.5 pm) using the spin coupled (modern valence bond) theory. In the disulfur dihalides, but not for HSSH, incipient hypercoordinate character is observed at sulfur, with two partial t-like interactions in approximately perpendicular planes, and some antibonding character in the S-X (X=F or Cl) bonds. In other words, it is the form of t-like orbitals that is most rele-... 

It is readily apparent that the three a bonds are capable of holding the six bonding electrons in the a t and e molecular orbitals. The possibility of some 7r-bonding is seen in the molecular orbital diagram as a result of the availability of the a2" orbital, and in fact there is some experimental evidence for this type of interaction. The sum of the covalent radii of boron and fluorine atoms is about 152 pm (1.52 A), but the experimental B-F bond distance in BF3 is about 129.5 pm (1.295 A). Part of this "bond shortening" may be due to partial double bonds resulting from the 7r-bonding. A way to show this is by means of the three resonance structures of the valence bond type that can be shown as... 

Sato et al. carried out detailed studies on the possibilities of transformation of tetrazolo[l,5-tf]pyrazines 54 to 2-aminopyrazines 56 < 1994S931 >. These authors found that the generally used methods for this conversion fail because the starting compound exists in the stable bicyclic form 54, whereas partial formation of the azide valence bond isomer 55 would be necessary for the success of the transformation. Application of special reaction conditions succeeded, however hydrogenation over palladium catalyst in the presence of ammonium hydroxide or treatment with stannous chloride in a mixture of methanol and hydrochloric acid solved this problem. Thus, a great number of derivatives of 54 was reduced to the corresponding 2-aminopyrazine 56 in medium to high yields (45-100%). 

It has become increasingly popular to represent [C5H5]- by the structure shown in Fig. 6d. This representation is a valence bond structure notation it is intended to represent the five equivalent resonance structures (three of these are shown as Figs. 6c, /, and g the other two are similar), in which the negative charge is located at each carbon in turn. Each double bond is thus only a partial double bond and, if the usual notation of writing a partial double bond by a dashed line were applied, the structure would be written as 6h. The solid circle (6d) is a rapid way to write the 66 structure. In a completely analogous... 

The r-delocalisation in the parent phospholide anion I (Fig. 3, R =R =H) can be expressed in the valence bond picture by resonance between the canonical structures lA-IC (and their mirror images). Phosphonio-sub-stituents (R =R =PH3 ) increase the weight of the 1,2-dipolaric canonical structure IB and induce thus, in essence, a partial r-bond localisation and a shift of r-electron density from the phosphorus to the adjacent carbon atoms [16]. Consequences of this effect are the decrease in delocalisation energy for reaction (1) depicted in Fig. 4, and lower C2-C3/C4-C5 and higher C3-C4 bond orders which are reproduced in concomitant variations of computed bond distances [16]. 

Asg ° by applying the valence bond concept (Fig. 9a). The bonding situation of the electron-deficient chains shown in Fig. 9b is not as trivial. In these cases, on one hand, multiple element-element bonding is discussed, and on the other hand, the possibility of partial protonation cannot be excluded completely, especially for compounds of the sometimes hydrogen-contaminated metal barium. 

Figure 2.21. The composition of the adsorbate orbitals of symmetry in a perturbational treatment for N2 adsorbed on Ni. Polarization is achieved through intergroup and intragroup mixing. In terms of a valence bonding model, this can be seen as the partial breaking of the N2tt bond, forming nitrogen radicals. From Ref. [3].

The valence-bond treatment described above involves neglect of the partial ionic character of the bonds in the benzene molecule, and the molecular-orbital treatment overemphasizes it.80... 

The distances found between platinum centers in these molecules have been correlated with the resonating valence bond theory of metals introduced by Pauling. The experimentally characterized partially oxidized one-dimensional platinum complexes fit a correlation of bond number vs. metal-metal distances, and evidence is presented that Pt—Pt bond formation in the one-dimensional chains is resonance stabilized to produce equivalent Pt—Pt distances.297 The band structure of the Pt(CN)2- chain has also been studied by the extended Huckel method. From the band structure and the density of states it is possible to derive an expression for the total energy per unit cell as a function of partial oxidation of the polymer. The equilibrium Pt-Pt separation estimated from this calculation decreases to less than 3 A for a loss of 0.3 electrons per platinum.298... 

I think that the theory of resonance is independent of the valence-bond method of approximate solution of the Schrodinger wave equation for molecules. I think that it was an accident in the development of the sciences of physics and chemistry that resonance theory was not completely formulated before quantum mechanics. It was, of course, partially formulated before quantum mechanics was discovered and the aspects of resonance theory that were introduced after quantum mechanics, and as a result of quantum mechanical argument, might well have been induced from chemical facts a number of years earlier. [25]... 

PDDO PRDDO RHF SAMO SCF SOGI STO STO-nG UA UHF VB VIP Projectors of Diatomic Differential Overlap Partial Retention of Diatomic Differential Overlap Restricted Hartree-Fock Simulated ab initio Method Self Consistent Field Spin Optimized GVB method Slater Type Orbital Slater Type Orbital expanded in terms of nGTO United Atom Unrestricted Hartree-Fock Valence Bond Vertical Ionization Potential... 

It is noteworthy that among the formulae proposed for benzene in the 19 century, only the first one, that of Loschmidt, is not far from being correct. The next acceptable formula only appeared with Thiele s suggestion of fractional carbon-carbon bonds (partial valences) in 1899-1900. This formalism did not explain why cyclooctatetraene is not aromatic, however, as shown experimentally (for historical accounts, see refs. [15, 16]). 

Finally we discuss the Generalized Valence Bond (GVB) method as proposed by Hunt, Hay and Goddard./180,181,182/ This is a method aimed at improving some of the HF procedures, for instance to obtain the correct dissociation limit, and to partially include correlation effects. The basic idea is that the type... 

The complete theory takes into account valence bond angles and other considerations, and invokes partial 3-center bonding and resonance forms to provide detailed... 

---