Variable valency

What I have done is to take a dummy centre halfway along the C3-C4 bond defined the oxygen atom by its distance from the dummy atom and work fronj there. The lengths and angles used in a Z-matrix do not have to correspond to chemical bond distances or angles, although the normal valence variables are said to be the best choice for geometry optimization. 

VALENCY-VARIABLE CATION Co-DOPING AND Sn-Cl-DOPING IN Bi-SYSTEM 2223 PHASE SUPERCONDUCTOR... 

Doping of valence-variable cation Co speeds up the formation of the high T, 2223 phase in Bi-system. Doping with both cation Sn and anion Cf speeds up the formation of high Tc 2223 phase further. There are Cu, Cu, Bi and Bi cations in the high Tc 2223 phase, but Cu is not found. 

It is significant that the electronic state of Sc is variable, and sometimes controversial. Both experimental and theoretical results recommend the divalent state for Sc in mono-metallofullerenes and di-metallofullerenes [ 119], but it was determined that the three Sc atoms in Sc3N Cso take the - -3 valency. Variable electronic states of Tm in different EMFs were also found. For example, Tm was found to be -1-2 in Tm Cs2, but -1-3 in Tm3N Cso [120]. It is worth noting that a purely ionic picture is not valid to describe the electronic structure of the encaged metals because the charges are not observable quantities [2]. 

Common Designation UNS w/Constant Valence Variable Valence Equivalent Weight Variable Valence Equivalent Wei t Element/ Val ice Equivalen Weight... 

A double-zeta (DZ) basis in which twice as many STOs or CGTOs are used as there are core and valence AOs. The use of more basis functions is motivated by a desire to provide additional variational flexibility so the LCAO-MO process can generate MOs of variable difhiseness as the local electronegativity of the atom varies. 

In practice, each CSF is a Slater determinant of molecular orbitals, which are divided into three types inactive (doubly occupied), virtual (unoccupied), and active (variable occupancy). The active orbitals are used to build up the various CSFs, and so introduce flexibility into the wave function by including configurations that can describe different situations. Approximate electronic-state wave functions are then provided by the eigenfunctions of the electronic Flamiltonian in the CSF basis. This contrasts to standard FIF theory in which only a single determinant is used, without active orbitals. The use of CSFs, gives the MCSCF wave function a structure that can be interpreted using chemical pictures of electronic configurations [229]. An interpretation in terms of valence bond sti uctures has also been developed, which is very useful for description of a chemical process (see the appendix in [230] and references cited therein). 

The transition elements are often said to exhibit variable valency. Because they so readily form complex compounds, it is better to use the term variety of oxidation states . The states usually found for the elements Sc-Zn are ... 

Cerium is especially intereshng because of its variable electronic structure. The energy of the inner 4f level is nearly the same as that of the outer or valence electrons, and only small amounts of energy are required to change the relahve occupancy of these electronic levels. This gives rise to dual valency states. 

Processes in which solids play a rate-determining role have as their principal kinetic factors the existence of chemical potential gradients, and diffusive mass and heat transfer in materials with rigid structures. The atomic structures of the phases involved in any process and their thermodynamic stabilities have important effects on drese properties, since they result from tire distribution of electrons and ions during tire process. In metallic phases it is the diffusive and thermal capacities of the ion cores which are prevalent, the electrons determining the thermal conduction, whereas it is the ionic charge and the valencies of tire species involved in iron-metallic systems which are important in the diffusive and the electronic behaviour of these solids, especially in the case of variable valency ions, while the ions determine the rate of heat conduction. 

Here and below, T , 1, , and e, i, j = 1,. . . , 5, denote atomic position vectors, atom-atom distances, and the corresponding unit vectors, respectively. In order to construct a correctly closed conformation, variables qi,. . . , q4 are considered independent, and the last valence angle q is computed from Eq. (7) as follows. Variables qi,.. ., q4 determine the orientation of the plane of q specified by vector 634 and an in-plane unit vector 6345 orthogonal to it. In the basis of these two vectors, condition (7) results in... 

Transition elements, for which variable valency is energetically feasible, frequently show non-stoichiometric behaviour (variable composition) in their oxides, sulfides and related binary compounds. For small deviations from stoichiometry a thermodynamic approach is instructive, but for larger deviations structural considerations supervene, and the possibility of thermodynamically unstable but kinetically isolable phases must be considered. These ideas will be expanded in the following paragraphs but more detailed treatment must be sought elsewhere. " ... 

All three elements combine readily with most metals and many non-metals to form binary chalcogenides. Indeed, selenides and tellurides are the most common mineral forms of these elements (p. 748). Nonstoichiometry abounds, particularly for compounds with the transition elements (where electronegativity differences are minimal and variable valency is favoured), and many of the chalcogenides can be considered... 

The MNDO, AMI and PM3 methods are parameterizations of the NDDO model, where the parameterization is in terms of atomic variables, i.e. referring only to the nature of a single atom. MNDO, AMI and PM3 are derived from the same basic approximations (NDDO), and differ only in the way the core-core repulsion is treated, and how the parameters are assigned. Each method considers only the valence s- and p-functions, which are taken as Slater type orbitals with corresponding exponents, (s and... 

Transition-metal oxides are particularly effective decomposition and burning-rate catalysts. The metal elements can demonstrate variable valence or oxidation states. 

The biological chemistry of gold a metallo-drug and heavy atom label with variable valency. P. J. Sadler, Struct. Bonding (Berlin), 1976, 29,170-214 (142). 

When a d-metal atom loses electrons to form a cation, it first loses its outer s-electrons. However, most transition metals form ions with different oxidation Variable valence is discussed further states, because the ( -electrons have similar energies and a variable number can be... 

Many metallic elements in the p and d blocks, have atoms that can lose a variable number of electrons. As we saw in Section 1.19, the inert-pair effect implies that the elements listed in Fig. 1.57 can lose either their valence p-electrons alone or all their valence p- and s-electrons. These elements and the d-block metals can form different compounds, such as tin(II) oxide, SnO, and tin(IV) oxide, Sn02, for tin. The ability of an element to form ions with different charges is called variable valence. 

Elements that can expand their valence shells commonly show variable covalence, the ability to form different numbers of covalent bonds. Elements that have variable covalence can form one number of bonds in some compounds and a different number in others. Phosphorus is an example. It reacts directly with a limited supply of chlorine to form the toxic, colorless liquid phosphorus trichloride ... 

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