Valences unsatisfied

Separation of covalently bonded atoms into QM and MM regions introduces an unsatisfied valence in the QM region this can be satisfied by several different methods. In the frozen-orbital approach a strictly localized hybrid sp2 bond orbital containing a single electron is used at the QM/MM junction [29]. Fro-... 

In all cases a significant fraction of the plutonium remained immobilised on the leaves, both citrate and nitrate showed a 60—90% loss in solubility. It is possible that the reduced leachability of the plutonium may have been due to the entrapment of the compounds in crevasses on the leaf surface, engulfment by expanding epidermal plates, or by adsorption onto unsatisfied valences. 

Inifiation. The trick here is to get the reaction started. Usually a catalyst is used, typically an organic peroxide such as ditertiary butyl hydroperoxide. Peroxide molecules are somewhat unstable, and when they re heated, they decompose and turn into highly reactive free radicals. As you ll recall, a radical is an almost-complete molecule, but all the valence requirements are not satisfied. So it is very anxious to meet up with some other molecule to satisfy its valence needs. The free radical, in the presence of an abundance of monomers, say a million to one ratio, will react with a monomer molecule. It becomes part of the molecule. In doing so, the unsatisfied valence condition now transfers to the end of the monomer. A new radical is formed. That s the start of the initiation step. 

Atoms that are rearranged, forming new compounds at the demand of the unsatisfied valences of the surface atoms. 

The simultaneous formation of 8 and 9 results in a situation in which all copper(I) ions are tetracoordinate and all of the ligands nitrogen atoms are bound to copper centers. Any other structures formed from this mixture of subcomponents would either contain more than one metal center (entropically disfavored) or have unsatisfied valences at either metal or ligand (enthalpically disfavored). 

When one thinks in terms of the many fused-ring isomers with unsatisfied valences at the edges that would naturally arise from a graphite fragmentation, this result seems impossible there is not much to choose between such isomers in terms of stability. If one tries to shift to a tetrahedral diamond structure, the entire surface of the cluster will be covered with unsatisfied valences. Thus a search was made for some other plausible structure which would satisfy all sp valences. Only a spheroidal structure appears likely to satisfy this criterion, and thus Buckminster Fuller s studies were consulted (see, for example, ref. 7). An unusually beautiful (and probably unique) choice is the truncated icosahedron depicted in Fig. 1. As mentioned above, all valences are satisfied with this structure, and the molecule appears to be aromatic. The structure has the symmetry of the icosahedral group. The inner and outer surfaces are covered with a sea of v electrons. The diameter of this C o molecule is 7 A, providing an inner cavity which appears to be capable of holding a variety of atoms. ... 

Charcoal is essentially graphitic in character but with a low degree of order, and because of its extensive internal surfaces and unsatisfied valences possesses the property of adsorbing gases and vapours to a remarkable extent and of catalysing... 

Besides the compounds of C and H described on pp. 172 et in which all the valences of tlie C atoms are satisfied either by the attachment of H atoms, or by the interchange of a single valence between neighboring C atoms, there exist many others in which the proportion of H to 0 is lesa These compounds are non- uraied, in this, that they are capable of unit-ing directly with atoms of other elements, or with radicals, to foim products of addition, while the composition of the saturated hydrocarl ns can only be modihed by substUutioH they are not, however, to be considered as containing any unsatisfied valence. 

Substances containing double or triple bonds often react rapidly with reagents in such a way that two reacting molecules combine to form a single molecule. As this is what happens when two substances each containing an atom with incompletely saturated valences are brought together, compounds with multiple bonds have been classed as unsaturated compounds. So much has this been the case that unsaturation has come to connote a multiple bond rather than an unsatisfied valence. 

Like the elements, the radicals possess different valences, depending upon the number of unsatisfied valences which they contain. Thus the radical (CH,) is univalent, because three of the four valences of the carbon atom are satisfied by atoms of hydrogen, leaving one free valence. The radical (PO) of phosphoric acid is trivalent, because two of the five valences of the phosphorus atom are satisfied by the two valences of the bivalent oxygen atom, leaving three free valences. 

Quantum yield. Quantum yield (QY) is a measure of the ratio of the photons absorbed with respect to the photons emitted for any given sample and determines the eflBciency of these systems for the purpose of various emission related appUcations." Clearly, the surface of the nanocrystal plays a major role in determining the QY of a material and is affected by (a) surface defects, (b) surface ligands as well as (c) medium parameters such as refractive index. The surface defects in QDs occur in the form of unsatisfied valencies ( dangling bonds ) that result in non radiative recombination events. This opens up an alternate channel for energy dissipation that competes with radiative decay. 

The other special feature about a surface is that it is apt to contain unsatisfied valencies, which, elsewhere in the solid, are used to bind the constituent atoms to one another. This is evident from the representation of a piece of carbon in Fig. 8.1. It is obvious that the finer the carbon is ground, the more residual valencies there will be, and the more active an adsorbent it will become. 

The formula can be drawn, so that there are no unsatisfied valencies, in the following manner ... 

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