Valency saturation

Our studies [46] of interaction of hydroxyl radicals with the surface of oxide semiconductors show that our reasoning on other radicals is also applicable to these particles as their chemical activity is sufficiently high. With radicals possessing low chemical activity the situation changes drastically becoming close to the adsorption of valence-saturated molecules. 

It is also of importance that among the various forms of chemisorption there are, besides valency-saturated, radical forms in which the chemisorbed... 

Figure la shows the weak (electrically neutral) form of chemisorption of a H atom the chemisorption bond, as can be illustrated, is provided in this case by an electron of the H atom which is drawn, to a greater or lesser extent, from the atom into the lattice this is the radical form of chemisorption. The strong acceptor and donor forms are presented in Fig. 1 (b and c, respectively) these are electrically charged and valency-saturated forms. 

The weak and strong acceptor forms of chemisorption of an 0 atom are shown in Fig. 2 (a and b, respectively). In the first case the chemisorbed particle, is a dipole with a negative pole directed outward (Fig. 2a) this is an electrically neutral formation as a whole, it being valency-saturated. In the second case (Fig. 2b) the chemisorbed particle is a negative ion radical. 

Figure 3 shows different forms of chemisorption for a C02 molecule. In the weak form of chemisorption the C02 molecule is bound to the surface by two valency bonds, as shown in Fig. 3a. This is an example of adsorption on a Mott exciton which is a pair of free valencies of opposite sign (i.e., an electron-hole pair). This may be either a free exciton wandering about the crystal or a virtual exciton generated in the very act of adsorption. As seen from Fig. 3a, in the case of the C02 molecule the weak form of chemisorption is a valency-saturated and electrically neutral form. As a result of electron capture, this form is transformed into a strong acceptor form shown in Fig. 3b, while as a result of hole capture it becomes a strong donor form shown in Fig. 3c. Both these forms are ion-radical ones. It should, however, be noted that the ion-radicals formed in these two cases are quite different and, having entered into a reaction, may cause it to proceed in different directions.

A free atom or a radical possesses an odd number of electrons, except atoms of noble gases. T ypically, a valence-saturated molecule has an even number of electrons. Therefore, the reaction of a radical or an atom with a molecule will inevitably give rise to another atom or radical [1-3] ... 

Thus, free valence persists whenever an atom or a radical undergoes a unimolecular reaction or interacts with valence-saturated molecules (possessing an even number of electrons). This is a natural consequence of conservation of the number of electrons in chemical reactions. Therefore, free valence cannot persist when a radical reacts with a radical. Both reactants have an odd numbers of electrons, and the product formed has an even number of electrons, for example,... 

The participation of a free valence of the surface in chemisorption leads to the transformation of a valence-saturated particle into an ion-radical and, vice versa, to the transformation of a radical into a valence-satuiated electrically charged formation. Thus, among the different coexistent forms... 

In both cases we may consider that the free valence of the Na atom is saturated by the (positive or, respectively, negative) valence of the surface. The mutual saturation of two valencies of the same sign (positive valence of Na atom + free positive valence of the surface) leads to the formation of a homopolar bond (Fig. 2b) the mutual saturation of two valencies of opposite sign (positive valence of Na atom -f- free negative valence of the surface) leads to the formation of an ionic bond (Fig. 2c). In the given case, the strong i-bond and the strong p-bond thus represent valence-saturated forms of chemisorption. They are symbolically depicted in Fig. 4b and, respectively. Fig. 4c. 

The reactivity of the chemisorbed particles, i.e., the probability of their being in radical or valence-saturated states. 

A carbon radical has seven valence electrons, one shy of the octet of a valence-saturated carbon atom. Typical carbon-centered radicals have three substituents (see below). In terms of electron count, they occupy an intermediate position between the carbenium ions, which have one electron less (a sextet and a positive charge), and the carbanions, which have one electron more (an octet and a negative charge). Since both C radicals and carbenium ions are electron deficient, they are more closely related to each other than to carbanions. Because of this, carbon radicals and carbenium ions are also stabilized or destabilized by the same substituents. 

All radical substitution reactions are chain reactions. Every chain reaction starts with an initiation reaction. In one or more steps, this reaction converts a valence-saturated compound into a radical, which is sometimes called an initiating radical (the reaction arrow with the circle means that the reaction takes place through several intermediates, which are not shown... 

Starting from the structure of the trityl radical, radicals were designed that can be obtained even in pure form as stable radicals (Figure 1.7).There are two reasons why these radicals are so stable. For one thing, they are exceptionally well resonance-stabilized. In addition, their dimerization to valence-saturated species has a considerably reduced driving force. In the case of the trityl radical, for example, dimerization... 

According to the definition, electrophiles are electron pair acceptors. They therefore contain either a deficiency in the valence electron shell of one of the atoms they consist of or they are indeed valence-saturated but contain an atom from which a bonding electron pair can be removed as part of a leaving group. Concomitantly this atom accepts the electron pair of the nucleophile. Electrophiles are therefore, as a rule, cations or neutral compounds but not anions. In this book electrophile is abbreviated as E+, regardless of charge. 

The Sb and Fe atoms are situated at steps , C in Fig. 15, causing layer corrugations (Fig. 16). They apparently create the internal valency imbalance in each layer set, requiring mutual valency saturation by regular alternation of the two types of layer. In addition, see Fig. 16, the sulphur atoms of the SnS2-like layers complete the complicated, irregular coordination polyhedra of the Pb, Sn and Sb atoms in the (100) galena-like layers which, within their own layers, possess deformed, half-octahedral (square pyramidal) coordination. Fig. 7b. 

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