Bond Order Conservation Principle

The dependence on electron locahzation energy can also be illustrated by the use of the bond order conservation principle. This principle gives an approximate recipe to estimate changes in bond strength when coordination of a surface atom or adsorbate attachment changes [5, 15]. 

The energy of adsorption on a surface atom increases with increasing coordinative unsaturation of the surface metal atom(s). This agrees with ideas proposed by the Bond Order Conservation Principle, which would indicate that the strength of the chemical bond increases when the number of atoms which share bonds to different adorbates decreases. 

Attractive or repulsive through-surface interactions are readily understood in terms of the Bond Order Conservation principles. When an adatom binds to a neighboring surface metal atom, the metal-metal bonds that form to the surface metal atom of interest are weakened. This increases the potential reactivity of the neighboring metal atoms since less of its electron density is tied to the metal atom involved in the surface-adatom bond. Thus, another adatom bound to the neighboring surface metal atom would have an increased interaction energy. Through-surface interactions are repulsive when two or more adsorbates share a metal atom, but attractive when the adsorbates sit at neighboring metal atom sites. These effects are illustrated in Fig. 3.54. 

When two (or more) adsorbed atoms bond to the same surface atom(s), they experience a repulsive interaction. When two adsorbed atoms bond to two different neighboring metal atoms that share a metal-metal bond, they tend to experience attractive interactions. These two rules can readily be deduced from the Bond Order Conservation principle which indicates that the atom-surface bond strength decreases with an increase in the number of adatoms bonded to the same surface metal atom. This change does not occur linearly with the number of neighboring atoms or molecules, but instead tends to vary exponentially. 

The activation energy for CO dissociation on rhodium is lower for the (100) than for the (111) surface. The (100) surface is the more open one of the two. Each rhodium atom misses four nearest neighbors in the first coordination shell as compared to a bulk atom, whereas an atom in the (111) surface misses three nearest neighbors. Hence, carbon and oxygen atoms bond more strongly with the (100) than with (111) surface and, similar to the situation in Figure 6.8, both thermodynamics and kinetics are more favorable for CO dissociation on the (100) surface. This is a manifestation of the bond order conservation principle The more the valence electrons of a metal atom at the surface become distributed in bonds with neighboring atoms, the weaker the individual bond becomes. 

The bond order conservation principle is conceptually useful but only approximately correct. Changes in ionization potential and electron affinity are also important and have to be included in more rigorous considerations. Nonetheless, to a first approximation changes in reactivity are primarily related to the coordination and geometry in the first coordination shell of surface atoms. 

The changes in distance illustrate the operation of the bond order conservation principle The more the electrons of an atom become distributed over bonds to neighboring atoms, the more each of these bonds weakens. 

H. Sellers,/. Phys. Chem., 98,968 (1994). Relationship Among Force Constants Implied by the Principle of Bond-Order Conservation in Chemisorbed Systems. 

The photoisomerization of all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K illustrates the need for a new mechanism to explain the observed behavior [150]. Upon irradiation, all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K undergoes conformational change from all-s-trans to 2-s-cis. Based on NEER principle (NonEquilibrium of Excited state Rotamers), that holds good in solution, the above transformation is not expected. NEER postulate and one bond flip mechanism allow only trans to cis conversion rotations of single bonds are prevented as the bond order between the original C C bonds increases in the excited state. However, the above simple photochemical reaction is explainable based on a hula-twist process. The free volume available for the all-s-trans-all-trans 1,3,5,7-octatetraene in the //-octane matrix at 4.3 K is very small and under such conditions, the only volume conserving process that this molecule can undergo is hula-twist at carbon-2. 

The principle of conservation of free valence is fulfilled only in reactions of a first order with respect to the free radical. In reactions involving two free radicals or atoms, the free valence, as a rule, is saturated to form molecules as reaction products. Such reactions are exothermic because new bonds are formed in them and they occur with high rate constants. For example, the recombination and disproportionation of all l radicals in solution occur with the rate constant of diffiisional collisions (10 -lO l/(mol s)). In the gas phase, atoms recombine with the frequency of triple colli-... 

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