System constraints imposed upon

For Cu 7Cu systems, the electron transfer takes place with a large rearrangement of the inner coordination sphere, as Cu generally prefers distorted octahedral or square pyramidal environments whereas Cu likes better a tetrahedral geometry. This consideration implies that, in unconstrained systems, one or two Cu-S bonds must be broken and bond angles must be drastically modified upon Cu reduction. Therefore, any steric constraint imposed by the coordinated ligand could strongly affect the kinetics of the Cu 7Cu electron transfer process. 

The stress imposed upon the environment results primarily from the fact that the western way of life is dependent on high energy utilization but continued unlimited growth in energy dissipation is incompatible with maintenance of ecological stability and high quality of life. The ecological constraints demand the alteration of human, social, and economic systems toward a stationary state, where resources and materials have to be recycled as much as possible. 

Studies on chemical models show that the stereospecific formation of cis-CHD=CHD is not a consequence of any constraints imposed by the enzyme. It has been shown that [Fe4S4(SPh)4]3- reduces acetylene to ethylene in the presence of a proton source (vide supra), and when the reaction is performed in the presence of deutero-acid stereoselective formation of cis-CHD=CHD [97] is observed. Indeed, even simpler systems show that even in structurally very symmetrical complexes, such as [V(t 5-C5H5)2(ti -RCCR )], alkynes are converted to m-alkenes upon protonation [98,99]. In some cases this transformation has been shown to operate through the intermediate formation of a hydride. 

Every state of equihbrium is subject to certain constraints which are imposed upon the system. In fact, constraints and variables of constraint are in... 

However, for other problems, notably those of statistical mechanics, entropy describes a force of nature. In those cases, constraints such as averages over a row of probabilities are not only numbers that are known to an observer, they are also physical constraints that are imposed upon a physical system from outside (see Chapter 10). In this case, the Principle of Fair Apportionment is a description of nature s symmetries. When there is an underlying symmetry in a problem (such as t outcomes that are equivalent), then Fair Apportionment says that external constraints are shared equally between all the possible states that the system can occupy (grid cells, in our earlier examples). In this case, entropy is more than a description about an observ-er. It describes a tendency of nature that is independent of an observer. 

Any other physical constraint that may be imposed upon the system under consideration. 

On the other hand, regulation, imposes a direct constraint upon technological change. Certain regulatory systems are in-... 

It is clear, then, that a more sophisticated artificial photosynthetic system must include some ordering principle which imposes constraints upon the spatial relationships of the small organic molecules, and perhaps modifies their environment in other ways as well. In the natural reaction center, the membrane protein provides this function. In artificial systems, covalent linkages can perform a similar service. 

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