Chemical reactivity criterion

The bonding of all cycZo-M H molecules is characterized by a common ring-shaped electron density, which is constructed by highly delocalized a-, n-, and 8- type MOs and associated with their aromatic character. The aromaticity of all cyclo-M U (M = Cu, Ag, Au n = 3-6) molecules was estimated by making use of several criteria for aromaticity, such as the NICS(O) parameter, the relative hardness, At], and electrophilicity index, o). It was further verified on the grounds of a chemical reactivity criterion of aromaticity, that of the interaction of the aromatics with electrophiles, such as Li" ", T1+, or Ag" ". 

The general criterion of chemical reaction equiUbria is the same as that for phase equiUbria, namely that the total Gibbs energy of a closed system be a minimum at constant, uniform T and P (eq. 212). If the T and P of a siagle-phase, chemically reactive system are constant, then the quantities capable of change are the mole numbers, n. The iadependentiy variable quantities are just the r reaction coordinates, and thus the equiUbrium state is characterized by the rnecessary derivative conditions (and subject to the material balance constraints of equation 235) where j = 1,11,.. ., r ... 

Similarly, only selected cyclic systems containing more than one sulfoxide or sulfone groups have been included and discussed here, primarily in the thietane (i.e. 1,2- and 1,3-dithietanes) and thiane (i.e. 1,2-, 1,3- and 1,4-dithianes) series. The criterion for the inclusion of these multifunctional heterocycles was their contribution to the understanding of the physical properties and chemical reactivity of cyclic sulfones and sulfoxides, and the effects of these groups on either their immediate vicinity or on the behavior of the whole molecule. 

In a review, Gorelik51 has shown that magnetic, structural, and energetic properties are determined by the electronic structure of cyclic conjugated systems, which are stabilized by a cyclic delocalization of electrons. Chemical reactivity cannot serve satisfactorily as a general criterion of aromaticity. 

Another general criterion for softness is increased chemical reactivity. Certainly excited-state, or energetic, molecules are more reactive than ground-state molecules. The more excess energy they have, the more reactive they are. What other properties of excited-state molecules may be related to increased softness A soft molecule, or atom, is usually thought of as one with a more diffuse, or less compact, electron cloud than a similar hard molecule or atom. 

Of course, water solubility alone is not an adequate criterion for soil movement, and must be tempered with a knowledge of soil sorption, volatility, and chemical reactivity, competing processes that can remove a chemical from the soil water phase, and the method of application to soil. An attempt to build in the important factors which govern leaching (as well as a similar approach to volatilization from soil) has been described (22). Their leaching index" can be calculated from the simple ratio ... 

Up to a few years ago chemical reactivity was discussed in term of reactivity indexes. These approaches, although valuable, will not be discussed here, since they have been frequently reviewed in the past40-44). Nor will we discuss the perturbation molecular orbital theory for reactants, which has been the subject of extensive reviews 45—47) Extensions of this method can be found in papers by Klopman 48 5°) and Dougherty 51). I shall now mention some methods which have not yet found wide popularity but seem very promising. I mean the criterion of maxi-... 

This criterion may, however, provide different answers from considerations of chemical reactivity. Thermodynamic stability is determined by the energies of all the filled orbitals, whereas reactivity is determined by a number of factors, one of them, however, the energy of the HOMO of the molecule, and these factors do not necessarily run hand in hand. A good example is provided by the... 

The term aromatic was coined in 1855 by August Wilhelm von Hofmann (1818-1892), before the physical mechanism determining so-called aromaticity was unraveled. Originally, this definition was derived from a characteristic property, namely a sweet scent that was associated with some aromatic compounds. Subsequently, it was shown that arenes are unsaturated compounds, but showed a chemical reactivity which differed from that of both aUcenes or aUcynes. This unique reactivity pattern of arenes was used at the end of the nineteenth century as the only criterion of aromaticity. At the start of the twentieth century, additional... 

The energy released from a bulk metal upon addition of one electron is equal to the energy required to remove an electron. Therefore, the experimental and theoretical determination of ionization potentials (IP) or electron affinities (EA) represents a powerful criterion for the degree of convergence between a metal cluster and a metallic particle. [Ill] Moreover, both the IP and the EA are very important properties for determining, in a rather direct way, the chemical reactivity of a cluster towards adsorbed or interacting molecules. Accurate experimental determinations of cluster IP s have been recently reported. For metals, the (first) IP varies dramatically with the cluster size and typically decreases by 2-3 eV (or more) as one goes from the atom to the bulk. [55]... 

Traditionally, the desired monoclonal antibody is selected on the basis of binding affinity to the TSA. This approach led to a multitude of effective catalytic antibodies, the rate acceleration of which, however, is usually orders of magnitude below that of comparable enzymes. Furthermore, detailed mechanistic investigations often revealed a different catalysis mechanism than originally assumed. A different approach for the selection of catalytic antibodies is the reactive immunization where the selection criterion from simple binding is changed to chemical reactivity. 

These equivalent relations for the mixed Fukui functions represent the local criterion of selection of the chemical reactivity in a system where also the spin effects are considered. 

Reactivity was an early criterion for aromaticity and was one of the first applied in studies of antiaromaticity. This includes both qualitative and quantitative studies of reactivity in forming or destroying antiaromatic systems, for example, in the resistance to forming systems such as cyclobutadiene, and their high chemical reactivity if made. 

If one takes the bond length pattern (Table 4.1) as a topological criterion for radialene-Uke character, one may find it to some extent in the geometry of coran-nulene (154). This is supported by calculations of the molecular electrostatic potential, which show five minima at the bonds exocychc to the central ring and another five at the peripheral double bonds of the six-membered rings [115]. These structural features, with rather localized double bonds, are also reflected in fullerene Cgg, which consists of three corannulene substructures. Notably, the chemical reactivity of Cgg is dominated by addition reactions at a 6-6 bond (connecting two six-membered rings) [116], and it fits into this picture that the first addition of dichlorocarbene to corannulene also occurs at a radial (6-6) C=C bond [117]. 

In general, aromatic compounds tend to react in such a fashion as to retain the aromatic nucleus by undergoing substitution rather than addition reactions. This unusual (for unsaturated systems) pattern of reactivity is taken as an indication of the special stability of aromatics. Chemical reactivity is not a particularly good criterion for aromaticity as it is difficult to quantify and for many aromatics the preferred reaction motif is addition. 

Keywords Bond information probes Bond localization Chemical bonds Chemical reactivity Contra-gradience criterion Covalent/ionic bond components Direct/indirect bond multiplicities Entropic bond indices Fisher information Information theory Molecular information channels Orbital... 

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