Activation hardness

This idea can be quantitatively expressed by defining activation hardness as the difference between the LUMO-HOMO gap for the reactant and that for the rr-complex intermedi-... 

Scheme 10.3. Activation Hardness for Aromatic and Heteroaromatic Compounds ...

A quantitative scale of reactivity for aromatic substrates (fused, heterocyclic, and substituted rings) has been devised, based on the hard-soft concept (p. 338). From MO theory, a quantity, called activation hardness, can be calculated for each position of an aromatic ring. The smaller the activation hardness, the faster the attack at that position hence the treatment predicts the most likely orientations for incoming groups. 

Hard filter elimination. The list of solution scores is used for the elimination of solutions with values outside the range allowed by the corresponding active hard filters defined by the user. 

Shchukin et al. [288—290] confirmed that iron antimonates consist of a mixture of FeSb04 with either Sb204 or a-Fe203, and report that a maximum selectivity of 90% occurs between Fe/Sb = 0.06 and 1.7 while the activity hardly changes in this region. As with propene oxidation, a high selectivity thus requires an excess of Sb over Fe. 

An attempt has been made to analyse whether the electrophilicity index is a reliable descriptor of the kinetic behaviour. Relative experimental rates of Friedel-Crafts benzylation, acetylation, and benzoylation reactions were found to correlate well with the corresponding calculated electrophilicity values. In the case of chlorination of various substituted ethylenes and nitration of toluene and chlorobenzene, the correlation was generally poor but somewhat better in the case of the experimental and the calculated activation energies for selected Markovnikov and anti-Markovnikov addition reactions. Reaction electrophilicity, local electrophilicity, and activation hardness were used together to provide a transparent picture of reaction rates and also the orientation of aromatic electrophilic substitution reactions. Ambiguity in the definition of the electrophilicity was highlighted.15... 

The examples provided in this Chapter demonstrate that directed evolution resembles a very useful tool to create enzyme activities hardly accessible by means of rational protein design (Table 14.1). Even if the desired substrate specificity is known from other biocatalysts - e.g. phospholipase A1 activity - the advantage of the directed evolution approach resides in the already achieved functional expression of a particular protein. Thus bottlenecks arising from the identification of enzymes by traditional screening and cultivation methods can be circumvented. In addition, directed evolution can dramatically reduce the time required for the provision of a suitable tailor-made enzyme, also because cloning and functional expression of the biocatalyst has already been achieved. 

Our investigations show that catalyst composition and architecture can have a significant effect on the initial quantity of adsorbed hydrocarbons, i.e. soft delta coke, as well as on stripping rate (table 4). A high zeolite content usually results in a high soft coke make, but not necessarily in a low stripping rate (cat. A vs B). At a constant activity, hard and soft coke make, the stripping rate increases with accessibility ( cat. B vs C). 

The activation hardness is a measure of -+ dynamic reactivity obtained as the difference between absolute hardness of reactant (R) and transition states (T) ... 

The activation hardness is sensitive to the reactivity at different molecule sites and to orientation effects. 

GAP is an important stability index, a large GAP being related to the high stability of a molecule with its low reactivity in chemical reactions. It is an approximation of the lowest excitation energy of the molecule and can be used for the definition of absolute and activation hardness. 

The question to be answered is whether changes in p, or p, on going from reactant to transition state, offer any clue as to the magnitude of the activation energy. The complex C is assumed to be close in energy to the transition state. If we define the activation hardness as... 

What Zhou and Parr found was that the smaller the activation hardness is, the faster is the reaction." Thus is a reactivity index. Table 3.18 shows the results for the amounts of ortho-, para- and weZ -substitution in the nitration of substituted benzenes. Similar good results were found for the site selectivity in a large number of condensed-ring hydrocarbons and heterocyclic molecules. 

It can be seen in Table 3.18 that there is no correlation between reactivity and the activation hardness, if the molecule is changed. Thus benzoic acid has a smaller Ar/ than benzene, but is much less reactive. Again, changes in the cr-bonding have become important, similarly to the case of the cation localization energy. Actually both L+ and At/ do correlate with reactivity, if only the condensed-ring hydrocarbons are compared with each other. 

Zhou Z, Parr RG. Activation hardness. New index for describing the orientation of electrophilic aromatic substitution. J Am Chem Soc 1990 112 5720-5724. 

Simple HMO theory has been used to calculate Atj for several benzenoid hydrocarbons, substituted benzenes, and heterocycles. The resulting values are in qualitative agreement with reactivity trends. Scheme 9.3 gives some of the data. The less positive the number, the more reactive the position. Although there are some discrepancies between structural groups, within groups the Atj values correlate well with position selectivity. The most glaring discrepancy is the smaller activation hardness for deactivated compared with activated benzenes. In particular, benzaldehyde and benzoic acid have At values that are lower than that of benzene, which is counter to their relative reactivity. However, the preference for meta substitution of the deactivated benzenes is predicted correctly. The deactivation of pyridine, relative to benzene, is also not indicated by the At value. 

The potential to describe the chemical reaction path is emerging from the DF theory. Pearson [69] and Parr et al. [70] have proposed a principle of maximum hardness stable molecules arrange themselves as to be as hard as possible. Zhou and Parr introduced the activation hardness parameter for the electrophilic aromatic substitution [71], The same authors have shown a correlation between the absolute hardness of a molecule and aromacity [72]. Nalewajski et al. studied the protonation reaction and described the relation between the interaction energy and charge sensitivities hardness, softness, Fukui function [28, 38]. 

Finally, we mention the correlation between the Hammett a-parameters and the recently introduced activation hardnesses [46], both also being well suited for describing site preferences in electrophilic aromatic substitution. A fundamental study of the link between such longstanding concepts and the present ones is highly recommended. 

An inverse relationship between the HOMO-LUMO gap and the magnetic susceptibility exaltations has also been reported. Polarizability molecular electrostatic potential , activation hardness, average local... 

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