Activation hardness, and

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... 

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. 

Cardenas-Jiron Gl, Lahsen J, Toro-Labbe A (1995) Hardness profile and activation hardness for rotational isomerization processes. 1. application to nitrous acid and hydrogen persulfide. J Phys Chem 99(15) 5325-5330... 

Quats are usually moderately soluble ia water, but this varies widely owiag to the range of groups bonded to the nitrogen. They are fundamentally nonreactive but act as surface—active cations. Compatibility with anionic detergents and activity ia the presence of hard water have been claimed for some quats (19). 

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. 

Methane is a stable molecule and therefore hard to activate. As a result the sticking probability for dissociative chemisorption is small, of the order of 10 only, and ruthenium is more reactive than nickel. However, a stretched overlayer of nickel is significantly more active than nickel in its common form, in agreement with expectation. 

If an external body is engulfed, it can enrich the star with the original interstellar medium abundances of 6Li, 7Li, 9Be and 10,11B (written here in increasing order of hardness to be destroyed by thermonuclear reactions). This mechanism is then supposed to produce stellar enrichment of these elements up to the maximum meteoritic value. Also, the engulfing star will suffer a rotational increase due to the gain of the planet momentum and a thermal expansion phenomenon due to the penetration of the body provoking mass loss phenomena (Siess Livio 1999). An extension to this scenario has been proposed by Denissenkov Weiss (2000) in order to explain supermeteoritic Li abundance values, via a combination of stellar rotation and activation of the 7Be mechanism at the base of the convective layer produced by the penetration of the external body. 

Although the transfer reaction can occur at 100 °C, the anthraphos iridium complex does not begin to show catalytic activity until 150 °C and continues to be stable to 250 °C. Therefore, we have made temperature corrections to 150°C (423 K) in Table VI and to 250°C (523 K) in Table VII. Compared to STP values, the free-energy barriers for this reaction increase by 5.6 kcal/mol for 423 K and 10.0 kcal/mol for 523 K. As expected, the enthalpies (AH and AH) hardly change (< 0.5 kcal/mol). One can also make corrections for the fact that the... 

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