Hard interaction

The theory predicts high stabilities for hard acid - hard base complexes, mainly resulting from electrostatic interactions and for soft acid - soft base complexes, where covalent bonding is also important Hard acid - soft base and hard base - soft acid complexes usually have low stability. Unfortunately, in a quantitative sense, the predictive value of the HSAB theory is limited. Thermodynamic analysis clearly shows a difference between hard-hard interactions and soft-soft interactions. In water hard-hard interactions are usually endothermic and occur only as a result of a gain in entropy, originating from a liberation of water molecules from the hydration shells of the... 

These concepts play an important role in the Hard and Soft Acid and Base (HSAB) principle, which states that hard acids prefer to react with hard bases, and vice versa. By means of Koopmann s theorem (Section 3.4) the hardness is related to the HOMO-LUMO energy difference, i.e. a small gap indicates a soft molecule. From second-order perturbation theory it also follows that a small gap between occupied and unoccupied orbitals will give a large contribution to the polarizability (Section 10.6), i.e. softness is a measure of how easily the electron density can be distorted by external fields, for example those generated by another molecule. In terms of the perturbation equation (15.1), a hard-hard interaction is primarily charge controlled, while a soft-soft interaction is orbital controlled. Both FMO and HSAB theories may be considered as being limiting cases of chemical reactivity described by the Fukui ftinction. 

This reaction is efficient because of the soft-soft interaction of CH3 and I and the hard-hard interaction that leads to the formation of water. As in the case of some other catalytic processes, the idealized... 

For instance, in structure 12-e, the C-X and C-0 dipole moments are additive, leading to a destabilization of the molecule by increasing the energy. In structure 12-a, offset of the C-X and C-0 dipole moments minimizes electrostatic interactions, thus leading to a more stable conformation. This electrostatic model was supported by the observed increase of the percentage of the equatorial conformation of 2-methoxy tetrahydropyran (14) when moving from a non-polar to a polar solvent (Table 3).12 In this model, the polar groups are not polarizable and lead to dipole/dipole (hard/hard) interactions. 

Fig. 8 Interpretation of the conformational endo-anomeric effect according to Edwards (hard/hard interactions).

The combination of the dual electrophilic character of DMC with its reaction products allows two consecutive steps to occur in a selective way for what concerns both reaction sequence and yields first, the hard-hard reaction occurs and produces a soft anion only and second, a soft-soft nucleophilic displacement leads to the final product. Since hard-soft and soft-hard interactions are inhibited, double methylation and double carboxymethylation do not occur. 

The central feature of the mechanism is the 3-cuprio(III) enolate Cpop, of an open, dimeric nature, as shown by comparison of theory with experimentation involving NMR and KIEs [80, 81]. This species serves as the direct precursor to the product (Scheme 10.5, top box). In this critical CPop complex, copper/olefin (soft/soft) and a lithium/carbonyl (hard/hard) interactions are present. The open complex may be formed directly, by way of an open cluster (bottom left of Scheme 10.5), or by complexation of a closed cluster with the enone (CPcl). Experiments have shown that the enone/lithium complex (top left of Scheme 10.11) is a deadend species [60, 74]. 

Luminous matter has revealed dark matter, but the new substance remains obscure. What is it made from Is it perhaps composed of known forms of matter Only partly Is dark matter made up of microscopic particles If the answer is affirmative, we may suppose that this unknown form of energy penetrates and permeates the galaxies, the Solar System and even our own bodies, just as neutrinos pass through us every second without affecting us in any way. And like the neutrinos, these unknown particles would hardly interact at all with ordinary matter made from atoms. To absorb its own neutrinos, a star with the same density as the Sun would have to measure a billion solar radii in diameter. Luminous and radiating matter is a mere glimmer to dark matter. 

General Thermodynamic Characteristics of Soft-Soft and Hard-Hard Interactions. 167... 

The values of AH for the thallium (III) halide systems becomes less exothermic as complex formation proceeds. There are no steps with about the same value of AH , in marked contrast to e.g. Hg2+ and Pd2+. The trend of AH is in fact opposite to that found for several t)q)ical hard-hard interactions, e.g. iron (III) fluoride, lanthanum sulphate and yttrium acetate (Table 1). An even more striking feature of the thallium (III) halides is that AS°n is approximately constant for all steps. This is indeed different not only from ions such as In +, Cd2+ and Zn +, where reversals of the decreasing trend of AS°n occur for certain steps, but also from Hg2+ and Pd + where the higher steps have a much lower value of ASn than the earlier ones. 

Figure 10 shows the (111) substrate and the neighboring sites around a central bridge-bonded sulfate (in black). We model the interactions between the adsorbates in two different ways. First, we consider a shell of purely hard interactions, in which the simultaneous bonding of two anions to neighboring sites is simply excluded. These excluded neighboring sites are displayed in white in Figure 10. Next, we consider a second shell of neighboring sites with either finite attractive or finite repulsive interactions. These are displayed in grey in Figure 10.

For both nucleophiles, 2,5-dinitrofuran is the most active substrate, the thiophene derivative follows. On the other hand, the relative reactivity of 1-methyl-2,5-dinitropyrrole and 1,4-dinitrobenzene depends on the nature of the nucleophile. For the 4-MeC6H4S anion, the former is more active by about two powers of ten, but in the piperidinolysis reaction the 1,4-benzene is superior. These phenomena appear to be caused by differences in the polarizability of both substrate and nucleophiles. p-Tolylthiolate anion is a softer nucleophile in comparison with piperidine and the pyrrole system is certainly more polarizable than the benzene molecule. Therefore soft-soft interaction of 1-methyl-2,5-dinitropyrrole with 4-MeC6H4S and hard-hard interaction of 1,4-dinitrobenzene with piperidine should occur easier than interactions between reagents with opposite types of softness and hardness. 

The HSAB rule works and the reaction is exothermic as written. If we look at the individual heats of atomization of the species (from bond energies. Appendix E) we find BeF2 = +1264 HgFa = +536 Bel, = +578 Hgl, = +291 kJ moT. The driving force in Eq. 9.82 is almost entirely the strong bonding in the hard-hard interaction. 

The idea of equating hard-hard interactions with electrostatics has probably been overemphasized. It is natural, since a typical hard-hard interaction is U F But the isoelcctronic series Li—F, Be—0, B—N, C—C till form strong bonds. The Li—F bond is the strongest since ii is a resonance hybrid of LihF Li—F. Some... 

Detailed studies have been carried out with metal-boron (Ni-B, Pd-B, Pt-B) and metal-phosphorous (Ni-P, Pd-P) films prepared by radiofrequency sputtering269-272. Pt hardly interacts with boron and shows the low selectivity of the pure metal269. Interaction between Ni or Pd and the metalloids results in a change in the electron density of... 

There is no doubt that Pearson did not suggest something entirely new, as we have already seen, but he generalized the concepts derived from the mass-action law (formation constants and Bransted acidity) in a variety of new ways. Many chemists have felt that hard-hard interactions are a new name for electrovalent (ionic) bonding and soft-soft interactions for covalent bonding37. This is also a part of the truth, but other aspects are far more sophisticated and deserve detailed discussion. Other chemists sharply criticize the short and colloquial words hard and soft . In the writer s... 

Whereas the hard-hard interactions in Pearson s Dual Rule essentially are Coulombic, the soft-soft interactions frequently invite the comment that they represent polarizability. This word has connotations for the chemists which are not very different... 

The HSAB principle states that hard acids (Fe surface) prefer to coordinate with hard bases (oxygen, phosphates). Hard interactions are normally ionic. Iron oxides can be readily formed and the anti wear mechanism starts to interact with the polymeric zinc metaphosphate, Zn(P03)2. 

March J., Advanced Organic Chemistry, 4th edn, John Wiley Sons, Inc., New York, 1992, p. 573. 146Certain semi-empirical calculations (extended Hiickel, CNDO, etc.) orient the large lobes away from each other in the o orbital. Such an orbital is nonbonding rather than antibonding, because the two AOs hardly interact. 

This way of generating the EDM (-> IDM) collective modes guarantees their one-to-one correspondence to the respective IDM of reactants and, as such, may provide an alternative, convenient framework for describing the CT processes. Like the IDM, such localized EDM preserve the memory of the reactant interaction in M + and should lead to substantial hardness decoupling. This expectation is due to their resemblance to the PNM (IDM) of reactants, for which the diagonal (reactant) blocks of the relevant hardness matrix are exactly diagonal. Thus, with no external hardness interaction between the A and B subsets of EDM, it comes as no surprise that these collective, delocalized charge-displacement modes also bear some similarity to the PNM of M as a whole. 

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