Bond softness

From elemental sulfur to selenium and tellurium, intermolecular interactions (,secondary bonds, soft-soft interactions) play an increasing role. According to N. W. Alcock,1 the term secondary bond describes interatomic distances longer than covalent single bonds but shorter than van der Waals interatomic distances.1 In many cases secondary bonds can also be described as coordinative Lewis base - Lewis acid or charge transfer (donor-acceptor) types of interactions. 

Wilson AJ. Non-covalent polymer assembly using arrays of hydrogen-bonds. Soft Mater 2007 3 409-425. 

Adams, St. (2001). Relationship between bond valence and bond softness of alkali halides and chalcogenides. Acta Cryst. B57, 278 -87. 

Molecular Dispersion forces, dipole-dipole forces, hydrogen bonds Soft, low-melting, nonconducting H20, Bt2, co2, ch4... 

Model Definition. The HSAB model classifies Lewis acids (electrophiles) and bases (nucleophiles) as either "hard" or "soft." Hard acids and bases are relatively small, and exhibit low polarizability and a comparatively low tendency to form covalent bonds. Soft acids and bases have the opposite characteristics (24). Stated simply, the model postulates that hard acids react most readily with hard bases, and soft acids react most readily with soft bases (26). 

Furthermore, a good correlation between the orbital softness and the hydrogen Mulliken net charge (qH) exists. Namely, as it is expected, the o-bond softness increases with increasing of qH- Similarly, the f(j and fto of water and H2S indicates that the latter would be more reactive. 

In general, a C—H bond will have a higher energy LUMO than a C-halogen bond. Soft nucleophiles, therefore, are more likely to do an S 2 reaction than to initiate elimination. On the other hand, in an elimination reaction, the proton which is breaking free of the C—H bond will, because it is so small, have a relatively concentrated partial positive charge. This will make hard nucleophiles attack it rather than the carbon atom of the C—Br bond. This is the usual observation the harder the base, the more elimination there is relative to substitution. 

The larger the value of k /Xs at a BCP, the softer or fuzzier a bond is. This idea of bond softness is related to metallic character, so the softer a bond the more metallic it is. Though none of the interactions in this study are metallic, (ice VI shows evidence of semi-metallic character), it is useful to be able to observe the comparative insulating character of the different interactions in this study. We can define ... 

Figure 2 A plot of% rh) against the bond softness X,il /X-3 the data points to the right of the vertical dashed line all correspond to hydrogen bond critical points, whilst those to the left are for O—O interaction BCPs. The curve was fitted to (th) = A exp[B(Ihl/Xs)] where A =0.165 and B = 4.105 and the correlation was 0.96.

Organometallic reagents exhibit their expected typical reaction pattern towards a, S-unsaturated carbonyl compounds. Whereas methyllithium reacted cleanly at — 10°C with 3-cyclo-propylidenebutan-2-one (3) to give 3-cyclopropylidene-2-methylbutan-2-ol (4) in 80% yield via 1,2-addition, methylmagnesium iodide converted the ketone 3 to a 60 40 mixture of the 1,2- and 1,4-addition products 4 and 5 in 55% yield. The ratio depended on the substrate structure and was totally reversed when a phenyl substituent was present on the double bond. Soft methylcuprates only gave the 1,4-adducts 2l-o upon reaction with 1 in diethyl ether for 20 minutes at — 10°C to — 30 °C and subsequent warming to 20 °C (Table 1). 

Metallic Metallic bond Soft to hard, low to high melting point, good conductor of heat and electricity All metallic elements for example, Na, Mg, Fe, Cu... 

Hard Lewis acids and bases have inflexible electron orbitals that form ionic bonds. The electron orbitals of soft Lewis acids and bases are more polarizable and more likely to form covalent bonds. Soft Lewis acids and bases are also called covalent-bonding ions and are siderophile (sulfur-loving) ions in the geology literature. Organic ligands and soil organic matter range from hard to soft Lewis bases. ... 

Metallic Atoms Metallic bond Soft to hard, low to very high mp, excellent thermal and electrical conductors, malleable and ductile Na [97.8] Zn [420] Fe [1535]... 

A cordless drill housing molded from PC/ABS combines impact strength, UV resistance, process-abUity, chemical resistance and cost effectiveness. It also provides a molded-in and chemically bonded soft polyurethane grip that transfers less vibration to the operator. The material retains 70% of its original properties following 1,000-hr accelerated weathering. 

Methyl parathion hydrolysis has also been observed to be catalyzed by (Mabey et al., 1984). Additionally, the affected the reaction product distribution. As expected, the neutral (pH<8) hydrolysis of methyl parathion favors cleavage of the C-O bond (soft-soft interaction) (pathway a, 2.106), whereas the base-catalyzed (pH >8) hydrolysis favors cleavage of the P-O bond (hard-hard interaction (pathway b, 2.106). In the presence of Cu +, however, heterolytic cleavage of the P-O bond resulting in the formation of nitrophenol is the dominant process even at low pH (pathway b, 2.106). 

The numerical precision of the bond-valence concept can be further improved if the universality of the scaling length (B = 0.37 A) is abandoned, thus leaving one more parameter for fitting the experimental data. Research by Adams [45] indeed shows that B depends somewhat on the chemical nature of the atoms involved (e.g., ionization energy, electron affinity, polarizability, etc.), and a corresponding set of improved bond-valence parameters has been proposed, taking into account the bond softness. The latter idea is taken from the theory of (absolute) chemical hardness by Pearson, later covered in Section 2.14. 

Nair KP, Breedveld V, Week M (2011) Modulating mechanical properties of self-assembled polymer networks by multi-functional complementary hydrogen bonding. Soft Matter 7... 

The arrows represent here dative bonds (see Section 1.2.1. and Chapter 3) but secondary bonds (soft-soft interactions) can also play a similar role (see Section 1.2.2. and Chapter 4). The latter are characterized by interatomic distances larger than covalent bond lengths but shorter than van der Waals distances. 

Metallic Atoms Metallic bonds Soft to very hard, low to very high melting point, excellent thermal and electrical conduction, malleable and ductile AU metallic elements— for example, Cu, Fe, AbPt... 

Cavity-directed chemical transformations represent one of the most important features of three-dimensional hosts. Cavities isolated from the exterior by the frameworks of host complexes offer unusual chemical environment that is different from solution state and serve as unique reaction space as molecular flasks. In 1997, the pioneering work of such reactions was conducted by Rebek et al. They utilized the hydrogen-bonded soft ball ... 

Figure 17 Acceleration of a Diels-Alder reaction within the hydrogen-bonded soft baU capsule 24.

The results of Eqs. (3.232) and (3.233) are nevertheless a quarter of the electronic sharing indices within the additive model of atoms-in-mole-cules, (3.217) and (3.220), respectively, modeling therefore weaker bonds. Nevertheless, the geometrical mean model still prescribes the sharing index as behaving like the bonding softness, within equilibrated atoms-in-molecule, i.e., the states ( ), being this way superior to the additive... 

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