Hydrogen covalent bond formation

Many biological processes involve an associa tion between two species in a step prior to some subsequent transformation This asso ciation can take many forms It can be a weak associ ation of the attractive van der Waals type or a stronger interaction such as a hydrogen bond It can be an electrostatic attraction between a positively charged atom of one molecule and a negatively charged atom of another Covalent bond formation between two species of complementary chemical re activity represents an extreme kind of association It often occurs in biological processes in which aide hydes or ketones react with amines via imine inter mediates... 

Coupling to a mineral surface requires the presence of active hydroxyls on the substrate. The coupling reaction is a multi-step process that proceeds from a state of physisorption through hydrogen bond formation to actual covalent bond formation through condensation of surface hydroxyls with silanols ... 

The basic requirement for cellulose dissolution is that the solvent is capable of interacting with the hydroxyl groups of the AGU, so as to eliminate, at least partially, the strong inter-molecular hydrogen-bonding between the polymer chains. There are two basic schemes for cellulose dissolution (i) Where it results from physical interactions between cellulose and the solvent (ii) where it is achieved via a chemical reaction, leading to covalent bond formation derivatizing solvents . Both routes are addressed in details below. 

Jr Carbene Formation Reactive Hydrogen Containing Compound Or Insertion with Covalent Bond Formation (Reaction 56)... 

If counter ions are adsorbed only by electrostatic attraction, they are called indifferent electrolytes. On the other hand, some ions exhibit surface activity in addition to electrostatic attraction because of such phenomena as covalent bond formation, hydrogen bonding, hydrophobic and solvation effects, etc. Because of their surface activity, such counter ions may be able to reverse the sign of because the charge of such ions adsorbed exceeds the surface charge. 

Likewise, when the elementary process of Eq. (8.6) [H30(tq) reduction] is done in the presence of molecules (X) that stabilize hydrogen atoms (H-) via covalent bond formation, the shift in the reduction potential is a measure of the X— H bond energy. For example,4... 

Some of the considerations for electron-transfer processes that have been discussed in previous chapters are fundamental to the electrochemistry of these examples. Thus, reductive processes always involve the most electrophilic (acidic, positive-charge density) center (substrate or substrate-matrix combination) that produces the least basic (nucleophilic) product. Under acidic conditions the primary reactant often is the hydronium ion (H30+) to give a hydrogen atom that couples with the substrate via covalent bond formation for instance... 

Nitric Oxide (-NO). Reduction of the oxides of nitrogen (-NO, -N02, and N20) usually involves the addition of hydrogen atoms that are electrogenerated. Figure 11.6 illustrates the chronopotentiometric reduction of -NO in aqueous media at pH 7.0 and pH 5.0.9 The use of a mercury electrode inhibits the reduction of H30+ to H2 (is0, —2.2 V vs. SCE at pH 5), but allows formation of H when it couples with a substrate via covalent-bond formation ... 

However, science evolves More will be seen of the quantum mechanical approach to solvation and in particular in nonaqueous solutions when there is more chance of interactions involving overlap of the orbitals of transition-metal ions and those of organic solvent molecules. Covalent bond formation enters little into the aqueous calculations because the bonding orbitals in water are taken up in the bonds to hydrogen. With organic solvents, the quantum mechanical approach to bonding may be essential. 

Because the oxygen atom in a water molecnle is essentially without charge, these reduction processes represent the facilitated reduction of hydroninm ions by stabihzation of the product H atom through strong covalent bond formation with atomic oxygen. Thus, the standard state reduction of protons to hydrogen atoms (equation 32) is shifted to a much more favored process in the presence of atomic oxygen (equation 31). 

Although Bronsted proton transfer reactions appear to belong to a unique category not described by Scheme 14, they are examples of polar-group transfer reactions and are not different in principle from nucleophilic displacement reactions. Deprotonation by hydroxide ion can be regarded as the shift of an electron from HO to the Bronsted acid synchronously with the transfer of a hydrogen atom from the Bronsted acid to the incipient HO- radical, with the reaction driven by covalent bond formation between the HO- radical and the H- atom to form water (equation 161). 

Within the last 5 years a number of quantum mechanical calculations have been carried out, in particular for hydrogen-bonded systems such as HgO, NHg, and HF as solvents. Contrary to electrostatic models, MO calculations allow for the possibility of covalent bond formation and, consequently, constitute a fundamentally better approach to ion-molecule interactions. Some interesting results of recent model calculations, although qualitative in nature, are discussed in Section V. 

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