Hydrogen bonding structural effects

The reasons for the above phenomena are to be found in differing configurations of hydrogen bonds, the effect of pH, differences in the structures of fluorescence indicators and binders and differences in surface area. For example, silica gel 60 possesses a surface area of 500 m /g [211] while that of Si 50 000 lies below 5 m /g [212],... 

The photodecomposition and thermodecomposition of nitromethane have been extensively studied as model systems in combustion, explosion and atmosphere pollution processes[l]. On another hand, nitromethane was selected as a model solvent in experiments aimed at examining non hydrogen-bonded solvent effects in a general acid-base theory of organic molecules [2.3]. This selection is based on the electronic and structural characteristics of nitromethane that has a high dielectric constant, and at the same time cannot form hydrogen bonds with solute molecules. 

S Molecular structure Environment (electron density, hydrogen bonding, solvent effect, hydration etc.)... 

The explanation for the above is twofold. Firstly there is the effect of increasing cavita-tional collapse energy via a lowering in vapour pressure as the temperature is reduced (see above). This does not adequately explain the effect of the change in solvent. The primary process is unlikely to occur inside the cavitation bubbles and a radical pathway should be discarded. The most likely explanation is that the disruption induced by cavitation bubble collapse in the aqueous ethanolic media is able to break the weak intermolecular forces in the solvents. This will alter the solvation of the reactive species present. Significantly the maximum effect is found in 50 % w/w solvent composition - the solvent composition very close to the maximum hydrogen bonded structure. 

Hydrogen bonds between fluorinated substrates and biological macromolecules have been postulated in some enzyme-substrate complexes. However, it is rather difficult to determine if these hydrogen bonds really exist other factors may stabilize the conformation corresponding to the short H- F interatomic distance observed. Indeed, this conformation can be favored by other factors (e.g., other stronger hydrogen bonds, gauche effect), without participation of an H- F interaction to stabilize the supramolecular structure. The existence and possible... 

This effect, possibly because of its complexity, has not been adequately treated —-unless I am mistaken—in quantum theoretical accounts of the hydrogen bond. I feel that this may lead to error, as a particular case where it cannot be safely neglected occurs in the case of the most classical hydrogen-bonded structure— that of ice. 

The second approach that has been rather popular with mixed aqueous solvents is to assume that the mixture is more or less structured than that of pure water. There is much evidence to show that the particular hydrogen-bonded structure of water influences many of the properties of electrolytes in water (15). If nonelectrolytes can modify the structure of water (15), they can have an indirect effect on the properties of electrolytes. This explanation has been particularly successful in the case of U + W mixtures (1,2). Such a simple approach is not as successful with hydrophobic cosolvents. For example, AHe°(W — W + TBA) are positive for both alkali halides (16) and tetraalkylammonium bro-... 

To measure the effect of the presence of ions on the hydrogen-bond structure of bulk water, we studied the concentration dependence of the orientational relaxation of bulk water in aqueous solutions of Mg(C104)2. We chose Mg(C104)2 because Mg2+ is considered to be a strong structure-making ion the viscosity of 1 M Mg(C104)2 is about 30 %... 

A substrate binds an enzyme at the active site. Substrate-enzyme binding is based on weak intermolecular attractions contact forces, dipole forces, and hydrogen bonding. Steric effects also play an important role because the substrate must physically fit into the active site. Some enzymes have confined active sites, while others are open and accessible. A restricted active site can lead to high selectivity for a specific substrate. Low specificity can be advantageous for some enzymes, particularly metabolic and digestive enzymes that need to process a broad range of compounds with a variety of structures. Because enzymes are composed of chiral amino acids, enzymes interact differently with stereoisomers, whether diastereomers or enantiomers. 

M. Pietrzak et al., Counteranion and solvent effects on scalar couplings and hydrogen bond structures of protonated sponges, to be published... 

Sicinska D, Truhlar DG, Paneth P (2005) Dependence of transition state structure on substrate the intrinsic C-13 kinetic isotope effect is different for physiological and slow substrates of the ornithine decarboxylase reaction because of different hydrogen bonding structures. J. Am. Chem. Soc. 127 5414-5422... 

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