Chemical environment hydrogen bonding

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

The most striking feature of the earth, and one lacking from the neighboring planets, is the extensive hydrosphere. Water is the solvent and transport medium, participant, and catalyst in nearly all chemical reactions occurring in the environment. It is a necessary condition for life and represents a necessary resource for humans. It is an extraordinarily complex substance. Stmctural models of Hquid water depend on concepts of the electronic stmcture of the water molecule and the stmcture of ice. Hydrogen bonding between H2O molecules has an effect on almost every physical property of Hquid water. 

All of these functions are made possible by the characteristic chemical features of carbohydrates (1) the existence of at least one and often two or more asymmetric centers, (2) the ability to exist either in linear or ring structures, (3) the capacity to form polymeric structures via glyeosidie bonds, and (4) the potential to form multiple hydrogen bonds with water or other molecules in their environment. 

A high diffusion coefficient increases the rate of diffusion, all else being the same. The diffusion coefficient is determined in part by molecular size and shape. Small molecules tend to have high diffusion coefficients, which is one reason why formaldehyde penetrates faster than glutaraldehyde. In addition, interactions between the chemical and its environment will influence the diffusion coefficient. Thus, if the chemical hydrogen bonds to the water around it, the diffusion coefficient will be lower and the rate of diffusion will be reduced. 

A. A. Kossiakoff, M. Ultsch, S. White, C. Eigenbrot, Neutron Structure of Subtilisin BPN Effects of Chemical Environment on Hydrogen-Bonding Geometries and the Pattern of Hydrogen-Deuterium Exchange in Secondary Structure Elements , Biochemistry 1991, 30, 1211-1221. 

It is very instructive to compare the kinetics and plausible mechanisms of reactions catalyzed by the same or related catalyst(s) in aqueous and non-aqueous systems. A catalyst which is sufficiently soluble both in aqueous and in organic solvents (a rather rare situation) can be used in both environments without chemical modifications which could alter its catalytic properties. Even then there may be important differences in the rate and selectivity of a catalytic reaction on going from an organic to an aqueous phase. TTie most important characteristics of water in this context are the following polarity, capability of hydrogen bonding, and self-ionization (amphoteric acid-base nature). 

In a polarizable environment, the absolute minimum structure of HFIP carries the OH synclinal (sc) or almost synperiplanar (sp) to the adjacent CH (Fig. 9) [49]. On the basis of quantum-chemical considerations as well as single-crystal X-ray structures in which HFIP acts as hydrogen bond donor, HFIP always takes on such an sc or even sp conformation. In this conformation, the hydrogen bond donor ability of HFIP is significantly increased (Figs. 10 and 11) [49]. 

---