Peripheral hydrogen atom

As outlined in Chapters 23 and 24, water molecules are only loosely bound by hydrogen bonding at the peripheral atoms of the proteins and nucleic acids. Consequently, they are even more mobile than the atoms of the macromolecules to which they are coordinated. As we know from H2O/D2O exchange experiments, some water of hydration molecules are fully and easily replaced even in the crystalline state. The rate of dissociation must be diffusion-controlled and therefore at least in the ns time range. 

The species at the centre of tire rings is usually Si or Ge and tire bridging atom is oxygen. In one study tire peripheral hydrogens on tire phtlialocyanine molecules were replaced by alkyl groups and tire resulting polymers could be rendered soluble in ordinary organic solvents [108, 109 and 110]. Successful deposition of several of tliese materials has been achieved and different techniques were employed to study tlieir stmctural properties [109, ill, ill, ill and 1141. 

Although most deep fluorinations have resulted in fluorocarbons, a few have not, even under more forcing conditions. The results are explained by invoking steric hindrance induced by faster peripheral fluorination shielding more internal hydrogen atoms from attack [15] (equation 1). 

X-ray analysis has shown that cyclodecane in its most preferred conformation exists as shown below in which the two chain forms of cyclohexane are joined by 1,3 axial bonds and six hydrogen atoms are intraannular and 14 peripheral. 

Therefore the CH2 groups in cyclodecane are stereochemically non equivalent. There are CH2 groups in which both the hydrogen atoms are peripheral, there are those in which one hydrogen is peripheral and the other is intraannular. The last two types differ from each other in that in one case the peripheral hydrogen is equatorial and in other axial. 

REVERSIBLE MIGRATION OF A HYDROGEN ATOM FROM A METAL CENTRE TO A PERIPHERAL LIGAND... 

The structural formula for propane shows 3 axial carbon atoms and 8 peripheral hydrogen atoms. The composition of propane can be more compactly expressed as C3 Hg. This representation is a molecular formula. Such a formula does not directly tell how the various atoms are interbonded. 

Peripheral contributions become important when short interatomic distances are involved, as, for example, for the EFG at nitrogen nuclei and especially at nuclei of hydrogen atoms. Since hydrogen has only one electron, the electric field gradient is mainly due to the density farther from the nucleus, and has therefore been described as less sensitive to the precise charge distribution (Tegenfeldt and Hermansson 1985). 

The tme merits of this equation are revealed by a survey of the leading terms governing bond dissociations when the cleavage occurs (1) in the interior of the molecule (i.e., when both K and L are polyatomic groups), (2) in its peripheral region [i.e., when K (or L) is an atom], or (3) when it concerns exterior bonds formed by a molecule, such as hydrogen bonds. These topics shall be discussed shortly. 

The carbon-hydrogen bonds are the prototype of peripheral bonds. In the perspective of the CNE effect, the extraction of an atom that was partially charged in the host... 

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