Weak hydrogen bonds

Two factors are important when w e discuss the weaker interactions. The first is that they become important in the absence of stronger interactions, and secondly that they can make up in number what they lack in energy. Thus, even in the presence of a strong hydrogen bond, the weaker forces may determine the overall structure if they are more numerous. 

The 7t-7i and n-cj interactions, which are also called phenyl factors in some systems, occurs between aromatic n-systems. The interaction is the result of a dispersion interaction and a columbic polarization. In energy terms, 

In a few cases, particularly when the two anions are chemically equivalent or when steric constraints force the two anions close together as in certain organic molecules, the hydrogen bond can be symmetric. The hydrated ion, H5O2, discussed in Section 7.6, is one example, but such cases are rare since they involve considerable strain in the H-O bonds. 

Decreasing the 0-H...0 angle exposes the H+ ion to other neighbouring anions with which it can form one or more additional very weak bonds (Ah 0.05 vu, H... O 250 pm with O-H... O angles 130°). While such very weak bonds individually make only a small contribution to the valence sums, in compounds such as the perchloric acid hydrates they are so numerous that 

A particularly interesting question is what happens to free ions when placed 

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If tetramethylammonium chloride is dissolved in hydrochloric acid, the unstable salt [(CH3)4N] [HClj], can be crystallised out here chlorine is showing weak hydrogen bonding (cf. F H—F which is stable, and Cl - H—Cl which is unstable). 

The unexpected preference for the interfacial region at lower concentrations of benzene has prompted speculation. It has been demonstrated that aromatic compounds are capable of forming weak hydrogen bonds with water. This ability favours uptake in the aqueous interface over solubilisation in the interior. Alternatively, some authors have attributed the binding behaviour of benzene to its... 

In this case the parameters C and Q are of order of unity, and therefore they correspond to the intermediate situation between the sudden and adiabatic tunneling regimes. Examples are mal-onaldehyde, tropolon and its derivatives, and the hydrogen-oxalate anion discussed above. For intermolecular transfer, corresponding to a weak hydrogen bond, the parameters C, Q and b are typically much smaller than unity, and the sudden approximation is valid. In particular, carbonic acids fulfill this condition, as was illustrated by Makri and Miller [1989]. 

Bands at 1000 and 1035 cm-1 have been assigned (43) to a spectrum approximating to structure (I), a very weak hydrogen bond, and bands at 1010 cm-1 and 1035 cm-1 to a spectrum approximating to structure (II) in which the proton has become completely attached to the nitrogen atom. 

Collagen forms a triple helix, where three chains of connected amino acids form weak hydrogen bonds between the double-bonded oxygen atoms and the hydrogen atoms attached to the adjacent chain s nitrogens. The three chains then twist together like three cords in a rope. 

It is a lipophilic compound which removes intercellular lipids that are covalently linked to the cornified envelope surrounding epithelial cells [3]. It also enhances penetration of other agents. Resorcinol (m-dihydroxy benzene) is structurally and chemically similar to phenol. It disrupts the weak hydrogen bonds of keratin [4]. Lactic acid is an alpha hydroxy acid which causes corneocyte detachment and subsequent desquamation of the stratum corneum [5]. 

These results at the interfaces between the PVA gel and quartz surfaces, with and without modification by OTS, suggest that the weakly hydrogen bonded, that is, liquid-like , water plays an important role for the low friction at the PVA gel/quartz interface. 

Scatena, L. F., Brown, M. G. and Richmond, G. L. (2001) Water at hydrophobic surfaces weak hydrogen bonding and strong orientation effects. Science, 292, 908-912. 

Glendening, E. D., Streitwieser, A., 1994, Natural Energy Decomposition Analysis An Energy Partitioning Procedure for Molecular Interactions With Apphcation to Weak Hydrogen Bonding, Strong Ionic, and Moderate Donor-Acceptor Interactions , J. Chem. Phys., 100, 2900. 

Desiraju GR, Steiner T (1991) The weak hydrogen bond. Oxford University Press, Oxford... 

The binding between the substrate and the enzyme is often provided by relatively weak hydrogen bonds. The E-S complexes have equilibrium constants ranging from one hundredth to one hundred millionth M, which corresponds to interaction free energies of -3 to -12 kcal/mol much lower than the value of -450 kJ/mol characteristic for a covalent bond. 

INFRARED LINESHAPES OF WEAK HYDROGEN BONDS RECENT QUANTUM DEVELOPMENTS... 

The general potential U (8) has not been used before 1999 [52] because its numerical matrix representation requires huge basis sets, incompatible with the common computers. In order to avoid this situation, an approximation has been undertaken in previous studies the adiabatic approximation [54,55], Following an idea of Stepanov [56], Marechal and Witkowski assumed that the fast mode follows adiabatically the slow intermonomer motions, just as the electrons are assumed to follow adiabatically the motions of the nuclei in a molecule. It has been shown [57] that the adiabatic approximation is only suitable for very weak hydrogen bonds, as discussed in the next section. 

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