Effect of Hydrogen Bonding on Molecular Structure

Changes in covalent nonhydrogen bond length as a consequence of hydrogen-bond formation is predicted theoretically and observed experimentally in particular examples. Apart from the hydrogen-bonded water polymers discussed in the preceding chapter, the simple carboxylic acids provide one of the few examples where both monomers and hydrogen-bonded dimers can be studied by the same 

The results, shown in Thble 5.1, clearly demonstrate the lengthening of the acceptor C=0 bond and the shortening of the donor C-OH bond on hydrogen-bond formation. This polarization of the carboxylic acid group can be represented in valence-bond resonance terms as a significant contribution from 

Some high precision neutron diffraction studies of other carboxylic acids provide evidence that the shortening in the C-OH bond length is proportional to the length of the hydrogen bond, as shown in Thble 5.2. The evidence relating to a cor- 

For NH 0=C hydrogen bonds, changes in the C-N and C=0 bond lengths due to hydrogen-bond formation are observed experimentally and predicted theoretically. The simplest molecule containing the peptide C-N bond is form-amide. The structure has been studied both experimentally as the monomer by gas-phase diffraction [374], and theoretically as the monomer and as both the cyclic and open hydrogen-bonded dimers [298,299] 1 and 2. 

The shortening of the C-N bond implies that hydrogen bonding confers more rigidity into polypeptide chains, not only by reason of the hydrogen bonds, per se, but also because of the additional Tt-bond character in the peptide bond. In terms of valence-bond resonance, it increases the importance of the representation shown below  

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Specific solute-solvent interactions, such as hydrogen bonding or protonation, may be included in the calculation of the shielding of solute nuclei by a supermolecule approach. The appropriate structure of the solute-solvent supermolecule may be obtained by the use of molecular mechanics simulations. At the semi-empirical MO level this approach has been successfully used to describe the effects of hydrogen bonding on the nuclear shielding of small molecules. 

The dependence of the principal components of the nuclear magnetic resonance (NMR) chemical shift tensor of non-hydrogen nuclei in model dipeptides is investigated. It is observed that the principal axis system of the chemical shift tensors of the carbonyl carbon and the amide nitrogen are intimately linked to the amide plane. On the other hand, there is no clear relationship between the alpha carbon chemical shift tensor and the molecular framework. However, the projection of this tensor on the C-H vector reveals interesting trends that one may use in peptide secondary structure determination. Effects of hydrogen bonding on the chemical shift tensor will also be discussed. The dependence of the chemical shift on ionic distance has also been studied in Rb halides and mixed halides. Lastly, the presence of motion can have dramatic effects on the observed NMR chemical shift tensor as illustrated by a nitrosyl meso-tetraphenyl porphinato cobalt (III) complex. 

Pugh WJ, Roberts MS, Hadgraft J. (1996) Epidermal permeability-penetrant structure relationships 3. The effect of hydrogen bonding interactions and molecular size on diffusion across the stratum comeum. Int J Pharm 138 149-165. 

Noncovalent interactions play a key role in biodisciplines. A celebrated example is the secondary structure of proteins. The 20 natural amino acids are each characterized by different structures with more or less acidic or basic, hydrophilic or hydrophobic functionalities and thus capable of different intermolecular interactions. Due to the formation of hydrogen bonds between nearby C=0 and N-H groups, protein polypeptide backbones can be twisted into a-helixes, even in the gas phase in the absence of any solvent." A protein function is determined more directly by its three-dimensional structure and dynamics than by its sequence of amino acids. Three-dimensional structures are strongly influenced by weak non-covalent interactions between side functionalities, but the central importance of these weak interactions is by no means limited to structural effects. Life relies on biological specificity, which arises from the fact that individual biomolecules communicate through non-covalent interactions." " Molecular and chiral recognition rely on... 

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