Amides hydrogen bonding

Figure 8.15 Centrosymmetric 8 membered hydrogen bonded amide ring supramolecular synthon found in the crystal structure of saccharin.

Figure 3.17 Cyclic array of hydrogen-bonded amides resulting in cyclo-enantiomerism. 

Aspidofiline (CXXXVI), one of the simplest members of the group, was the first alkaloid to be isolated from A. pyrifolium (79). It was readily separated from the accompanying pyrifoline by virtue of its alkali solubility. The phenolic hydroxyl group so indicated is confirmed by the bathochromic shift of the UV-spectrum observed on addition of alkali. The spectrum is characteristic of a 17-hydroxy-V-acyldihydroindole and the IR-spectrum shows a hydrogen-bonded amide carbonyl band at... 

The FT-IR spectrum of ot-chitin shows two absorption bands at approximately 1625 and 1655 cm , characteristic of hydrogen-bonded amide groups. The DA of chitin can be determined by the ratios of different IR absorption bands (Fig. 2.27), as these bands disappear upon deacetylation of chitin. 

Subsequently we have made much use of the cobaltocenium fragment in combination with hydrogen-bonding amide substituents for the production of functional anion sensors. These hosts will be discussed in Section V.D. 

Clearly, the most satisfactory way to decide between conflicting concepts of the structure and nature of the hydrogen bond is to treat quantum-mechani-cally a hydrogen-bonded complex as a single large molecule entity with no truncation and to compare the results obtained for this supermolecule to those obtained for the separated molecules treated in the same approximation. This mode of approach is now possible, and a number of such computations using both all-valence electrons methods and the SCF MO non-empirical procedure have recently appeared. The references pertinent to biochemistry have been listed in Tables I and II. These concern only various hydrogen-bonded amides and the base pairs of the nucleic acids. 

A. All-Valence Electrons Computations on Hydrogen-Bonded Amides... 

Simulated coherent energy transfer in a hydrogen bonded amide chain arising from Fermi resonance has been modelled by Clarke and Collins . This interesting study is related to the Davydov soliton model which has been proposed for explaining energy transport in proteins. The role of similar nonlinear effects in simple organised chemical systems has yet to be established. 

The results and conclusions summarized here concerning lone-pair directionality in doubly hydrogen-bonded amide systems, based on molecular packing and potential energy calculations, match those of Thylor, Kennard and Versichel [108,151], who used a completely different approach. But the results presented here for single hydrogen-bond acceptors in trans amides do not show a pronounced preference for lone-pair directionality (0 130°). 

The principal hydrogen-bonding amide solvents are formamide, acetamide, iV-methylformamide (NMF), and A-methylacetamide (NMA). All four are basic solvents. For example, the medium effect for transfer of the proton from water to formamide (Appendix 3.3.2) is —1.2 the redox acidity function i o(H) is 4.7 in molten acetamide at 98°C. These solvents are most notable for their very high dielectric constants, which range from 109.5 for formamide to 182.4 for NMF at 25°C. Their autoprotolysis constants are comparable with those of water and the alcohols, e.g. pATs of formamide is 17.0 (Appendix 3.3.3), so that a wide range of acid strengths can be investigated. 

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