Hydrogen-bonding techniques

DFT calculations offer a good compromise between speed and accuracy. They are well suited for problem molecules such as transition metal complexes. This feature has revolutionized computational inorganic chemistry. DFT often underestimates activation energies and many functionals reproduce hydrogen bonds poorly. Weak van der Waals interactions (dispersion) are not reproduced by DFT a weakness that is shared with current semi-empirical MO techniques. 

Intimate information about the nature of the H bond has come from vibrational spectro.scopy (infrared and Raman), proton nmr spectroscopy, and diffraction techniques (X-ray and neutron). In vibrational spectroscopy the presence of a hydrogen bond A-H B is manifest by the following effects ... 

The physical properties of the pyridopyrimidines closely resemble those of their nearest A-heteroeyclie neighbors the quinazolines and the pteridines. Thus, in common with the pteridines, the presence of groups capable of hydrogen-bonding markedly raises the melting point and lowers the solubility. - The acid dissociation constants (pif a values) and ultraviolet absorption spectra of all four parent pyridopyrimidines have been determined by Armarego in a comprehensive study of covalent hydration in these heterocyclic systems. The importance of these techniques in the study of covalent hydration, and... 

This extensive hydrogen bonding bears on several aspects of the chemistry and applications of cellulose. For instance, being a semi-crystalline polymer, cellulose cannot be processed by the techniques most frequently employed for synthetic polymers, namely, injection molding and extrusion from the melt. The reason is that its presumably lies above the temperature of its thermal... 

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