Hydrogen bond Electrostatic interpretation

This interpretation is based on the fact that the variation of the overall charge which occurs when the metal complex undergoes a redox process influences the number of solvent molecules surrounding the complex (which are hence electrostatically orientated), as well as the number of solvent molecules which form hydrogen bonds with the complex. The change in the number of solvent molecules near the metal complex can be translated into an increase or decrease of the molecular order of the entire system and, hence, into an increase or decrease of entropy. 

In 1959, Eberson (1959, 1992) found that a family of derivatives of succinic acid shows a remarkably large negative cooperativity, i.e., gjj < < 1, which is difficult to explain on the basis of electrostatic theories only. We shall discuss these compounds in Subsection 4.8.6. At present, there is no satisfactory molecular interpretation of these findings. One of the more popular ideas, originally suggested by Jones and Soper (1936) and further elaborated upon by McDaniel and Brown (1953), is that an intramolecular hydrogen bond would facilitate the first dissociation of the proton, i.e., Kj becomes smaller (or Kj becomes larger). Also, the second proton will dissociate with more difficulty. The net effect would be a... 

In view of these difficulties, experimental evidence should at least be given either to support this interpretation of the polarization arising from hydrogen bond formation or to sbow its need owing to some inadequacy of a (logically applied) purely electrostatic interpretation of the hydrogen bond. 

A. Burawoy (Manchester) There is no unambiguous experimental evidence (from infrared spectra or other properties) that the bridge hydrogen atoms in diborane are in the centre of the B—H. .. B bonds formed (cf. Burawoy A. Nature Land. 1946 328 Lord Nielson J. Chem.Phys. 1981 19 1) The criticisms against the interpretation of the diborane bridge as an electrostatic hydrogen bond (Cartnell Fowles Valency and Molecular Structure p. 223 Butterworth London 1956) do not take sufficient account of the exceptionally... 

Alkylation Alkylation of the phenylindanone 31 with catalyst 3a by the Merck group demonstrates the reward that can accompany a careful and systematic study of a particular phase-transfer reaction (Scheme 10.3) [5d,5f,9,36], The numerous reaction variables were optimized and the kinetics and mechanism of the reaction were studied in detail. It has been proposed that the chiral induction step involves an ion-pair in which the enolate anion fits on top of the catalyst and is positioned by electrostatic and hydrogen-bonding effects as well as 71—71 stacking interactions between the aromatic rings in the catalyst and the enolate. The electrophile then preferentially approaches the ion-pair from the top (front) face, because the catalyst effectively shields the bottom-face approach. A crystal structure of the catalyst as well as calculations of the catalyst-enolate complex support this interpretation [9a,91]. Alkylations of related active methine compounds, such as 33 to 34 (Scheme 10.3), have also appeared [10,11]. 

The next step was made by Klebe et al. [50]. Two 3D-QSAR methods were applied to get three-dimensional quantitative structure-activity relationships using a training set of 72 inhibitors of the benzamidine type with respect to their binding affinities toward Factor Xa to yield statistically reliable models of good predictive power [51-54] the widely used CoMFA method (for steric and electrostatic properties) and the comparative molecular similarity index analysis (CoMSlA) method (for steric, electrostatic, hydrophobic, hydrogen bond donor, and hydrogen bond acceptor properties). These methods allowed the consideration of various physicochemical properties, and the resulting contribution maps could be intuitively interpreted. 

In addition to the steric and electrostatic descriptors, it was proposed to use other 3D molecular fields characterized by the sampling over the rectangular grid - in particular, the hydrophobic field/molecular lipophilic potential (MLP), ° hydrogen bonding and quantum-chemical parameters, e.g., orbital densities.Descriptor selection techniques are often recommended to enhance the stability, predictivity and interpretability of the CoMFA models. ... 

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