Criteria for H bonds

Type of bond XH---A Observed range of H - - A distances (A) Covalent R (neutron va Criteria for H-bonds based on sums of van der Waals radii Top line < W +WA — R —h Bottom line [Pg.30]

Operational Criteria for H Bond Formation. In these well ... 

Cannon tries to distinguish between dipole interaction and H bonding (349). In a more recent paper (347) he proposes criteria for H bond formation ... 

One of the important criteria for dihydrogen bonding is the loss of charge of hydrogen atoms participating in the interaction. This tendency is observed clearly in calculations of atomic charges. The charges on atoms H(6), H(7), H(8 ), and H(12 ) in the dimer strnctnre have been calcnlated as 0.254, 0.258,-0.022, and 0.01 e, respectively, versns 0.260, 0.260, 0.003, and 0.033 for the same atoms in a monomer structure. 

AIM topological analysis of the electron density performed for two complexes and for isolated components is shown in Table 6.15. The bond critical points found in the H H directions are characterized by the small electronic density with Pc = 0.002 and 0.009 au in the CILj- HF and SilLj- HF systems, respectively. The Laplacian, V pc, is also small but takes positive values in accordance with the AIM criteria for dihydrogen bonding. 

H bond formation has dramatically obvious and unusual effects on the IR and Raman spectra. The most prominent of the spectral changes occur in the region of v. near 3500 cm (3/u). These changes have properly eissumed the importance of qualitative criteria for H... 

In capillary electrophoresis (CE), several criteria can be applied to classify solvents [e.g., for practical purposes based on the solution ability for analytes, on ultraviolet (UV) absorbance (for suitability to the UV detector), toxicity, etc.]. Another parameter could be the viscosity of the solvent, a property that influences the mobilities of analytes and that of the electro-osmotic flow (EOF) and restricts handling of the background electrolyte (BGE). For more fundamental reasons, the dielectric constant (the relative permittivity) is a well-recognized parameter for classification. It was initially considered to interpret the change of ionization constants of acids and bases according to Born s approach. This approach has lost importance in this respect because it is based on too simple assumptions limited to electrostatic interactions. Indeed, a more appropriate concept reflects solvation effects, the ability for H-bonding, or the acido-base property of the solvent. 

Thus, the statishcal criteria for Eqs. (5) and (6) show that the approach provides a reasonably reliable method to calculate H-bond enthalpy and free energy. 

Usually aquatic toxicity of chemicals with general narcosis mechanism of action is described by the octanol/water partition coefficient [73]. However, log is a composite descriptor which has components of molecular volume and H-bond acceptor terms. Raevsky and Dearden [74] therefore used molecular polarizabihty (as a volume-related term) and the H-bond acceptor factor instead of log to model aquatic toxicity (log LC50) to the guppy for 90 chemicals with general narcosis mechanisms. This excellent correlation has statistical criteria better than that obtained for the same data using log Pofy, ... 

The three extreme cases of bonding which can be considered, correspond to the adsorption of hydrogen as H" " ions, covalently bonded H atoms or H ions. Dowden (109) has considered, from a theoretical standpoint, the factors favoring each of these types of adsorption. The criteria for negative ion formation are opposite to and readily distinguishable from the other two the distinction between the criteria for the formation of covalent bonds... 

The space arrangement of the important feature is also to be considered. Indeed, many SAR studies include length modifications in scaffold side-chains to estimate the influence of flexibility and steric tolerance of the binding site (see, for example, [29, 30]). Chirality of the model is another spatial criterion that is necessary if two enantiomers of different activity level are to be discriminated. At least four points are necessary, but not sufficient to assure enantio-selectivity. The use of directional features (H-bond acceptor/donor) or shape criteria can help in obtaining chiral models. 

---