Van der Waals molecular

Initial attempts to correlate the affinity of ligands toward cucurbituril using independently estimable parameters such as van der Waals molecular surface area or molecular volume of the guests were relatively unsua sful. The difficulty apparently is that the interior of the receptor has a definite shape and distribution of polarity, so that complementarity between cucurbituril and its ligand depends more subtly upon structure of the bound entity. Consequently we opted for an empirical treatment of our data, which would yield an indication of how particular regions of the interior of cucurbituril interact with ligands. 

Van der Waals molecular volume is the volume contained by van der Waals surface of a molecule which is defined as the surface of the intersection of spheres each of which is centered at the equilibrium position of the atomic nucleus with van der Waals radius of each atom 62). Since the van der Waals radius of an atom is the distance at which the repulsive force balances the attraction forces between two non-bonded atoms, van der Waals molecular volume is regarded as the volume impenetrable for other molecules with thermal energies at ordinary temperatures. 

The combined effect of attraction and repulsion forces has been treated by many investigators in terms borrowed from theories of colloidal stability (Weiss, 1972). These theories treat the adhesion of colloidal particles by taking into account three types of forces (a) electrostatic repulsion force (Hogg, Healy Fuerstenau, 1966) (b) London-Van der Waals molecular attraction force (Hamaker, 1937) (c) gravity force. The electrostatic repulsion force is due to the negative charges that exist on the cell membrane and on the deposition surface because of the development of electrostatic double layers when they are in contact with a solution. The London attraction force is due to the time distribution of the movement of electrons in each molecule and, therefore, it exists between each pair of molecules and consequently between each pair of particles. For example, this force is responsible, among other phenomena, for the condensation of vapors to liquids. 

With respect to the molecular interactions the simplest asymmetric films are these from saturated hydrocarbons on a water surface. Electrostatic interaction is absent in them (or is negligible). Hence, of all possible interactions only the van der Waals molecular attraction forces (molecular component of disjoining pressure) can be considered in the explanation of the stability of these films. For films of thickness less than 15-20 nm, the retardation effect can be neglected and the disjoining pressure can be expressed with Eq. (3.76) where n = 3. When Hamaker s constants are negative the condition of stability is fulfilled within the whole range of thicknesses. 

Liquid-like densities of supercritical gases result in liquid-like solvent powers this property and faster diffusion characteristics due to low-gas viscosity make supercritical fluids attractive extraction agents. Solubility of substances in supercritical gases derives from van der Waals molecular attractive forces and increases with increasing pressure at a constant temperature. The temperature influences the solution equilibria in a more complicated way than does the pressure. Compounds can be selectively dissolved by changing the density of the gas, i.e., pressure and temperature conditions. 

Bondi volume -> molecular surface (O van der Waals molecular surface) bond number (B) ( edge counting bond count)... 

The van der Waals surface area 5Avdw (also known as Total molecular Surface Area, TSA) is then defined as the exterior surface of the union of all such spheres in the molecule, i.e. the area of the van der Waals molecular surface. 

Analogously, the van der Waals volume (Vvdw)> also called intrinsic molecular volume Vi, is the volume of the space within the van der Waals molecular surface. The... 

The parent phenazine (P) is a typical van der Waals molecular crystal. Its CT complexes have long been known, especially with its 5.10-dihydrogenated form, H2P in Table 1, the so-called phenazhydrinesP acts as a weak n-acceptor... 

In the hard-sphere model [Ciubotariu, Medeleanu et al., 2004], the van der Waals molecular surface SA " (also known as Total molecular Surface Area, TSA) is then defined as the exterior surface of the union of all such spheres in the molecule, that is, the area of the van der Waals molecular surface. It can be calculated by generating a uniform grid around each sphere of the molecule atoms, followed by the counting of the number of points generated on the surface n, consisting in the points that satisfy at least one of the following equalities ... 

Ciubotariu, D., Gogonea, V. and Medeleanu, M. (2001) van der Waals molecular descriptors. Minimal steric difference, in QSPR/QSAR Studies by Molecular Descriptors (ed. M.V. Diudea), Nova Science, Huntington, NY, pp. 281-361. 

Theoretical and van der Waals Molecular Volumes" Calculated Mean Number of Electrons... 

The curves refer to H2O (a), NHs(b), aziridine (c) (solid curves). For ease of visualization, rj is reported on a logarithmic scale, while distances are measured in fractions of the medium van der Waals molecular radius of the molecule considered. The rj values rapidly decrease with increasing distance and, in the case of the two small molecules, they are less than 10 % at a distance equal to 2 VdW. With aziridine, it is necessary to go to distances larger than 1.4 i VdW in order to have comparable rj values. Fig. 48 gives analogous results obtained from a many-center expansion of the b) type (dashed curves). As was to be expected, at... 

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