Normal van der Waals interaction

Although interactions between vicinal atoms are nominally treated as nonbonded interactions, most of the force fields treat these somewhat differently from normal 1-5 and greater nonbonded interactions. HyperChem allows each of these nonbonded interactions to be scaled down by a scale factor <1.0 with AMBER or OPLS. For BlO-t the electrostatic may be scaled and different parameters may be used for 1 van der Waals interactions. Th e AMBER force field, for exam p le, norm ally u ses a scalin g factor of 0.5 for both van der Waals and electrostatic interactions. 

The 1-4 van der Waals interactions cannot be scaled in CHARMM but in newer CHARMM parameter sets some atom types (usually united atoms) use different parameters for 1 interactions. These are specified for BlO-t in the file pointed to by the 6-12Atom 14VDW entry, usually called nbd.txt(dbf). If an atom type is absent in the 6-12Atom 14VDW file, the normal parameters are used. The format of the 6-12Atom 14VDW file is also specified by the 6-12AtomVDWFormat entry for the parameter set. 

FlC. 20.—Parallel packing arrangement of 6-fold, curdlan I (17) helices, (a) Stereo view of two unit cells approximately normal to the 6c-plane. The helix is stabilized by intrachain 4-0H---0-5 hydrogen bonds. There are only van der Waals interactions between the helices. 

The surface force apparatus (SFA) is a device that detects the variations of normal and tangential forces resulting from the molecule interactions, as a function of normal distance between two curved surfaces in relative motion. SFA has been successfully used over the past years for investigating various surface phenomena, such as adhesion, rheology of confined liquid and polymers, colloid stability, and boundary friction. The first SFA was invented in 1969 by Tabor and Winterton [23] and was further developed in 1972 by Israela-chivili and Tabor [24]. The device was employed for direct measurement of the van der Waals forces in the air or vacuum between molecularly smooth mica surfaces in the distance range of 1.5-130 nm. The results confirmed the prediction of the Lifshitz theory on van der Waals interactions down to the separations as small as 1.5 nm. 

The relation between UV- and PES-spectra is quite similar as found for planar aromatic compounds. All chemical shifts in NMR-spectra can well be explained by normal ring-current effects and Van der Waals interactions. Polarographic data do not deviate from those for planar compounds. 

In this last section of the chapter other van der Waals interactions in which —H bonds and/or carbon atoms or, more correctly, electron density, are involved will be discussed. —H- - interactions are considered to be present when the hydrogen atom of a —H bond interacts with an aromatic ring in a nearly normal fashion and with H centroid distances shorter than about 3 A. Interactions between clouds of electron density of neighboring nearly parallel aromatic rings that lead to intercentroid distances below 3.5 A are considered as effective - interactions. Although these contacts are not reported very often, they may play an important role in the packing forces of metal complexes. 

The crystals of dithiazolyl radical 28 consist of planar (to within 0.03 A) undimerized radicals aligned in a slipped ji-stack arrangement running parallel to the x axis <2002CC1872>. There are no S- S intermolecular interactions between the radicals that are inside the normal van der Waals contact of 3.6 A. The closest S- -S interactions outside this range are the head-to-head contact (3.843 A), the head-to-tail contact (3.626 A), and the jt-stacking contact (3.707 A). 

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