Chlorine atoms, interaction between

The range of systems that have been studied by force field methods is extremely varied. Some force fields liave been developed to study just one atomic or molecular sp>ecies under a wider range of conditions. For example, the chlorine model of Rodger, Stone and TUdesley [Rodger et al 1988] can be used to study the solid, liquid and gaseous phases. This is an anisotropic site model, in which the interaction between a pair of sites on two molecules dep>ends not only upon the separation between the sites (as in an isotropic model such as the Lennard-Jones model) but also upon the orientation of the site-site vector with resp>ect to the bond vectors of the two molecules. The model includes an electrostatic component which contciins dipwle-dipole, dipole-quadrupole and quadrupole-quadrupole terms, and the van der Waals contribution is modelled using a Buckingham-like function. 

The archetype of the ionic ceramic is sodium chloride ("rocksalt"), NaCl, shown in Fig. 16.1(a). Each sodium atom loses an electron to a chlorine atom it is the electrostatic attraction between the Na ions and the CF ions that holds the crystal together. To achieve the maximum electrostatic interaction, each Na has 6 CF neighbours and no Na neighbours (and vice versa) there is no way of arranging single-charged ions that does better than this. So most of the simple ionic ceramics with the formula AB have the rocksalt structure. 

London23 has treated the case of the attractive force between anisotropic molecules on the dipole-dipole interaction basis as well as on the monopole basis mentioned above. The small anisotropy found for the chlorine atom makes the dipole-dipole formulation appropriate. For the symmetrical orientation in the Cl2 molecule the London formula is... 

A certain interaction between tin and chlorine atoms through ji- and a-electron systems has been confirmed by spectral structural studies of chloroorganotin adducts and model compounds35). 

Figure 5.3 Steric interaction between the ortho chlorine atoms in biphenyl prevents the two rings from adopting a coplanar orientation.

Here there is no formal electronic bar to interaction between the electrons, i.e. pairing to form the diamagnetic species (137) but this does not in fact happen, because the bulky chlorine atoms in the o-positions prevent the benzene rings from attaining a conformation close enough to coplanarity to allow of sufficient p orbital overlap for electron-pairing to occur. 

Zheng et al. [1] postulated that the driving force for placing Zr and B on the same carbon might stem from interactions between the zirconium and oxygen or boron and chlorine atoms. However, an X-ray analysis of 22 revealed that there are no intra- or intermo-lecular interactions between any of these atoms [35]. Compound 22 was also unambiguously characterized by 1H-1H double quantum filtered COSY [36] and 13C-1H heteronuc-lear chemical shift correlation NMR spectroscopy [37,38]. Considerable differences in the chemical shifts of the diastereotopic Cp groups were found in both the XH and 13C NMR spectra. The NMR study unequivocally showed that the methine proton was at-... 

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