Molecule centered moments, intermolecular

Fig. 12.5 Illustration of the orientation angles used in the Stockmayer intermolecular potential. Molecule j consists of atoms A and B, and molecule i consists of atoms C and D. The vector ry runs from the center of mass of molecule i to the center of mass of molecule j. The vector JTJ gives the orientation and magnitude of the dipole moment of molecule i, with a similar definition for JTj. A ghost copy of molecule j is shifted to left to more easily visualize the orientation angle ifr. See Eqs. 12.11 to 12.13 and accompanying text for definition and description of these angles.

The calculated Euler angles (a = 50°, /3 = 60°, and y = 40°), which determine the relative orientation between the principal-axis system of the rotational diffusion tensor and that of the moment of inertia tensor, indicate a significant shift between the two tensors. This result is expected because of the fact that molecule 31 contains a number of polar groups and hydrogen-bonding centers, leading to strong intermolecular interactions. 

A molecule with its center of positive charge in a location different from its center of negative charge is said to be a polar molecule or to have a dipole or a dipole moment. One effect of such asymmetry of charge is intermolecular attraction (Section 13.6). Where do these dipoles come from Do all molecules with polar bonds (Section 13.3) have dipoles ... 

Indeed, the dipole moment appearing in eqn (5.45) is not the dipole moment in vacuum but an effective dipole moment found by taking into account the internal field correction. In the isotropic medium this is pa = [(e + 2)/3 p°J which yields precisely eqn (5.69) if we take into account eqn (5.68). This conclusion is valid for the local centers of any nature in the nonconducting medium. We mean here media in which the intermolecular interaction does not violate the neutrality of molecules. The specific effects found in ionic crystals are discussed by Smith and Dexter (27). 

Static ( eie)> polarization ( poi), van der Waals ( vdw)> and total interacting energies were calculated when a fluorine ion approaches the hexafluoroben-zene molecule perpendicular to the center of the aromatic ring. The obtained results point out the importance of the polarization component, which is similar to the electrostatic term in the 2.0 to 3.0 A range, the equihbrium distance for the fluoride aryl centroid complex is 2.6 A. The authors mention the importance of the quadrupole moment for understanding intermolecular interactions of aromatic system but they do not elaborate on this issue in this work. 

Only pure dispersion forces (second-order perturbation terms of pure Coulomb interactions). Thus, it is assumed that the interactions between permanent electrical dipole (and higher harmonics) moments, as intermolecular interactions, are important only for the arrangements of the centers of gravity of the molecules and for the energy content of the isotropic distribution along the axes however, they are not particularly important for the orientational order. 

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