Intermolecular shielding surfaces

The intermolecular shielding surface and the effects of electric fields on shielding (shielding polarizabilities and hyperpolarizabilities) are useful in the interpretation of long-range intramolecular effects and in intermolecular effects on shielding. 

Shielding Surfaces for Two Interacting Molecules. In the Maryland meeting Jameson and de Dios reported the first ab initio calculations of the rare gas pair intermolecular shielding surfaces for Ar2, ArNe, Ne2, and NeHe (55). With these shielding surfaces it was possible to calculate the second virial coefficient for nuclear shielding as a function of temperature, using the well established intermolecular potential functions V(R) for the pair. 

Fig. 8. The intermolecular shielding values (AS) due to the ring current in the N(1)H and N(9)H tautomers of purine (in a plane 3.4 A distant from the molecular surface).

There is substantial overlap between discussing intermolecular effects and shielding surfaces in the solid state. Identification of the nature of these interactions, however, can remain separate. Hydrogen bonding appears to be the major factor influencing and shielding tensors in the two... 

Jameson, C. J. de Dios, A. C. "The Nuclear Shielding Surface The Shielding as a Function of Molecular Geometry and Intermolecular Separation", Nuclear Magnetic Shieldings and Molecular Structure (NATO AST. Series C Mathematical and Physical Sciences, Vol. 386) Tossell, J. A., Ed., Kluwer the Netherlands, 1993, pp 95-116. 

The surface tension of most liquids decreases with increasing temperature in a nearly linear fashion (some metal melts being exceptional in this respect) and becomes very small in the region of the critical temperature, when the intermolecular cohesive forces approach zero. A number of empirical equations have been suggested which relate surface tension and temperature, one of the most satisfactory being that of Ramsay and Shields ... 

Very recently, an even more directly useful presentation of the results has been produced by a direct evaluation of the intermolecular nuclear shielding values for protons of purines. Figure 8 represents such values (in ppm) due to the ring current in the N(9)H and N(1)H tautomers of purine in a plane 3.4 A distant from the molecular surface of the purine (which is the mean intermolecular distance in complexes involving purines with other conjugated rings). The diagrams for the N(7)H and N(3)H tautomers are very similar to the ones for N(9)H and N(1)H. 

Several theories have been developed to account for the observed characteristics of the plasticization process Daniels has recently published a review of plasticization mechanisms and theories [8]. Although most mechanistic studies of plasticization have focused on PVC, much of this information can be adapted to other polymer systems. The lubricating theory of plasticization holds that plasticizers act as molecular lubricants to facilitate polymer chain movement when a force is applied to the plastic. It starts with the assumption that the unplasticized polymer chains do not move freely because of surface irregularities and van der Waals attractive forces. As the system is heated and mixed, the plasticizer molecules diffuse into the polymer and weaken the polymer-polymer interactions. Portions of the plasticizer molecule are strongly attracted to the polymer while other parts of the plasticizer molecule can shield the polymer chain and act as a lubricant. This reduction in intermolecular or van der Waals forces among the polymer chains increases the flexibility, softness, and elongation of the polymer. 

---