Shielding intermolecular effects

The paramagnetic term, apara, derives from the excitation of p-elec-trons by the external held, and its impact is opposite to that of diamagnetic shielding. The term,, derives from the effect of neighboring groups, which can increase or decrease the held at the nucleus, a can also be affected by intermolecular effects, in most cases deriving from interaction of the solvent. 

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. 

M. Kubiszewski, W. Makulski and K. Jackowski, Intermolecular effects on spin-spin coupling and magnetic shielding constants in gaseous difluoromethane.. Mol. Struct., 2004, 704,211-214. 

The goal of the present work is to obtain a consistent structural model for a microemulsion system. In particular, we are interested in carrying this model down to the molecular level so that the intermolecular effects which are responsible for the stability of these systems can be elucidated. We have studied the system consisting of water, SLS and MMA with and without n-hexanol or n-pentanol. We have determined the phase boundaries of the isotropic microemulsion and Lj phases and determined how these are affected by surfactant concentration and alcohol chain length. Measurements were also made of the vapor pressure of MMA over these systems to determine the concentration of MMA in the water surrounding the microemulsion droplets. From these data, the energetics of transfer of the MMA from aqueous to micellar solution were determined. Finally, a 1,C NMR chemical shielding study was performed to find how the MMA and the alcohol were distributed within the microemulsion. 

The 13C chemical shielding study, together with the headspace results, allowed a somewhat more detailed picture of these systems to be drawn. The chemical shielding experienced by a nucleus can be directly related to its surrounding electronic environment. In the absence of susceptibility or anisotropy effects, the measured chemical shift is an accurate expression of this chemical shielding. The dependence of the chemical shift on intermolecular effects can be used to sense how a molecule s environment changes when it is placed... 

Other studies during this reporting period that involve intermolecular effects on shielding include a correlation between H, N, C shifts and acid strengths of Bronsted acidic zeolites, as probed by an acetonitrile molecule. From this study, it is unraveled that the isotropic shifts of H and N in acetonitrile correlate very well with acid strength while C shifts are more sensitive to the... 

Yonker, C.R. Palmer, B.J. Investigation of C02/Fluorine Interactions through the Intermolecular Effects on the IH and 19F Shielding of CH3F and CHF3 at Various Temperatures and Pressures. J. Phys. Chem. A 2001, 105, 308. 

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... 

Irrespective of the subtle differenee in the all-trans or trans-syn structures that PDMS possesses at a local level, its overall structure is believed to have a regular coiled helical conformation where the backbone Si-0 bonds are effectively shielded from outside influence by the umbrella-type protection afforded by the lipophilic sheath of methyl groups [35]. Since the methyl groups are nonpolar, intermolecular interactions are minimiun and this also accounts for the low melting temperature seen for poly(dimethylsiloxane) (-40 °C). 

Conformational studies on poly(dimethylsiloxane) suggests that it adopts an all-trans (or trans-syn) local structure and that it has an overall regular helical coiled conformation. Within these coils the inner Si-0 backbone is effectively shielded by the outer lipophilic methyl side-groups. Lack of intermolecular interaction between the methyl groups also accounts for low crystallinity in these polymers. 

---