Intermolecular Effects on Nuclear Shielding

In the presence of a paramagnetic species, pseudocontact shifts can provide additional restraints since these shifts are both distance and orientation dependent. Gaponenko et a/. have attached a Co + paramagnetic probe to the N-terminal domain of the protein STAT4. The attachment is achieved by modifying a cysteine residue into a thiol-reactive chelator. With the additional restraints derived from pseudocontact shifts, the NMR structures are improved based on the rmsd value of the family of structures obtained. This approach is likewise illustrated in the work of Barbieri et on the protein Calbindin D9k. Here, the ions employed are Ce, Yb , and Dy .  

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 

A variety of methods have been applied to calculations of shielding for 

Comparative calculations of the shielding constants in water dimer reveal the following. SCF calculations with a medium-size basis set (as in 6-31++G ) are reliable for proton shieldings. On the other hand, for 0, the requirements are much more stringent. First, larger basis sets, at least triple- 

Gas phase NMR measurements at various densities have long been applied to provide the shielding for an isolated molecule that permits closer comparisons with theoretical values. Jackowski et have done gas phase 

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The TB MO calculation on the 15N chemical shift of polypyrrole in the solid state allows useful information to be extracted from the observed spectra, namely that the two peaks obtained are correctly assigned to the quinoid and aromatic structures.(l 1,38) ( The quinoid structure is closely to the electric conductivity.) A decrease in the band gap leads to a downfleld shift. These results on conducting polymers demonstrate that the chemical shift behavior provides information about the band gap which, in turn, is a measure of the electric conductivity. It can be said that TB MO calculations offer useful perspectives in interpreting the results of NMR nuclear shieldings in polymers, both in terms of the structure in the solid state and in understanding the effect of intermolecular interactions on nuclear shieldings. The latter are shown to operate through the electronic structures of the polymers considered. 

Analysing these data, the most interesting result is the possibility of obtaining experimental values of 33S chemical shift at infinite dilution, i.e. a measure of nuclear shielding in the absence of intermolecular interactions. These values can be compared with the ones obtained in condensed phases, providing an estimate of the effect of intermolecular interactions on nuclear shielding. 

Solvent effects on nuclear magnetic properties are well known, and have been studied for a long time. Both the NMR shielding constant and the nuclear spin-spin coupling constant depend on the electronic structure of the whole system. This means that both are sensitive to the weak intermolecular interactions between solute and solvent molecules. 

Older methods based on solubility changes upon complexation, or on partition coefficients between aqueous solutions and hydrophobic solvents, have been shown to lead to gross errors as compared to spectroscopic techniques (40) that are also less sensitive to the formation of emulsions, micelles, and so on. The traditional X-ray analysis of inclusion compounds is of limited significance for establishing complexation between lipophilic substrates and macrocyclic host, particularly in aqueous solution. The essential hydrophobic driving force for complexation, of course, is nonexistent in the crystal. The future development of NMR methods including shielding calculations and measurements of intermolecular nuclear Overhauser effects is expected to provide the most reliable information on intercavity inclusion complexes in solution as the basis for catalytic applications. 

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