In the first part, the theory of molecules in a magnetic field is reviewed with special attention to chemical shifts. The central role of the current density is pointed out. The gauge origin problem and its solution by means of the IGLO method is explained. Various methods for the calculation of chemical shifts are compared and the IGLO method is presented in detail both as far as computations and interpretation of the result are concerned.

The IGLO-Method: Ab-initio Calculation and Interpretation of NMR Chemical Shifts and Magnetic Susceptibilities

A theory of NMR shielding tensors is derived from Ramsey’s expressions, using the framework of the random phase approximation (RPA) and localized molecular orbitals. By expanding angular momentum terms relative to a local origin for each orbital and using properties of the RPA solutions, we arrive at shielding expressions that contain no reference to an overall gauge origin and that lead to appropriate damping of basis set errors in contributions from distant groups. The expressions allow an analysis of the shielding into intrinsic bond and bond–bond coupling contributions. The resulting method is a variant of the coupled‐Hartree–Fock approach. Ab initio results are presented for 13C isotropic shieldings and shielding tensors in a number of organic molecules ranging in size up to benzene. These results agree very well with experiment, for both isotropic shieldings and principal tensor components, even in double‐zeta basis sets. For some low symmetry molecules we study the asymmetry of the shielding tensor...

Localized orbital/local origin method for calculation and analysis of NMR shieldings. Applications to 13C shielding tensors

Conventions for the reporting of nuclear magnetic shielding (or shift) tensors suggested by participants in the NATO ARW on NMR shielding constants at the University of Maryland, College Park, July 1992.

The conformation and motion of the choline head group in lipid bilayers above and below the gel-to-liquid crystal transition point are studied by means of deuterium and phosphorus magnetic resonance. For this purpose dipalmitoyl-3-sn-phosphatidylcholine is selectively deuterated at various positions on the choline and glycerol constituents. The residual deuteron quadrupole couplings and the phosphorus chemical-shift anisotropy of the corresponding lipid-water mixtures yield quantitative information on the segmental motions. The choline methyl group is only slightly hindered in its movement, but the motional freedom becomes increasingly restricted the closer the segment is located to the glycerol backbone. The average value of the OC-CN bond rotation angle changes with temperature. Increasing the temperature rotates the choline methyl group into the vicinity of the phosphorus atom. The choline group as a whole is thus characterized by a flexible, temperature-dependent structure. Its orientation in space is not fixed, either parallel or perpendicular to the bilayer surface. Instead all segments execute angular oscillations with varying degrees of restriction around the normal on the bilayer surface. The gel-to-liquid crystal phase transition at 41 degrees is clearly reflected in the deuterium and phosphorus resonance spectra of the choline moiety, while no change is observed at 34 degrees. The calorimetric pretransition at 34 degrees seems not to be associated with a conformational change in the choline group.

Conformation and motion of the choline head group in bilayers of dipalmitoyl-3-sn-phosphatidylcholine.

Calculations of chemical shifts have been carried out using “locally dense” basis sets for the resonant atom of interest, and smaller, attenuated sets on other atoms in the molecule. For carbon, calculations involving a 6-311G(d) triply split valence set with polarization on the resonant atom and 3-21G atomic bases on other heavy atoms result in good agreement with experiment, and are virtually identical to those found employing the larger basis on all atoms. For species such as nitrogen, oxygen, and fluorine where standard balanced basis sets do not agree well with experiment, use of attenuated sets fail as well. The use of locally dense basis sets permits calculations previously impractical, and the successful application to carbon suggests that the chemical shift is most dependent on the local basis set, and less so on whether or not a balanced or unbalanced calculation is being carried out.

Locally dense basis sets for chemical shift calculations

Magnetic Shielding and Susceptibility Anisotropies

Fluorine shielding anisotrophy in CFCl3

The fluorine‐19 shielding anisotropies of CF3Cl, CF3Br, CF3I, and CF3CCl3 have been determined from analyses of fluorine NMR spectra in partially ordered smectic liquid crystal solutions. The high degree of orientation and increased solubility produced in the smectic phase leads to measurements of σ∥−σ⊥ for each of these systems to relatively high accuracy. The anisotropies obtained for the trifluorohalomethanes are found to increase in the order: CF3Cl, −13.4±0.5 ppm; CF3Br, 2.7±0.3 ppm; CF3I, 16.8±0.2 ppm. The quantity σ∥ is approximately constant for each of the CF3X molecules studied. This feature permits an estimate of the shielding and spin–rotation constants of CF4.

Bernard R. Appleman

Papers

Magnetic Shielding and Susceptibility Anisotropies

Fluorine shielding anisotrophy in CFCl3

Fluorine‐19 FT–NMR studies of CF3‐containing systems dissolved in smectic liquid crystal solutions

Carbon-13 shielding tensors of CCl3X systems from studies of liquid-crystal solutions