IGLO calculations, shift tensors

The principal values of the calculated chemical shift tensors are compared to experimental values and IGLO ab initio calculations taken from literature in Table I. The experimental and most of the calculated values are given in the order 5, / 522 / 833. However in cases where the orientation of the principal axes was known from symmetry considerations or ab initio calculations, and this order was not adequately reproduced in our calculations the components were sorted according to their corresponding tensor orientation in the local coordinate system. In many of these small molecules the tensor axes are defined by the symmetry of the molecules. In other cases the axes are at least loosely defined by nearly symmetric local bond surroundings. For example the 533 component of the CH3 chemical shift tensor is close to the direction of the non CH bond or the 833 component of a sp2 carbon is perpendicular to the plane of the 71 system. 

The earliest calculations of the chemical-shift tensors for some purine bases employed an IGLO method of the HF type." The general findings agree well with the results obtained recently by a combination of experimental solid-state measurements and GIAO calculations. ... 

In fact, the IGLO method provides the chemical shift tensors for all atoms in a single calculation, but only the values of the central atoms will be used for the simulation. 

Using the capacity of IGLO to calculate the full chemical shift tensor, first tentative results on the chemical shift anisotropy in atactic poly(propylene) indicate that the tensorial character of the chemical shift may be used as source of structural information, too. 

After the introduction of IGLO, molecules of chemical interest became accessible to ab initio calculations. Early applications included the ab initio derivation of an increment system for hydrocarbons, a definite confirmation of the non-classical structure of the 2-norbomylcation, the assignment of the shift tensors of cyclopropane, bicyclobutane, and [1.1.1]-propellane, and the prediction of the F shift in SF2 prior to its measurement. These applications have been reviewed (see NMR Chemical Shift Computation Structural Applications). 

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