Bond polarization theory tensors

Extremely Fast Calculation of UC Chemical Shift Tensors Using the Bond Polarization Theory... 

The semi-empirical bond polarization theory provides a fast method for the calculation of 13C chemical shift tensors with an accuracy level comparable to ab initio methods for most molecules. There are no restrictions in the size of the molecular system, so that the method can be applied to large molecules or crystal lattices. Chemical shift calculations were carried out for several small molecules and compared to ab initio results. Additionally the calculated chemical shift tensor for the fullerence Qo is presented. 

The authors of this work proposed a semi-empirical scheme for the calculation of 13C chemical shift tensors based on the bond polarization theory (5). This method can reproduce 13C chemical shift tensors with deviations from experiment comparable to the errors of the ab initio methods. One major advantage is that the calculations can be performed for large molecular systems with hundreds of atoms even on a PC computer. In contrast to the ab initio method a set of empirical parameters is needed for the calculations. In the case of the bond polarization theory these parameters can be estimated directly from experimental chemical shifts solving a set of linear equations. 

The semiclassical theories described so far are aimed mostly at interpreting the experimentally determined vibrational absorption intensities of molecules in terms of quantities associated with the charge distribution and dynamics. Fewer attempts have been made for quantitative predictions of intensities based on transferable intensity parameters. Successful predictions are difficult to achieve because transferability properties are not so well expressed as in the case of force constants. This is determined by a number of factors (1) the high sensitivity of vibrational intensities associated with particular modes to changes in molecular environment (2) the physical limitations of the approximate point-charge models and (3) mathematical difficulties in applying non-approximate models such as polar tensors or bond polar parameters for larger molecules. 

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