Chemical Shifts with Other Molecular Properties

CORRELATIONS OF CHEMICAL SHIFTS WITH OTHER MOLECULAR PROPERTIES... 

As already outlined in Chapter 3, the variation in 33S chemical shift with molecular structure has been quite extensively studied in sulphinyl, -SO-, sulphonyl, -S02-, and sulphonic, — SO3, moieties. It has been found that the 33S chemical shift is much more sensitive to structural variation than the chemical shift of other nuclei, such as 13C, 15N or 170. For this reason, 33S NMR is a powerful method for structural elucidation and in many cases a unique probe for assessing the properties of the electronic distribution around the sulphur atom. 

On the other hand, several attempts have been made to connect chemical shifts with charge distributions given by semiempirical methods whose parameters have been adjusted to other molecular properties. A linear condition of the type a = — AQc — BQn+C has been found to be quite satisfactory for the chemical shift of the proton in aminoacids >. For 0 the results of similar attempts have not been equally encouraging ), the reason possibly being the charge definition... 

The convergence properties of the density matrix-based equations, i.e., the number of iterations to converge P , are similar to the ones encountered for a solution in the MO space, so that the advantage of using sparse multiplications within the density-based approach allows us to reduce the scaling property of the computational effort in an efficient manner. In this way, NMR chemical shift calculations with linear-scaling effort become possible and systems with 1000 and more atoms can be treated at the HF or DFT level on today s computers.Extensions to other molecular properties can be formulated in a similar fashion. 

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