Effects of the Atomic Basis, Endgroups, and Empirical Force Field

Effects of the Atomic Basis, Endgroups, and Empirical Force Field 

As in the other systems, the absolute value of the chemical shift is not well reproduced by the quantum chemical calculation. This is an artifact of the small atomic basis as can be seen from Fig. 47, where the same set of geometries has been recalculated with different atomic bases (II, IF, DZ, and another double zeta basis DZ ). It is seen, that the geometry-induced variation is well reproduced by the small atomic basis sets whereas the absolute values change with the basis. 

Qualitatively, the two force fields yield the same results as can be seen from the comparison of the simulated spectra (Fig. 48). The qualitative results, upon which we put the greatest emphasis in the present study, are unaffected by the choice of the force field. A closer look, however, reveals that there is no 1 1-relation between AMBER chemical shifts and CVFF chemical shifts (Fig. 49). Especially for the methyl region a considerable scatter is observed. Thus, for finer details, the pecularities of the force field might be important. In fact, the correlation of the chemical shift with the lengths of the adjacent carbon bonds (cf. Fig. 45) is much weaker in the AMBER geometries. Again, to get rid of these uncertainties, reliable ab initio geometries are needed which will become available with the advent of more powerful computers. 

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