Isotope effects on nuclear magnetic shieldings calculated by including zero-point vibration corrections: the VMF approach

As a minimal dynamic correction the 'zero-point vibration', ZPV, was included in the ab initio calculation of the isotropic magnetic shielding of C-13, F-19, Si-29, P-31, and (CI)-C-35 in some molecules including the references of NMR spectroscopy (f.i. CFCl3, CH3-NO2, and TMS). In contrast to most previous works the ZPV correction is not based on gradients of internal coordinates, but uses derivatives obtained by 'vibration mode following' (VMF, the coordinates of the gradients describing the magnetic shielding hypersurface are the normal coordinates of the ground state vibration motion). The vibrational corrections obtained at the applied DFT level of theory are slightly smaller than those reported for MP2 calculations. Isotope effects computed with the ZPV/VMF correction are in reasonable agreement with experimental data. Contributions from individual vibration modes to the zero-point correction show that rotational motions have a large effect. (C) 2003 Elsevier B.V. All rights reserved.