PROXIMITY EFFECTS ON THE NUCLEAR MAGNETIC SHIELDING TENSOR .1. AN ABINITIO STUDY OF O-17 AND F-19 CHEMICAL-SHIFTS IN THE PROXIMITY OF A METHYL-GROUP

The deshielding effect produced by repulsive interactions between prox imate groups was studied from a theoretical point of view, calculating the magnetic shielding constants for the following nuclei: O-17 in di -coordinated and carbonyl oxygen atoms; F-19 and C-13. The ''super mol ecule'' approach was used to study the proximity effect in the followi ng three model systems: water-methane; formaldehyde-methane; methyl fl uoride-methane, calculating the chemical shieldings for various interm olecular distances. Calculations were carried out using the GIAO appro ach with an ab initio ground state wavefunction calculated with the 6- 31G* basis set. Linear correlations are found between the intermolecu lar effects on the magnetic shielding constants and the interaction en ergies, yielding an increase in the deshielding effect when increasing the repulsion between the two molecules.