An approach for modeling long range deuterium isotope effects on
C-13 chemical shifts, based on ab initio calculations of the differences of atomic charges between parent and deuterated molecules, is proposed. The total atomic charges were calculated for a series of molecules consisting of two benzene rings linked direct1yor via different groups, the so called binudear aromatic molecules: biphenyl, trans-stilbene, cis-stilbene, diphenylacetylene, diphenylbutadiene and their 4-deuterated derivatives. The basis sets used were: STO-3G, 6-31G, 6-31G** and MIDI. The deuteration site, i.e. C-D bond was simulated by the shortening of the corresponding C-H bond length by 0.012 A or by the reduction of CCH in-plane or out-of-plane bending angle by 1.3° and 2°, respectively. Calculations were performed
both for the X-ray geometries and 6-31G fully optimized geometries. The charge differences from Lčwdin but not from Mulliken population analysis, calculated with 6-31G, 6-31G**and MIDI basis sets, showed good correlaation with the measured longrange deuterium isotope effects on C-13 chemical shifts of C-atoms, five, six, seven, eight, ten and twelve bonds away from the site of deuteration. This correlation holds only for bond shortening but not for bending angle changes, which corresponds to the predominance of stretching over bending mode contributions to isotope effects. The vibrational analysis (6-31G) revealed no coupling of C-D stretching with the vibrations
of remote C-atoms. Therefore, we assume that the symmetry
breaking due to unsymmetrical deuterium substitution produces a small electric dipole moment by an unbalance in the normal vibrations which in turn polarizes n-electrons, giving rise to charge changes throughout the molecule. Calculated charge redistribution due to deuterium, experimentally also observed by microwave spectroscopy, might induce long-range deuterium isotope effects on C-13 chemical shifts in agreement with the postulated C-C n-bond polarization contribution to C-13 chemical shif
