LOWDIN ATOMIC CHARGES FOR MODELING LONG-RANGE DEUTERIUM-ISOTOPE SHIFTS IN C-13 NMR-SPECTRA OF BINUCLEAR AROMATIC-COMPOUNDS

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 con sisting of two benzene rings linked directly or via different groups, the so called binuclear aromatic molecules: biphenyl, trans-stilbene, cis-stilbene, diphenylacetylene, diphenylbutadiene and their I-deutera ted 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 shorten ing of the corresponding C-H bond length by 0.012 Angstrom or by the r eduction of CCH in-plane or out-of-plane bending angle by 1.3 degrees and 2 degrees, respectively. Calculations were performed both for the X-ray geometries and 6-31G fully optimized geometries. The charge diff erences from Lowdin but not from Mulliken population analysis, calcula ted with 6-31G, 6-31G* and MIDI basis sets, showed good correlation w ith 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 sho rtening but not for bending angle changes, which corresponds to the pr edominance of stretching over bending mode contributions to isotope ef fects. The vibrational analysis (6-31G) revealed no coupling of C-D st retching with the vibrations of remote C-atoms. Therefore, we assume t hat 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 pi-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 sh ifts in agreement with the postulated C-C pi-bond polarization contrib ution to C-13 chemical shifts.