Lowdin Atornic Charges for Modeling Long Range Deuteriurn Isotope Shifts in C-13 NMR Spectra of Binuclear Arornatic Compounđs

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

The Synthesis of some B6 Vitamin Halophosphates

A series of new halo-derivates of pyridoxine-3-O- or 5’-0-phosphates and pyridoxal-3-O- or 5’-0-phosphates have been synthesized. In the reaction of partially protected pyridoxine and pyridoxal (3-5) with phosphorusoxychlo- ride in the presence of triethylamine, the following dichlorophosphates were obtained: 3,4’-0-isopropylidenepyridoxine-5’-0-dichlorophosphate (6), 4,5’-0- isobutilidenepyridoxine-3-O-dichlorophosphate (7), and monoethylacetal-3-O- dicholorophosphate (8). Dichlorophosphates 6-8 reacted with NaF under catalytic action of 18-crown-6-ether, giving the corresponding difluorophos- phates 14-16. The 3,4’-0-isopropyiidene pyridoxine-5’-0-difluorophosphate (14), 4’,5’-isobutilydene-3-0-difluoro-phosphate (15) and monoethylacetal-3-O-di- fluorophosphate (16), in the reaction with aniline, yielded the corresponding monofluorophosphates in the form of aniline salts (17-19). The direct fluor- ination of pyridoxal-5’-phosphate with dinitrofluorobenzene yielded pyridox- al-5’-0-monofluorophosphate (I) which was isolated as cyclohexylamine salt (2)

On the models for deuterium long-range isotope effects in13C NMR spectroscopy

The long-range deuterium isotope effects on13C nuclear shielding are physically not yet completely understood. Two existing models for explaining these effects, vibrational and substituent, are compared here. The vibrational model is based on the Born-Oppenheimer approximation, but it can explain only one-bond deuterium effects. To the contrary, the substituent model may explain many long-range isotope effects, but it is controversial due to the assumption of some distinct electronic properties of isotopes. We explain how long-range deuterium isotope effects may be rationalized by the subtle electronic changes induced by isotope substitution, which does not violate the Born-Oppenheimer approximation

The synthesis of some B₆ vitamin halophosphates

A series of new halo-derivates of pyridoxine-3-O- or 5’-0-phosphates and pyridoxal-3-O- or 5’-0-phosphates have been synthesized. In the reaction of partially protected pyridoxine and pyridoxal (3-5) with phosphorusoxychlo- ride in the presence of triethylamine, the following dichlorophosphates were obtained: 3,4’-0-isopropylidenepyridoxine-5’-0-dichlorophosphate (6), 4,5’-0- isobutilidenepyridoxine-3-O-dichlorophosphate (7), and monoethylacetal-3-O- dicholorophosphate (8). Dichlorophosphates 6-8 reacted with NaF under catalytic action of 18-crown-6-ether, giving the corresponding difluorophos- phates 14-16. The 3,4’-0-isopropyiidene pyridoxine-5’-0-difluorophosphate (14), 4’,5’-isobutilydene-3-0-difluoro-phosphate (15) and monoethylacetal-3-O-di- fluorophosphate (16), in the reaction with aniline, yielded the corresponding monofluorophosphates in the form of aniline salts (17-19). The direct fluor- ination of pyridoxal-5’-phosphate with dinitrofluorobenzene yielded pyridox- al-5’-0-monofluorophosphate (I) which was isolated as cyclohexylamine salt (2)