QUANTUM-MECHANICAL PM3 CALCULATIONS AND HIGH-FIELD H-1 AND C-13 NMR-STUDIES ON CHALCANTHRENES, C12H8XY (X, Y = O, S, SE, TE IN ALL COMBINATIONS)

The complete series of ten chalcanthrenes (C12H8XY), with X, Y = O, S, Se, Te in all combinations, was investigated using quantum mechanical PM3 calculations and two-dimensional H-1 and C-13 NMR spectroscopy. The former located single flat conformational minima and low interconversion energetics. The X,Y=Se, Te and Te, Te compounds had a more sharp minimum, and accordingly, this well reproduced the conformations attained in the solid state. The IPs were calculated and discussed in relation to the experimental data. The conformations in the gas were found in line with the calculated bonding situation determined by the effects of increasing, along the series, metallic character of the HOMOs. In the most folded molecules X, Y = Se, Te and Te, Te the p(z)-pi interactions decrease with increasing localization of the HOMO on the heavier heteroatoms. Unambiguous spectral H-1 and C-13 NMR assignments on correlative bases revealed combined effects of anisotropic and paramagnetic contributions by the heavy atom. The measured 1J(Se,C), 1J(Te,c), 2J(Se,c), 2J(Te,c), 3J(Se,c), and 3J(Te,C) values were interpreted and discussed in terms of possible coupling interactions.