Molecular orbital computations and 1H nuclear magnetic resonance measurements of the bending motion of xanthene in the gas and in solution

Abstract

STO-3G and 4-31G MO computations are reported for a range of values of the folding angle in xanthene, the dihedral angle between the benzene planes. Unlike 9,10-dihydroanthracene but like dibenzo-p-dioxin, its "parent" molecules, the inversion or puckering potential for xanthene is calculated to be rather flat. The molecular energies between a folding angle of 180° (planar molecule) and 120° are reproduced by analytical functions of [Formula: see text], θ being the folding angle. The long-range spin–spin coupling constants between the methylene protons and the aromatic protons at 300 K are reported for xanthene dissolved in a CS2/C6D12 solvent mixture and in acetone-d6 solution. These conformationally sensitive coupling constants are consistent with the theoretical puckering potentials and therefore with substantial "butterfly" motion at ambient temperatures. The computed geometries of xanthene are also given and briefly discussed. Keywords: xanthene, MO computations on inversion; xanthene, long-range spin–spin coupling constants; xanthene, internal motion; xanthene, inversion potentials.