Conformational Studies on Xanthene, Thioxanthene and Acridan

A study of the dipole moments of xanthone, thioxanthone and their halogeno‐derivatives leads to the conclusion that these molecules are not planar. In the xanthene and thioxanthene series, the dipole moments as well as the NMR spectra can be explained by a planar or a non‐planar but rapidly inverting model. In the acridan series, the NMR spectra of N‐acylacridans show that the molecule is non‐planar and inverts. The barrier is 11.7, 13.1 and 13.1 Kcal/mole for the N‐acetyl‐, N‐chloroacetyl‐ and N‐iodoacetyl derivatives, respectively. Equally, it has been observed that at low temperature the rotation around the XCH2 CO bond in the halogenoacetylacridans is hindered.