STRUCTURES OF 9,10-DIHYDRO-9-HETEROANTHRACENES AND 9,10-DIHYDRO-9-HETEROANTHRACENIDE ANIONS - AN AB-INITIO MO CALCULATION

The structural parameters and charge distributions of 9,10-dihydroanth racene (DHA) la, xanthene Ib, N-methylacridane Ic and thioxanthene Id, and their monoanions 2a-d have been calculated by an ab initio MO met hod at the Hartree-Fock SCF level using 6-31G(d) basis set, The calcul ated dihedral angles (alpha) indicate that xanthene (alpha = 180 degre es) is planar, while DHA (alpha = 142.2 degrees), N-methylacridane (al pha = 142.7 degrees) and thioxanthene (alpha = 133.3 degrees) have non -planar geometries, Deprotonation of 1a-d at C-9 leads to a completely hat structure for xanthenide 2b (alpha = 180 degrees) and significant ly flattened molecules for DHA(-)2a (alpha = 153.3 degrees), N-methyla cridanide 2c (alpha = 157.6 degrees) and thioxanthenide anion 2d (alph a = 150.0 degrees). The calculated charge distributions of these anion s are found to correlate well with the experimentally determined C-13 NMR chemical shifts, The combined data from net charge distributions a nd carbon chemical shifts indicate that the heteroanthracenide anions 2b-d may be characterized as extensively delocalized 16 pi-electron sy stems in which oxygen and nitrogen anions on the one hand and a sulfur analogue on the other exhibit some fundamentally different charge dis tribution pattern, While the extent of overall delocalization of the b enzylic negative charge increases in the order of O < N < S, the rever se order S < N < O is observed with regard to the extent of charge del ocalization into the benzenoid moieties, This observed discrepancy may be explained by the second-row nature of the sulfur atom and its abil ity to stabilize an adjacent negative charge.