Structures of 9,10-dihydro-9-heteroanthracenes and 9,10-dihydro-9-heteroanthracenide anions; an ab initio MO calculation

Abstract

The structural parameters and charge distributions of 9,10-dihydroanthracene (DHA) 1a, xanthene 1b, N-methylacridane 1c and thioxanthene 1d, and their monoanions 2a–d have been calculated by an ab initio MO method at the Hartree–Fock SCF level using 6-31G(d) basis set. The calculated dihedral angles (α) indicate that xanthene (α = 180°) is planar, while DHA (α = 142.2°), N-methylacridane (α = 142.7°) and thioxanthene (α = 133.3°) have non-planar geometries. Deprotonation of 1a–d at C⁹ leads to a completely flat structure for xanthenide 2b (α = 180°) and significantly flattened molecules for DHA^- 2a (α = 153.3°), N-methylacridanide 2c (α = 157.6°) and thioxanthenide anion 2d (α = 150.0°). The calculated charge distributions of these anions are found to correlate well with the experimentally determined ¹³C NMR chemical shifts. The combined data from net charge distributions and carbon chemical shifts indicate that the heteroanthracenide anions 2b–d may be characterized as extensively delocalized 16 π-electron systems in which oxygen and nitrogen anions on the one hand and a sulfur analogue on the other exhibit some fundamentally different charge distribution pattern. While the extent of overall delocalization of the benzylic negative charge increases in the order of O < N < S, the reverse order S < N < O is observed with regard to the extent of charge delocalization into the benzenoid moieties. This observed discrepancy may be explained by the second-row nature of the sulfur atom and its ability to stabilize an adjacent negative charge.