Structures of 9,10-Dihydro-9-Heteroanthracenes and 9,10-Dihydro-9-Heteroanthracenide Anions; An Ab Initio MO Calculation

Document Type

Article

Publication Date

1-1-1997

Description

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 (a) indicate that xanthene (a = 180°) is planar, while DHA (a = 142.2°), N-methylacridane (a = 142.7°) and thioxanthene (a = 133.3°) have non-planar geometries. Deprotonation of 1a-d at C9 leads to a completely flat structure for xanthenide 2b (a = 180°) and significantly flattened molecules for DHA- 2a (a = 153.3°), N-methylacridanide 2c (a = 157.6°) and thioxanthenide anion 2d (a = 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.

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