Acidity of dibasic carbon acids. Part 2. Geometry and electronic structure of mono- and di-anions of 9, 10-dihydroanthracene and its derivatives in tetrahydrofuran

Abstract

The geometric structure, charge distribution and heats of formation of 9-R¹-10-R²-9, 10-dihydroanthracenes (R¹= R²= H; R¹= Me, R²= H; R¹= R²= Me; R¹= Ph, R²= H; R¹= R²= Ph; R¹= CN, R²= H), their mono- and di-anions, deprotonated at positions 9 and 10 have been calculated by the AM-1 method. The ¹³C chemical shifts of the mono- and di-alkali metal salts have been determined in [²H₈]THF. The structures of the mono- and di-anions are nearly planar., the dianions being more planar than the monoanions. The hybridization of the deprotonated carbon atoms is nearly sp². Phenyl substituents subtend angles of 56.5° and 45.0° to the anthracene plane in the mono- and di-anions, respectively. With the exception of the cyano group, there is little participation by the substituents in the delocalization of negative charge. There is a correlation between the π-electron densities and the change in ¹³C chemical shift upon anion formation from the neutral species. The values of the proportionality constants are equal to 120.3 and 133.1 ppm/electron for mono- and di-anions, respectively. The pattern of negative charge distribution in the monoanions of 9, 10-dihydroanthracenes is similar to that of substituted diphenylmethanes. The first and second acidity constants of substituted 9,10-dihydroanthracenes in the liquid phase follow the patterns indicated by the gas phase calculations of their relative thermodynamic stabilities.