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.