HOMOLYTIC BOND-DISSOCIATION ENERGIES OF THE BENZYLIC C-H BONDS IN RADICAL-ANIONS AND RADICAL CATIONS DERIVED FROM FLUORENES, TRIPHENYLMETHANES, AND RELATED-COMPOUNDS

A method for estimating the homolytic bond dissociation energies of the benzylic C-H bonds in radical anions, BDE(HA.-), has been devised. The method, which is potentially applicable to all weak acids, has been applied to the radical anions derived from 19 fluorenes, 15 triphenylmethanes, p-nitrotoluene, diphenylmethane, bis(4-nitrophenyl)methane, and three xanthenes. (In this abstract and in the text, the C-H bonds referred to are all of the benzylic type.) The BDE(HA.-) values for the C-H bonds in the radical anions derived from fluorene and 18 of its 2- and 9-substituted derivatives averaged 35 kcal/mol compared to an average of 75 kcal/mol for the BDE(HA) values of the corresponding C-H bonds in the parent fluorenes. The average BDE(HA.-) values for the C-H bonds in the radical anions derived from 9-(dialkylamino)- and 9-(cycloalkylamino)fluorenes of 36 +/- 3 kcal/mol were found to be close to the average value of the homolytic bond dissociation energies of the C-H bonds in the corresponding radical cations, BDE(HA.+) = 39 +/- 4 kcal/mol. The BDE(HA.-) values for the C-H bonds in the radical anions derived from diphenylmethane and four meta- and para-substituted triphenylmethanes averaged 46 +/- 3 kcal/mol compared to 81 +/- 1 kcal/mol for the BDE(HA) values of the C-H bonds in the corresponding parent molecules. The BDE(HA.-) value for 4-PhCOC6H4CHPh2 was 8 kcal/mol higher than the average for the triphenylmethanes, and the average BDE(HA.-) value for 4-NO2C6H4CH3, (4-NO2C6H4)2CH2, (4-NO2C6H4)3CH, and five 4-NO2-substituted triphenylmethanes was 16 kcal/mol higher, i.e., 62 kcal/mol. (The BDE(HA) values for the C-H bonds in these nitro compounds were, for the most part, near 81 kcal/mol). In Contrast, the average BDE(HA.+) value for the C-H bonds in the radical cations derived from (4-NO2C6H4)2CH2, 4-PhCOC6H4CHPh2, 4-NO2C6H4CHPh2, and (4-NO2C6H4)3CH is only 27 +/- 1 kcal/mol.