Acid-base properties of fulleropyrrolidines: Experimental and theoretical investigations

The acid-base equilibria of 2-(n-alkyl)fulleropyrrolidines and N-methyl-2-(n-alkyl)fulleropyrrolidines in aqueous micellar media of a sodium dodecyl sulfate surfactant are examined experimentally and modeled theoretically by using ab initio 3-21G(*) methods. The pK(a) values, for 2-(n-alkyl)fulleropyrrolidines and N-methyl-2-(n-alkyl)fulleropyrrolidines; determined both by HCl and NaOH titration with potentiometric detection of the end point, are equal to 6.3 +/- 0.1 and 7.5 +/- 0.1, respectively, and are independent of the alkyl chain length within a range of C1 to C8, The obtained results indicate that the fullerene cage fused to a pyrrolidine ring increases the acidity of the protonated pyrrolidine nitrogen mainly due to the induced electronic effects and, to a smaller extent, by structural effects. The 3-21G(*) estimated free energy difference, Delta E (with solvation corrections incorporated based on the Cramer-Truhlar SM5.4 solvation model), between the protonated and unprotonated fulleropyrrolidines, and the experimental pK(a) values have been correlated for 37 different nitrogenous bases, A 0 < pK(a) < 4 range was covered with these bases which include the presently investigated fulleropyrrolidines. Correlation between the calculated Delta E and experimental pK(a) values is good (Delta E = 2.3126 pK(a) + 280.49, correlation coefficient is equal to 0.95) and indicates that the effect on pK(a) of the micellar environment, utilized in the present study for solubilization of the fulleropyrrolidines, is negligible. Comparison of the calculated HOMO and LUMO energy levels for the protonated and free-base forms of the investigated fulleropyrrolidines indicates that electron deficiency of the C-60 cage and the HOMO-LUMO energy gap caused by N-protonation are increased. This result agrees well with the earlier electrochemical results for fulleropyrrolidinium cations.