Quantum chemical calculations of the pK a of C60H2 in DMSO, toluene—DMSO mixture, and water
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Using quantum chemical calculations, the first pK a values of dihydrofullerene C60H2 and test carbon compounds were determined in three solvents. The gas-phase free energies were calculated at the DFT/B3LYP/6-31G(d)//B3LYP/6-311++G(2d,p) level, while the free energies of solvation were found by HF and B3LYP methods combined with the polarizable continuum model (PCM). The PCM parameters for fullerene C60 and its derivatives were obtained from the calibration procedure. The R(C) radii of the solvation cavity of the fullerene C atoms were estimated taking into account the AO hybridization in the non-planar aromatic system of the fullerene C60 and from the calculation data of constant electron density contours. The R(C) values for the C60H− anion were derived from consideration of the thermodynamic cycle of pK a changes in DMSO and in a toluene—DMSO system for saccharin and dihydrofullerene. The calculations carried out for test carboxylic acids reflect rather accurately the gas-phase basicities and pK a in water and DMSO. The calculations for solutions demonstrated that C60H2 has much lower pK a than the test aromatic molecules, which is mainly caused by the contribution of the gas-phase basicity. The absolute value of pK a of the dihydrofullerene C60H2 in DMSO obtained in this way is 3.8, while in water, it is ∼5 units higher.
Key wordsfullerene C60 dihydrofullerene C60H2 free energy of solvation protonation constants pKa polarizable continuum model density functional theory B3LYP functional Hartree-Fock method
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