Are there reliable DFT approaches for C-13 NMR chemical shift predictions of fullerene C-60 derivatives?

摘要

The relationships between experimental and theoretical C-13 NMR chemical shifts of a pristine fullerene C-60, monoadducts from [2+n] cycloaddition (n=1-3), and one [2+1] bis-adduct are systematically analyzed for the first time by using diverse quantum-chemical levels of theory. These levels involved B3LYP, B3PW91, B97-2, mPW1PW91, PBE1PBE, and X3LYP hybrid functionals combined with 3-21G, 6-31G, 6-31G(d), 6-31G(d,p), 6-31G(d,2p), LanL2DZ, and SDDAll basis sets. X3LYP/6-31G approach is determined to have the lowest deviations from the C-13 NMR experimental data compared to the other methods for all the fullerene compounds (mean absolute error value is 0.856 ppm and root mean squared error value is 1.197 ppm). The highest deviations are characteristic for (sp(2) C2/C5/C8/C10) and (sp(2) C6/C7/C11/C12) carbon atoms relative to a functionalization site and for those (sp(3) C1/C9) directly attached with a side fragment in the [2+n] monoadducts (n=1-3). A probable reason of such deviation is that the approaches do not take into account a contribution of paramagnetic ring currents to C-13 NMR chemical shifts. The results will be useful in design of novel fullerene derivatives and in performing unambiguous C-13 NMR chemical shift assignments with modern quantum chemistry calculations.