How regioisomeric fullerene C60 bis-cycloadducts can be distinguished with 13C NMR? Quantum-chemical assessment and empirical correction

Abstract

In this work, we propose a theoretical approach to distinguish fullerene C60 bis-cycloadducts with identical addends giving the same number of 13C NMR signals of sp2 fullerene carbon atoms. Trans-1 C60 bis-cycloadducts, including those that bear different addends, may be assigned from their unique number of 13C NMR signals of sp2 fullerenyl carbon atoms (6 × 8C + 2 × 4C) compared with the remaining bis-cycloadducts. The same is typical of equatorial C60 bis-cycloadducts (27 × 2C and 2 × 1C). A distinctive feature to distinguish trans-2, trans-3, and cis-3 C60 bis-cycloadducts with identical addends, giving equal number of 13C NMR signals of sp2 fullerenyl carbon atoms, is a high correlation coefficient obtained after cross-comparison between their experimental and theoretical 13C NMR CSs. Similar cross-comparison may be applied to trans-4, cis-1, and cis-3 C60 bis-cycloadducts with identical addends. The RMSE values after empirical correction to theoretical 13C NMR CSs of sp2 fullerenyl carbons decreased from 3.45 to 1.41 ppm. This is an effective way to decrease the error of GIAO-X3LYP/6-31G method, which probably does not fully take into account the contribution of magnetic ring currents to the magnetic shielding of sp2 fullerenyl carbon atoms. The proposed cross-comparison was verified on a test set of regiosiomeric C60 bis-cycloadducts not involved into the training set, which led to unambiguous structural assignments.