Search

• We propose theoretical approach to distinguish C 60 bis-cycloadducts. • Cross-comparison approach allows distinguishing between C 60 bis-cycloadducts. • RMSE values after empirical corrections decreased from 3.45 to 1.41 ppm. In this work, we propose a theoretical approach to distinguish fullerene C 60 bis-cycloadducts with identical addends giving the same number of 13C NMR signals of sp2 fullerene carbon atoms. Trans -1 C 60 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 C 60 bis-cycloadducts (27 × 2C and 2 × 1C). A distinctive feature to distinguish trans -2, trans -3, and cis -3 C 60 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 C 60 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 C 60 bis-cycloadducts not involved into the training set, which led to unambiguous structural assignments. [ABSTRACT FROM AUTHOR]

Abstract: The correlation analysis of the calculated by means of GIAO PBE/3ζ method and experimental (2D NMR INADEQUATE) 13C NMR chemical shifts of C60 derivatives was performed. It was shown that the computational method with sufficient accuracy (r.m.s.∼5ppm) reproduces the experimental 13C NMR chemical shifts of the sp 2-fullerene carbon atoms uncoupled with the sp 3-fullerene carbons. By quality description the GIAO method in PBE/3ζ approach best reflects the experimental 13C NMR chemical shifts of studied in this work fullerene derivatives such as hydrofullerene C60H2, methanofullerenes, fullerene adducts of [2+2]- and [2+3]-cycloaddition, as well as azaheterofullerene. [ABSTRACT FROM AUTHOR]