A systematic study of the accuracy factors for the computation of 15 N NMR chemical shifts in comparison with available experiment in the series of 72 diverse heterocyclic azines substituted with a classical series of substituents (CH3 , F, Cl, Br, NH2 , OCH3 , SCH3 , COCH3 , CONH2 , COOH, and CN) providing marked electronic σ- and π-electronic effects and strongly affecting 15 N NMR chemical shifts is performed. The best computational scheme for heterocyclic azines at the DFT level was found to be KT3/pcS-3//pc-2 (IEF-PCM). A vast amount of unknown 15 N NMR chemical shifts was predicted using the best computational protocol for substituted heterocyclic azines, especially for trizine, tetrazine, and pentazine where experimental 15 N NMR chemical shifts are almost totally unknown throughout the series. It was found that substitution effects in the classical series of substituents providing typical σ- and π-electronic effects followed the expected trends, as derived from the correlations of experimental and calculated 15 N NMR chemical shifts with Swain-Lupton's F and R constants.
Keywords: 15N NMR; DFT functionals; GIAO-DFT; basis sets; heterocyclic azines; locally dense basis set scheme; substitution effects.
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