Computational 1 H and 13 C NMR of the trimeric monoterpenoid indole alkaloid strychnohexamine: Selected spectral updates.

Very large trimeric indole alkaloid strychnohexamine, with empirical formula C59 H60 N6 O (66 second-row atoms and 60 protons), has been subjected to the state-of-the-art computation of the 1 H and 13 C nuclear magnetic resonance (NMR) chemical shifts of its configurational isomers at each of the 14 asymmetric centers. Several spectral reassignments and corrections of 1 H and 13 C NMR spectra of this alkaloid were suggested based on the PBE0/pcSseg-2//pcseg-2 calculation of its NMR chemical shifts. Thus, all pairs of diastereotopic protons were assigned together with four aromatic carbon resonances of C-9 and C-11, C-9″, and C-11″. In addition, the unassigned chemical shifts of carbon C-23″ and proton at C-3' in, accordingly, 13 C and 1 H NMR spectra were predicted.