Computational NMR of natural products: On the way to super large molecules exemplified with alasmontamine A.

1 H and 13 C nuclear magnetic resonance (NMR) chemical shifts of a tetrakis monoterpene indole alkaloid alasmontamine A with a molecular formula of C 84 H 91 N 8 O 12 have been calculated at the PBE0/pcSseg-2//pcseg-2 level of theory on M06-2X/aug-cc-pVDZ geometry. In the course of the preliminary conformational search, six true minimum energy conformers were identified that can contribute to the actual conformation of this huge alkaloid. Calculated chemical shifts generally demonstrated a good agreement with available experimental data characterized with a corrected mean absolute error of 0.10 ppm for the range of about 7 ppm for protons and 1.1 ppm for the range of about 160 ppm for carbons.