The natural product haplophytine and its half-molecules, aspidophytine and the still elusive 'left domain', exhibit unique structural skeletons and unusual spectroscopic features. These issues have been tackled with density functional calculations. The congested framework of haplophytine consists of two hetero-polycyclic segments connected with a sterically demanding single bond and incorporates two protonation sites of different basicity and a relatively strong hydrogen bond O-H center dot center dot center dot O, responsible for the acidic activity. The molecular and electronic structure of these alkaloids has been elucidated by analysing the carbonyl vibrations, C-13 NMR chemical shifts, and UV absorption spectra that are most sensitive to stereoelectronic factors. (C) 2010 Elsevier B.V. All rights reserved.