The molecular and electronic structure of three pairs of diastereomeric quinuclidine derivatives (1-6) have been investigated by quantum chemical calculations and UV photoelectron (PE) spectroscopy. Compounds 1-6 each possess a hydroxymethyl group at C2 and an ethyl (1, 2), vinyl (3, 4) or ethynyl group (5, 6) at C5. The diastereomeric pairs differ only in the configuration of carbon C2 to which the hydroxymethyl group is attached. The 1-azabicyclo[2.2.2]octane cages of 1-6 are slightly twisted. The torsion angles show opposite signs in the pseudoenantiomeric pairs and vary only little with the degree of unsaturation in the C5 substituent. In all compounds the hydroxymethyl group forms an intramolecular hydrogen bond with the nitrogen atom of the bicycle in the gas phase (in contrast to the crystalline and liquid state). The PE spectra of the diastereomers are nearly identical. Compared to unsubstituted quinuclidine, the orbital of the nitrogen lone pair electrons n(N) is stabilized by 0.3-0.4 eV, which is a result of partially compensating contributions of the OH...N hydrogen bond and substituent effects. Among these a substantial through-bond interaction of the n(N) with a pi(CC) orbital is detected for 3-6. The marked differences of the properties of 1-6 in the gas phase versus solid state and solution are emphasised. In solution the ethynyl azabicyclics 5 and 6 are more polar than the vinyl analogues 3 and 4.