State-selected dissociation of dehydrovincamine alkaloid stereo-isomers

Abstract

We have obtained results on alkaloid stereoisomers that show strongly stereoselective fragmentation processes. An original technique for such molecules, i.e. photoionisation by synchrotron radiation in the VUV range coupled to the photoelectron–photoion coincidence technique, is used to prepare state-selected parent ions with a very wide internal energy range. Mass spectra obtained after photoionisation, and metastable spectra obtained after electron impact ionisation, exhibit stereospecific fragmentations leading either to water loss and subsequent fragmentations in dehydrovincamine, or to carbomethoxy and ethyl loss in epi-dehydrovincamine. The results are in agreement with previous measurements obtained on the protonated samples, or on the similar vincamine molecule. Appearance energies for major fragmentations were recorded as well as a state-selected breakdown diagram for the epi-dehydrovincamine sample. They show that thresholds for the fragmentation processes are almost identical in the two isomers, indicating very similar thermodynamics. Very slow fragmentation decay for the first dissociation channels leads to the coexistence of parent and fragment ions in a very wide internal energy range of the breakdown diagram. The results show that the fragmentation of the system is most probably driven by strong steric factors. Further modelling with statistical theories should give the precise transition states and lead to a better understanding of this stereospecificity.