M. A. Almoster Ferreira

UVPES of some aliphatic azides. Part I

Abstract Ab initio SCF calculations for 2-chloroethyl azide (ClCH2CH2N3), ethyl azidoacetate (N3CH2COOC2H(5) and 3-azido-2-butanone (CH3CHN3COCH3) were performed in order that their UV photoelectron spectra could be reliably interpreted. Good agreement was found when comparing the ionisation energies given by the calculations with the bands of the photoelectron spectra of the above mentioned molecules.

A PEPICO STUDY OF FIVE ENERGY-SELECTED C6H12+. ISOMERIC MOLECULAR IONS. PART 1

Abstract The breakdown diagrams of five isomeric C 6 H 12 ions were obtained by energy-selected molecular ions with photoelectron photoion coincidence (PEPICO) spectroscopy. The breakdown diagrams of cyclohexane, 1-hexene and 3-hexene are quite similar showing that the lower dissociation paths are the losses of C 2 H 4 and CH 3 leading to the C 4 H 8 +. and C 5 H 9 + fragment ions, respectively. The three molecular ions were found to dissociate slowly near the threshold, the time of flight distributions of the C 4 H 8 +. ion having been analysed in terms of two exponential decay rates while the time of flight distributions of the C 5 H 9 + ion could be fitted considering a single component. The breakdown diagrams of the 2-methyl-2-pentene and 2,3-dimethyl-1-butene molecular ions are identical showing as dominant dissociation channel the elimination of CH 3 . to form the C 5 H 9 + fragment ion. The loss of CH 3 . was found to be a fast reaction and accurate dissociation onsets were determined from the crossover energy in the breakdown diagrams. The results indicate that the cyclohexane and the two open chain isomers follow a distinct and more complex dissociation mechanism than the two branched open chain isomers.

Mass spectrometric study of the amipurimycin hexopyranosidic sugar moiety

Amipurimycin is a natural nucleoside that displays a remarkable activity in vitro and in vivo against Pyricularia oryzae, which is responsible for the rice blast disease. Six important precursors for the synthesis of the Amipurimycin sugar moiety were prepared. In order to obtain structural information a mass spectrometric study of these compounds was performed using liquid secondary ion mass spectrometry (LSIMS) with high-energy collision-induced dissociation (CID) experiments on a four-sector instrument. Elimination of methanol is the preferential fragmentation path for five of the six protonated molecules, indicating that protonation plays an important role in the process, whilst for the other protonated molecule loss of water is the main fragmentation. Examination of the [M + Li]+ precursor ion spectra shows that loss of methanol does not occur in three of them, but in the other three gives rise to fairly abundant ions, indicating that in this case methanol elimination implies intramolecular hydrogen transfer, the resulting ion having a very stable structure.