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A photoionisation mass spectrometric study of some stereoisomers Akhtar, Zinat Mahal (Mia)


Photoionisation mass spectrometry has been used to study a number of stereoisomeric systems. Low resolution spectra were obtained for the epimers of 4-t-butylcyclohexanol, 4-t-butylcyclohexyl acetate, 4-t-butylcyclohexyl methyl ether and the 4, 3, and 2 methyl substituted cyclohexanols using the He 584 Å radiation and the Lyman a radiation for ionisation. Analogues labelled with deuterium at specific positions were also prepared for all the molecules studied. The major fragments in the low resolution spectra were mass measured to determine the elemental composition. Possible fragmentation pathways are postulated for the different molecules studied making use of the information obtained from the labelled analogues, together with the high resolution results and any observed ions due to metastable transitions. The epimers of 4-t-butylcyclohexanol and 4-t-butylcyclohexyl methyl ether give rise to mass spectra which differ in the relative abundances of both the molecular ion and other major ions. The observed differences are explained in terms of the stereochemical arrangement of the atoms and groups in the respective isomers. The spectra obtained for cis and trans 4-t-butylcyclohexyl acetate are almost identical. The similarity is proposed to be due to the intervention of different mechanisms from that observed in the corresponding alcohols and ethers. An attempt is also made to discuss the results obtained for the various methyl cyclohexanols both in terms of the spatial arrangement and the position of the substituent.

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