The Abundances of Fragment Ions Formed via Skeletal Rearrangements from 2,5-Disubstituted-1,3,4-Oxadiazoles and Their Theoretical Calculated Stabilities

Abstract

Protonated molecules of 2,5-di-R₁,R₂-substituted-1,3,4-oxadiazoles lose isocyanic acid (loss of mass 43) via skeletal rearrangement. This was observed in low-energy CID mass spectra recorded by using electrospray ionisation (ESI). On electron ionisation- induced decomposition, these compounds also reveal complex skeletal rearrangement, consisting on N₂ and CO elimination, leading to the formation of fluorene or indene type ions. It was found that abundances of fragment ions formed in these processes are in good agreement with their theoretically calculated stabilities. In the case of ESI use, the fragment ion abundances were calculated in relation to [M + H]⁺ ion abundances and for EI use, where extensive fragmentations proceed, the sums of the abundances of [M – N₂ – CO]^+• ions and abundances of fragment ions derived from them were expressed as a percentage of total-ion current.

Keywords

1,3,4-oxadiazoles fragmentation pathways skeletal rearrangements electrospray ionisation electron ionisation

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