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.

Mass spectrometric decomposition of N-arylbenzonitrilium ions

“…Our findings based on accurate masses have to be considered a complement of previous papers concerning HNCO loss from protonated 1,3,4-oxadiazoles [8][9][10][11] and the reactions between nitrobenzenes and benzonitrile radical cations [7].…”

“…In addition, theoretical calculations performed for various 2,5-diaryl-1,3,4-oxadiazoles have shown that such groups like methoxy or amino (electron-donor groups) at 4 -position favor the protonation of N(3) atom of oxadiazole ring [9,11]. Therefore, formation of N-methoxyphenylbenzonitrilium ion A seems to be favored over the formation of its isomeric ion B (Scheme 1).…”

“…Recently, a number of solution properties of these ions have been reported by Ruane et al [6]. As for the gas phase, Narylbenzonitrilium ions are formed by ion-molecule reaction between nitrobenzenes and benzonitrile radical cations (detection of nitroaromatic compounds) [7], and they are products of mass spectrometric decomposition of protonated 2,5-diaryl-1,3,4-oxadiazoles (unusual skeletal rearrangement) [8][9][10][11]. In order to determine the structure of these ions, we report the mass spectrometric decomposition of benzonitrilium ion derived from 2-(4 -methoxy)phenyl-5-phenyl-1,3,4-oxadiazole and from N-(4 -methoxyphenyl)-benzamide and 4-methoxy-N-phenyl-benzamide (compounds 1, 2 and 3, respectively).…”

Fragmentation and skeletal rearrangements of 2‐arylylamino‐5‐aryl‐1,3,4‐oxadiazoles and their noncovalent complexes with cobalt cation and cyclodextrin studied by mass spectrometry

“…Five‐member heterocyclics containing oxygen and nitrogen atoms reveal interesting mass spectrometric fragmentation pathways 1. Among this class of compounds, 2‐amino‐5‐aryl‐1,3,4‐oxadiazole derivatives are of special interest because they show different biological activities 2–10.…”

Differentiation of isomeric para‐ and meta‐substituted 2,5‐diphenyl‐1,3,4‐oxadiazole derivatives of anthracene by electrospray ionization tandem mass spectrometry

“…[11] A literature survey showed that few reports are available on mass spectral fragmentation of oxadiazole derivatives. [12][13][14][15][16][17][18][19] As these compounds belong to the category of organic light-emitting diodes (OLEDs) containing an oxadiazole group, the fragmentation behavior of these compounds under mass spectrometric conditions assumes importance and is desirable. These mass spectral studies may help in understanding the degradation process of these materials and in turn their stability and life.…”