Mass spectrometry of heterocyclic compounds. V—Substituent effects on the fragmentation pathways of 3,5-diphenyl-1,2,4-oxadiazole derivatives

The 70 eV mass spectra of a number of derivatives of 3,5-diphenul-1,2,4-oxadiazole substituted either in the 5-phenyl (fifteen) or in the 3-phenyl (four) position, have been studied using exact mass measurements and metastable determinations by the defocusing technique. The substituent effects on the heterocyclic cleavage are not very important for electron-withdrawing and weak electron-donating groups. The main cleavage is the formal retro 1,3-dipolar cycloaddition with the positive charge retained by the C₇H₅NO (orYC₇H₄NO in the case of the 3-substituted derivatives) fragment specifically containing the 3-phenyl, as oreviously observed for 3,5-diphenyl-1,2,4-oxadiazole using labelling experiments. Two other minor primary processes leading to benzoyl and nitrile ions, both containing the 5-phenyl, become important in the case of the electron-donating substituted compounds. Correlation of the abundance of these fragmednts and of the molecular ions with σ⁺ (para) Brown constants is discussed. Proximity effets are shown by some ortho derivatives.