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

Abstract

Abstract 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 C 7 H 5 NO (orYC 7 H 4 NO 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.