Mass spectral fragmentation pattern of 5-methyl-4-[(phenylamino)methylene]-2,4-dihydro-3h-pyrazol-3-one and its 2-methyl and 2-phenyl derivatives

The mass spectral fragmentation patterns of 5-methyl-4-(phenylamino)methylene]-2,4-dihydro-3H-pyrazol-3-one and its 2-methyl and 2-phenyl derivatives have been elucidated. The principal initial fragmentation route involves rupture of the exocyclic CH-NH bond. Minor routes involve loss of H, OH and C₆H₅ from the molecular ion and rupture of the pyrazolone ring.