Mass spectrometric study of methyl-substituted 4-azaphenanthrenes and their nitration products

Abstract

The mass spectral behavior of five derivatives of the 4-azaphenanthrene series — 1,3-dimethyl-(I), 2,3-dimethyl-(II), 1,2,3,-trimethyl-(III), 1,2,3-trimethyl-8-nitro-(IV), and 1,3-dimethyl-6,7-dinitro-4-azaphenanthrene (V) — was studied. The stabilities of the molecular ions with respect to gragmentation (WM) are higher by a factor of two or more for the methyl-substituted I–III than for nitro derivatives IV and V. The intensity of the [M-H]+ ion peak in the mass spectra of I–V does not depend on the number of methyl groups but only on their positions: the presence of a CH3 group in the 2 position leads to an [M-H]+ ion that is 1.5 times more intense than when there is a methyl group in the 1 position. The molecular ions of I–V do not eliminate HCN molecules; this constitutes evidence for the absence of randomization of their methyl groups. The presence of a CH3 substituent in the 1 or 2 position does not affect the intensity of the [M-CH3]+ ion peaks, while the simultaneous presence of CH3 groups attached to the C1 and C2 atoms increases the intensity of the [M-CH3]+ fragment peak by a factor of two. In the mass spectra of nitro derivatives IV and V, [M-O]+, [M-OH]+, [M-NO]+, and [M-NO2]+ fragments are observed in the first step of the fragmentation of the M+ ion, whereas the [M-CO]+ ion peak characteristic for the dissociative ionization of 1-nitronaphthalene is also observed for 8-nitro-substituted IV.