On the formation and decomposition of the [M  HNO] + ion from o ‐nitrobenzaldoxime

The molecular ion of o‐nitrobenzaldoxime appears to eliminate a molecule of HNO, the latter as shown by deuterium labelling containing the hydroxyl hydrogen atom. This reaction may be followed either by loss of CO2 or C2O2 (2 × CO), which requires complicated skeletal rearrangements. To determine the structure of the [M HNO]+ ion, the kinetic energies, released in the loss of CO2 and of C2O2 from this ion, have been measured. Similar measurements have been made on the appropriate metastable ions from o‐nitrosobenzaldehyde, 2,1‐benzisoxazoline‐3‐one, 2‐benzoxazolinone and 3‐hydroxy‐1,2‐benzisoxazole, whose molecular ions have the same elemental composition as the [M HNO]+ ion from o‐nitrobenzaldoxime. In the loss of HNO from the molecular ions of o‐nitrobenzaldoxime o‐nitrosobenzaldehyde molecular ions are generated first, which then, at least upon decomposition, rearrange further to 2,1‐benzisoxazoline‐3‐one molecular ions. Part of the latter finally rearrange to 2‐benzoxazolinone molecular ions, which eliminate CO via two routes. One of these corresponds to a low energy process and the other to a higher energy path whose excess energy induces the subsequent rapid loss of a second CO molecule. The observed effect of the rate‐determining isomerizations of the molecular ions of o‐nitrobenzaldoxime, o‐nitrosobenzaldehyde, 2,1‐benzisoxazoline‐3‐one and 3‐hydroxy‐1,2‐benzisoxazole to 2‐benzoxazolinone on the intensity and shape of the metastable peak for the low energy process for CO expulsion from the molecular ions of 2‐benzoxazolinone supports the proposed rearrangements.