Conformation dependence in the chemical ionization mass spectra of 3‐pinanones

The conformation of the rigid isomer pair of cis ‐ and trans ‐3‐pinanones was investigated using methane, isobutane, and ammonia chemical ionization (CI) mass spectrometry as a tool to study the dependence of the mass spectrometric fragmentation patterns. The methane and the isobutane CI mass spectra of the cis and trans isomers were found to be different enough to permit the differentiation of the isomers. Each isomer exhibited distinctive fragmentation paths. The isobutane mass spectra of the cis isomer was dominated by the loss of H 2 O from the pseudo‐molecular ion, whereas the loss of C 2 H 4 O was the predominant fragment for the trans isomer. Molecular modeling and quantum‐chemical computations were used to calculate the conformers of lowest energy for the two isomers. The theoretical calculations were then used to explain the differences observed in the chemical ionization mass spectra. In addition, high resolution mass spectrometry measurements, deuterium labeling experiments and energy calculations of the hypothetical transition states and ionized species were helpful and were used to propose mechanisms for the CI mass spectral fragmentation.