The origins of the base peak in the electron impact spectrum of limonene

The origins and nature of the C₅H₈]⁺? ions which form the base peak in the electron impact spectrum of limonene, at nominal electron energies greater than 11 eV, have been investigated. Linked scan techniques were used to study unimolecular and collision induced fragmentation reactions. No fragmentation pathway leading to C₅H₈]⁺? could be found. Measurement of ionization efficiency curves indicated that the threshold for formation of C₅H₈⁺?] lies above the range of internal energies deposited in incident ions by collisional activation. By a combination of comparisons of collisionally activated spectra and energetic considerations, the C₅H₈]⁺? ions formed from limonene were shown to resemble those of the molecular ion of isoprene, while the neutral fragment is most likely isoprene also. Deuterium labelling experiments yielded evidence of extensive scrambling prior to fragmentation. The most probable mechanism of formation of C₅H₈]⁺? appears to involve a retro Diels–Alder reaction of a structurally intact molecular ion of limonene.