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The origins of the base peak in the electron impact spectrum of limonene
Authors:Debi Harris  Stewart McKinnon  Robert K Boyd
Abstract: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.
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