Photoionization and dissociation of the monoterpene limonene: Mass spectrometric and computational investigation

The photoionization of the monoterpene limonene has been studied using tunable vacuum ultraviolet synchrotron radiation in the region from the threshold for ionization of the parent molecule up to 15.5 eV. The adiabatic ionization energy of limonene is derived from photoionization efficiency spectrum and found to be 8.27 eV, compared with the density functional theory calculations which yields a value of 8.08 eV (B3LYP/6-311++G). Primary dissociation pathways of the parent molecule ions are investigated by experimental observations and theoretical calculations. Most of the fragmentation channels occur via a rearrangement reaction prior to dissociation. Transition structures and intermediates for those isomerization processes are also determined.