Bimolecular and Unimolecular Contributions to the Disparate Self-Chemical Ionizations of α-Pinene and Camphene Isomers

The contributions of molecular and fragment ions toward the disparate self-chemical ionization (SCI) of α-pinene and camphene isomers were investigated. A kinetic model was constructed to predict the SCI outcomes for these two C10H16 isomers. A major portion of the camphene molecular ions (isolated 500 ms after the 10 ms EI event at 24 eV) unimolecularly dissociated within 200 s of the ionization event. Conversely, under similar experimental conditions, the α-pinene molecular ions as well as the major fragment ions of α-pinene and camphene showed no unimolecular dissociation. The α-pinene and camphene molecular ions yielded product ions through two different reaction mechanisms (direct charge-transfer {CT} and indirect proton transfer {PT}). The isolated terpene fragment ions at m/z 93 and 121 reacted with their respective neutrals to produce [M + H]+. Proton affinity (PA) bracketing experiments, PA additivity schemes, and alkene PA versus adiabatic ionization energy (IE) linear correlation indicated that the PAs of camphene and α-pinene were comparable (∼210 ± 2 kcal · mol−1). The observed [M + H]+ SCI terpene ions were mainly the products of various fragment ion reactions.