Exploring the Unimolecular Chemistry of Representative Terpenes Under Protonated and Pyrolysis Conditions

Abstract

The art of combining experiment and theory together allows for the painting of a complete chemical picture. In this work, the two concepts were married to investigate the unimolecular gasphase decomposition of representative terpene molecules under protonated and extreme thermal conditions. Protonation studies were carried out experimentally using tandem mass spectrometry with collision-induced dissociation. Protonated isoprene was dominated by the neutral loss of C2H4, while its derivative prenol had competition between the neutral losses of C3H6, CH2O, CH3OH, and H2O. The dissociation of the monoterpenes showed abundant neutral losses of C3H6 and C4H8, with minor appearance of C2H4 and C3H8 in some examples. The monoterpenoid derivatives showed similar dissociation products but introduced the neutral losses of oxygenated compounds. Density functional theory calculations were used to unravel the minimum energy reaction pathways that describe the mechanisms by which each given protonated terpene was converted to the observed product ions in the experiment. In nearly all cases, the initial site of protonation was a key driver for the observed dissociation chemistry of the protonated terpenes. The decomposition of terpenes under extreme thermal conditions was investigated using flash pyrolysis at the Swiss Light Source synchrotron using the iPEPICO beamline. This experiment enabled the identification of novel isomeric products that have not been previously described in the literature when experiments were performed with isoprene. Density functional theory calculations revealed a new mechanism that could feasibly produce the newly observed cyclopentene intermediate on the pathway towards cyclopentadiene, a previously described pyrolysis product. These calculations were also used to help confirm product identities with Franck-Condon simulations and were used to reveal the mechanisms by which these products were formed. Similar experimental and theoretical methodology was applied for representative monoterpenes. Here, there was a clear differentiation in the product distribution that was observed for α-pinene, dominated by sequential losses of methyl radicals to yield substituted benzenes, a process not observed in β-pinene or limonene, which were shown to have some degree of overlap between their decomposition chemistries. Unique to the former was the appearance of abundant propargyl radicals, while the latter was shown to be dominated by intermolecular cleavage to yield two molecules of isoprene. In the end, when experiment and theory are combined to provide the same picture, we can have more confidence that the individual images were captured correctly.