Exploring the unimolecular chemistry of protonated limonene and -terpineol

Terpenes are an incredibly diverse category of chemical compounds, featuring a variety of structural features and modifications. This work focuses on the monocyclic monoterpene (C10H16) limonene and one of its hydrated derivatives alpha-terpineol. Previous experiments have shown that these compounds may become protonated under atmospheric conditions. It was of interest to provide greater detail in how these compounds would break down following protonation, where previous studies lacked mechanistic detail. To overcome these shortcomings, our lab applied combined experimental and theoretical approaches. Tandem mass spectrometry experiments were used to yield the protonated compounds, and collision induced dissociation to break them apart, identifying underlying unimolecular reaction channels of the protonated target. The experimental data is supplemented with density functional theory calculations, which are used to model the individual transformations involved in each of the experimentally observed reaction channels. The mass spectrometry experiments revealed four fragmentation channels in the case of protonated limonene, two major ones and two minor ones, in terms of relative abundance. The neutral loss of C4H8 was the most abundant product ion channel, followed by the neutral loss of C3H6. The other two fragmentation pathways were minor, and lead to the neutral losses of C2H4 and C3H8. Density functional theory and RRKM calculations revealed the mechanisms by which these reactions occurred and were shown to be consistent with the experimental data. In the case of protonated alpha-terpineol, a single fragmentation channel was observed, leading to the neutral loss of water, although sequential fragmentation that was similar to protonated limonene was observed. These observations were justified through calculations, showing that a relatively stable alpha-terpineol ion-molecule complex is formed in the source, leading to the observed chemistry.