The Site of Protonation Affects the Dissociation of Protonated α- and β-Pinene Ions

In electrospray ionization and atmospheric pressure chemical ionization, the protonation site directly guides the ion's dissociation. But what if the site of protonation is ambiguous? In this study, we explored the unimolecular reactions of protonated α- and β-pinene ions with a combination of tandem mass spectrometry and theory. Each has multiple potential protonation sites that influence their chemistry.

Atmospheric pressure chemical ionization was employed to form the protonated pinene isomers. The unimolecular chemistry of these ions was explored with a Waters Ultima triple-quadrupole mass spectrometer using energy-resolved collision-induced dissociation with argon collision gas. Reaction mechanisms were calculated with CBS-QB3 single-point energy calculations on B3LYP/6-311+G(d,p) optimized structures.

The two main dissociation reactions in each ion lead to the loss of neutral propene and isobutene. Both ions were found to dissociate over the same minimum energy reaction pathway, the only difference being the site of initial protonation. α-Pinene preferentially protonates at the bridging carbon, while β-pinene can only significantly protonate at the exocyclic double bond. This leads to a lower appearance energy for loss of isobutene, and thus relatively greater m/z 81 fragment ion abundance for β-pinene.

The distinct sites of initial protonation result in the subtle differences observed in the CID of α- and β-pinene. The work highlights that it is not necessarily the "lowest energy" ion that will be formed in the ion source, and any distribution of initial structures must be accounted for when examining CID mass spectra.