Buenger Edgar White, Mayer Paul M
Department of Chemistry and Biomolecular Sciences, University of Ottawa, Ottawa, Canada.
Rapid Commun Mass Spectrom. 2025 Mar 30;39(6):e9978. doi: 10.1002/rcm.9978.
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.
在电喷雾电离和大气压化学电离中，质子化位点直接引导离子的解离。但如果质子化位点不明确会怎样？在本研究中，我们结合串联质谱和理论方法，探索了质子化的α-蒎烯和β-蒎烯离子的单分子反应。每种离子都有多个潜在的质子化位点，这些位点会影响它们的化学性质。
采用大气压化学电离法形成质子化的蒎烯异构体。使用沃特世Ultima三重四极杆质谱仪，通过与氩碰撞气体进行能量分辨碰撞诱导解离，研究这些离子的单分子化学性质。利用CBS-QB3单点能量计算方法，对在B3LYP/6-311+G(d,p)优化结构上的反应机理进行计算。
每个离子中的两个主要解离反应都会导致中性丙烯和异丁烯的损失。发现两种离子都通过相同的最低能量反应途径解离，唯一的区别在于初始质子化位点。α-蒎烯优先在桥连碳处质子化，而β-蒎烯只能在外环双键处显著质子化。这导致异丁烯损失的出现能量较低，因此β-蒎烯的相对m/z 81碎片离子丰度较高。
初始质子化位点的不同导致了在α-蒎烯和β-蒎烯的碰撞诱导解离中观察到的细微差异。这项工作强调，在离子源中形成的不一定是“能量最低”的离子，在检查碰撞诱导解离质谱时，必须考虑初始结构的任何分布情况。
