The vacuum ultraviolet (VUV) photochemistry of biogenic volatile organic compounds (BVOCs) plays a crucial role in atmospheric processes, particularly in the formation of secondary organic aerosols and the overall oxidative capacity of the atmosphere. Among these compounds, 2-methyl-3-buten-2-ol (MBO232), an isoprenoid alcohol with significant emission rates from pine species, has garnered attention due to its high reactivity and contribution to regional VOC budgets. This study investigates the dissociative photoionization pathways of MBO232 under tunable VUV synchrotron radiation (8.0–15.0 eV) using time-of-flight mass spectrometry combined with high-level quantum chemical calculations.
Photoionization mass spectra recorded at 9.5, 12.0, and 15.0 eV reveal the formation of multiple fragment ions, including C4H7O+, C3H7O+, C5H9+, C3H6O+, CH3CO+, CH3O+, C4H5+, and C3H5+. The adiabatic ionization energy (AIE) of MBO232 was determined experimentally as 9.43 ± 0.05 eV, consistent with theoretical prediction (9.45 eV). Appearance energies (AEs) for each fragment were measured and found to align well with calculated values, validating the proposed dissociation mechanisms. Notably, the dominant fragmentation channel involves direct CH3 loss from the cationic MBO232, yielding a resonance-stabilized radical cation (C4H7O+), which accounts for the largest branching ratio across all photon energies.
Beyond direct bond cleavage, the data reveal unexpected intramolecular CH3 migration, a rare phenomenon in metal-free organic systems under VUV excitation. Quantum chemical calculations identify five indirect channels involving transition states and intermediates, where the methyl group migrates toward the cationic center or oxygen atom.Goat Anti-Chicken IgY H&L supplier These migrations are driven by electron delocalization via hyperconjugation and enhanced by the weakened C2–C4 bond upon ionization. Electron localization function (ELF) analysis confirms increased electron delocalization and reduced bond order at C2–C4, facilitating methyl transfer. A roaming mechanism is proposed for the CH3 migration to C1, lowering the effective barrier by 0.12 eV compared to conventional transition states, thus enabling early onset of fragment formation.
Kinetic modeling via RRKM theory shows that reaction rate constants, rather than appearance energies alone, govern the competition between channels. While some fragments such as P1 (C4H7O+) dominate due to high rate constants, others like P4 (C3H6O+) exhibit delayed appearance despite low AE, attributed to kinetic bottlenecks.Cadherin-17 Proteinmedchemexpress The sequential decomposition of P1 into P5 (CH3CO+) is confirmed by coincident branching ratio changes at ~11.PMID:35240227 65 eV.
In summary, this work uncovers a unique interplay between direct dissociation and CH3 migration in MBO232, revealing a non-conventional reaction landscape shaped by electronic structure and roaming dynamics. These findings underscore the importance of incorporating complex rearrangement pathways in atmospheric models of BVOCs, especially those involving resonance-stabilized radicals and intramolecular transport processes.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com