A Comprehensive Investigation of the Unimolecular Decomposition of Guaiacol and its Derivatives: A Comparison of Neutral, Protonated, and Radical Cation Forms

Abstract

This research addresses a critical knowledge gap regarding the atmospheric fate of methoxyphenols (guaiacols), semi-volatile aromatic oxygenated compounds derived primarily from biomass pyrolysis that serve as precursors to secondary organic aerosol formation. The research objective is to elucidate the unimolecular dissociation mechanisms of guaiacol and its derivatives through a direct comparison of their behaviour across three key environmental forms: the neutral molecule (dominant in the gas phase), the protonated species (relevant to aqueous media like cloud/fog droplets), and the radical cation (relevant to high-energy combustion and photoionization). This work integrates a multifaceted methodology combining tandem mass spectrometry (using electrospray ionization) and density functional theory (DFT) for the protonated ions, mass-analyzed ion kinetic energy (MIKE) spectrometry and high-resolution imaging photoelectron photoion coincidence spectroscopy (iPEPICO) combined with RRKM (Rice-Ramsperger-Kassel-Marcus) kinetic modelling for the radical cations, and systematic low-pressure SiC microreactor pyrolysis coupled with iPEPICO for the neutral molecules.

In Chapter 4, the thermal decomposition of neutral 4-ethylguaiacol and eugenol was probed using a low-pressure pyrolysis SiC microreactor at an approximation of 1000 ºC coupled to iPEPICO spectroscopy. Pyrolysis products were identified using photoion mass-selected threshold photoelectron spectroscopy (ms-TPES) based on their m/z values, supported by ionization energy calculations and Franck-Condon simulations. DFT calculations were conducted to elucidate the detailed reaction mechanisms of the thermal degradation pathways, and microcanonical unimolecular rate constants were evaluated using RRKM theory to assess competing reaction channels. In contrast to prior investigations conducted at atmospheric pressure and lower temperatures, which predominantly invoked free-radical mechanisms, these findings under low-pressure conditions indicate that dissociation can also occur via the loss of closed-shell species. The results establish a mechanistic framework for neutral dissociation in alkylated guaiacols and provide fundamental insight into the thermal behaviour of lignin model compounds under high-temperature, low-pressure conditions.

Chapter 5 explores the dissociation of the protonated ions of guaiacol and its substituted derivatives (creosol, 4-ethylguaiacol, 4-vinylguaiacol, eugenol, and vanillin) and hence, potentially, their atmospheric fate. Owing to their volatility, guaiacol and its derivatives can interact with atmospheric water, forming protonated methoxyphenols via proton transfer. Tandem mass spectrometry (ESI-MS/MS) was employed to analyze the unimolecular dissociation of the protonated forms of these methoxyphenols. DFT calculations were applied to determine the observed minimum energy reaction pathways, and reliable energetics were acquired using CBS-QB3 single-point energy calculations. All the protonated ions, except for vanillin, exhibit the loss of CH₃OH via a series of hydrogen transfers, followed by ring contraction to lose CO. In contrast, vanillin first exhibits the loss of CO, followed by sequential losses of CH₃OH and CO to generate a cyclopentadienyl ion. Altogether, the protonated ions primarily lose CH₃OH and CO as neutral molecules, generating a cyclopentadienyl ion as a dissociation product.

Chapter 6 assesses the gas-phase acid-mediated degradation of guaiacol using reactive tandem mass spectrometry. The ion-molecule reactions between HSO₄⁻ and NO₃⁻ with neutral guaiacol vapour were studied in a modified triple quadrupole mass spectrometer. In both cases, an exothermic reaction was observed, forming neutral benzene and HSO₄(HCOOH)⁻ and NO₃(HCOOH)⁻. Also observed were the formate anion, deprotonated guaiacol and the deprotonated guaiacol dimer ion. Deuterium exchange experiments demonstrated intracluster hydrogen transfer, supporting a mechanism that proceeds via a stabilized encounter complex rather than direct deprotonation. Density functional theory calculations of the reaction pathways demonstrated exothermic reactions in each case. These results establish a mechanistically viable, acid-guided route for the C-O bond cleavage in methoxyphenols under low-solvation, acid-mediated conditions. The study demonstrates that strong Brønsted-like centers can facilitate the catalytic extraction of oxygen as small oxygenates (formic acid) while preserving the aromatic hydrocarbon ring, a finding highly relevant to understanding the role of acidity in bio-oil upgrading catalysts.

Chapter 7 investigates the ion chemistry of radical cations to determine structural and mechanistic similarities with neutral thermal degradation. MIKE spectrometry was employed to investigate the unimolecular dissociation of radical cations, complemented by DFT calculations to elucidate the minimum energy reaction pathways. All the radical cations, apart from vanillin, exhibit the loss of a methyl radical (•CH₃). Methanol (CH₃OH) loss is also observed in selected guaiacol derivatives, with the relative intensities of the methanol-loss fragment ions being lower than that for •CH₃ loss. Conversely, vanillin only displayed CO loss. The observed radical cation chemistry of guaiacol correlates well with previous studies of its neutral thermal decomposition, suggesting that the dissociation of the radical cations of the derivatives may also be analogous to their neutral thermal degradation.

Chapter 8 explores the dissociative ionization pathways of 4-ethylguaiacol and eugenol using iPEPICO spectroscopy and RRKM modelling. Threshold photoelectron spectra (TPES) for both species were recorded and analyzed with Franck-Condon simulations. Breakdown diagrams were analyzed using RRKM theory. 4-Ethylguaiacol dominantly loses a •CH₃ group at low energies, consistent with our prior MIKE study (Chapter 7), although traces of methanol loss are also seen. Nonetheless, the discrepancy between the RRKM-fitted methyl-loss E₀ of 1.88 eV and the previously proposed theoretical value (2.15 eV) led us to find a new reaction pathway involving sequential hydrogen shifts and structural rearrangements consistent with the experimental results. The eugenol radical cation was found to dissociate by the loss of •CH₃ and CH₃OH, in agreement with MIKE results. The energy barriers derived from the RRKM analysis (1.60 eV and 1.52 eV) were again significantly lower than the previous computational reaction barriers of 2.63 eV and 3.21 eV, respectively. Alternative, lower-energy, isomerization-fragmentation mechanisms, analogous to those of 4-ethylguaiacol, were found to be active. This study highlights the critical role of experimental techniques in validating and refining computational models and demonstrates how quantitative spectroscopic data can uncover previously unidentified reaction mechanisms.