Bridging Experiment and Computation: Unveiling Novel Dissociation Pathways of 4-Ethylguaiacol and Eugenol Radical Cations Using iPEPICO Spectroscopy

The unimolecular dissociative ionization pathways of 4-ethylguaiacol and eugenol were explored using imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. Threshold photoelectron spectra (TPES) for both species were recorded and analyzed with Franck-Condon simulations. Experimental adiabatic ionization energies (IE) are reported for 4-ethylguaiacol (7.65 ± 0.05 eV) and eugenol (7.67 ± 0.05 eV), the latter of which agrees with previous measurements. The first excited state of the 4-ethylguaiacol radical cation and the first two excited states for the eugenol radical cation are also discussed in detail. Breakdown diagrams were analyzed using Rice-Ramsperger-Kassel-Marcus (RRKM) theory. 4-Ethylguaiacol dominantly loses a ^•CH₃ group at low energies, consistent with our prior mass-analyzed ion kinetic energy (MIKE) study, although traces of methanol loss are also seen. Nonetheless, the discrepancy between the RRKM-fitted methyl-loss <i>E</i>₀ 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 and 1.52 eV) were, again, significantly lower than previous computational reaction barriers of 2.63 and 3.21 eV, respectively. Alternative, lower-energy, isomerization-fragmentation mechanisms, analogous to those of 4-ethylguaiacol, were found to be active. Furthermore, an additional fragment ion was observed at <i>m</i>/<i>z</i> 104. 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.