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 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 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 m/z 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.