Does the dissociation of guaiacol family radical cations mimic their thermal decomposition?

Abstract

Guaiacol (2-methoxyphenol) and its derivatives are naturally occurring phenolic compounds which originate primarily from wood lignin. Guaiacol pyrolysis has been extensively studied where degradation products have been identified and reaction schemes have been proposed illustrating their formation. The aim of the present work is to study the dissociation of the radical cations of guaiacol and its derivatives and explore whether there is any similarity with the thermal degradation of the neutral compounds. Tandem mass spectrometry was employed to explore the unimolecular dissociation of the radical cations of guaiacol (1), creosol (2), 4-ethylguaiacol (3), 4-vinylguaiacol (4), eugenol (5) and vanillin (6). Calculations were performed at the CBS-QB3(sp)// B3LYP/6–311 + G(d,p) level of theory and minimum energy reaction pathways are presented. All the radical cations, with the exception of vanillin, exhibit the loss of a methyl radical (CH3•) with the favored CH3• loss pathway being via bond cleavage from the methoxy group for 1, 2 and 4. Methanol (CH3OH) loss is also observed in 2, 3 with the relative intensities of the methanol-loss fragment ions in 2 and 3 being lower than that for CH3• loss. Conversely, vanillin only displayed CO loss. Altogether, the radical cation chemistry for guaiacol agrees well with previous studies on its neutral thermal decomposition, which suggests that the dissociation of the radical cations of the derivatives may also be analogous to their neutral thermal degradation.