Methylparaben Isolated in Solid Argon: Structural Characterization and UV-Induced Conversion into Methylparaben Radical and Isomeric Ketenes

Abstract

Methylparaben (methyl p-hydroxybenzoic acid; MP) is a widely used antimicrobial preservative, being the most frequently used antimicrobial preservative in cosmetics. The generalized use of MP has become controversial, with several recent reports of dangerous side effects. For example, the presence of MP in human breast tumors and its harmful effects on human skin exposed to the sunlight have been demonstrated. In spite of the important practical relevance of the compound and of the controversy about its practical use, its structural and photochemical characterization had not been undertaken hitherto. To fill this gap, in the present study, MP was isolated in solid argon (T = 15 K) and structurally characterized by a combined infrared spectroscopy/quantum chemistry approach. The potential energy surface (PES) of the molecule was investigated in detail, revealing the existence of two almost isoenergetic (ΔE(0) = 0.37 kJ mol(-1)) s-cis carboxylic ester low-energy conformers, with an estimated population ratio in the gas phase at room temperature (∼298 K) of ca. 0.83. The calculations also predicted the existence of two high-energy (ΔE(0) = ∼50 kJ mol(-1)) s-trans carboxylic ester conformers of MP. Upon isolation of the compound in an argon matrix, only the lowest energy conformer was found to survive, due to occurrence of extensive conformational cooling during matrix deposition. The infrared spectrum of this conformer was obtained and interpreted. In addition, the chemical processes resulting from in situ irradiation of the matrix-isolated MP with a broadband UV source (λ > 234 nm) were investigated, revealing extensive conversion of MP into highly reactive methylparaben radical and isomeric ketenes. These observations support the recent concerns regarding uses of MP, in particular when the compound has to be exposed to UV light.