EPR study of methyl and ethyl acrylate radical cations and their transformations in low-temperature matrices

Abstract

Using low-temperature EPR spectroscopy, transformations of radical cations of methyl (MeA) and ethyl acrylate (EtA) radiolytically generated in freon (CFCl2CF2Cl, CF3CCl3 and CFCl3) and in argon matrices were investigated. The assignment of the EPR spectra was made with the help of partially deuterated acrylates, namely ethyl-d2 (EtA-d2) and ethyl-d5 acrylates (EtA-d5) and methyl-d3 acrylate (MeA-d3). In addition, quantum chemical calculations were performed to obtain information on the electronic structure, the hfs constants and energy levels of the transient species observed. The primary radical cations show broad singlet spectra (ΔHpp ≈ 1.4 mT) and transform quickly by hydrogen transfer from the ester group to the carbonyl oxygen leading to distonic radical cations. This transformation can be observed directly at 77 K in the case of the MeA leading to CH2CH–(COH)+–OCH2˙ (a(2Hα) = 2.24 mT, first-order rate constant in CF3CCl3: 1 h−1). The primary cation of EtA could be trapped below 40 K in a CFCl3 matrix only. In all the freon matrices studied, the intramolecular rearrangement of this cation yields the CH2CH–(COH)+–OCH2CH2˙ terminal-type distonic radical cation. The conformation of the latter species in polycrystalline CFCl3 and CF3CCl3 matrices corresponds to the calculated minimum close to the transition state geometry. In the case of CF3CCl3 matrix, warming the samples to temperatures above 130 K results in the simultaneous formation of two new species, which were assigned to six- and five-member ring structures (a(H)/mT: 2.36 (Hα), 5.13 (Hβ1), 1.9 (Hβ2) and 2.27 (2Hα), 2.6 (Hβ), respectively) formed by intramolecular cycloaddition of the terminal radical to the vinyl double bond. The formation of the propagating radical –CH2–CH˙–R due to an ion-molecule reaction is observed in CFCl2CF2Cl at temperatures above 98 K; this process was also detected in other freons under the conditions of matrix softening. The primary radical cation of EtA is not trapped in an argon matrix even at 16 K due to the realisation of the “high-energy” reaction paths yielding methyl radicals and, probably, rearrangement products.