Unexpected methyl migrations of ethanol dimer under synchrotron VUV radiation

While methyl transfer is well known to occur in the enzyme- and metal-catalyzed reactions, the methyl transfer in the metal-free organic molecules induced by the photon ionization has been less concerned. Herein, vacuum ultraviolet single photon ionization and dissociation of ethanol dimer are investigated with synchrotron radiation photoionization mass spectroscopy and theoretical methods. Besides the protonated clusters cation (C₂H₅OH) ⋅ H⁺ (m/z = 47) and the β-carbon-carbon bond cleavage fragment CH₂O ⋅ (C₂H₅OH)H⁺ (m/z = 77), the measured mass spectra revealed that a new fragment (C₂H₅OH) ⋅ (CH₃)⁺ (m/z = 61) appeared at the photon energy of 12.1 and 15.0 eV, where the neutral dimer could be vertically ionized to higher ionic state. Thereafter, the generated carbonium ions are followed by a Wagner-Meerwein rearrangement and then dissociate to produce this new fragment, which is considered to generate after surmounting a few barriers including intra- and inter-molecular methyl migrations by the aid of theoretical calculations. The appearance energy of this new fragment is measured as 11.55 ± 0.05 eV by scanning photoionization efficiency curve. While the signal intensity of fragment m/z = 61 starts to increase, the fragments m/z = 47 and 77 tend to slowly incline around 11.55 eV photon energy. This suggests that the additional fragment channels other than (C₂H₅OH) ⋅ H⁺ and CH₂O ⋅ (C₂H₅OH)H⁺ have also been opened, which consume some dimer cations. The present report provides a clear description of the photoionization and dissociation processes of the ethanol dimer in the range of the photon energy 12-15 eV