Unexpected methyl migrations of ethanol dimer under synchrotron VUV radiation

Abstract

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.

Rotamers and Migration: Investigating the Dissociative Photoionization of Ethylenediamine

“…The interconversion barrier between the two isomers was calculated by Knowles et al 67,68 at 1.83 eV (from • CH 2 NH 2 ) to ascertain the stability of these radicals as they relate to possible amino acid precursors in space. This 1,2-hydrogen transition loosely resembles the 1,2-hydrogen shift from structure [32] to the EN +• isomer [34], suggesting potential future interest in EN and [34] in astrophysical research.…”

“…Several studies have reported ionic isomerization pathways prior to dissociation. 32,33 In light of the work by Yue and Baer 28 with ethylene glycol and Wei et al 29 with EN, the quantum chemical calculations regarding the rotational barriers (from [1] to [5] in Figure 2) and known isomerization processes were necessary for the current PEPICO investigation. A specific lowenergy isomer was reported to be involved in the m/z = 43 and 44 formation channels of EN photodissociation 29 and was reproduced with CBS-QB3.…”

“…This served as a motivation to revisit the dissociation of the ethylenediamine ion via TPEPICO, using the analysis by Wei et al as a starting point for the current investigation. With TPEPICO and computational analysis we further characterize the rotation, H-migration, and functional group migration processes involved in EN dissociative photoionization, as these are of potential interest to the scientific community − and can be applied, for example, to combustion diagnostics …”

Communication: Protonation process of formic acid from the ionization and fragmentation of dimers induced by synchrotron radiation in the valence region

“…The ionization and fragmentation of small organic molecules, as well as their dimers and clusters, have been extensively studied in the valence and in the core level regions. Several experimental techniques, ionization sources, and quantum chemical calculations have been employed to obtain and interpret mass spectra of simple organic molecules from various functional groups such as carboxylic acids, 1-6 alcohols, 7,8 ethers, 9 amides, 10,11 and aldehydes. 12,13 There is an increasing interest in the dimers and small clusters of molecules involving multiple hydrogen bonds, because of their fundamental importance in the formation of biomolecules, 6 such as DNA base pairs.…”

Acetone Dimer Hydrogenation under Vacuum Ultraviolet: An Intracluster Trimolecular Dissociation Mechanism

“…Actually, jet-cooled clusters have been well recognized to be ideal systems for investigating the intermolecular proton transfer as the cluster structures, and rovibronic states are certain before and after the reactions without the interference of the solvent environment. − On the other hand, photoionization studies are often used to study thermodynamic and bonding information on hydrogen bonded clusters. − For example, under vacuum ultraviolet (VUV) radiation, photoionization and photoinduced dissociation of an ethanol dimer were studied, where carbonium ions were observed and followed a Wagner–Meerwein rearrangement after surmounting the barriers of intra- and intermolecular methyl migrations. Also, dimethyluracil was found to readily dissociate in VUV radiation and produce a protonated monomer, where the proton transfer occurred from the methyl groups instead of the aromatic C–H groups .…”