Photoionization of Two Potential Biofuel Additives: γ-Valerolactone and Methyl Butyrate

Abstract

The photoionization of two potential biofuel additives, γ-valerolactone (GVL, C 5 H 8 O 2 ) and methyl butyrate (MB, C 5 H 10 O 2 ) has been studied by imaging photoelectron photoion coincidence spectroscopy (iPEPICO) at the VUV beamline of the Swiss Light Source (SLS). The vibrational fine structure in the photoelectron spectrum is compared with a Franck−Condon simulation for the electronic ground-state band of the GVL cation. In the lowest energy dissociative photoionization channel of GVL, CO 2 is lost, resulting in a 1-butene fragment ion with a 0 K appearance energy of E 0 = 10.35 ± 0.01 eV. A newly calculated 1-butene ionization energy of 9.595 ± 0.015 eV establishes the reverse barrier height to CO 2 loss as 66.6 ± 4.3 kJ mol −1 . Methyl butyrate cations undergo McLafferty rearrangement, which explains the missing ion signal at the computed adiabatic ionization energy of 9.25 eV. After H transfer, ethylene is lost in the lowest energy dissociation channel to yield the methyl acetate enol ion at E 0 = 10.24 ± 0.04 eV. This value connects the energetics of methyl butyrate with that of methyl acetate enol ion, which is established atParallel to ethylene loss, methyl loss is also observed from the enol tautomer of the parent ion. Both samples exhibit low-energy nonstatistical dissociative ionization channels. In GVL, the methyl-loss abundance rises quickly but levels off suddenly in the energy range of the first electronically excited states, indicating nonstatistical competition between CH 3 and CO 2 loss. In MB, the major parallel dissociation channel is the loss of a methoxy radical. Calculations indicate that McLafferty rearrangement is inhibited on the excited-state surface. Indeed, breakdown curve modeling of this and a sequential CO-loss channel confirms a second statistical regime in dissociative photoionization, decoupled from ethylene loss.

Isomer-Dependent Threshold Photoelectron Spectroscopy and Dissociative Photoionization Mechanism of Anisaldehyde

“…This goes on to lose CH 3 and forms the m/z 92 In the first step, the methylene attaches to the benzene ring over a 0.97 eV barrier and forms bicyclic [16] at 11.85 eV, which then ring contracts over [17] ‡ to yield [18]. The three-membered ring in [18] opens over the transition state [19] ‡ to form the ketene-loss precursor [20] at 11.23 eV, which yields m/z 65 [21] at 13.67 eV above the neutral.…”

“…Nonetheless, the mass spectral assignment is greatly facilitated if reference mass spectra are known. Cations of similar size as anisaldehyde, for example, of halocyclohexanes or methyl butyrate, often exhibit a varied and quickly changing fragmentation pattern as a function of the internal energy, which is difficult to predict theoretically. Therefore, a detailed investigation of anisaldehyde valence photoionization is desirable to lay the groundwork for using photoionization mass spectrometric approaches to study the oxidative depolymerization of lignin.…”

Discriminating between the dissociative photoionization and thermal decomposition products of ethylene glycol by synchrotron VUV photoionization mass spectrometry and theoretical calculations

“…21,22 Numerous studies have therefore been carried out to study the ionizationinduced reactions of biofuels. 4,19,[23][24][25][26] It has been reported that protonated methanol (CH 3 OH 2…”

“…3 Typical biofuels such as alcohols, ethers, esters, and lactones contain oxygen in the form of hydroxyl, carboxyl, aldehyde, or ketone functional groups. 4 The variation of the composition can lead to different chemical properties, thereby leading to different combustion performances and emissions of harmful substances. 1 For better evaluating the combustion performance and identifying the recurring reaction patterns of biofuels, it is of great significance to study the pyrolysis, oxidation, and combustion chemistry of prototypical variants of potential biofuels.…”

“…21,22 Numerous studies have therefore been carried out to study the ionization-induced reactions of biofuels. 4,19,23–26 It has been reported that protonated methanol (CH 3 OH 2 + ) is formed upon ionization of EG with a photon energy higher than 10.4 eV, which is near the vertical ionization energy (VIE) of EG. 27,28 Interestingly, to generate protonated methanol, a double proton transfer reaction should principally occur prior to the ionization-induced isomerization/dissociation.…”