To roam or not to roam, that is the question for the methyl group in isopropanol cations

Covert, Kyle J.; Bödi, András; Torma, Krisztián G.; Voronova, Krisztina; Baer, Tomas; Sztáray, Bálint

doi:10.1016/j.ijms.2020.116469

Cited by 6 publications

(8 citation statements)

References 34 publications

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“…The complementary approach of experimental data analysis, statistical modeling, and potential energy surface calculations can provide most insights into the fragmentation mechanism. ,, Potential energy surface exploration is guided by the experimental data to reveal reaction coordinates consistent with the observed fragmentation channels as a function of parent ion internal energy. We have explored multiple bond breaking and isomerization pathways and only report those that appear likely or possible in comparison with the experimental observations.…”

Section: Resultsmentioning

confidence: 99%

Valence Photoionization and Energetics of Vanillin, a Sustainable Feedstock Candidate

Zhou

Bjelić

et al. 2021

J. Phys. Chem. A

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We studied the valence photoionization of vanillin by photoelectron photoion coincidence spectroscopy in the 8.20−19.80 eV photon energy range. Vertical ionization energies by EOM-IP-CCSD calculations reproduce the photoelectron spectral features. Composite method calculations and Franck−Condon simulation of the weak, ground-state band yield the adiabatic ionization energy of the most stable vanillin conformer as 8.306(20) eV. The lowest energy dissociative photoionization channels correspond to hydrogen atom, carbon monoxide, and methyl losses, which form the dominant C 8 H 7 O 3 + (m/z 151) and the less intense C 7 H 8 O 2 + (m/z 124) and C 7 H 5 O 3+ (m/z 137) fragment ions in parallel dissociation channels at modeled 0 K appearance energies of 10.13(1), 10.40(3), and 10.58(10) eV, respectively. On the basis of the breakdown diagram, we explore the energetics of sequential methyl and carbon monoxide loss channels, which dominate the fragmentation mechanism at higher photon energies. The 0 K appearance energy for sequential CO loss from the m/z 151 fragment to C 7 H 7 O 2 + (m/z 123) is 12.99(10) eV, and for sequential CH 3 loss from the m/z 123 fragment to C 6 H 4 O 2 + (m/z 108), it is 15.40(20) eV based on the model. Finally, we review the thermochemistry of the bi-and trifunctionalized benzene derivatives guaiacol, hydroxybenzaldehyde, anisaldehyde, and vanillin. On the basis of isodesmic functional group exchange reactions, we propose new enthalpies of formations, among them Δ f H°2 98K (vanillin, g) = −383.5 ± 2.9 kJ mol −1 . These mechanistic insights and ab initio thermochemistry results will support analytical works to study lignin conversion involving vanillin.

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Section: Resultsmentioning

confidence: 99%

Valence Photoionization and Energetics of Vanillin, a Sustainable Feedstock Candidate

Zhou

Bjelić

et al. 2021

J. Phys. Chem. A

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“…Thus, the precursor ion to SF 3 + must be the parent ion itself, and the observed CF 3 + , SF 5 + , and SF 3 + fragment ions are the result of a parallel fragmentation mechanism of CF 3 SF 5 + . Similar to the breakdown diagram of isopropanol, which implies statistical OH loss although it is not, this is another example that cursory analysis of the breakdown diagram can be misleading regarding the reaction mechanism.…”

Section: Resultsmentioning

confidence: 83%

From Energetics to Intracluster Chemistry: Valence Photoionization of Trifluoromethylsulfur Pentafluoride (CF₃SF₅) by Double Velocity Map Imaging

2021

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Trifluoromethylsulfur pentafluoride (CF3SF5) was valence threshold photoionized in a double imaging photoelectron photoion coincidence spectrometer using vacuum ultraviolet synchrotron radiation. In the 12.5–16.4 eV photon energy range, CF3 +, SF5 +, and SF3 + cations were observed in both room temperature (RT) and molecular beam (MB) experiments. Their fractional abundances exhibited differences beyond the sample temperature. Kinetic energy analysis of the fragment ions confirmed the difference in the dissociative photoionization mechanism. In the RT experiment, the CF3 + kinetic energies were extrapolated to a 11.84 ± 0.15 eV threshold, which was used in an ion cycle to determine the enthalpy of formation of CF3SF5 as Δf H°298K(CF3SF5) = −1593 ± 16 kJ mol–1. We also updated the enthalpy of formation of the sulfur pentafluoride radical as Δf H°298K(SF5) = −854 ± 7 kJ mol–1 and discuss the discrepancy between the CF3 ionization energy based on the Active Thermochemical Tables and the value anchored to the CF ionization energy. A computed reaction enthalpy network optimization resulted in Δf H°298K(CF3SF5) = −1608 ± 20 kJ mol–1. Both values for Δf H°298K(CF3SF5) agree with previous ab initio ones in contrast to the original, experimental determination. SF3 + is formed by F-transfer processes both in the RT and MB experiments. Although the same peaks were observed in both experiments, the lower SF3 + onset energy and the more slowly rising CF3 + kinetic energy release in the MB experiment revealed clustering and intracluster F-transfer reactions upon ionization. The monomer and dimer cation potential energy surfaces were explored to rationalize the observations. In the dimer cation, the observer CF3SF5 catalyzes fluorine transfer and promotes CF4 formation, which ultimately leads to the SF3 + fragment ion.

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“…23 The breakdown diagram of MB is also analyzed in the statistical framework, which not only reveals sequential and parallel dissociation channels but also indicates that methoxy radical loss takes place in an excited electronic state, similar to OH loss in methanol 36 and 2-propanol. 37 In this excited state, γ-hydrogen transfer to the carbonyl group is kinetically inhibited.…”

Section: Introductionmentioning

confidence: 99%

“…Thanks to parent ion internal energy selection by monochromatic VUV single-photon ionization and photoelectron kinetic energy analysis, as well as the measurement of unimolecular dissociation rates in the 10 4 s –1 < k ( E ) < 10 7 s –1 range, we will be able to revisit the structure of CO 2 -loss product ion in GVL, formed over a sizable reverse barrier, and identify it as the 1-butene cation, instead of the methylcyclopropane structure proposed previously . The breakdown diagram of MB is also analyzed in the statistical framework, which not only reveals sequential and parallel dissociation channels but also indicates that methoxy radical loss takes place in an excited electronic state, similar to OH loss in methanol and 2-propanol . In this excited state, γ-hydrogen transfer to the carbonyl group is kinetically inhibited.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2021

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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.

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To roam or not to roam, that is the question for the methyl group in isopropanol cations

Cited by 6 publications

References 34 publications

Valence Photoionization and Energetics of Vanillin, a Sustainable Feedstock Candidate

Valence Photoionization and Energetics of Vanillin, a Sustainable Feedstock Candidate

From Energetics to Intracluster Chemistry: Valence Photoionization of Trifluoromethylsulfur Pentafluoride (CF₃SF₅) by Double Velocity Map Imaging

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

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To roam or not to roam, that is the question for the methyl group in isopropanol cations

Cited by 6 publications

References 34 publications

Valence Photoionization and Energetics of Vanillin, a Sustainable Feedstock Candidate

Valence Photoionization and Energetics of Vanillin, a Sustainable Feedstock Candidate

From Energetics to Intracluster Chemistry: Valence Photoionization of Trifluoromethylsulfur Pentafluoride (CF3SF5) by Double Velocity Map Imaging

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

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From Energetics to Intracluster Chemistry: Valence Photoionization of Trifluoromethylsulfur Pentafluoride (CF₃SF₅) by Double Velocity Map Imaging