Photoelectron–photofragment coincidence (PPC) spectroscopy is a powerful technique for studying the decarboxylation dynamics of carboxyl radicals. Measurement of photoelectron and photofragment kinetic energies in coincidence provides a kinematically complete measure of the dissociative photodetachment (DPD) dynamics of carboxylate anions. PPC spectroscopy studies of methanoate, ethanoate, propanoate, 2-butenoate, benzoate, p -coumarate and the oxalate monoanion are reviewed. All of the systems studied undergo decarboxylation via a two-body DPD channel i.e. , driven by the thermodynamic stability of CO 2 . Additionally, decarboxylation is observed via a three-body ionic photodissociation channel for p -coumarate. In some cases photodetachment also results in a stable carboxyl radical (RCO 2 ). The branching ratio for DPD, the threshold detachment energy and the peak of the kinetic energy release spectrum are compared for different carboxylates, as a probe of the character of the potential energy landscape in the Franck–Condon region.
more »
« less
Photoelectron photofragment coincidence spectroscopy of aromatic carboxylates: benzoate and p -coumarate
Photoelectron–photofragment coincidence spectroscopy was used to study the dissociation dynamics of the conjugate bases of benzoic acid and p -coumaric acid. Upon photodetachment at 266 nm (4.66 eV) both aromatic carboxylates undergo decarboxylation, as well as the formation of stable carboxyl radicals. The key energetics are computed using high-level electronic structure methods. The dissociation dynamics of benzoate were dominated by a two-body DPD channel resulting in CO 2 + C 6 H 5 + e − , with a very small amount of stable C 6 H 5 CO 2 showing that the radical ground state is stable and the excited states are dissociative. For p -coumarate ( p -CA − ) the dominant channel is photodetachment resulting in a stable radical and a photoelectron with electron kinetic energy (eKE) <2 eV. We also observed a minor two-body dissociative photodetachment (DPD) channel resulting in CO 2 + HOC 6 H 4 CHCH + e − , characterized by eKE <0.8 eV. Evidence was also found for a three-body ionic photodissociation channel producing HOC 6 H 5 + HCC − + CO 2 . The ion beam contained both the phenolate and carboxylate isomers of p -CA − , but DPD only occurred from the carboxylate form. For both species DPD is seen from the first and second excited states of the radical, where vibrational excitation is required for decarboxylation from the first excited radical state.
more »
« less
- NSF-PAR ID:
- 10293638
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 23
- Issue:
- 34
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 18414 to 18424
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The anionic products following (H + H + ) abstraction from o -, m -, and p -methylphenol (cresol) are investigated using flowing afterglow-selected ion flow tube (FA-SIFT) mass spectrometry and anion photoelectron spectroscopy (PES). The PES of the multiple anion isomers formed in this reaction are reported, including those for the most abundant isomers, o -, m - and p -methylenephenoxide distonic radical anions. The electron affinity (EA) of the ground triplet electronic state of neutral m -methylenephenoxyl diradical was measured to be 2.227 ± 0.008 eV. However, the ground singlet electronic states of o - and p -methylenephenoxyl were found to be significantly stabilized by their resonance forms as a substituted cyclohexadienone, resulting in measured EAs of 1.217 ± 0.012 and 1.096 ± 0.007 eV, respectively. Upon electron photodetachment, the resulting neutral molecules were shown to have Franck–Condon active ring distortion vibrational modes with measured frequencies of 570 ± 180 and 450 ± 80 cm −1 for the ortho and para isomers, respectively. Photodetachment to excited electronic states was also investigated for all isomers, where similar vibrational modes were found to be Franck–Condon active, and singlet–triplet splittings are reported. The thermochemistry of these molecules was investigated using FA-SIFT combined with the acid bracketing technique to yield values of 341.4 ± 4.3, 349.1 ± 3.0, and 341.4 ± 4.3 kcal mol −1 for the o -, m -, and p -methylenephenol radicals, respectively. Construction of a thermodynamic cycle allowed for an experimental determination of the bond dissociation energy of the O–H bond of m -methylenephenol radical to be 86 ± 4 kcal mol −1 , while this bond is significantly weaker for the ortho and para isomers at 55 ± 5 and 52 ± 5 kcal mol −1 , respectively. Additional EAs and vibrational frequencies are reported for several methylphenyloxyl diradical isomers, the negative ions of which are also formed by the reaction of cresol with O − .more » « less
-
The valence photoionization of light and deuterated methanol dimers was studied by imaging photoelectron photoion coincidence spectroscopy in the 10.00–10.35 eV photon energy range. Methanol clusters were generated in a rich methanol beam in nitrogen after expansion into vacuum. They generally photoionize dissociatively to protonated methanol cluster cations, (CH 3 OH) n H + . However, the stable dimer parent ion (CH 3 OH) 2 + is readily detected below the dissociation threshold to yield the dominant CH 3 OH 2 + fragment ion. In addition to protonated methanol, we could also detect the water- and methyl-loss fragment ions of the methanol dimer cation for the first time. These newly revealed fragmentation channels are slow and cannot compete with protonated methanol cation formation at higher internal energies. In fact, the water- and methyl-loss fragment ions appear together and disappear at a ca. 150 meV higher energy in the breakdown diagram. Experiments with selectively deuterated methanol samples showed H scrambling involving two hydroxyl and one methyl hydrogens prior to protonated methanol formation. These insights guided the potential energy surface exploration to rationalize the dissociative photoionization mechanism. The potential energy surface was further validated by a statistical model including isotope effects to fit the experiment for the light and the perdeuterated methanol dimers simultaneously. The (CH 3 OH) 2 + parent ion dissociates via five parallel channels at low internal energies. The loss of both CH 2 OH and CH 3 O neutral fragments leads to protonated methanol. However, the latter, direct dissociation channel is energetically forbidden at low energies. Instead, an isomerization transition state is followed by proton transfer from a methyl group, which leads to the CH 3 (H)OH + ⋯CH 2 OH ion, the precursor to the CH 2 OH-, H 2 O-, and CH 3 -loss fragments after further isomerization steps, in part by a roaming mechanism. Water loss yields the ethanol cation, and two paths are proposed to account for m/z 49 fragment ions after CH 3 loss. The roaming pathways are quickly outcompeted by hydrogen bond breaking to yield CH 3 OH 2 + , which explains the dominance of the protonated methanol fragment ion in the mass spectrum.more » « less
-
The dissociative photoionization processes of methyl hydroperoxide (CH 3 OOH) have been studied by imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy experiments as well as quantum-chemical and statistical rate calculations. Energy selected CH 3 OOH + ions dissociate into CH 2 OOH + , HCO + , CH 3 + , and H 3 O + ions in the 11.4–14.0 eV photon energy range. The lowest-energy dissociation channel is the formation of the cation of the smallest “QOOH” radical, CH 2 OOH + . An extended RRKM model fitted to the experimental data yields a 0 K appearance energy of 11.647 ± 0.005 eV for the CH 2 OOH + ion, and a 74.2 ± 2.6 kJ mol –1 mixed experimental-theoretical 0 K heat of formation for the CH 2 OOH radical. The proton affinity of the Criegee intermediate, CH 2 OO, was also obtained from the heat of formation of CH 2 OOH + (792.8 ± 0.9 kJ mol –1 ) to be 847.7 ± 1.1 kJ mol –1 , reducing the uncertainty of the previously available computational value by a factor of 4. RRKM modeling of the complex web of possible rearrangement-dissociation processes were used to model the higher-energy fragmentation. Supported by Born–Oppenheimer molecular dynamics simulations, we found that the HCO + fragment ion is produced through a roaming transition state followed by a low barrier. H 3 O + is formed in a consecutive process from the CH 2 OOH + fragment ion, while direct C–O fission of the molecular ion leads to the methyl cation.more » « less
-
Photoelectron angular distributions (PADs) in SO − photodetachment using linearly polarized 355 nm (3.49 eV), 532 nm (2.33 eV), and 611 nm (2.03 eV) light were investigated via photoelectron imaging spectroscopy. The measurements at 532 and 611 nm access the X 3 Σ − and a 1 Δ electronic states of SO, whereas the measurements at 355 nm also access the b 1 Σ + state. In aggregate, the photoelectron anisotropy parameter values follow the general trend with respect to electron kinetic energy (eKE) expected for π*-orbital photodetachment. The trend is similar to O 2 − , but the minimum of the SO − curve is shifted to smaller eKE. This shift is mainly attributed to the exit-channel interactions of the departing electron with the dipole moment of the neutral SO core, rather than the differing shapes of the SO − and O 2 − molecular orbitals. Of the several ab initio models considered, two approaches yield good agreement with the experiment: one representing the departing electron as a superposition of eigenfunctions of a point dipole-field Hamiltonian, and another describing the outgoing electron in terms of Coulomb waves originating from two separated charge centers, with a partial positive charge on the sulfur and an equal negative charge on the oxygen. These fundamentally related approaches support the conclusion that electron–dipole interactions in the exit channel of SO − photodetachment play an important role in shaping the PADs. While a similar conclusion was previously reached for photodetachment from σ orbitals of CN − (Hart, Lyle, Spellberg, Krylov, Mabbs, J. Phys. Chem. Lett. , 2021, 12 , 10086–10092), the present work includes the first extension of the dipole-field model to detachment from π* orbitals.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d1cp02972j
