Concerted Fragmentation Research Articles

Native frames: An approach for separating sequential and concerted three-body fragmentation

Native frames: An approach for separating sequential and concerted three-body fragmentation

The Effect of β-Hydrogens on the Tropospheric Photochemistry of Aldehydes: Norrish Type 1, Triple Fragmentation, and Methylketene Formation from Propanal.

Wavelength and pressure dependent quantum yields (ϕ, QYs) of propanal photolysis have been measured for photolysis wavelengths, λ = 300-330 nm, and buffer gases of 3-10 Torr propanal and 0-757 Torr N2. Following laser photolysis, three photochemical pathways were established, using Fourier transform infrared spectroscopy of the stable end-products. Photolysis is dominated by the Norrish Type 1 reaction, which has been reported previously, but with inconsistent quantum yields. The propanal α-hydrogen leads to a 4-center elimination of H2, as observed in CH3CHO, here leading to methylketene. The presence of hydrogen attached to the β-carbon allows a new photochemical pathway: concerted triple fragmentation into CO + H2 + C2H4 via a 5-center transition state. Neither of these channels has been reported previously. No evidence for the previously reported C2H6 + CO, C2H4 + H2CO or CH3 + CH2CHO channels, nor for phototautomerization to 1-propenol (CH3CH═CHOH) was found. Modeling of the wavelength, pressure and collision partner dependence of the QYs allows us to reconcile the previous NT1a results and make recommendations for the quantum yields of all three channels under tropospheric conditions. The general impact of β-hydrogen atoms in the photochemistry of aldehydes is to open up new pathways from cyclic transition states and to reduce the importance of other photolysis or isomerization channels.

Coherence mapping to identify the intermediates of multi-channel dissociative ionization

Identifying the short-lived intermediates and reaction mechanisms of multi-channel radical cation fragmentation processes remains a current and important challenge to understanding and predicting mass spectra. We find that coherent oscillations in the femtosecond time-dependent yields of several product ions following ultrafast strong-field ionization represent spectroscopic signatures that elucidate their mechanism of formation and identify the intermediate(s) they originate from. Experiments on endo-dicyclopentadiene show that vibrational frequencies from various intermediates are mapped onto their resulting products. Aided by ab initio methods, we identify the vibrational modes of both the cleaved and intact molecular ion intermediates. These results confirm stepwise and concerted fragmentation pathways of the dicyclopentadiene ion. This study highlights the power of tracking the femtosecond dynamics of all product ions simultaneously and sheds further light onto one of the fundamental reaction mechanisms in mass spectrometry, the retro-Diels Alder reaction.

Probing H_{2} Double Ionization with Bicircular Laser Fields.

We present a kinematically complete study on strong-field double ionization of H_{2} molecules in two-color bicircular laser fields. The releasing times of electrons and protons are recorded with the double-hand attoclock. We observe the relative emission angles of two electrons oscillate with the kinetic energy release of protons, indicating the internal concerted four-body fragmentation. Using a three-dimensional molecular semiclassical ensemble model, we have disentangled the attosecond correlated electron emission in H_{2} double ionization. This work reveals the strong electron-nuclear coupling in the molecular bond breaking and may open up a new approach to experimentally accessing the intramolecular electron and bond dynamics with bicircular fields.

Steric Perturbation of the Grob-like Final Step of Ethylene-Forming Enzyme Enables 3-Hydroxypropionate and Propylene Production.

Ethylene-forming enzyme (EFE) is an iron(II)-dependent dioxygenase that fragments 2-oxoglutarate (2OG) to ethylene (from C3 and C4) and 3 equivs of carbon dioxide (from C1, C2, and C5). This major ethylene-forming pathway requires l-arginine as the effector and competes with a minor pathway that merely decarboxylates 2OG to succinate as it oxidatively fragments l-arginine. We previously proposed that ethylene forms in a polar-concerted (Grob-like) fragmentation of a (2-carboxyethyl)carbonatoiron(II) intermediate, formed by the coupling of a C3-C5-derived propion-3-yl radical to a C1-derived carbonate coordinated to the Fe(III) cofactor. Replacement of one or both C4 hydrogens of 2OG by fluorine, methyl, or hydroxyl favored the elimination products 2-(F1-2/Me/OH)-3-hydroxypropionate and CO2 over the expected olefin or carbonyl products, implying strict stereoelectronic requirements in the final step, as is known for Grob fragmentations. Here, we substituted active-site residues expected to interact sterically with the proposed Grob intermediate, aiming to disrupt or enable the antiperiplanar disposition of the carboxylate electrofuge and carbonate nucleofuge required for concerted fragmentation. The bulk-increasing A198L substitution barely affects the first partition between the major and minor pathways but then, as intended, markedly diminishes ethylene production in favor of 3-hydroxypropionate. Conversely, the bulk-diminishing L206V substitution enables propylene formation from (4R)-methyl-2OG, presumably by allowing the otherwise sterically disfavored antiperiplanar conformation of the Grob intermediate bearing the extra methyl group. The results provide additional evidence for a polar-concerted ethylene-yielding step and thus for the proposed radical-polar crossover via substrate-radical coupling to the Fe(III)-coordinated carbonate.

Three-body fragmentation dynamics of BrCNq+ (q=3–6) induced by 1-keV electron impact

The three-body fragmentation dynamics of $\mathrm{BrC}{\mathrm{N}}^{q+} (q=3--6)$ produced by 1-keV electron impact has been investigated using the ion momentum imaging technique. Up to 11 three-body Coulomb explosion channels were identified and analyzed. The corresponding kinetic energy release (KER) distributions were obtained and compared with the predictions of the Coulomb explosion model. By means of the Dalitz plot, Newton diagram, and native frame method, we have studied the concerted and sequential fragmentation mechanisms for channels leading to ${\mathrm{Br}}^{l+}+{\mathrm{C}}^{+}+{\mathrm{N}}^{+} (l=1--3)$. The KER for the intermediate dications $\mathrm{Br}{\mathrm{C}}^{2+}$ and $\mathrm{C}{\mathrm{N}}^{2+}$ from the sequential mechanism were determined and the electronic states of the intermediate molecular ion $\mathrm{C}{\mathrm{N}}^{2+}$ were discussed. For the channels leading to higher charge states of the carbon and nitrogen ions, only the concerted fragmentation mechanism was observed.

Fragmentation of SO2 q+ (q = 2-4) induced by 1keV electron collision.

We report an investigation on the fragmentation dynamics of SO2 q+ (q = 2-4) induced by 1keV electron collision utilizing an ion momentum imaging spectrometer. Six complete Coulomb explosion channels were observed using the time-of-flight correlation map. The kinetic energy release distributions for these channels were obtained and compared with those available in the literature. The fragmentation mechanisms of the three-body dissociation channels were analyzed by the Dalitz plots and Newton diagrams. Both concerted breakup and sequential fragmentation pathways were identified in the channel SO2 3+ → O+ + O+ + S+, whereas only the concerted breakup mechanism was confirmed for the channels SO2 4+ → O+ + O+ + S2+ and SO2 4+ → O2+ + O+ + S+. Using the Coulomb explosion model, we determined the molecular geometry from the concerted fragmentation channels, and the obtained bond lengths and angles from the higher kinetic energy release peaks are close to that of the neutral SO2 obtained by high-level quantum chemical calculation. The present results indicate that the electron impact experiment is a potential tool for the Coulomb explosion imaging of small molecules.

Fragmentation dynamics of acetylene in collision with highly charged ions: Concerted and sequential breakage of CH and CC bonds

The three-body fragmentation of ${\mathrm{C}}_{2}\mathrm{H}{{}_{2}}^{3+}$ to ${\mathrm{H}}^{+}+{\mathrm{C}}^{+}+{\mathrm{CH}}^{+}$ as a consequence of one CH and one CC bond breaking is investigated by 50-keV/u ${\mathrm{Ne}}^{8+}$ impact. All three fragments are detected in coincidence with a scattered projectile (either ${\mathrm{Ne}}^{7+}$ or ${\mathrm{Ne}}^{6+}$) employing a reaction microscope, and their momentum vectors as well as the kinetic energies were obtained. Four distinguished structures are observed in the energy correlation spectra, indicating that abundant fragmentation mechanisms contribute to the ${\mathrm{H}}^{+}+{\mathrm{C}}^{+}+{\mathrm{CH}}^{+}$ channel. The Newton diagrams and Dalitz plots are employed to trace fragmentation mechanisms. We found that both the concerted fragmentation and the sequential pathway with CH bond breaking prior to CC contribute to this channel. The possible electronic states of the ${\mathrm{C}}_{2}\mathrm{H}{{}_{2}}^{3+}$ precursor that may contribute to the identified fragmentation mechanisms are analyzed with the help of quantum chemical calculations. Furthermore, the influence of the collision dynamics between the projectile and the target to the dissociation mechanisms is discussed by comparing the contributions from the reaction channel with transferring one electron while ionizing the other two, i.e., T1I2, and the reversed channel T2I1. The T2I1 channel is observed to be more efficient to initiate fragmentation mechanisms leading to higher kinetic-energy release.

Concerted and sequential three-body fragmentation of deep-core-ionized carbon disulfide.

Momentum vector correlation is a powerful tool to study molecular dissociation. We have studied the three-body fragmentation of carbon disulfide after sulfur 1s photoionization by means of momentum imaging techniques. Concerted and sequential pathways are disentangled in three-body fragmentation using adapted analysis strategies. In particular, we introduce various data visualization schemes that are proved to be particularly efficient to determine dissociation dynamics.

Dissociative multi-ionization of N2O molecules in strong femtosecond laser field.

Multi-ionization and subsequent Coulomb explosion (CE) of the N2O molecule irradiated by a linearly polarized 800nm laser field is investigated by a reaction microscope, where a number of CE channels of N2Oq+ with q ≤ 5 for two-body fragmentation and q ≤ 8 for three-body fragmentation were observed. For two-body CE, by analyzing the internuclear separations extracted from kinetic energy releases (KERs), dissociation branching fractions, and laser intensity dependence, interestingly, we found that fragmentation N2O5+ → N3+ + NO2+ is produced directly from dissociating N2O3+ via non-sequential stairstep ionization, whereas most of the others result from the sequential stairstep ionization. For three-body CE, 25 fragmentation channels of N2Oq+ (q = 3-8) are distinguished in the present charge-encoded multi-photoion coincidence plot, and the concerted fragmentation mechanism is nominated in a typical Dalitz plot. With the help of the numerical computation with the measured KERs and momentum correlation angles, the geometric structures of molecular ions prior to fragmentation are reconstructed, which display the bending motion and simultaneous two-bond stretching before the CE. Increasing of the bond length for high charged N2Oq+ indicates the dominating stairstep ionization in the three-body fragmentation.

Fragmentation Dynamics of a Carbon Dioxide Dication Produced by Ion Impact.

The response of carbon dioxide to radiolysis is crucial for understanding the atmospheric chemistry of planets. Here, we present a combined experimental and theoretical investigation of the three-body fragmentation dynamics of to C+ + O+ + O initiated by 1 keV/u Ar2+ impact. Taking advantage of the kinematic complete measurement employing a reaction microscope, three dissociation mechanisms are distinguished, and their branching ratios are determined. The concerted fragmentation with two C-O bonds breaking simultaneously is dominant, while the sequential pathway with CO+ as the intermediate also makes a significant contribution. Also, a novel isomerization pathway with transitory formation of is identified. The identified mechanisms can contribute to O+ and O escaping from the Martian atmosphere, since the kinetic energies of most of the fragments are observed to be higher than the escape energy of oxygen.

Three-body fragmentation dynamics of CH3CCH3+ investigated by 50-keV/u Ne8+ impact: Comparison with its isomer ion CH2CCH23+

The three-body fragmentation dynamics of propyne $({\mathrm{CH}}_{3}\mathrm{C}\mathrm{C}\mathrm{H})$ to ${\mathrm{H}}^{+}$ $+$ $\mathrm{C}{\mathrm{H}}^{+}$ $+$ ${\mathrm{C}}_{2}{\mathrm{H}}_{2}{}^{+}$ and ${\mathrm{H}}^{+}$ $+$ ${\mathrm{CH}}_{2}{}^{+}$ $+$ ${\mathrm{C}}_{2}{\mathrm{H}}^{+}$ is investigated by 50-keV/u ${\mathrm{Ne}}^{8+}$ ion impact. Employing the reaction microscope, all three ionic fragments are detected in coincidence, and the momentum vector as well as kinetic energy of each fragment is obtained. By analyzing the momentum and kinetic energy correlation between different fragments, various fragmentation mechanisms are identified, and the relative ratio of each mechanism is determined. We find that the concerted fragmentation with CH and CC bonds breaking simultaneously and the sequential pathway with CH breakage prior to the CC breakage are the major contributions to both channels. In contrast, the sequential pathway with CC breakage prior to CH breakage was found to make a minor contribution only to the ${\mathrm{H}}^{+}$ $+$ $\mathrm{C}{\mathrm{H}}^{+}$ $+$ ${\mathrm{C}}_{2}{\mathrm{H}}_{2}{}^{+}$ channel. In addition, we compare the present results for ${\mathrm{CH}}_{3}\mathrm{C}\mathrm{C}\mathrm{H}$ with ${\mathrm{CH}}_{2}{\mathrm{CCH}}_{2}$ published in Ma et al. [C. Ma, S. Xu, D. Zhao, D. Guo, S. Yan, W. Feng, X. Zhu, and X. Ma, Phys. Rev. A 101, 052701 (2020)]. The location of the CH and CC bonds' breakage, either from distinct C atoms or from the same C atom, leads to similar dissociation mechanisms but different ionic fragments for the two isomers.

Two- and three-body fragmentation of multiply charged tribromomethane by ultrafast laser pulses.

We investigate the two- and three-body fragmentation of tribromomethane (bromoform, CHBr3) resulting from multiple ionization by 28-femtosecond near-infrared laser pulses with a peak intensity of 6 × 1014 W cm-2. The analysis focuses on channels consisting exclusively of ionic fragments, which are measured by coincidence momentum imaging. The dominant two-body fragmentation channel is found to be Br+ + CHBr2+. Weaker HBr+ + CBr2+, CHBr+ + Br2+, CHBr2+ + Br2+, and Br+ + CHBr22+ channels, some of which require bond rearrangement prior to or during the fragmentation, are also observed. The dominant three-body fragmentation channel is found to be Br+ + Br+ + CHBr+. This channel includes both concerted and sequential fragmentation pathways, which we identify using the native frames analysis method. We compare the measured kinetic energy release and momentum correlations with the results of classical Coulomb explosion simulations and discuss the possible isomerization of CHBr3 to BrCHBr-Br (iso-CHBr3) prior to the fragmentation.

Investigation of the carbon monoxide dication lifetime using (CO) 2 dimer fragmentation

The fragmentation of carbon monoxide dimers induced by collisions with low-energy ${\mathrm{Ar}}^{9+}$ ions is investigated using the cold target recoil ion momentum spectroscopy technique. The presence of a neighbor molecule in the dimer serves here as a diagnostic tool to probe the lifetimes of the $\mathrm{C}{\mathrm{O}}^{2+}$ molecular dications resulting from the collision. The existence of metastable states with lifetimes ranging from 2 ps to 200 ns is clearly evidenced experimentally through a sequential three-body fragmentation of the dimer, whereas fast dissociation channels are observed in a so-called concerted three-body fragmentation process. The fast fragmentation process leads to a kinetic energy release distribution also observed in collisions with monomer CO targets. This is found in contradiction with the conclusions of a previous study attributing this fast process to the perturbation induced by the neighbor molecular ion.

Dissociation dynamics in chloroform molecule induced by ion impact

The ion impact multiple ionization and subsequent dissociation of Chloroform (CHCl3) is investigated with Ar7+ ion beam having an energy of 1 MeV. Using a linear time of flight mass spectrometer coupled with a position sensitive detector, complete as well as incomplete Coulomb explosion pathways for CHCl32+ and CHCl33+ ions are identified and studied. Possible modes of fragmentation pathway for all the identified channels are investigated with the help of kinetic energy release and momentum distributions of fragmented ions. Although concerted fragmentation is the most dominant pathway within the identified channels, we have discussed the possibilities of sequential processes which may occur. A positive slope between the coincidence partners of H+ and C+ probably features sequentiality in the breakup dynamics but remain inconclusive due to lack of complete information. Formation of Cl2+ from the doubly charged CHCl32+ is a direct evidence of bond rearrangement reaction following a structural change during charge separation.

Three-body fragmentation mechanism of C2H43+ produced by 18−keV/uNe8+ impact

In this article we present the three-body fragmentation dynamics of ${\mathrm{C}}_{2}{{\mathrm{H}}_{4}}^{3+}$ caused by $18\text{\ensuremath{-}}\mathrm{keV}/\mathrm{u}\phantom{\rule{0.16em}{0ex}}{\mathrm{Ne}}^{8+}$ ion impact. Utilizing the cold target recoil ion momentum spectroscopy, the complete kinematical information of three fragmentation channels, i.e., ${\mathrm{C}}_{2}{{\mathrm{H}}_{4}}^{3+}\ensuremath{\rightarrow}{\mathrm{H}}^{+}+{\mathrm{H}}^{+}+{\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{+}, {\mathrm{H}}^{+}+\mathrm{C}{\mathrm{H}}^{+}+\mathrm{C}{{\mathrm{H}}_{2}}^{+}$, and ${\mathrm{H}}^{+}+{{\mathrm{H}}_{2}}^{+}+{\mathrm{C}}_{2}{\mathrm{H}}^{+}$ is obtained. For each channel, both concerted and sequential fragmentation mechanisms are observed and differentiated through the momentum correlation of resultant fragments visualized by the Dalitz plot and the Newton diagram. In particular, for the channel ${\mathrm{C}}_{2}{{\mathrm{H}}_{4}}^{3+}\ensuremath{\rightarrow}{\mathrm{H}}^{+}+{\mathrm{H}}^{+}+{\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{+}$, it is found in addition to the sequential pathway that the two protons can be emitted concertedly in two ways, i.e., from the same carbon atom or from the two carbon atoms. The present results show a more diverse molecular fragmentation triggered by ion collisions in contrast to a previous study using intense laser field ionization.

Fragmentation dynamics of CO24+ : Contributions of different electronic states

Fragmentation of ${\mathrm{CO}}_{2}^{4+}$, created by the impact of slow, highly charged ions $(96 \mathrm{keV} {\mathrm{Ar}}^{q+};$ $8\ensuremath{\le}q\ensuremath{\le}14)$ on ${\mathrm{CO}}_{2}$, has been studied by recoil ion momentum spectrometry. ${\mathrm{CO}}_{2}^{4+}$ was found to dissociate into three ionic fragments through two channels: ${\mathrm{CO}}_{2}^{4+}\ensuremath{\rightarrow}{\mathrm{O}}^{+}+{\mathrm{C}}^{2+}+{\mathrm{O}}^{+}$ [the (1,2,1) channel] and ${\mathrm{CO}}_{2}^{4+}\ensuremath{\rightarrow}{\mathrm{O}}^{2+}+{\mathrm{C}}^{+}+{\mathrm{O}}^{+}$ [the (2,1,1) channel]. The kinetic energy of each fragment ion and the total kinetic energy release (KER) distributions for these channels were derived. Ab initio quantum chemical calculations at the multiconfiguration self-consistent-field configuration-interaction level of theory were carried out to obtain the potential energy curves of ${\mathrm{CO}}_{2}^{4+}$, from which the expected KER values were derived. A comparison of the experimental KER distributions with the expected KER values for different excited states enabled the estimation of the relative probabilities of accessing different electronic states of ${\mathrm{CO}}_{2}^{4+}$. These probabilities were found to depend on the projectile charge $q$. The fragmentation was visualized using the Dalitz plots, which revealed that the (1,2,1) channel arises exclusively via concerted fragmentation, while the (2,1,1) channel also has a small contribution from sequential fragmentation.

The "Hidden" Reductive [2+2+1]-Cycloaddition Chemistry of 2-Phosphaethynolate Revealed by Reduction of a Th-OCP Linkage.

The reduction chemistry of the newly emerging 2‐phosphaethynolate (OCP)− is not well explored, and many unanswered questions remain about this ligand in this context. We report that reduction of [Th(TrenTIPS)(OCP)] (2, TrenTIPS=[N(CH2CH2NSiPri 3)]3−), with RbC8 via [2+2+1] cycloaddition, produces an unprecedented hexathorium complex [{Th(TrenTIPS)}6(μ‐OC2P3)2(μ‐OC2P3H)2Rb4] (5) featuring four five‐membered [C2P3] phosphorus heterocycles, which can be converted to a rare oxo complex [{Th(TrenTIPS)(μ‐ORb)}2] (6) and the known cyclometallated complex [Th{N(CH2CH2NSiPri 3)2(CH2CH2SiPri 2CHMeCH2)}] (4) by thermolysis; thereby, providing an unprecedented example of reductive cycloaddition reactivity in the chemistry of 2‐phosphaethynolate. This has permitted us to isolate intermediates that might normally remain unseen. We have debunked an erroneous assumption of a concerted fragmentation process for (OCP)−, rather than cycloaddition products that then decompose with [Th(TrenTIPS)O]− essentially acting as a protecting then leaving group. In contrast, when KC8 or CsC8 were used the phosphinidiide C−H bond activation product [{Th(TrenTIPS)}Th{N(CH2CH2NSiPri 3)2[CH2CH2SiPri 2CH(Me)CH2C(O)μ‐P]}] (3) and the oxo complex [{Th(TrenTIPS)(μ‐OCs)}2] (7) were isolated.

The “Hidden” Reductive [2+2+1]‐Cycloaddition Chemistry of 2‐Phosphaethynolate Revealed by Reduction of a Th‐OCP Linkage

AbstractThe reduction chemistry of the newly emerging 2‐phosphaethynolate (OCP)− is not well explored, and many unanswered questions remain about this ligand in this context. We report that reduction of [Th(TrenTIPS)(OCP)] (2, TrenTIPS=[N(CH2CH2NSiPri3)]3−), with RbC8 via [2+2+1] cycloaddition, produces an unprecedented hexathorium complex [{Th(TrenTIPS)}6(μ‐OC2P3)2(μ‐OC2P3H)2Rb4] (5) featuring four five‐membered [C2P3] phosphorus heterocycles, which can be converted to a rare oxo complex [{Th(TrenTIPS)(μ‐ORb)}2] (6) and the known cyclometallated complex [Th{N(CH2CH2NSiPri3)2(CH2CH2SiPri2CHMeCH2)}] (4) by thermolysis; thereby, providing an unprecedented example of reductive cycloaddition reactivity in the chemistry of 2‐phosphaethynolate. This has permitted us to isolate intermediates that might normally remain unseen. We have debunked an erroneous assumption of a concerted fragmentation process for (OCP)−, rather than cycloaddition products that then decompose with [Th(TrenTIPS)O]− essentially acting as a protecting then leaving group. In contrast, when KC8 or CsC8 were used the phosphinidiide C−H bond activation product [{Th(TrenTIPS)}Th{N(CH2CH2NSiPri3)2[CH2CH2SiPri2CH(Me)CH2C(O)μ‐P]}] (3) and the oxo complex [{Th(TrenTIPS)(μ‐OCs)}2] (7) were isolated.

A novel interaction mechanism in lignin pyrolysis: Phenolics-assisted hydrogen transfer for the decomposition of the β-O-4 linkage

Abstract Pyrolysis of lignin involves complex unimolecular decomposition and bimolecular interaction mechanisms. Unimolecular decomposition of lignin model compounds has been widely investigated, while bimolecular interactions are far from well elucidated. Currently, only two radical reaction-based interactions, i.e., hydrogen abstraction and hydrogen-bond-induced homolysis mechanisms, have been proposed to explain the interactions in the lignin pyrolysis process, which ignores the concerted reaction-based interactions involving the pyrolytic products of lignin. Particularly, a large number of phenolics are produced in the pyrolysis of lignin, which can further affect the concerted pyrolysis reactions of lignin. In the present work, a novel interaction mechanism, phenolics-assisted hydrogen transfer (phenolics-AHT) process for the concerted reactions, is confirmed and carefully investigated. Phenylethyl phenyl ether (PPE), a typical β-O-4 lignin dimer, is used as the model compound to reveal the phenolics-AHT mechanism in the lignin pyrolysis system. Density functional theory (DFT) calculation, electrostatic potential (ESP) analysis, and fast pyrolysis experiment results are combined to reveal that the phenolic products participate in the hydrogen transfer process of the concerted Maccoll elimination reaction by means of their phenolic hydroxyl structures and significantly reduce the energy barrier, thus promoting the cleavage of the β-O-4 linkage. On the contrary, phenolics-AHT has no obvious effect on the concerted retro-ene fragmentation. At medium pyrolysis temperatures, the hydrogen transfer processes induced by phenolic products play a more significant role in the dissociation of the β-O-4 linkage than the hydrogen abstraction induced by radicals, due to the limited amount of free radicals. In addition, DFT calculation results further suggest that decarbonylation, decarboxylation, dehydration, tautomerism, and other concerted reactions can also be promoted by phenolics-AHT.