Radical Chemistry in the Thermal Decomposition of Anisole and Deuterated Anisoles: An Investigation of Aromatic Growth

Advancement of lignin into bioactive compounds through selective organic synthesis methods

The conversion of lignin into bioactive compounds through selective organic synthesis methods represents a promising frontier in the pursuit of sustainable raw materials and green chemistry. This review explores the versatility of lignin-derived bioactive compounds, ranging from their application in drug discovery to their role in the development of biodegradable materials. Despite notable advancements, the synthesis routes and yields of highly bioactive molecules from lignin still require further exploration and improvement. This review provides an in-depth examination of the progress made in understanding the complex structure of lignin and developing innovative approaches to exploit its potential. Specifically, the types of lignins covered include softwood Kraft lignin, hardwood organosolv lignin, and soda lignin. This work is divided into three parts: first, the transformation of lignin into bioactive molecules with chemically active centres and functionalised hydroxyl groups through depolymerisation; second, kinetic modelling techniques essential for understanding the chemical kinetics of lignin and enabling significant scaling up in the conversion of organic molecules; third, efficient catalytic pathways for synthesising molecules with anticancer and antibacterial properties. In conclusion, this comprehensive review spurs further investigations into lignin-derived bioactive compounds, their applications, and the advancement of sustainable processes.

Rotational Spectrum of the Phenoxy Radical

We report the hyperfine-resolved rotational spectrum of the gas-phase phenoxy radical in the 8-25 GHz frequency range using cavity Fourier transform microwave spectroscopy. A complete assignment of its complex but well-resolved fine and hyperfine splittings yielded a precisely determined set of rotational constants, spin-rotation parameters, and nuclear hyperfine coupling constants. These results are interpreted with support from high-level quantum chemical calculations to gain detailed insight into the distribution of the unpaired π electron in this prototypical resonance-stabilized radical. The accurate laboratory rest frequencies enable studies of the chemistry of phenoxy in both the laboratory and space. The prospects of extending the present experimental and theoretical techniques to investigate the rotational spectra of isotopic variants and structural isomers of phenoxy and other important gas-phase radical intermediates that are yet undetected at radio wavelengths are discussed.

IR/UV Double Resonance Study of the 2-Phenylallyl Radical and its Pyrolysis Products

Isolated 2-phenylallyl radicals (2-PA), generated by pyrolysis from a nitrite precursor, have been investigated by IR/UV ion dip spectroscopy using free electron laser radiation. 2-PA is a resonance-stabilized radical that is considered to be involved in the formation of polycyclic aromatic hydrocarbons (PAH) in combustion, but also in interstellar space. The radical is identified based on its gas-phase IR spectrum. Furthermore, a number of bimolecular reaction products are identified, showing that the self-reaction as well as reactions with unimolecular decomposition products of 2-PA form several PAH efficiently. Possible mechanisms are discussed and the chemistry of 2-PA is compared with the one of the related 2-methylallyl and phenylpropargyl radicals.

Probing the Pyrolysis of Guaiacol and Dimethoxybenzenes Using Collision-Induced Dissociation Charge-Remote Fragmentation Mass Spectrometry

The dissociation of lignin model compounds has been examined using mass spectrometry and collision-induced dissociation charge-remote fragmentation (CID-CRF). The model compounds guaiacol and o- and m-dimethoxybenzene containing a remote sulfonate (SO₃^-) charge group undergo CID by dissociation without the involvement of the anionic group. The first dissociation for all three compounds is loss of methyl radical to form phenoxy radicals. Subsequent dissociation pathways depend on the specific structures being examined The dissociation pathways are compared to those observed upon gas-phase pyrolysis that have been reported previously. While the pathways are largely similar, there are some important differences that are explained by changes in dissociation barriers due to the effect of adding the charged group. This work shows that CID-CRF is an effective approach for tracking the thermolysis of lignin model compounds while eliminating secondary reactions that normally convolute such studies.

Probing the Reaction Mechanisms of 3,5-Difluoro-2,4,6-Trinitroanisole (DFTNAN) through a Comparative Study with Trinitroanisole (TNAN)

To probe the thermal decomposition mechanisms of a novel fluorinated low-melting-point explosive 3,5-difluoro-2,4,6-trinitroanisole (DFTNAN), a comparative study with trinitroanisole (TNAN) was performed under different heating conditions. The thermal decomposition processes and initial reactions were monitored by DSC-TG-FTIR-MS and T-jump-PyGC-MS coupling analyses, respectively. The results show that fluorine decreased the thermal stability of the molecular structure, and the trigger bond was transferred from the ortho-nitro group of the ether to the para-nitro group. The possible reaction pathway of DFTNAN after the initial bond breakage is the rupture of the dissociative nitro group with massive heat release, which induces the ring opening of benzene. Major side reactions include the generation of polycyclic compounds and fluorine atom migration. Fluorine affects the thermal stability and changes the reaction pathway, and fluorinated products appear in the form of fluorocarbons due to the high stability of the C-F bond.

The gas-phase infrared spectra of the 2-methylallyl radical and its high-temperature reaction products

The resonance-stabilized 2-methylallyl radical, 2-MA, is considered as a possible intermediate in the formation of polycyclic aromatic hydrocarbons (PAHs) in combustion processes. In this work, we report on its contribution to molecular growth in a high-temperature microreactor and provide mass-selective IR/UV ion dip spectra of the radical, as well as the various jet-cooled reaction products, employing free electron laser radiation in the mid-infrared region. Small (aromatic) hydrocarbons such as fulvene, benzene, styrene, or para-xylene, as well as polycyclic molecules, like (methylated) naphthalene, were identified with the aid of ab initio DFT computations. Several reaction products differ by one or more methyl groups, suggesting that molecular growth is dominated by (de)methylation in the reactor.

A Conical Intersection Influences the Ground State Rearrangement of Fulvene to Benzene

The rearrangement of fulvene to benzene is believed to play an important role in the formation of soot during hydrocarbon combustion. Previous work has identified two possible mechanisms for the rearrangement─a unimolecular path and a hydrogen-atom-assisted, bimolecular path. Computational results to date have suggested that the unimolecular mechanism faces a barrier of about 74 kcal/mol, which makes it unable to compete with the bimolecular mechanism under typical combustion conditions. This computed barrier is about 10 kcal/mol higher than the experimental value, which is an unusually large discrepancy for modern electronic structure theory. In the present work, we have reinvestigated the unimolecular mechanism computationally, and we have found a second transition state that is approximately 10 kcal/mol lower in energy than the previously identified one and, therefore, in excellent agreement with the experimental value. The existence of two transition states for the same rearrangement arises because there is a conical intersection between the two lowest singlet states which occurs in the vicinity of the reaction coordinates. The two possible paths around the cone on the lower adiabatic surface give rise to the two distinct saddle points. The lower barrier for the unimolecular mechanism now makes it competitive with the bimolecular one, according to our calculations. In support of this conclusion, we have reanalyzed some previous experimental results on anisole pyrolysis, which leads to benzene as a significant product and have shown that the unimolecular and bimolecular mechanisms for fulvene → benzene must be occurring competitively in that system. Finally, we have identified that similar conical intersections arise during the isomerizations of benzofulvene and isobenzofulvene to naphthalene.

Experimental study on light volatile products from thermal decomposition of lignin monomer model compounds: effect of temperature, residence time and methoxyl group

In order to investigate the effects of temperature, residence time (RT) and methoxyl (OCH₃) on the product distribution and vapor phase reactions during pyrolysis of complex solid fuels, three model phenolic representatives, phenol, guaiacol and syringol, were pyrolyzed at a residence time of 0.7 s, over a temperature range of 400 °C–950 °C, and at temperatures of 650 °C and 750 °C, in a RT region of 0.1 s–4.2 s. Increasing yields of CO and C₁–C₅ light hydrocarbons (LHs) with RT at 650 °C and 750 °C indicated that ring-reduction/CO elimination of phenolic compounds happened at 650 °C, and dramatically at 750 °C. The addition of OCH₃ affects the product distribution and ring-reduction pathways: C₅ LHs from phenol, C₂ LHs, C₄ LHs and C₅ LHs from guaiacol, and C₁–C₂ LHs from syringol. CO₂ yields increase with the addition of OCH₃. CO₂ was formed via benzoyl and a four-membered ring, which would compete with the CO formation. The addition of OCH₃ promotes the formation of coke and tar. The decomposition pathways are discussed, based on the experimental data, focusing on ring-reduction reactions and the formation of CO/CO₂ and C₁–C₅ LHs.

Effects of temperature, residence time and methoxyl on the decomposition of phenol, guaiacol and syringol, were investigated. Thermal decomposition pathways of the three model compounds were discussed based on ring reduction/CO elimination reactions.

Mapping the intrinsic absorption properties and photodegradation pathways of the protonated and deprotonated forms of the sunscreen oxybenzone

Sunscreens provide vital protection against the photodamaging effects of UV radiation, however, many fundamental questions remain about the detailed mechanisms by which they dissipate UV energy. One such issue is the extent to which the pH environment of an organic sunscreen molecule alters its effectiveness, both in terms of ability to absorb UV radiation, and also its potential to photodegrade. Here, we use gas-phase laser photodissociation spectroscopy for the first time to measure the intrinsic UVA-UVC absorption spectra and associated photodegradation products of protonated and deprotonated oxybenzone, away from the complications of bulk mixtures. Our results reveal that protonation state has a dramatic effect on the absorption and photodissociation properties of this sunscreen. While the UV absorption profile of oxybenzone is only modestly affected by protonation across the range from 400-216 nm, deprotonated oxybenzone displays a significantly modified absorption spectrum, with very low photoabsorption between 370-330 nm. Protonated oxybenzone primarily photofragments by rupture of the bonds on either side of the central carbonyl group, producing cationic fragments with m/z 151 and 105. Additional lower mass photofragments (e.g. m/z 95 and 77) are also observed. The production spectra for the photofragments from protonated oxybenzone fall into two distinct categories, which we discuss in the context of different excited state decay pathways. For deprotonated oxybenzone, the major photofragments observed are m/z 211 and 212, which are associated with the ejection of methane and the methyl free radical from the parent ion, respectively. Implications for the suitability of oxybenzone in its protonated and deprotonated forms as an optimum sunscreen molecule are discussed.

Self-Reaction of ortho-Benzyne at High Temperatures Investigated by Infrared and Photoelectron Spectroscopy

ortho-Benzyne, a Kekulé-type biradical is considered to be a key intermediate in the formation of polycyclic aromatic hydrocarbons (PAH) and soot. In the present work we study the ortho-benzyne self-reactions in a hot microreactor and identify the high-temperature products by IR/UV spectroscopy and by photoion mass-selected threshold photoelectron spectroscopy (ms-TPES) in a free jet. Ms-TPES confirms formation of ortho-benzyne as generated from benzocyclobutenedione, as well as benzene, biphenylene, diacetylene, and acetylene, originating from the reaction o-C₆H₄ → HCC-CCH + C₂H₂, and CH₃. PAH molecules like naphthalene, 2-ethynylnaphthalene, fluorene, phenanthrene, and triphenylene are identified based on their IR/UV spectra. By comparison with recent computations their formation starting from o-benzyne can be readily understood and supports the importance of the biradical addition (1,4-cycloaddition followed by fragmentation) pathway to PAH molecules, recently proposed by Comandini et al.

Dimerization of the Benzyl Radical in a High-Temperature Pyrolysis Reactor Investigated by IR/UV Ion Dip Spectroscopy

We investigate the self-reaction of benzyl, C₇ H₇ , in a high-temperature pyrolysis reactor. The work is motivated by the observation that resonance-stabilized benzyl radicals can accumulate in reactive environments and contribute to the formation of polycyclic aromatic hydrocarbons (PAHs) and soot. Reaction products are detected by IR/UV ion dip spectroscopy, using infrared radiation from the free electron laser FELIX, and are identified by comparison with computed spectra. Among the reaction products identified by their IR absorption are several PAHs linked to toluene combustion such as bibenzyl, phenanthrene, diphenylmethane, and fluorene. The identification of 9,10-dihydrophenanthrene provides evidence for a mechanism of phenanthrene formation from bibenzyl that proceeds by initial cyclization rather than an initial hydrogen loss to stilbene.

Reaction-time-resolved measurements of laser-induced fluorescence in a shock tube with a single laser pulse

Shock tubes allow for the study of ultra-fast gas-phase reactions on the microsecond time scale. Because the repetition rate of the experiments is low, it is crucial to gain as much information as possible from each individual measurement. While reaction-time-resolved species concentration and temperature measurements with fast absorption methods are established, conventional laser-induced fluorescence (LIF) measurements with pulsed lasers provide data only at a single reaction time. Therefore, fluorescence methods have rarely been used in shock-tube diagnostics. In this paper, a novel experimental concept is presented that allows reaction-time-resolved LIF measurements with one single laser pulse using a test section that is equipped with several optical ports. After the passage of the shock wave, the reactive mixture is excited along the center of the tube with a 266-nm laser beam directed through a window in the end wall of the shock tube. The emitted LIF signal is collected through elongated sidewall windows and focused onto the entrance slit of an imaging spectrometer coupled to an intensified CCD camera. The one-dimensional spatial resolution of the measurement translates into a reaction-time-resolved measurement while the species information can be gained from the spectral axis of the detected two-dimensional image. Anisole pyrolysis was selected as the benchmark reaction to demonstrate the new apparatus.

Thermal Decomposition of Potential Ester Biofuels. Part I: Methyl Acetate and Methyl Butanoate

Two methyl esters were examined as models for the pyrolysis of biofuels. Dilute samples (0.06-0.13%) of methyl acetate (CH₃COOCH₃) and methyl butanoate (CH₃CH₂CH₂COOCH₃) were entrained in (He, Ar) carrier gas and decomposed in a set of flash-pyrolysis microreactors. The pyrolysis products resulting from the methyl esters were detected and identified by vacuum ultraviolet photoionization mass spectrometry. Complementary product identification was provided by matrix infrared absorption spectroscopy. Pyrolysis pressures in the pulsed microreactor were about 20 Torr and residence times through the reactors were roughly 25-150 μs. Reactor temperatures of 300-1600 K were explored. Decomposition of CH₃COOCH₃ commences at 1000 K, and the initial products are (CH₂═C═O and CH₃OH). As the microreactor is heated to 1300 K, a mixture of CH₂═C═O and CH₃OH, CH₃, CH₂═O, H, CO, and CO₂ appears. The thermal cracking of CH₃CH₂CH₂COOCH₃ begins at 800 K with the formation of CH₃CH₂CH═C═O and CH₃OH. By 1300 K, the pyrolysis of methyl butanoate yields a complex mixture of CH₃CH₂CH═C═O, CH₃OH, CH₃, CH₂═O, CO, CO₂, CH₃CH═CH₂, CH₂CHCH₂, CH₂═C═CH₂, HCCCH₂, CH₂═C═C═O, CH₂═CH₂, HC≡CH, and CH₂═C═O. On the basis of the results from the thermal cracking of methyl acetate and methyl butanoate, we predict several important decomposition channels for the pyrolysis of fatty acid methyl esters, R-CH₂-COOCH₃. The lowest-energy fragmentation will be a 4-center elimination of methanol to form the ketene RCH═C═O. At higher temperatures, concerted fragmentation to radicals will ensue to produce a mixture of species: (RCH₂ + CO₂ + CH₃) and (RCH₂ + CO + CH₂═O + H). Thermal cracking of the β C-C bond of the methyl ester will generate the radicals (R and H) as well as CH₂═C═O + CH₂═O. The thermochemistry of methyl acetate and its fragmentation products were obtained via the Active Thermochemical Tables (ATcT) approach, resulting in Δ_fH₂₉₈(CH₃COOCH₃) = -98.7 ± 0.2 kcal mol^-1, Δ_fH₂₉₈(CH₃CO₂) = -45.7 ± 0.3 kcal mol^-1, and Δ_fH₂₉₈(COOCH₃) = -38.3 ± 0.4 kcal mol^-1.

Does a Nitrogen Lone Pair Lead to Two Centered-Three Electron (2c-3e) Interactions in Pyridyl Radical Isomers?

Each of the three isomeric pyridyl radicals (2-, 3-, and 4-dehydropyridines) contains a lone pair and an unpaired electron. As a result, a potential two centered-three electron interaction between the radical electron and the lone pair through-space (TS) and/or through-bond (TB) can exist that may influence the stability of the radicals. Due to the change in geometrical positions relative to each other, the strength of interaction can be varied. In this study, we investigated the structural and stability aspects of pyridyl radical isomers with a major emphasis on the interaction of a nitrogen lone pair with the radical center. In order to obtain evidence for such interactions, protonated and N-oxide analogues of the corresponding isomeric pyridyl radicals have been considered in such a way to understand the consequences due to unavailability of the lone pair. Similarly, electron attachment and detachment energies at the radical center (vertical detachment energy, VDE, of corresponding anions and vertical ionization energy, VIE, of radical isomers) have been calculated to find out the interaction trend upon modification at the radical center. Different levels of theory including (U)B3LYP/cc-pVTZ, (U)M06/cc-pVTZ, CBS-QB3, single-point energy calculations at (U)CCSD(T)/cc-pVTZ, and multireference CASSCF/cc-pVTZ methods have been employed in this regard. A closer inspection of geometries, relative stability order, spin density, electrostatic potential, molecular orbitals, NBO analysis, and vibrational analysis have showed a strong and stabilizing TS interaction between the radical center and the lone pair in the case of the 2-pyridyl radical. On the other hand, the 4-pyridyl radical showed stabilizing interactions only via TB coupling, whereas the TS interaction is nonexistent. Despite the presence of both interactions in the case of the 3-pyridyl radical, their overall influence is less effective toward stability.

Are the three hydroxyphenyl radical isomers created equal?--The role of the phenoxy radical

We have investigated the thermal decomposition of the three hydroxyphenyl radicals (˙C6H4OH) in a heated microtubular reactor. Intermediates and products were identified isomer-selectively applying photoion mass-selected threshold photoelectron spectroscopy with vacuum ultraviolet synchrotron radiation. Similarly to the phenoxy radical (C6H5-O˙), hydroxyphenyl decomposition yields cyclopentadienyl (c-C5H5) radicals in a decarbonylation reaction at elevated temperatures. This finding suggests that all hydroxyphenyl isomers first rearrange to form phenoxy species, which subsequently decarbonylate, a mechanism which we also investigate computationally. Meta- and para-radicals were selectively produced and spectroscopically detectable, whereas the ortho isomer could not be traced due to its fast rethermalization and rapid decomposition in the reactor. A smaller barrier to isomerization to phenoxy was found to be the reason for this observation. Since hydroxyphenyl species may be present under typical sooting conditions in flames, the resonantly stabilized cyclopentadienyl radical adds to the hydrocarbon pool and can contribute to the formation of polycyclic aromatic hydrocarbons, which are precursors in soot formation.

On the formation of cyclopentadiene in the C3H5˙ + C2H2 reaction

The reaction between the allyl radical (C3H5˙) and acetylene (C2H2) in a heated microtubular reactor has been studied at the VUV beamline of the Swiss Light Source. The reaction products are sampled from the reactor and identified by their photoion mass-selected threshold photoelectron spectra (ms-TPES) by means of imaging photoelectron photoion coincidence spectroscopy. Cyclopentadiene is identified as the sole reaction product by comparison of the measured photoelectron spectrum with that of cyclopentadiene. With the help of quantum-chemical computations of the C5H7 potential energy surface, the C2H2 + C3H5˙ association reaction is confirmed to be the rate determining step, after which H-elimination to form C5H6 is prompt in the absence of re-thermalization at low pressures. The formation of cyclopentadiene as the sole product from the allyl + acetylene reaction offers a direct path to the formation of cyclic hydrocarbons under combustion relevant conditions. Subsequent reactions of cyclopentadiene may lead to the formation of the smallest polycyclic aromatic molecule, naphthalene.

Formation of polycyclic aromatic hydrocarbons from bimolecular reactions of phenyl radicals at high temperatures

The self-reaction of the phenyl radical is one of the key reactions in combustion chemistry.

Photofragmentation of Gas-Phase Lanthanide Cyclopentadienyl Complexes: Experimental and Time-Dependent Excited-State Molecular Dynamics

Unimolecular gas-phase laser-photodissociation reaction mechanisms of open-shell lanthanide cyclopentadienyl complexes, Ln(Cp)₃ and Ln(TMCp)₃, are analyzed from experimental and computational perspectives. The most probable pathways for the photoreactions are inferred from photoionization time-of-flight mass spectrometry (PI-TOF-MS), which provides the sequence of reaction intermediates and the distribution of final products. Time-dependent excited-state molecular dynamics (TDESMD) calculations provide insight into the electronic mechanisms for the individual steps of the laser-driven photoreactions for Ln(Cp)₃. Computational analysis correctly predicts several key reaction products as well as the observed branching between two reaction pathways: (1) ligand ejection and (2) ligand cracking. Simulations support our previous assertion that both reaction pathways are initiated via a ligand-to-metal charge-transfer (LMCT) process. For the more complex chemistry of the tetramethylcyclopentadienyl complexes Ln(TMCp)₃, TMESMD is less tractable, but computational geometry optimization reveals the structures of intermediates deduced from PI-TOF-MS, including several classic “tuck-in” structures and products of Cp ring expansion. The results have important implications for metal–organic catalysis and laser-assisted metal–organic chemical vapor deposition (LCVD) of insulators with high dielectric constants.

Laser ablation with resonance-enhanced multiphoton ionization time-of-flight mass spectrometry for determining aromatic lignin volatilization products from biomass

We have designed and developed a laser ablation∕pulsed sample introduction∕mass spectrometry platform that integrates pyrolysis (py) and∕or laser ablation (LA) with resonance-enhanced multiphoton ionization (REMPI) reflectron time-of-flight mass spectrometry (TOFMS). Using this apparatus, we measured lignin volatilization products of untreated biomass materials. Biomass vapors are produced by either a custom-built hot stage pyrolysis reactor or laser ablation using the third harmonic of an Nd:YAG laser (355 nm). The resulting vapors are entrained in a free jet expansion of He, then skimmed and introduced into an ionization region. One color resonance-enhanced multiphoton ionization (1+1 REMPI) is used, resulting in highly selective detection of lignin subunits from complex vapors of biomass materials. The spectra obtained by py-REMPI-TOFMS and LA-REMPI-TOFMS display high selectivity and decreased fragmentation compared to spectra recorded by an electron impact ionization molecular beam mass spectrometer (EI-MBMS). The laser ablation method demonstrates the ability to selectively isolate and volatilize specific tissues within the same plant material and then detect lignin-based products from the vapors with enhanced sensitivity. The identification of select products observed in the LA-REMPI-TOFMS experiment is confirmed by comparing their REMPI wavelength scans with that of known standards.