Gas-Phase Infrared Spectra of the C7H5 Radical and Its Bimolecular Reaction Products.

Resonance-stabilized radicals are considered as possible intermediates in the formation of polycyclic aromatic hydrocarbons (PAHs) in interstellar space. Here, we investigate the fulvenallenyl radical, the most stable C7H5 isomer by IR/UV ion dip spectroscopy employing free electron laser radiation in the mid-infrared region between 550 and 1750 cm-1. The radical is generated by pyrolysis from phthalide. Various jet-cooled reaction products are identified by their mass-selective IR spectra in the fingerprint region, based on a comparison with computed spectra. Interestingly, benzyl is present as a second resonance-stabilized radical. It is connected to fulvenallenyl by a sequence of two H atom losses or additions. Among the identified aromatic hydrocarbons are toluene and styrene, as well as polycyclic molecules, such as indene, naphthalene, fluorene and phenanthrene. Mechanisms for the formation of PAH from C7H5 have already been suggested in previous computational work. In particular, the radical/radical reaction of two fulvenallenyl radicals provides an efficient route to phenanthrene in one bimolecular step and might be relevant for PAH formation under astrochemical conditions.

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.

Do Xylylenes Isomerize in Pyrolysis?

We report infrared spectra of xylylene isomers in the gas phase, using free electron laser (FEL) radiation. All xylylenes were generated by flash pyrolysis. The IR spectra were obtained by monitoring the ion dip signal, using a IR/UV double resonance scheme. A gas phase IR spectrum of para-xylylene  was recorded, whereas ortho- and meta-xylylene were found to partially rearrange to benzocyclobutene and styrene. Computations of the UV oscillator strength  for all molecules were carried out and provde an explanation for the observation of the isomerization products.

Reduced above-ground growth and wood density but increased wood chemical concentrations of Scots pine on relict charcoal hearths.

Deciphering the drivers of tree growth is a central aim of dendroecology. In this context, soil conditions may play a crucial role, since they determine the availability of water and nutrients for trees. Yet, effects of systematically differing soil conditions on tree growth render a marginally studied topic. In this context, relict charcoal hearths (RCH) - a widespread legacy of anthropogenic charcoal production - render a valuable 'natural' experiment to study possible effects of artificially altered soil conditions on tree growth. We hypothesize, that the differing physico-chemical properties of RCH result in differing wood properties if compared to trees growing on unmodified soils. To test this hypothesis, we analyzed tree-growth, wood density, and wood elemental concentrations of Scots pine as well as physico-chemical soil properties. We applied a classic control-treatment design to compare RCH with unmodified soils. Our analyses identified significantly lower above-ground wood production but systematically higher wood elemental concentrations in RCH-trees compared to control trees. Since we could not identify treatment-specific growth patterns, we hypothesize the observed lower above-ground productivity of Scots pine to indicate an increased root-shoot ratio to compensate for a potentially lower plant water availability on RCH-sites. The observed higher wood elemental concentrations likely reflect higher soil elemental concentrations of Fe, Ca, K, and Mn in RCH soils. In conclusion, our study highlights diverse effects of RCH on tree growth and wood properties and strengthens the value of dendro-chemistry to use the tree-ring archive as proxy for soil conditions within a dendro-ecological context.

The Gas-Phase Infrared Spectra of Xylyl Radicals.

The three isomers of the xylyl radical, C8H9, are possible intermediates in the formation of soot and polycyclic aromatic hydrocarbons (PAH). Their infrared spectra have been recorded by IR/UV ion dip spectroscopy using free electron laser radiation. The radicals were generated by flash pyrolysis from the corresponding nitrites and resonantly ionized via the D3 ← D0 transition around 310 nm. Mid-infrared spectra of the three xylyl isomers were recorded between 550 and 1700 cm-1 and are in excellent agreement with computations, provided that overtones and combination bands are included in the simulation. The results show that the three xylyl isomers can be distinguished by their infrared spectra and that no isomerization occurs in the pyrolysis reactor. The IR spectra obtained at m/z = 208 indicate that dimerization of xylyl radicals leads to substituted stilbenes, which has not been observed for benzyl.

Carbon monoxide insertion at a heavy p-block element: unprecedented formation of a cationic bismuth carbamoyl.

Major advances in the chemistry of 5th and 6th row heavy p-block element compounds have recently uncovered intriguing reactivity patterns towards small molecules such as H2, CO2, and ethylene. However, well-defined, homogeneous insertion reactions with carbon monoxide, one of the benchmark substrates in this field, have not been reported to date. We demonstrate here, that a cationic bismuth amide undergoes facile insertion of CO into the Bi-N bond under mild conditions. This approach grants direct access to the first cationic bismuth carbamoyl species. Its characterization by NMR, IR, and UV/vis spectroscopy, elemental analysis, single-crystal X-ray analysis, cyclic voltammetry, and DFT calculations revealed intriguing properties, such as a reversible electron transfer at the bismuth center and an absorption feature at 353 nm ascribed to a transition involving σ- and π-type orbitals of the bismuth-carbamoyl functionality. A combined experimental and theoretical approach provided insight into the mechanism of CO insertion. The substrate scope could be extended to isonitriles.

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.

Excited state dynamics and time-resolved photoelectron spectroscopy of para-xylylene.

We investigated the excited-state dynamics of para-xylylene using a combination of field-induced surface hopping (FISH) simulations and time-resolved ionisation experiments. Our simulations predict an ultrafast decay of the initially excited bright state (S2/S3) to the S1 state on a sub-100 fs time scale, followed by return to the ground state within ∼1 ps. This is accompanied by a transient change of the biradical character of the molecule, as monitored by calculating natural orbital occupation numbers. Specifically, the initially low biradicality is increased by electronic excitation as well as by vibrational activation. Experimentally, para-xylylene was generated by pyrolysis from [2,2]paracyclophane and excited with 266 nm radiation into the S2/S3 bright state. The subsequent dynamics were followed using ionisation as the probe step, with both mass spectra and photoelectron spectra recorded as a function of pump-probe delay. The observed decay of photoelectron and photoion intensities closely matches the theoretical predictions and is consistent with the sequential mechanism found in the simulations. This mechanism exhibits characteristic signatures in both time-resolved mass and photoelectron spectra, in particular in the appearance of fragment ions that are exclusively generated from the S1 state. This allows for a separation of the S2 and S1 dynamics in the photoelectron and mass spectra. An excellent agreement between the observed and the simulated ion signal is observed.

Stimulus-Triggered Formation of an Anion-Cation Exciplex in Copper(I) Complexes as a Mechanism for Mechanochromic Phosphorescence.

The investigation of the mechanisms of mechanochromic luminescence is of fundamental importance for the development of materials for photonic sensors, data storage, and luminescence switches. The structural origin of this phenomenon in phosphorescent molecular systems is rarely known and thus the formulation of structure-property relationships remains challenging. Changes in the M-M interactions have been proposed as the main mechanism with d10 coinage metal compounds. Herein, we describe a new mechanism-a mechanically induced reversible formation of a cation-anion exciplex based on Cu-F interactions-that leads to highly efficient mechanochromic phosphorescence and unusual large emission shifts from UV-blue to yellow for CuI complexes. The low-energy luminescence is thermo- and vaporesponsive, thus allowing the generation of white light as well as for recovering the original UV-blue emission.

Impact of crystalline and amorphous iron- and aluminum hydroxides on mechanisms of phosphate adsorption and desorption.

Fourier-transform infrared (FT-IR) spectroscopic experiments were carried out during phosphate adsorption on highly crystalline gibbsite, poorly crystalline 2-line-ferrihydrite and amorphous iron-aluminum-hydroxide mixtures in the molar ratio 1:0, 10:1, 5:1, 1:1, 1:5, 1:10 and 0:1. The OH stretching vibrational bands were utilized to analyze changes in structural and surface OH groups during adsorption, because the position of characteristic PO vibrational bands can shift depending on reaction conditions, pH or adsorbed phosphate content. Adsorption and desorption kinetics were studied at pH6 and different initial phosphate concentrations to achieve varying phosphate coverage on the mineral surfaces. For gibbsite the formation of AlHPO4 and Al2HPO4 can be assumed, while for ferrihydrite, a FeHPO4 or Fe2PO4 complex and the precipitation of FePO4 with longer equilibration time were proposed. Fe2HPO4 or a Fe2PO4 surface complex was deduced for Fe-hydroxides, an AlH2PO4 surface complex was identified for Al-hydroxide, and both displayed either hydrogen bonds to neighboring hydroxyl groups or hydrogen bonds to outer-sphere complexes. Fe:Al-hydroxide mixtures with high Al ratios showed a low phosphate desorption rate, while ferrihydrite and the Fe:Al-hydroxide mixtures with high Fe ratios had almost negligible desorption rates. It was concluded that within the weakly associated amorphous FeO(OH) materials, FePO4 precipitated, which was bound by outer-sphere hydrogen bonds. With high Al ratios, desorption increased, which indicated weaker phosphate binding of both inner-sphere and outer-sphere complexes and hence, either no or minor quantities of precipitate. Ferrihydrite showed a more rigid structure and a lower extent of precipitation compared to amorphous Fe-hydroxide.

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.

Well-controlled in-situ growth of 2D WO3 rectangular sheets on reduced graphene oxide with strong photocatalytic and antibacterial properties.

Finding the materials, which help to control the water pollution caused by organic and bacterial pollutants is one of the challenging tasks for the scientific community. 2D sheets of WO3 and composite of WO3 and reduced graphene oxide (rGO) have been synthesized in a well-controlled way using a hydrothermal method. The as synthesized 2D sheet of WO3 and rGO-WO3 composite were characterized by various techniques. The 2D sheets of WO3 and rGO-WO3 composite are efficiently utilized for the photocatalytic degradation of methylene blue (MB) and Rhodamine B (RhB) dyes under sunlight. The rGO-WO3 composite reveals excellent photocatalytic degradation of RhB dye by degrading it upto 85% under sunlight. However, the MB dye was degraded by 32%. The greater degradation of RhB dye was explained in terms of the molecular electrostatic potential. We found that RhB has a more positive potential compared to MB dye where O2- and OH̊ radicals interact more strongly, resulting in a greater degradation of the RhB dye. The antibacterial activity of the 2D sheets of WO3 and rGO-WO3composite was also investigated on gram positive (B. subtilis) and gram negative (P. aeroginosa) microbes for the first time.