Photodissociation of Dibromobenzenes at 266 nm by the Velocity Imaging Technique

A velocity imaging technique combined with (2+1) resonance‐enhanced multiphoton ionization (REMPI) is used to detect the primary Br(²P_3/2) fragment in the photodissociation of o‐, m‐, and p‐dibromobenzene at 266 nm. The obtained translational energy distributions suggest that the Br fragments are produced via two dissociation channels. For o‐ and m‐dibromobenzene, the slow channel that yields an anisotropy parameter close to zero is proposed to stem from excitation of the lowest excited singlet (π,π*) state followed by predissociation along a repulsive triplet (n,σ*) state localized on the C? Br bond. The fast channel that gives rise to an anisotropy parameter of 0.53–0.73 is attributed to a bound triplet state with smaller dissociation barrier. For p‐dibromobenzene, the dissociation rates are reversed, because the barrier for the bound triplet state becomes higher than the singlet–triplet crossing energy. The fractions of translational energy release are determined to be 6–8 and 29–40 % for the slow and fast channels, respectively; the quantum yields are 0.2 and 0.8, and are insensitive to the position of the substituent. The Br fragmentation from bromobenzene and bromofluorobenzenes at the same photolyzing wavelength is also compared to understand the effect of the number of halogen atoms on the phenyl ring.     

(1+1) Resonance‐Enhanced Multiphoton Ionization and Photodissociation Study of CS2 via the 1B2 State

(1+1) resonance‐enhanced multiphoton ionization (REMPI) spectra of CS₂ and molecular dissociation dynamics are investigated using a time‐of‐flight mass spectrometer equipped with velocity imaging detection. The REMPI spectra via a linear‐bent →¹B₂( ) transition are acquired in the wavelength range of 208–217 nm. Each ro‐vibrational band profile of the ¹B₂( ) state is deconvoluted to yield the corresponding predissociative lifetime from 0.3 to 3 ps. Upon excitation at 210.25 and 212.54 nm, the resulting images of S⁺ and CS⁺ fragments are analyzed to give individual translational energy distributions, which are resolved into two components corresponding to the CS+S(³P) and CS+S(¹D) channels. The product branching ratios of S(³P)/S(¹D) are evaluated to be 5.7±1.0 and 9.6±2.5 at 210.25 and 212.54 nm, respectively. Despite the difficulty avoiding the effect of multiphoton absorption, the molecular dissociation channel is verified to prevail over the dissociative ionization channel of CS₂. The anisotropy parameters for the triplet and singlet channels are determined to be ～0.8 and 1.1–1.3, respectively, suggesting that the predissociative state should have a bent configuration with a short lifetime.     

Electrons Mediate the Gas‐Phase Oxidation of Formic Acid with Ozone

Gas‐phase reactions of CO₃^.? with formic acid are studied using Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometry. Signal loss indicates the release of a free electron, with the formation of neutral reaction products. This is corroborated by adding traces of SF₆ to the reaction gas, which scavenges 38 % of the electrons. Quantum chemical calculations of the reaction potential energy surface provide a reaction path for the formation of neutral carbon dioxide and water as the thermochemically favored products. From the literature, it is known that free electrons in the troposphere attach to O₂, which in turn transfer the electron to O₃. O₃^.? reacts with CO₂ to form CO₃^.?. The reaction reported here formally closes the catalytic cycle for the oxidation of formic acid with ozone, catalyzed by free electrons.     

Quantum Control of Gas‐Phase and Liquid‐Phase Femtochemistry

Active control of chemical reactions on a microscopic (molecular) level, that is, the selective breaking or making of chemical bonds, is an old dream. However, conventional control agents used in chemical synthesis are macroscopic variables such as temperature, pressure or concentration, which gives no direct access to the quantum‐mechanical reaction pathway. In quantum control, by contrast, molecular dynamics are guided with specifically designed light fields. Thus it is possible to efficiently and selectively reach user‐defined reaction channels. In the last years, experimental techniques were developed by which many breakthroughs in this field were achieved. Femtosecond laser pulses are manipulated in so‐called pulse shapers to generate electric field profiles which are specifically adapted to a given quantum system and control objective. The search for optimal fields is guided by an automated learning loop, which employs direct feedback from experimental output. Thereby quantum control over gas‐phase as well as liquid‐phase femtochemical processes has become possible. In this review, we first discuss the theoretical and experimental background for many of the recent experiments treated in the literature. Examples from our own research are then used to illustrate several fundamental and practical aspects in gas‐phase as well as liquid‐phase quantum control. Some additional technological applications and developments are also described, such as the automated optimization of the output from commercial femtosecond laser systems, or the control over the polarization state of light on an ultrashort timescale. The increasing number of successful implementations of adaptive learning techniques points at the great versatility of computer‐guided optimization methods. The general approach to active control of light–matter interaction has also applications in many other areas of modern physics and related disciplines.     

Gas‐Phase Structure Determination of Dihydroxycarbene,One of the Smallest Stable Singlet Carbenes

Carbenes are reactive molecules of the form R¹? C?? R² that play a role in topics ranging from organic synthesis to gas‐phase oxidation chemistry. We report the first experimental structure determination of dihydroxycarbene (HO? C?? OH), one of the smallest stable singlet carbenes, using a combination of microwave rotational spectroscopy and high‐level coupled‐cluster calculations. The semi‐experimental equilibrium structure derived from five isotopic variants of HO? C?? OH contains two very short CO single bonds (ca. 1.32 Å). Detection of HO? C?? OH in the gas phase firmly establishes that it is stable to isomerization, yet it has been underrepresented in discussions of the CH₂O₂ chemical system and its atmospherically relevant isomers: formic acid and the Criegee intermediate CH₂OO.     

Gas‐Phase Photodissociation of CH3CHBrCOCl at 248 nm: Detection of Molecular Fragments by Time‐Resolved FT‐IR Spectroscopy

By employing time‐resolved Fourier transform infrared emission spectroscopy, the fragments HCl (v=1–3), HBr (v=1), and CO (v=1‐3) are detected in one‐photon dissociation of 2‐bromopropionyl chloride (CH₃CHBrCOCl) at 248 nm. Ar gas is added to induce internal conversion and to enhance the fragment yields. The time‐resolved high‐resolution spectra of HCl and CO were analyzed to determine the rovibrational energy deposition of 10.0±0.2 and 7.4±0.6 kcal mol^?1, respectively, while the rotational energy in HBr is evaluated to be 0.9±0.1 kcal mol^?1. The branching ratio of HCl(v>0)/HBr(v>0) is estimated to be 1:0.53. The bond selectivity of halide formation in the photolysis follows the same trend as the halogen atom elimination. The probability of HCl contribution from a hot Cl reaction with the precursor is negligible according to the measurements of HCl amount by adding an active reagent, Br₂, in the system. The HCl elimination channel under Ar addition is verified to be slower by two orders of magnitude than the Cl elimination channel. With the aid of ab initio calculations, the observed fragments are dissociated from the hot ground state CH₃CHBrCOCl. A two‐body dissociation channel is favored leading to either HCl+CH₃CBrCO or HBr+CH₂CHCOCl, in which the CH₃CBrCO moiety may further undergo secondary dissociation to release CO.     

A Puckered Singlet Cyclopentane‐1,3‐diyl: Detection of the Third Isomer in Homolysis

In the photochemical denitrogenation of 1,4‐diaryl‐2,3‐diazabicyclo[2.2.1]heptane ( AZ6 ) bearing sterically hindered substituents, a curious new absorption band at about 450 nm was observed under low‐temperature matrix conditions, together with the previously well‐characterized planar singlet diradical pl‐¹ DR6 with λ_max=≈580 nm. The 450 nm species was electron paramagnetic resonance (EPR)‐silent. Instead of generating the planar diradical pl‐¹ DR6 and the precursor azoalkane AZ6 upon warming, the ring‐closed bicyclo[2.1.0]pentane derivative SB6 , that is, the AZ6 denitrogenation product was identified. Based on product analysis, low‐temperature spectroscopic observations, high‐level quantum‐mechanical computations, viscosity effect, and laser‐flash photolysis, the puckered singlet diradicaloid puc‐¹ DR6 was assigned to the new 450 nm absorption. The latter was detected experimentally at the same time as the planar singlet diradical pl‐¹ DR6 . Sterically demanding substituents as well as viscosity impediments were essential for the detection of the experimentally hitherto unknown puckered singlet cyclopentane‐1,3‐diyl diradicaloid puc‐¹ DR6 , that is, the third isomer in homolysis. The present findings should stimulate future work on the mechanistically fascinating stereoselectivity documented in the formation of bicyclo[2.1.0]pentanes during the 2,3‐diazabicyclo[2.2.1]heptane denitrogenation.     

Perspective on Mechanism Development and Structure‐Activity Relationships for Gas‐Phase Atmospheric Chemistry

This perspective gives our views on general aspects and future directions of gas‐phase atmospheric chemical kinetic mechanism development, emphasizing on the work needed for the sustainable development of chemically detailed mechanisms that reflect current kinetic, mechanistic, and theoretical knowledge. Current and future mechanism development efforts and research needs are discussed, including software‐aided autogeneration and maintenance of kinetic models as a future‐proof approach for atmospheric model development. There is an overarching need for the evaluation and extension of structure‐activity relationships (SARs) that predict the properties and reactions of the many multifunctionalized compounds in the atmosphere that are at the core of detailed mechanisms, but for which no direct chemical data are available. Here, we discuss the experimental and theoretical data needed to support the development of mechanisms and SARs, the types of SARs relevant to atmospheric chemistry, the current status and limitations of SARs for various types of atmospheric reactions, the status of thermochemical estimates needed for mechanism development, and our outlook for the future. The authors have recently formed a SAR evaluation working group to address these issues.     

Photoelectrocyclization as an Activation Mechanism for Organelle‐Specific Live‐Cell Imaging Probes

Photoactivatable fluorophores are useful tools in live‐cell imaging owing to their potential for precise spatial and temporal control. In this report, a new photoactivatable organelle‐specific live‐cell imaging probe based on a 6π electrocyclization/oxidation mechanism is described. It is shown that this new probe is water‐soluble, non‐cytotoxic, cell‐permeable, and useful for mitochondrial imaging. The probe displays large Stokes shifts in both pre‐activated and activated forms, allowing simultaneous use with common dyes and fluorescent proteins. Sequential single‐cell activation experiments in dense cellular environments demonstrate high spatial precision and utility in single‐ or multi‐cell labeling experiments.     

Photolysis of 1‐C4H9I and 2‐C4H9I at 266 nm: Direct Observation of the Effect of Branching on the Photodissociation Mechanism

Velocity ion imaging (see figure) is used to analyze the photodissociation mechanism of 1‐C₄H₉I (left image) and 2‐C₄H₉I (right image) at 266 nm. For 2‐C₄H₉I, a new channel for formation of I and I* atoms is observed. It is attributed to the repulsive mode along the C? I stretch, coupled with some bending motions.

     

Room‐Temperature and Aqueous‐Phase Synthesis of Plasmonic Molybdenum Oxide Nanoparticles for Visible‐Light‐Enhanced Hydrogen Generation

A straightforward aqueous synthesis of MoO_3?x nanoparticles at room temperature was developed by using (NH₄)₆Mo₇O₂₄?4 H₂O and MoCl₅ as precursors in the absence of reductants, inert gas, and organic solvents. SEM and TEM images indicate the as‐prepared products are nanoparticles with diameters of 90–180 nm. The diffuse reflectance UV‐visible‐near‐IR spectra of the samples indicate localized surface plasmon resonance (LSPR) properties generated by the introduction of oxygen vacancies. Owing to its strong plasmonic absorption in the visible‐light and near‐infrared region, such nanostructures exhibit an enhancement of activity toward visible‐light catalytic hydrogen generation. MoO_3?x nanoparticles synthesized with a molar ratio of Mo^VI/Mo^V 1:1 show the highest yield of H₂ evolution. The cycling catalytic performance has been investigated to indicate the structural and chemical stability of the as‐prepared plasmonic MoO_3?x nanoparticles, which reveals its potential application in visible‐light catalytic hydrogen production.     

UV Laser‐Induced Gas‐Phase Copolymerization of Carbon Disulfide and Ethene

Summary: The laser irradiation at 193 nm of a gaseous mixture of carbon disulfide and ethene induces the copolymerization of both compounds and affords the chemical vapour deposition of a C/S/H polymer, the composition of which indicates the reaction between two to three CS₂ molecules and one C₂H₄ molecule. Polymer structure is interpreted on the basis of X‐ray photoelectron and FT‐IR spectra as consisting of >CS, >CC<,  CH₂ CH₂ , (CC)S_nC_{4 − n},  C (CS) S ,  S (CS) S , and C S S C configurations. The gas‐phase copolymerization of carbon disulfide and ethene represents the first example of such a reaction between carbon disulfide and a common monomer.

Scheme showing the expected reaction of excited CS₂ molecules with other CS₂ molecules to form dimers, which then react with another CS₂ molecule or add to ethene.     

Two-photon dissociation spectroscopy of state-selected HCl+ and DCl+ ions.

HCl+ and DCl+ ions were formed via the R(1) pump line of the f3delta2(v'=0)<--sigma+(v'=0) REMPI process. For these ions, the two-photon dissociation spectroscopy, resonance-enhanced via the A2sigma+(v')<--pi3/2(v'=0) transition, was investigated for various intermediate states of HCl+ (v'=4,5,6) and DCl+ (v'=6,7,8,9). From the analysis of the data, spectroscopic parameters of the X and the A states were derived (including the lambda-doubling in the X state and the spin-rotation coupling in the A state). Some of the parameters deviate considerably from literature data. The spectra provide clear evidence that the REMPI process employed for forming the ions has a very high rotational selectivity.