Rate Coefficients for the Gas‐Phase Reaction of Ozone with C5 and C6 Unsaturated Aldehydes

Emissions of biogenic volatile organic compounds are higher than those from anthropogenic sources. In this work, we studied the kinetics of the reaction of three unsaturated aldehydes (trans‐2‐pentenal, trans‐2‐hexenal, and 2‐methyl‐2‐pentenal) with ozone in a rigid atmospheric simulation chamber coupled to an FTIR spectrometer at four different temperatures (273, 298, 333, and 353 K). Reaction rate constants (× 10⁻¹⁸ cm³ molecule⁻¹ s⁻¹) at 298 K are 1.24 ± 0.06 for trans‐2‐pentenal (t‐2P), 1.37 ± 0.03 for trans‐2‐hexenal (t‐2H), and 1.58 ± 0.20) for 2‐methyl‐2‐pentenal (2M2P). The following Arrhenius expressions were deduced (cm³ molecule⁻¹ s⁻¹): The obtained data are presented and compared to those reported in the literature at room temperature, as well as to homologous alkenes. The atmospheric lifetimes with respect to ozone, derived from this study, are estimated to vary between 7 and 10 days.     

Radical Formation in the Gas‐Phase Ozonolysis of Deprotonated Cysteine

Although the deleterious effects of ozone on the human respiratory system are well‐known, many of the precise chemical mechanisms that both cause damage and afford protection in the pulmonary epithelial lining fluid are poorly understood. As a key first step to elucidating the intrinsic reactivity of ozone with proteins, its reactions with deprotonated cysteine [Cys?H]^? are examined in the gas phase. Reaction proceeds at near the collision limit to give a rich set of products including 1) sequential oxygen atom abstraction reactions to yield cysteine sulfenate, sulfinate and sulfonate anions, and significantly 2) sulfenate radical anions formed by ejection of a hydroperoxy radical. The free‐radical pathway occurs only when both thiol and carboxylate moieties are available, implicating electron‐transfer as a key step in this reaction. This novel and facile reaction is also observed in small cys‐containing peptides indicating a possible role for this chemistry in protein ozonolysis.     

Hydroxylamine Reactions with Peroxide Products of Alkenes Ozonolysis

Reactions were studied of peroxide ozonolysis products obtained from linear and cyclic alkenes with hydroxylamine prepared in situ from NH₂OH·HCl by hydrogen chloride neutralization with sodium acetate. A one-pot reactions sequence was performed: alkene oxidation with ozone → reduction to a carbonyl compound with hydroxylamine → condensation of the carbonyl compound with hydroxylamine providing a possibility of direct transformation of alkenes in keto- and aldoximes excluding the stage of preparation and isolation of the carbonyl compound.     

Rate Coefficients for the Gas‐Phase Reactions of Chlorine Atoms with Cyclic Ethers at 298 K

Rate coefficients of reactions of Cl atoms with cyclic ethers, tetrahydropyran (THP), tetrahydrofuran (THF), and dihydrofurans (2,5‐DHF and 2,3‐DHF) have been measured at 298 K using a relative rate method. The relative rate ratios for THP and THF are 0.80 ± 0.05 and 0.80 ± 0.08, respectively, with n‐hexane as the reference molecule. The relative rate ratios for THF and 2,5‐DHF with n‐pentane as the reference molecule are 0.95 ± 0.07 and 1.73 ± 0.06, respectively, and for 2,5‐DHF with 1‐butene as reference is 1.38 ± 0.05. The average values of the rate coefficients are (2.52 ± 0.36), (2.50 ± 0.39), and (4.48 ± 0.59) × 10^?10 cm³ molecule^?1 s^?1 for THP, THF, and 2,5‐DHF, respectively. The errors quoted here for relative rate ratios are 2σ of the statistical variation in different sets of experiments. These errors, combined with the reported errors of the reference rate coefficients using the statistical error propagation equation, are the quoted errors for the rate coefficients. In the case of 2,3‐DHF, after correcting for the dark reaction with CH₃COCl and assuming no interference from other radical reactions, a relative rate ratio of 0.85 ± 0.16 is obtained with respect to cycloheptene, corresponding to a rate coefficient of (4.52 ± 0.99) × 10^?10 cm³ molecule^?1 s^?1. Unlike cyclic hydrocarbons, there is no increase with increasing number of CH₂ groups in these cyclic ethers whereas there is an increase in the rate coefficient with unsaturation in the ring. An attempt is also made to correlate the rate coefficients of cyclic hydrocarbons and ethers with the molecular size as well as HOMO energy.     

Single‐Step Gas‐Phase Polyperfluoroalkylation of Naphthalene Leads to Thermodynamic Products

High‐temperature gas‐phase, solvent‐ and catalyst‐free reaction of naphthalene with an excess of R_FI reagent (R_F?CF₃, C₂F₅, n‐C₃F₇, and n‐C₄F₉) was used for the first time to produce a series of highly perfluoroalkylated naphthalene products NAPH(R_F)_n with n=2–5. Four 95+ % pure 1,3,5,7‐NAPH(R_F)₄ with R_F?CF₃, C₂F₅, n‐C₃F₇, and n‐C₄F₉ were isolated using a simple chromatography‐free procedure. These new compounds were fully characterized by ¹⁹F and ¹H NMR spectroscopy, X‐ray crystallography (for R_F?CF₃ and C₂F₅), atmospheric‐pressure chemical ionization mass spectrometry, and cyclic and square‐wave voltammetry. DFT calculations confirm that the proposed synthesis yields the most stable isomers that have not been accessed by alternative preparation techniques.     

Gas‐Phase Reaction of Monomethylhydrazine with Ozone: Kinetics and OH Radical Formation

The gas‐phase reaction of monomethylhydrazine (CH₃NH? NH₂; MMH) with ozone was investigated in a flow tube at atmospheric pressure and a temperature of 295 ± 2 K using N₂/O₂ mixtures (3–30 vol% O₂) as the carrier gas. Proton transfer reaction–mass spectrometry (PTR‐MS) and long‐path FT‐IR spectroscopy served as the main analytical techniques. The kinetics of the title reaction was investigated with a relative rate technique yielding k_MMH+O3 = (4.3 ± 1.0) × 10^?15 cm³ molecule^?1 s^?1. Methyldiazene (CH₃N?NH; MeDia) has been identified as the main product in this reaction system as a result of PTR‐MS analysis. The reactivity of MeDia toward ozone was estimated relative to the reaction of MMH with ozone resulting in k_MeDia+O3 = (2.7 ± 1.6) × 10^?15 cm³ molecule^?1 s^?1. OH radicals were followed indirectly by phenol formation from the reaction of OH radicals with benzene. Increasing OH radical yields with increasing MMH conversion have been observed pointing to the importance of secondary processes for OH radical generation. Generally, the detected OH radical yields were definitely smaller than thought so far. The results of this study do not support the mechanism of OH radical formation from the reaction of MMH with ozone as proposed in the literature.     

Bulk Gas‐Phase Acidity

The capability of a gaseous Brønsted acid HB to deliver protons to a base is usually described by the gas‐phase acidity (GA) value of the acid. However, GA values are standard Gibbs energy differences and refer to individual gas pressures of 1 bar for acid HB, base B^?, and proton H⁺. We show that the GA value is not suited to describe the bulk acidity of a gaseous acid. Here the pressure dependence of the activities of HB, H(HB)_n⁺, and B(HB)_m^? that result from gaseous autoprotolysis have to be considered. In this work, the pressure‐dependent absolute chemical potential of the proton in the representative gaseous proton acids CH₄, NH₃, H₂O, HF, and HCl was worked out and the general theory to describe bulk gas phase acidity—that can directly be compared with solution acidity—was developed.     

Rate Coefficient for the Gas‐Phase OH + CHF=CF2 Reaction between 212 and 375 K

Rate coefficients, k(T), for the OH + CHF=CF₂ (trifluoroethylene, HFO‐1123) gas‐phase reaction were measured under pseudo–first‐order conditions using pulsed laser photolysis to produce OH radicals and pulsed laser induced fluorescence to measure the OH radical temporal profile. Rate coefficients were measured over the temperature range 212–375 K at total pressures between 20 and 500 Torr (He, N₂ bath gas). The rate coefficient was found to be independent of pressure over this range of pressure with a temperature dependence that is described by the Arrhenius expression (3.04 ± 0.30) × 10^–12 exp[(312 ± 25)/T] cm³ molecule^–1 s^–1 with k(296 K) measured to be (8.77 ± 0.80) × 10^–12 cm³ molecule^–1 s^–1 (quoted uncertainties are 2σ and include estimated systematic errors). Rate coefficients for the reaction of CHF=CF₂ with ¹⁸OH and OD were also measured as part of this study at 296 and 373 K and a total pressure of ～25 Torr (He). The isotope measurements were used to evaluate the observed OH radical regeneration. CHF=CF₂ is a very short‐lived substance with an atmospheric lifetime of ～1 day with respect to OH reactive loss, whereas the actual lifetime of CHF=CF₂ will depend on the time and location of its emission. The global warming potential for CHF=CF₂ on the 100‐year time horizon (GWP₁₀₀) was estimated using the present results and a lifetime correction factor to be 3.9 × 10^?3.     

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.     

Kinetics of the Gas‐Phase Elimination Reaction of Benzyl Chloroformate and Neopentyl Chloroformate

The gas‐phase eliminations of benzyl chloroformate (475–523 K, 31–103 Torr) and neopentyl chloroformate (563–622 K, 37–70 Torr), in a deactivated static reaction vessel, and in the presence of a free radical suppressor, are homogeneous, unimolecular, and follow a first‐order rate law. The rate coefficients are expressed by the following Arrhenius equations: Benzyl chloroformate Neopentyl chloroformate Formation of neopentyl chloride: Formation of 2‐methylbutenes: The derived kinetic and thermodynamic parameters for benzyl chloroformate decomposition indicate the reaction proceeds through a concerted four‐membered cyclic transition state to give benzyl chloride and CO₂ gas. Neopentyl chloroformate undergoes a parallel reaction, where neopentyl chloride formation may arise from a polar‐concerted four‐membered cyclic transition state, whereas the mixture of olefins, 2‐methyl‐2‐butene, and 2‐methyl‐1‐butene appears to be produced from a carbene intermediate. This intermediate seems to be originated from a concerted five‐membered cyclic transition state of the neopentyl substrate.     

Ozonolysis of Ricinolic Acid Derivatives and Transformations of the Ozonolysis Products under Barton Reaction Conditions

The possibility of functionalizing the alkyl part of ricinolic acid using the Barton reaction was investigated. __________ Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 529–534, November–December, 2005.     

Colossal Stability of Gas‐Phase Trianions: Super‐Pnictogens

Multiply charged negative ions are ubiquitous in nature. They are stable as crystals because of charge compensating cations; while in solutions, solvent molecules protect them. However, they are rarely stable in the gas phase because of strong electrostatic repulsion between the extra electrons. Therefore, understanding their stability without the influence of the environment has been of great interest to scientists for decades. While much of the past work has focused on dianions, work on triply charged negative ions is sparse and the search for the smallest trianion that is stable against spontaneous electron emission or fragmentation continues. Stability of BeB₁₁(X)₁₂³⁻ (X=CN, SCN, BO) trianions is demonstrated in the gas phase, with BeB₁₁(CN)₁₂³⁻ exhibiting colossal stability against electron emission by 2.65 eV and against its neutral adduct by 15.85 eV. The unusual stability of these trianions opens the door to a new class of super‐pnictogens with potential applications in aluminum‐ion batteries.     

Metallocene‐Catalyzed Gas‐Phase Ethylene Copolymerization: Kinetics and Polymer Properties

The influence of 1‐hexene is examined on the kinetics of ethylene copolymerization with a metallocene catalyst in gas phase. A model is derived, which is able to describe a large reaction rate increase due to a small amount of incorporated comonomer. This complexation model describes the measured reaction rates for ethylene and 1‐hexene, and the co‐monomer incorporation. Polymer properties were analyzed, such as comonomer weight fraction. The density, melting point, and molecular weight of the produced polymer decreased with increase in 1‐hexene gas concentration. The in situ 1‐hexene sorption is estimated and follows Henry's law, but seems much higher than reported in the literature.

     

Gas‐Phase versus Liquid‐Phase Structures by Electrospray Ionization Mass Spectrometry

Preferred protonation : Does electrospray ionization mass spectrometry produce gas‐phase or liquid‐phase structures? The preferred protonation site in p‐aminobenzoic acid depends upon the medium, and the structure of its conjugate acid varies with the solvent used during spraying.

     

CFD Analysis of Gas–Particle Heat Transfer in Gas‐Phase Olefin Polymerizations

Computational fluid dynamics (CFD) is used to study the gas–particle heat transfer in gas‐phase olefin polymerizations. Particularly, the effects of particle rotation on the gas–particle heat transfer coefficient and internal particle temperatures are evaluated, showing that particle rotation can exert a significant impact on observed temperature profiles, so that this effect should not be neglected during detailed CFD process simulations. As a consequence, particle rotation can lead to particle cooling and development of spherical gradient symmetry, validating the use of simpler modeling schemes that are based on reaction–diffusion in symmetrical spherical geometry.

     

Theoretical Study on the Gas‐Phase SN2 Reaction of Microhydrated Fluoride with Methyl Fluoride

The rate constants for the gas‐phase S_N2 reaction of F^?(H₂O) with CH₃F have been calculated using the dual‐level variational transition state theory including multidimensional tunneling from 50 to 500 K. Tunneling was found to dominate the reaction below 200 K. The deuterium, ¹³C, and ¹⁴C kinetic isotope effects (KIEs) and solvent (D₂O) isotope effects (SKIEs) were also calculated in the same temperature range. The results indicated that the deuterium and heavy water substitutions resulted in inverse KIEs (0.6～0.8 ) while the ¹³C and ¹⁴C substitutions resulted in normal KIEs (1.0～1.2) at room temperature. The calculated carbon KIEs increased significantly below 80 K due to the differences in the magnitude of the tunneling effects for different isotopic substitutions.     

Rate Constants for the Gas‐Phase Reactions of Ozone and Nitrate Radicals with the Sesquiterpenes: Valencene and Farnesol

Sesquiterpenes are constituents of a variety of essential oils that are used in flavorings, perfumes, personal care, and cleaning products. Two sesquiterpenes that are commonly used as indoor fragrances are valencene and farnesol. Knowing the reaction rate constants of these chemicals with ozone (O₃) and nitrate radical () is an important factor in determining their fate indoors. In this study, the bimolecular rate constants of , , , and were measured using the relative rate technique at 297 ± 3 K and 1 atm total pressure. Using the rate constants reported here and measured/modeled indoor concentrations of O₃ and (20 ppb and 1 ppt, respectively), pseudo–first‐order‐rate lifetimes , , , and were determined.