Electron‐Driven Self‐Assembly of Salt Nanocrystals in Liquid Helium

The self‐assembly of salt nanocrystals from chemical reactions inside liquid helium is reported for the first time. Reaction is initiated by an electron impacting a helium nanodroplet containing sodium atoms and SF₆ molecules, leading to preferential production of energetically favorable structures based on the unit cell of crystalline NaF. These favorable structures are observed as magic number ions (anomalously intense peaks) in mass spectra and are seen in both cationic and anionic channels in mass spectra, for example, (NaF)_nNa⁺ and (NaF)_nF^?. In the case of anions the self‐assembly is not directly initiated by electrons: the dominant process involves resonant electron‐induced production of metastable electronically excited He^? anions, which then initiate anionic chemistry by electron transfer.     

Multiple‐Well,multiple‐path unimolecular reaction systems. II. 2‐methylhexyl free radicals

Vibrationally excited 2‐methylhexyl radicals formed by shock wave activation or by chemical activation can isomerize by multiple pathways to form any of six stable isomers, can fragment by multiple C? H and C? C bond fission pathways, and can be collisionally stabilized. Master equation simulations of chemical activation and of shock wave activation are used to explore the generic behavior of this complicated coupled system. Selecting the argon pressure in chemical activation systems that produce the 2‐methyl‐1‐hexyl radical isomer ( 1 ) can control the yield of specific isomers. Shock heating of 1 also shows a highly regular sequence of isomer formation. This regular behavior is because the first isomerization steps are faster than subsequent steps. Other radical isomers, such as 2‐methyl‐3‐hexyl ( 3 ), do not show such regular behavior, because the first isomerization step is slower than subsequent steps. Incubation and unimolecular rate‐constant fall‐off are observed in the shock wave simulations. The unimolecular rate‐constant fall‐off for the coupled system produces low‐pressure limiting rate constants proportional to [M]ⁿ, where n can be greater than unity. The fact that n can be greater than unity is a natural feature of multichannel coupled unimolecular reaction systems, but detection of the effect in experiments may be very demanding. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 246–261, 2001     

Density functional theory study of the isomerization conversion of the cluster ConMoS (n = 1–5)

To investigate the isomerization transition of cluster Co_nMoS (n = 1–5), we employ density functional theory and transition state theory methods in this study. The cluster is optimized at the B3LYP/def2tzvp quantum chemical level. The results reveal eight isomerization reactions for the clusters Co_nMoS (n = 3, 5). Analyzing the activation energies shows a greater propensity for the isomerization transformation in the forward reaction compared to the reverse reaction. At room temperature, six isomerization transformation processes exhibit rapid conversion to the product configurations. Investigation of the equilibrium constants and application of the Arrhenius formula demonstrate that the cluster isomerization reactions are primarily driven by the forward reactions, with four reactions displaying efficient reactant to product conversion rates. Furthermore, there exists a consistent relationship between the structural complexity of the cluster and the change in entropy value. This study provides theoretical insights into reaction rates and optimization of reaction pathways, facilitating mutual validation and development between experimental and theoretical approaches.     

Initiation of Radical Chain Reactions of Thiol Compounds and Alkenes without any Added Initiator: Thiol‐Catalyzed cis/trans Isomerization of Methyl Oleate

A kinetic study of the dodecanethiol‐catalyzed cis/trans isomerization of methyl oleate (cis‐ 2 ) without added initiator was performed by focusing on the initiation of the radical chain reaction. The reaction orders of the rate of isomerization were 2 and 0.5 for 1 and cis‐ 2 , respectively, and an overall kinetic isotope effect k_H/k_D of 2.8 was found. The initiation was shown to be a complex reaction. The electron‐donor/‐acceptor (EDA) complex of dodecanethiol ( 1 ) and cis‐ 2 formed in a pre‐equilibrium reacts with thiol 1 to give a stearyl and a sulfuranyl radical through molecule‐assisted homolysis (MAH) of the sulfur–hydrogen bond. Fragmentation of the latter gives the thiyl radical, which catalyzes the cis/trans isomerization. A computational study of the EDA complex, MAH reaction, and the sulfuranyl radical calculated that the activation energy of the isomerization was in good agreement with the experimental result of E_A=82 kJ M ^?1. Overall, the results may explain that the thermal generation of thiyl radicals without any initiator is responsible for many well‐known thermally initiated addition reactions of thiol compounds to alkenes and their respective polymerizations and for the low shelf‐life stability of cis‐unsaturated thiol compounds and of mixtures of alkenes and thiol compounds.     

Dehydrogenative Coupling Reactions with Oxidized Guanidino‐Functionalized Aromatic Compounds: Novel Options for σ‐Bond Activation

We present a new option for metal‐free σ‐bond activation, making use of oxidized, guanidino‐functionalized aromatic compounds (GFAs). We demonstrate this new option by the homocoupling reactions of thiols and phosphines. The kinetics and the reaction pathway were studied by a number of experiments (including heterocoupling of thiols and phosphines), supported by quantum‐chemical computations. Reaction of the oxidized GFA with p‐dihydrobenzoquinone to give p‐benzoquinone shows that typical proton‐coupled electron‐transfer reactions are also possible.     

Isomer‐dependent reaction mechanisms of cyclic ether intermediates: cis‐2,3‐dimethyloxirane and trans‐2,3‐dimethyloxirane

Oxiranes are a class of cyclic ethers formed in abundance during low‐temperature combustion of hydrocarbons and biofuels, either via chain‐propagating steps that occur from unimolecular decomposition of β‐hydroperoxyalkyl radicals (β‐?QOOH) or from reactions of H?O with alkenes. The cis‐ and trans‐isomers of 2,3‐dimethyloxirane are intermediates of n‐butane oxidation, and while rate coefficients for β‐?QOOH → 2,3‐dimethyloxirane + ?OH are reported extensively, subsequent reaction mechanisms of the cyclic ethers are not. As a result, chemical kinetics mechanisms commonly adopt simplified chemistry to describe the consumption of 2,3‐dimethyloxirane by convoluting several elementary reactions into a single step, which may introduce mechanism truncation error—uncertainty derived from missing or incomplete chemistry. The present research examines the isomer dependence of 2,3‐dimethyloxirane reaction mechanisms in support of ongoing efforts to minimize mechanism truncation error. Reaction mechanisms are inferred via the detection of products from Cl‐initiated oxidation of both cis‐2,3‐dimethyloxirane and trans‐2,3‐dimethyloxirane using multiplexed photoionization mass spectrometry (MPIMS). The experiments were conducted at 10 Torr and temperatures of 650 K and 800 K. To complement the experiments, the enthalpies of stationary points on the ?R + O₂ surfaces were computed at the ccCA‐PS3 level of theory. In total, 28 barrier heights were computed on the 2,3‐dimethyloxiranylperoxy surfaces. Two notable aspects are low‐lying pathways that form resonance‐stabilized ketohydroperoxide‐type radicals caused by ?QOOH ring‐opening when the unpaired electron is localized adjacent to the ether group, and cis‐trans isomerization of ?R and ?QOOH radicals, via inversion, which enable reaction pathways otherwise restricted by stereochemistry. Several species were identified in the MPIMS experiments from ring opening of 2,3‐dimethyloxiranyl radicals. Neither of the two conjugate alkene isomers prototypical of ?R + O₂ reactions were detected. Products were also identified from decomposition of ketohydroperoxide‐type radicals. The present work provides the first analysis of 2,3‐dimethyloxirane oxidation chemistry and reveals that consumption pathways are complex and require the expansion of submechanisms in chemical kinetics mechanisms.     

Photochemical isomerization in the absence of a potential barrier

Different time resolved spectroscopic techniques have been used to investigate the photophysics of the isomerization reaction of 1,1′-diethyl-4,4′-cyanin. The molecule is characterized by a very short excited state lifetime, linear viscosity dependence over a wide viscosity range and no or negative temperature dependence of the reaction rate. The wavelength dependence of the ground state recovery experiment reported earlier (?kessonet al 1986,Chem. Phys. Lett. 126 385) has been shown to be the result of dependence mainly on the analyzing light. We believe that this molecule can be a representative of the barrierless reaction type (E₀ < 0) and that the probe wavelength dependence in the GSR experiment is due to the fact that different spectroscopic techniques may probe different physical events in the case of barrierless reactions, and suggest that it is a result of stimulated emission in combination with the resolution of the movement of the population on the excited state surface.     

Kinetics and mechanisms of transalkylation and disproportionation of meta‐diethylbenzene by triflic acid catalyst

The kinetics of transalkylation and isomerization of meta‐diethylbenzene in the presence of benzene using triflic acid as a catalyst has been investigated. High catalytic activity of the triflic acid catalyst was observed in homogeneous liquid‐phase reactions. On the basis of the product distribution obtained, transalkylation, disproportionation, and isomerization reactions have been considered and the main product of the reaction was ethylbenzene. These reactions are conducted in a closed liquid batch reactor with continuous stirring under dry nitrogen and atmospheric pressure over the temperature range of 288–308 K. The main transalkylation, disproportionation, and isomerization reactions occurred simultaneously and were considered as elementary reactions. The apparent activation energy of the transalkylation reaction was found to be 35.5 kJ/mol, while that of disproportionation reaction was 42.3 kJ/mol. The reproducibility of the experimental product distribution occurred with an average relative error of ±2%. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 555–563, 2003     

Mechanistic Study on Oxorhenium‐Catalyzed Deoxydehydration and Allylic Alcohol Isomerization

The reaction mechanism of 1,2×n‐deoxydehydration (DODH; n=1, 2, 3 …) reactions with 1‐butanol as a reductant in the presence of methyltrioxorhenium(VII) catalyst has been investigated by DFT. The reduced rhenium compound, methyloxodihydroxyrhenium(V), serves as the catalytically relevant species in both allylic alcohol isomerization and subsequent DODH processes. Compared with three‐step pathway A, involving [1,3]‐transposition of allylic alcohols, direct two‐step pathway B is an alternative option with lower activation barriers. The rate‐limiting step of the DODH reaction is the first hydrogen transfer in methyltrioxorhenium(VII) reduction. Moreover, the increase in the distance between two hydroxyl groups in direct 1,2×n‐DODH reactions for C4 and C6 diols results in a higher barrier height.     

Cu(0)/2,6‐bis(imino)pyridines catalyzed single‐electron transfer‐living radical polymerization of methyl methacrylate initiated with poly(vinylidene fluoride‐co‐chlorotrifluoroethylene)

A series of 2,6‐bis(imino)pyridines, as common ligands for late transition metal catalyst in ethylene coordination polymerization, were successfully employed in single‐electron transfer‐living radical polymerization (SET‐LRP) of methyl methacrylate (MMA) by using poly(vinylidene fluoride‐co‐chlorotrifluoroethylene) (P(VDF‐co‐CTFE)) as macroinitiator with low concentration of copper catalyst under relative mild‐reaction conditions. Well‐controlled polymerization features were observed under varied reaction conditions including reaction temperature, catalyst concentration, as well as monomer amount in feed. The typical side reactions including the chain‐transfer reaction and dehydrochlorination reaction happened on P(VDF‐co‐CTFE) in atom‐transfer radical polymerization process were avoided in current system. The relationship between the catalytic activity and the chemical structure of 2,6‐bis(imino)pyridine ligands was investigated by comparing both the electrochemical properties of Cu(II)/2,6‐bis(imino)pyridine and the kinetic results of SET‐LRP of MMA catalyzed with different ligands. The substitute groups onto N‐binding sites with proper steric bulk and electron donating are desirable for both high‐propagation reaction rate and C? Cl bonds activation capability on P(VDF‐co‐CTFE). The catalytic activity of Cu(0)/2,6‐bis(imino)pyridines is comparable with Cu(0)/2,2′‐bipyridine under the consistent reaction conditions. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4378–4388     

Polymerization of o‐quinodimethanes. V. Ionic polymerization of o‐quinodimethanes generated by thermal isomerization of corresponding benzocyclobutenes

The ionic polymerization of substituted o‐quinodimethanes via thermal isomerization of benzocyclobutenes is described. In the cationic polymerizations of 1‐methoxy‐o‐quinodimethane in the presence of various cationic initiators at 110 °C for 12 h, chain transfer reactions also considerably underwent besides the polymerization. Meanwhile, cationic polymerizations of 1‐trimethylsilyloxy‐o‐quinodimethane under the same conditions gave good yields of the corresponding polymer. Anionic polymerizations of 1‐cyano‐o‐quinodimethane in the presence of anionic initiators such as n‐BuLi or t‐BuOK were performed at various temperatures for 12 h. Good yields of hexane‐insoluble polymer, which was produced by anionic polymerization of corresponding o‐quinodimethane as an intermediate, were obtained above 120 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 844–850, 2008     

A model investigation of ab initio geometries for identity and nonidentity substitution with three‐center four‐ and three‐electron transition states

With the van't Hoff model derived from the dynamics of a regular tetrahedron in which the interstitial carbon changes its position from tetrahedral into a trigonal (bi)pyramidal configuration, the introduction of dimensionless ratio numbers (R) can serve to localize significant points on the principal reaction coordinate. These numbers are expressed as R(d) and R(n) based on transition state geometries and the number of electrons involved in the three‐center bonding, respectively. Using this concept, we obtain a model for the evaluation of different ab initio calculations based on identity and nonidentity substitution reactions for three‐center four‐ and three‐electron bonding transition states. Similar ratio numbers have been derived for proton exchange reactions. The reactions under investigation show clearly that in spite of the differences in chemical outcome the transition steps reduced to the first principles of chemical bonding are similar. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

How computational methods and relativistic effects influence the study of chemical reactions involving Ru‐NO complexes?

Two treatments of relativistic effects, namely effective core potentials (ECP) and all‐electron scalar relativistic effects (DKH2), are used to obtain geometries and chemical reaction energies for a series of ruthenium complexes in B3LYP/def2‐TZVP calculations. Specifically, the reaction energies of reduction ( A ‐ F ), isomerization ( G‐I ), and Cl⁻ negative trans influence in relation to NH₃ ( J ‐ L ) are considered. The ECP and DKH2 approaches provided geometric parameters close to experimental data and the same ordering for energy changes of reactions A ‐ L . From geometries optimized with ECP, the electronic energies are also determined by means of the same ECP and basis set combined with the computational methods: MP2, M06, BP86, and its derivatives, so as B2PLYP, LC‐wPBE, and CCSD(T) (reference method). For reactions A ‐ I , B2PLYP provides the best agreement with CCSD(T) results. Additionally, B3LYP gave the smallest error for the energies of reactions J ‐ L . © 2017 Wiley Periodicals, Inc.     

Consecutive Chemical Activation Steps in the OH‐Initiated Atmospheric Degradation of Isoprene: An Analysis with Coupled Master Equations

The influence of a two‐step chemical activation on 1,5‐H and 1,6‐H shift reactions of hydroxyl‐peroxy radicals formed in the atmospheric photooxidation of isoprene was investigated by means of a master equation analysis. To account for multiple chemical activation processes, three master equations were coupled. The general approach of this coupling is described, and consequences for steady‐state regimes are examined. The specific calculations show that chemical activation has no substantial influence on the rate coefficients of the above‐mentioned reactions under tropospheric conditions. However, it is demonstrated that high‐pressure limits of the thermal rate coefficients instead of the falloff‐corrected values have to be used for kinetic modeling. This is a consequence of the continuous population of the high‐energy part of the isoprene‐OH‐O₂ adduct distribution by the forming reactions under steady‐state conditions. The rate coefficients of the isomerization reactions at T = 298 K were calculated to be k_3a^∞ = 1.5 × 10^?3 s^?1 (1,5‐H‐shift of the 1,2‐isomer) and k_4a^∞ = 6.5 s^?1 (1,6‐H‐shift of the (Z)‐1,4‐isomer). The calculated value of k_4a^∞ is three orders of magnitude larger than a recently reported experimentally observed rate coefficient for the hydrogen shift reactions of the hydroxyl‐peroxy intermediates. It is shown that this discrepancy is in part due to the fact that the experiment does not distinguish between different structural isomers. A comparison of the experimentally determined isotope effect with the calculated value shows a reasonable agreement.     

Enhanced Reactivities of Iron(IV)‐Oxo Porphyrin π‐Cation Radicals in Oxygenation Reactions by Electron‐Donating Axial Ligands

The proximal axial ligand in heme iron enzymes plays an important role in tuning the reactivities of iron(IV)‐oxo porphyrin π‐cation radicals in oxidation reactions. The present study reports the effects of axial ligands in olefin epoxidation, aromatic hydroxylation, alcohol oxidation, and alkane hydroxylation, by [(tmp)^+. Fe^IV(O)(p‐Y‐PyO)]⁺ ( 1 ‐Y) (tmp=meso‐tetramesitylporphyrin, p‐Y‐PyO=para‐substituted pyridine N‐oxides, and Y=OCH₃, CH₃, H, Cl). In all of the oxidation reactions, the reactivities of 1 ‐Y are found to follow the order 1 ‐OCH₃ > 1 ‐CH₃ > 1 ‐H > 1 ‐Cl; negative Hammett ρ values of ?1.4 to ?2.7 were obtained by plotting the reaction rates against the σ_p values of the substituents of p‐Y‐PyO. These results, as well as previous ones on the effect of anionic nucleophiles, show that iron(IV)‐oxo porphyrin π‐cation radicals bearing electron‐donating axial ligands are more reactive in oxo‐transfer and hydrogen‐atom abstraction reactions. These results are counterintuitive since iron(IV)‐oxo porphyrin π‐cation radicals are electrophilic species. Theoretical calculations of anionic and neutral ligands reproduced the counterintuitive experimental findings and elucidated the root cause of the axial ligand effects. Thus, in the case of anionic ligands, as the ligand becomes a better electron donor, it strengthens the FeO? H bond and thereby enhances its H‐abstraction activity. In addition, it weakens the Fe?O bond and encourages oxo‐transfer reactivity. Both are Bell–Evans–Polanyi effects, however, in a series of neutral ligands like p‐Y‐PyO, there is a relatively weak trend that appears to originate in two‐state reactivity (TSR). This combination of experiment and theory enabled us to elucidate the factors that control the reactivity patterns of iron(IV)‐oxo porphyrin π‐cation radicals in oxidation reactions and to resolve an enigmatic and fundamental problem.     

Single‐Electron Transfer in σ‐Complexation Reactions of 2,6‐Dimethoxy‐3,5‐dinitropyridine with Para‐X‐phenoxide Anions in Aqueous Solution

Second‐order rate constants have been measured spectrophotometrically for reactions of 2,6‐dimethoxy‐3,5‐dinitropyridine 1 with 4‐X‐substituted phenoxide anions (X = OMe, Me, H, Cl, and CN) 2a–e in aqueous solution at various temperatures. The effect of phenoxide substituents on the reaction rate was examined quantitatively on the basis of kinetic measurements, leading to nonlinear correlations of Δ^≠H and Δ^≠S with Hammett's substituent constants (σ). Each Hammett plots exhibits two intersecting straight lines for the reactions of 1 with the phenoxide anions 2a–e , whereas the Yukawa–Tsuno plots for the same reactions are linear. The large negative ρ values (?4.03 to ?3.80) obtained for the reactions of 1 with the phenoxide anions possessing an electron‐donating group supports the proposal that the reactions proceed through a single‐electron transfer mechanism.     

Anionic ring‐opening polymerization of N‐glycidylphthalimide: Combination of phosphazene base and activated monomer mechanism

Anionic ring‐opening polymerization of glycidyl phthalimide, initiated with alcohol–phosphazene base systems and based on monomer activation with a Lewis acid (iBu₃Al), has been studied. No propagation occurred for initiator: iBu₃Al ratios less or equal to 1:3. For larger Lewis acid amounts, the first anionic ring‐opening polymerizations of glycidyl phthalimide were observed. Polymers were carefully characterized by NMR, MALDI‐TOF mass spectrometry, and size exclusion chromatography and particular attention was given to the detection of eventual transfer or side‐reactions. However, polymer precipitation and transfer reaction to aluminum derivative were detrimental to monomer conversion, polymerization control, and limited polymer chain molar masses. The influence of reaction temperature and solvent on polymer precipitation and transfer reactions was studied and reaction conditions have been optimized leading to afford end‐capped poly(glycidyl phthalimide) with narrow molar mass distributions. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1091–1099     

Frustrated Lewis Pair‐Catalyzed Cycloisomerization of 1,5‐Enynes via a 5‐endo‐dig Cyclization/Protodeborylation Sequence

The first frustrated Lewis pair‐catalyzed cycloisomerization of a series of 1,5‐enynes was developed. The reaction proceeds via the π‐activation of the alkyne and subsequent 5‐endo‐dig cyclization with the adjacent alkene. The presence of PPh₃ was of utmost importance on the one hand to prevent side reactions (for example, 1,1‐carboboration) and on the other hand for the efficient protodeborylation to achieve the catalytic turnover. The mechanism is explained on the basis of quantum‐chemical calculations, which are in full agreement with the experimental observations.     

电子效应调控丁烯二腈光分子开关的CASSCF和MS-CASPT2理论研究

采用多组态CASSCF方法和MS-CASPT2方法研究了丁烯二腈中性分子及阳离子和阴离子的顺-反异构化机理.结果表明,中性分子和离子态的光顺-反异构化反应经历不同的非绝热跃迁途径:中性丁烯二腈受光激发至S1态后,需克服一个不低于19.7 k J/mol的能垒才有机会到达基态和激发态间的圆锥交叉(S_1/S_0-CI),随后经非辐射跃迁回到基态,S_1/S_0-CI在结构上偏离C=C双键旋转路径,且能量较高,因此会降低旋转速度,阻碍旋转的单向性;丁烯二腈阳离子和阴离子自由基的D_0态和D_1态旋转势能面在90°处相交,优化的D_1/D_0-CI与D_1态中间体的结构和能量均相近,因此从D1态经由D_1/D_0-C_I无辐射跃迁到D_0态的过程无势垒,在此过程中C=C旋转方向性得到最大限度的保持.研究结果证实了电子诱导不仅能降低基态热旋转势垒,而且能够调控光旋转的非绝热跃迁机理.