Precise measurement of hfs constant of the muonium substituted radical in cis-polyacetylene

The state of the muon in cis-polyacetylene was studied by muon spin rotation method under high transverse magnetic field. The most of the muons are found to be in muonium substituted radical state. The hyperfine coupling constant was determined to be 91 MHz with a line width of 10 MHz.     

Spin relaxation studies of the muonium substituted ethyl radical in the gas phase

This short communication draws attention to the power of μSR and related measurements in providing an unusually complete characterisation of muonium substituted organic radicals in the gas phase. Spectroscopic information is available from muon spin rotation and muon level crossing resonance, giving all the nuclear hyperfine coupling constants, just as in the liquid phase. In addition, measurements of the relaxation time of the muon Zeeman energy become possible; these are potentially informative on the molecular collision dynamics. Demonstration results are presented in summary for the muonium substituted ethyl radical, ĊH₂CH₂Mu, in ethene gas.     

用交叉分子束研究氟原子与反式1，3-丁二烯分子的反应

利用改进型通用交叉分子束装置和脉冲直流放电产生脉冲氟原子束实验方法 ,研究了氟原子和 1,3 丁二烯分子的反应散射 .只有一个脱氢原子反应通道被观测到 ,没有观测到碳碳单键、碳碳双键断裂以及氟化氢分子的生成 .直接测量到反应产物的角度分布和飞行时间质谱 .通过把实验数据从实验室坐标系转化到质心坐标系 ,得到反应产物在不同质心角度下的平动能分布和角度分布 .从反应产物的三维速度分布 角度分布 通量图中 ,得出氟原子和 1,3 丁二烯反应生成氢原子的过程是通过形成了一个长寿命的中间体     

氟原子与反式1,3-丁二烯分子反应动力学的理论研究

在CCSD(T)／6-311g(d,p)／B3LYP／6-311g(d,p)的基础上对F原子与反式的1,3-丁二烯的反应体系进行了理论研究．计算中发现,这个反应体系有两个形成中间复合物的通道．同时,理论计算结果表明,在交叉分子束的实验中探测到的H原子是通过这两个形成长寿命的中间复合物的通道产生的．而通过同一个复合物通道,生成C2H3 C2H3F的速率要比生成氢原子的低,在交叉分子束实验中要探测到C2H3 C2H3F通道也是很困难的．理论分析的结果还表明, HF也可能是反应中的一个产物,它主要是通过直接抽取反应产生的．     

Formation of muonium substituted free radicals

SR experiments with mixtures of benzene with cyclohexane, 2,3-dimethyl-1,3-butadiene and carbontetrachloride allow the conclusion that the direct muonic precursor of the muonic cyclohexadienyl radical is thermal muonium. In neat benzene it has a life time of 10 ps and disappears by the addition reaction. Muonium is formed within about 1 ps by combination of the positive muon with an electron near the end of the track of the thermalizing muon.Support by the Swiss National Foundation for Scientific Research and by the Swiss Institute for Nuclear Research (SIN) are gratefully acknowledged.     

Low frequency vibrations of 1,3-butadiene, 1,3-butadiene-d4, and 1,3-butadiene-d6

The absorption bands in the far infrared due to the torsional and in-plane skeletal bending vibrations have been examined in detail together with some mid-infrared bands. Torsional energy level spacings up to v₁₃ = 5 for butadiene and up to v₁₃ = 4 and 3, respectively, for the d₄- and d₆-compounds have been measured accurately, and the data used to investigate the nature of the potential function for rotation about the CC single bond. A Coriolis interaction between the torsion and the in-plane bending mode ν₂₄ has been analyzed in detail.     

On the low-temperature chemistry of 1,3-butadiene

In this paper, species versus temperature profiles were measured during the oxidation of 1,3-butadiene in a jet-stirred reactor (JSR) at 1 atm, at different equivalence ratios (φ = 0.5, 1.0 and 2.0), in the temperature range 600 – 1020 K. Both synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC) methods were used to analyze the species. The experimental results show that a large proportion of the products are aldehydes (formaldehyde, acetaldehyde, acrolein, etc.) and ketenes (ketene, methyl-ketene), with acrolein being one of the major products. Moreover, furan, 1,3-cyclopentadiene and benzene are also present as intermediates in significant amounts. The reaction pathways leading to the formation of these species are discussed in detail. A new detailed mechanism, NUIGMech1.3, was developed to simulate these new data as well as other experimental data available in the literature. The validation results indicate that quantum calculations are also needed to explore the formation of some important species formed in the oxidation of 1,3-butadiene. Overall, the new 1,3-butadiene mechanism agrees well with various experimental data in the low- to high-temperature regimes and at different pressures. Flux and sensitivity analyses show that 1,3-butadiene shares some common reaction chemistry pathways with 1- and 2-butene via Ḣ atom and HȮ₂ radical addition to the C = C double bond in 1,3-butadiene, reactions which are important for both systems. The low temperature chemistry of 1,3-butadiene is mainly controlled by the reaction pathways of ȮH radical addition to the C = C double bond of the fuel molecule. The 1-buten-4-ol-3-yl radicals so formed subsequently add to O₂ and react via the Waddington mechanism, which is important in accurately simulating the oxidation and auto-ignition of 1,3-butadiene at engine relevant conditions.     

Exploration on laminar flame propagation of 3,3-dimethyl-1-butene, 2,3-dimethyl-1-butene and 2,3-dimethyl-2-butene

Laminar flame propagation of branched hexene isomers/air mixtures including 3,3-dimethyl-1-butene (NEC₆D3), 2,3-dimethyl-1-butene (XC₆D1) and 2,3-dimethyl-2-butene (XC₆D2) was investigated using a high-pressure constant-volume cylindrical combustion vessel at 1–10 atm, 373 K and equivalence ratios of 0.7–1.5. The measured laminar burning velocity (LBV) decreases in the order of NEC₆D3, XC₆D1 and XC₆D2, which indicates distinct fuel molecular structure effects. A kinetic model was constructed and examined using the new experimental data. Modeling analyses were performed to reveal fuel-specific flame chemistry of branched hexene isomers. In the NEC₆D3 and XC₆D1 flames, the allylic CC bond dissociation reaction plays the most crucial role in fuel decomposition under rich conditions, while its dominance is replaced by H-abstraction reactions under lean conditions. The H-abstraction and H-assisted isomerization reactions are concluded to govern fuel consumption in the XC₆D2 flame under all investigated conditions. Both C₀C₃ reactions and fuel-specific reactions are found to be influential to the laminar flame propagation of the three branched hexene isomers. Fuel molecular structure effects were analyzed with special attentions on key intermediates distributions and fuel-specific reactions in all flames. Due to the formation selectivity of key intermediates such as 2-methyl-1,3-butadiene and 2,3-dimethyl-1,3-butadiene, the production of reactive radicals especially H follows the order of NEC₆D3 > XC₆D1 > XC₆D2, which results in the same order of fuel reactivities and LBVs.     

Temperature dependence of the muon and proton hyperfine constants of an \alpha‐muonium‐substituted methyl radical

Muon hyperfine constants A_μ have been measured by transverse field μSR for (CH₃)₃Si\mbox\.CHMu in hexane from 167 K to 332 K. In addition, avoided level‐crossing resonance was used to determine \alpha‐proton coupling constants A_p over a similar range of temperatures. The two hyperfine constants can be described by a common temperature dependence, d|A_i|/ dT=1.4\times 10^-3 MHz\,K^-1, where A_i represents A_p or the reduced muon constant A^\prime_μ=0.3141A_μ. There is a small isotope effect (A^\prime_μ is 2.2 % larger than A_p) consistent with zero‐point motion in the anharmonic C–H bond stretch. The common temperature dependence is tentatively attributed to a coupled deviation of the C–H and C–Mu bonds out of the nodal plane of the p orbital containing the unpaired electron. This revised version was published online in August 2006 with corrections to the Cover Date.     

Finite-temperature properties of the muonium substituted ethyl radical CH2MuCH2: nuclear degrees of freedom and hyperfine splitting constants

The finite-temperature (T) properties of the muonium substituted ethyl radical CH₂MuCH₂ have been theoretically studied by Feynman path integral quantum Monte Carlo (PIMC) simulations. To derive the ensemble averaged expectation values we have combined the PIMC formalism with an efficient tight-binding (TB) Hamiltonian and a density functional operator of the B3LYP type in the EPRIII basis. The TB operator has been used to calculate the potential energy surface (PES) of the ethyl radical in the doublet ground state, the harmonic and anharmonic vibrational wave numbers as well as several probability density functions of the nuclei. The harmonic linear response approximation, which makes use of the Feynman centroid density, has been adopted to evaluate the anharmonic wave numbers. The large anharmonicities in the nuclear potential lead to bond lengths in thermal equilibrium which exceed the vibrationless parameters at the PES minimum. This enhancement is particularly strong for the C–Mu bond. It is responsible for the suppression of the intramolecular rotation for temperatures below room temperature. In C₂ H₅ the rotation is allowed down to 10?K. The dissimilar rotational dynamics for H₂MuCH₂ and C₂ H₅ has been studied with the help of TB-based probability density functions. The nuclear configurations of CH₂MuCH₂ and C₂ H₅, which are populated in thermal equilibrium, have been used to evaluate the isotropic and anisotropic hyperfine splitting (hfs) constants under explicit consideration of the nuclear vibrations and the internal rotation. The hfs constants have been determined with the help of the B3LYP-EPRIII Hamiltonian. The hindered low-temperature rotation in the Mu isomer is responsible for roto-vibrational corrections to the isotropic hfs constants which are smaller than the corrections in C₂ H₅. The shortcomings of single-configuration approaches for the evaluation of isotropic hfs constants have been demonstrated for both radicals. The ensemble corrections to the isotropic hfs parameters are correlated with fluctuations in the atomic spin densities. Differences in the absolute values of the isotropic hfs parameters in CH₂MuCH₂ and C₂ H₅ can be traced back to differences in the nuclear degrees of freedom. The ensemble shift for each isotropic hfs parameter can be explained by characteristic nuclear motions. For this discussion we make use of the distribution functions of the isotropic hfs constants. Roto-vibrational corrections to the anisotropic hfs constants are rather small. PIMC simulations have been performed between 25 and 1000?K, i.e. in a T interval that is large enough to consider nuclear effects beyond zero-point motions. The TB and B3LYP-EPRIII based physical quantities of CH₂MuCH₂ and C₂ H₅ have been compared with experimental findings whenever possible.     

Formation of muonium and a muonic radical in fullerene

Three distinct electronic states were detected for positive muons (⁺) after implantation into a C₆₀ powder sample. About 40% of the ⁺ remained in the bare (diamagnetic) state, essentially an interstitial charged point particle. The rest of the muons were found to thermalize predominantly in two muonium (Mu=⁺ e^–) atomic species. A vacuum Mu state, with hyperfine coupling close to that of free Mu, most likely at the molecular center, and a muonic substituted radical, i.e. a hydrogen-like Mu addition to double bonds on the carbon rings. This opens up a rich subfield of fullerene spectroscopy using muons.     

Torsional potential of 1,3-butadiene: ab initio calculations

Accurate torsional potentials and torsional barriers are derived for the 1,3-butadiene molecule using state-of-the-art coupled-cluster (CC) methods. The basis set effect, and the performance of different ab initio methodologies with respect to the CC calculations, have been carefully addressed. The CC results obtained here provide an excellent compromise between accuracy and computational cost, which can be extended to other systems.     

Polycyclic aromatic hydrocarbons from the co-pyrolysis of catechol and 1,3-butadiene

In order to better understand the reactions responsible for the formation and growth of polycyclic aromatic hydrocarbons (PAH) from solid fuels, we have performed pyrolysis experiments in an isothermal laminar-flow reactor (at temperatures of 600-1000 °C and a fixed residence time of 0.3 s) with catechol, a model fuel representative of the aromatic moieties in coal and biomass fuels; 1,3-butadiene, a major product of biomass pyrolysis; and with catechol and 1,3-butadiene together (in a catechol-to-1,3-butadiene molar ratio of 0.83). No PAH of ?3 rings are produced at temperatures <700 °C, but PAH production becomes significant at temperatures ?800 °C. Analysis of the higher-temperature reaction products by high-pressure liquid chromatography with diode-array ultraviolet-visible absorbance detection has led to the identification of over 100 PAH (ranging in size to 10 fused aromatic rings) - 47 of which have never before been reported as products of any phenol-type fuel. Quantification of the product yields shows that a much higher percentage of fed carbon is converted to PAH in the catechol-only pyrolysis experiments than in the 1,3-butadiene-only pyrolysis experiments - a result attributable to catechol’s relatively labile O-H bond and capacity for generating oxygen-containing radicals, which accelerate both fuel conversion and the pyrolysis reactions leading to 1- and 2-ring aromatics and PAH. When the two fuels are co-pyrolyzed, the percentage of the total fed carbon converting to PAH is more than two times higher than the amount calculated for the hypothetical case of the two fuels together behaving as a linear combination of the two fuels individually. This elevated production of PAH from the co-pyrolysis experiments reflects not only the reaction-accelerating role of the oxygen-containing radicals but also the efficacy, as growth agents, of the C₂ - and especially the C₄ - species abundantly present in the catechol/1,3-butadiene co-pyrolysis environment.     

Experimental counterflow ignition temperatures and reaction mechanisms of 1,3-butadiene

The ignition temperatures of nitrogen-diluted 1,3-butadiene by heated air in counterflow were experimentally determined for pressures up to 5 atmospheres and pressure-weighted strain rates from 100 to 250 s⁻¹. The experimental data were compared with computational results using the mechanism of Laskin et al. [A. Laskin, H. Wang and C.K. Law, Int. J. Chem. Kinet. 32 (10) (2000) 589-614], showing that while the overall prediction is approximately within the experimental uncertainty, the mechanism over-predicts ignition temperature by about 25-40 K, with the differences becoming larger at high pressure/low temperature region. Sensitivity analyses for the near-ignition states were performed for both reactions and diffusion, which identified the importance of H₂/CO chain reactions, three 1,3-butadiene reaction pathways, and the binary diffusion between 1,3-butadiene and N₂ on ignition. The detailed mechanism, consisting of 94 species and 614 reactions, was then simplified to a skeletal mechanism consisting of 46 species and 297 reactions by using a new reduction algorithm combining directed relation graph and sensitivity analysis. The skeletal mechanism was further simplified to a 30-step reduced mechanism by using computational singular perturbation and quasi-steady-state assumptions. Both the skeletal and reduced mechanisms mimic the performance of the detailed mechanism with good accuracy in both homogeneous and heterogeneous systems.     

Dissociative electron attachment to the explosive detection taggant 2,3-dimethyl-2,3-dinitrobutane (DMNB)

A detailed study on dissociative electron attachment (DEA) to 2,3-dimethyl-2,3-dinitrobutane (DMNB) in the gas phase is presented. Ion yields as a function of the incident electron energy from about 0 to 14 eV have been measured for the most dominant fragments including anions such as NO₂ ⁻, [M-NO₂]⁻ or N₂O₄ ⁻. To help identifying which anion and neutral fragments are formed upon electron attachment we calculated the thermodynamic thresholds using the G4(MP2) method.     

Ultrasonic pretreatment of liquid NaK metal catalyst for side-chain alkenylation of o-xylene with 1,3-butadiene

The effects of sonochemical treatment of a NaK eutectic mixture as catalyst on the side-chain alkenylation of o-xylene with 1,3-butadiene were studied. The parameters studied include ultrasound frequency, insonation time, sonication power as well as the reaction temperature. In addition, the effect of N,N,N',N'-tetramethylethylenediamine (TMEDA) on the reaction was also studied. The results showed that sonochemical treatment of this NaK eutectic mixture catalyst resulted in excellent conversion (up to 83.16%) under mild conditions. The introduction of TMEDA further increased the conversion to 89.4%.     

Reaction of 1,3-cyclohexadiene with the Ge(100) surface

The adsorption and reaction of 1,3-cyclohexadiene with the Ge(100)-(2×1) surface are investigated. The possibility of a surface Diels–Alder reaction ([4+2] cycloaddition), as well as a [2+2] cycloaddition reaction are explored. The surface reactions are followed by ultraviolet photoelectron and high-resolution electron energy loss spectroscopies. The vibrational spectroscopy results are compared with theoretically predicted frequencies for each of the possible surface reactions.     

IR Study of the Pressure Induced Solid State DI- and Polymerization in 1,3-butadiene

The pressure induced chemical reaction of butadiene was studied in the crystal phase by means of FTIR spectroscopy using a Membrane Diamond Anvil Cell (DAC). Two different reaction products, vinylcyclohexene (dimerization) and trans-polybutadiene (polymerization), were obtained. The phase diagram of butadiene was investigated and the existence of an orientationally disordered phase, between the liquid and the ordered solid phase, was identified. The analysis of the time evolution of the integrated absorption of the product bands provides information on the reaction mechanisms. Distinct models are necessary to explain the polymer and the dimer formation.     

Electron scattering from trans 1,3-butadiene molecule: cross-sections, oscillator strength and VUV photoabsorption cross-sections

Electron energy-loss spectra for the butadiene molecule were measured in the scattering angular range of 2.0° to 8.0°, in an energy-loss range from 2 to 50 eV, using 1000 eV incident electrons. The absolute generalized oscillator strength (GOS) and inelastic cross section have been determined for the \hbox{$\tilde{\rm X}^{1}$}?X1A _g  → 1¹B _u transition. The absolute elastic differential cross section was also determined spanning an angular range from 2.0° to 40.0°. From a small angle electron energy-loss spectrum, the optical oscillator distribution (photoabsorption spectrum) for the butadiene molecule was obtained in the 2 to 100 eV photon energy range. Accurate ab initio calculations have been performed, within the First Born Approximation, for generalized oscillator strength (GOS) and excitation energies for the \hbox{$\tilde{\rm X}^{1}$}?X1A _g  → 1¹B _u and \hbox{$\tilde{\rm X}^{1}$}?X1A _g  → 2¹A _g transitions. Our results emphasize the importance of using highly correlated wavefunctions and accurate methodologies in the calculation of the GOS for electron impact-induced electronic transitions in molecules.     

Rate constants for the reactions of cyclohexadienyl type radicals with quinones

Observation of substituted cyclohexadienyl type radicals by muon spin rotation in substituted benzene at different concentrations of added p-benzoquinone or duroquinone allows a selective determination of rate constants for the reaction of the radicals with oxidants. Methyl substitution has no effect, methoxy substitution enhances the rate constants for the ortho and para substituted isomers.Support by the Swiss National Foundation for Scientific Research, by the Swiss Institute for Nuclear Research (SIN), the Netherlands Organisation for the Advencement of Pure Research (Z.W.O.) and the Foundation for Fundamental Research on Matter (F.O.M.) are gratefully acknowledged.