Theoretical prediction of spectroscopic constants of 1-alkoxy radicals

Spectroscopic constants for the conformers of the 1-alkoxy radicals were computed by ab initio and density functional methods. These parameters include ground and excited state rotational constants, harmonic frequencies, and and excitation energies. Elements of the electronic spin–molecular rotation tensor have also been predicted for the ground electronic state utilizing a simple transformation. The transformation employed the experimentally observed spin-rotation tensor of the ethoxy radical as a reference and the calculated rotational constants of the ethoxy radical and the investigated 1-alkoxy radicals. The predicted spectroscopic parameters are in good agreement with the experimental ones where available, and found to be useful parameters for assigning the rotationally resolved spectral bands to given conformers.     

The Excited Triplet State of Azoalkanes: Electron Spin Polarization and Magnetic Field Effects During Triplet-Sensitized Photolysis of trans-Azocumene in Solution

The triplet-sensitized photodecomposition of azocumene into nitrogen and cumyl radicals is investigated by time-resolved electron paramagnetic resonance and absorption spectroscopy. The radicals are found to be created spin polarized with a yield depending on the strength of the applied magnetic field. The phenomenon arises because in triplet azocumene, the decay into radicals competes with a fast triplet-sublevel selective intersystem crossing back to the azocumene ground state. The size of the initial spin polarization of the radicals and the magnetic field effect on their yield are determined in solvents of different viscosities. Data analysis yields rate constants for the intersystem crossing and the cleavage reaction of triplet azocumene as well as its zero-field splitting D _ZFS. At room temperature in nonpolar solvents, the most probable values are: k _x ?=?k _y ?=?1.2?×?10¹¹?s^?? and k _z ?=?1.9?×?10¹⁰?s^?? for the intersystem crossing from the energetically lower and upper triplet substates, respectively, k _p ?=?1.6?×?10⁹?s^?? for the cleavage reaction and for the zero-field splitting D _ZFS?=???.4?×?10^10?s^?? (0.18?cm^??).     

Pulsed EPR measurements on reaction rate constants for addition of photo-generated radicals to double bonds of diethyl fumarate and diethyl maleate

Addition reaction of photo-generated radicals to double bonds of diethyl fumarate (deF) and diethyl maleate (deM), which are geometrical isomers, was studied by means of time-resolved- (TR-) and pulsed-electron paramagnetic resonance (EPR). Analysis of TR-EPR spectra indicates that adduct radicals from deF and deM should have the same structure. The double bonds of these monomers are converted to single ones by addition reaction, which allows hindered internal rotation to give the same structure of adduct radical. The rate constants for addition reaction of photo-generated radicals were determined by Stern–Volmer analysis of the decay time of electron spin-echo intensity of these radicals measured by the pulsed EPR method. Rate constants for deF were found to be larger than those for deM. This relation is in good consistent with efficiency of polymerisation of deF and deM. Experimentally determined rate constants were evaluated by introducing the addition reaction model on the basis of two important factors enthalpy and polar effects.     

卷烟烟气中活性自由基的自旋捕获ESR研究

卷烟烟气中的瞬态自由基对人体健康是相当有害的，而检测活性自由基最常用的方法是自旋捕获方法. 本研究中使用高效新型捕捉剂DEPMPO能够直接在水相中捕捉到烷基自由基与羟基自由基，并未发现烷氧自由基加合物. 考虑到DEPMPO对活性自由基的捕捉能力和加合物ESR谱图解析特征性都优于PBN与DMPO，且在有机溶剂中溶解较多的氧分子. 因此认为以往文献中所捕获的烷氧自由基来源于烷基自由基氧化后的次级自由基产物.     

Femtosecond dynamics of geminate recombination of radicals upon photodissociation of aromatic disulfides

The dynamics of the geminate recombination of thiyl radicals formed upon photodissociation of aromatic disulfides and the effect of the intramolecular relaxation on this recombination are studied using pico-and femtosecond kinetic spectroscopy. It is shown that, in terms of a phenomenological model, the geminate recombination of phenylthiyl radicals in neutral solvents can be satisfactorily described by a biexponential dependence. The model suggests the occurrence of primary geminate recombination in a solvent cage formed around an original molecule and secondary recombination controlled by the diffusional motion of the radicals of a pair. The primary geminate recombination, whose characteristic time (9 ps) is close to the characteristic times of intermolecular vibrational relaxation of complex molecules in solvents at room temperatures, masks the manifestation of thermalization processes in the time kinetics. The direct geminate recombination of aminophenylthiyl radicals with the formation of original molecules virtually does not occur because of the intramolecular charge transfer. In connection with this, the intermolecular vibrational relaxation manifests itself in the kinetics of the induced optical density in the region of the absorption band maximum of radicals as a growing component with a characteristic time of 6 ps.     

Chemiluminescence accompanying the co-oxidation of a mixture consisting of cumene and tetraline

The co-oxidation of a mixture of cumene with tetraline shows a minimum rate of oxygen consumption provided a small percentage of tetraline occurs in the cumene. At the same composition of the mixture the maximum chemiluminescence appears. The luminescence intensity of the mixture consisting of the above solvents changes with temperature much more markedly than that of one obtained experimentally with pure solvents. This result excludes the interpretation hitherto accepted, according to which the preferential termination of unequal peroxy radicals plays a decisive part in this process. The influence of the presence of tetraline on the fragmentation reaction of cumyl oxy radicals is discussed as an important cause of the phenomenon observed.     

Multichannel decomposition and isomerization of octyl radicals

The thermal cracking patterns from the decomposition and isomerization of octyl-1 radicals have been determined from the pyrolysis of n-octyl iodide in single pulse shock tube experiments at temperatures in the 850-1000 K range and pressures near 2 bar. Rate constants for the six beta bond scission and five of the six isomerization processes have been derived over all combustion conditions [0.1-100 bar, 700-1900 K]. Comparisons are made with previous studies on the decomposition of other primary radicals. Results are consistent with similar types of reactions having equal rate constants. The larger size of the octyl radicals makes contributions from secondary to secondary radical isomerization increasingly important. The results confirm that the 1-3 H-transfer process (involving a seven member cyclic transition state) have rate constants that are within a factor of 2 of those for the 1-4 process (six member cyclic transition state) It appears that rate constants for 1-2 H-transfer isomerization, involving an eight member cyclic transition state is unimportant in comparison to contributions from other isomerization processes. The strain energy does not appear to play an important role for these larger transition states. The implications of these results to larger fuel radicals will be discussed.     

Rate constant measurements for initial addition reactions of radicals at the propagation step of photo‐polymerization as studied by pulsed EPR spectroscopy

Pulsed EPR spectroscopy was employed to determine reaction rate constants at an early stage of addition reactions in radical polymerizations triggered by four initiator radicals, which were generated by photodissociation of four parent molecules. Two monomers (tert‐butylacrylate and tert‐butylmethacrylate) were examined as reactant. Stern–Volmer analysis on the measured decay time of electron spin echo intensity of reacting radicals provides rate constants for addition reactions. We focused on rate constants for the second step reaction between monomer and adduct radical that is produced by the first step addition reaction between initiator radical and monomer. The rate constant measured by pulsed EPR was evaluated by theoretical calculations in the light of (1) enthalpy difference between product radical and reactants and (2) charge transfer interaction between reacting radical and monomer. Copyright © 2016 John Wiley & Sons, Ltd.     

硅(磷)中心自由基与2-丁烯二酸二乙酯的加合反应机理

用ESR方法研究了硅(磷)中心自由基与2-丁烯二酸二乙酯的加合反应机理.具有顺式构型的马来酸二乙酯(DEM)与硅(磷)中心自由基反应,生成自由基加合物。DEM的聚合反应受到抑制。具有反式构型的富马酸二乙酯(DEF)易发生聚合反应,DEF与硅(磷)中心自由基的加合反应受到抑制.DEF与DEM在反应性能上的差异,主要来自DEF有较大的空间位阻效应.     

Hyperfine interaction in a strained free radical

The geometric parameters and the hyperfine coupling constants for simple free radicals chosen as model fragments of a middle macroradical in a loaded polymer are calculated. The changes revealed in the hyperfine coupling constants for the α, β, γ, and δ protons and the ¹³C nuclei due to the deformation of inter-atomic bonds and the longitudinal deformation of the carbon skeleton of the radicals are estimated.     

Sonoluminescence intensity and ultrasonic cavitation temperature in organic solvents: Effects of generated radicals

Ultrasonic cavitation in organic solvents remains poorly understood in contrast with aqueous systems, largely because of complexities related to solvent decomposition. In this study, we sonicated different types of organic solvents (i.e. linear alkanes, aliphatic alcohols, aromatic alcohols, and acetate esters) under argon saturation. The average temperature of the cavitation bubbles was estimated using the methyl radical recombination method. We also discuss the effects of the physical properties of the solvents, such as vapor pressure and viscosity, on the cavitation temperature. The average cavitation bubble temperature and sonoluminescence intensity were higher in organic solvents with lower vapor pressure; for aromatic alcohols, these values were particularly high. It was found that the specific high sonoluminescence intensities and average cavitation temperatures exhibited in aromatic alcohols are caused by the highly resonance-stable generated radicals. The results obtained in this study are very useful for acceleration of sonochemical reaction in organic solvents, which are indispensable for organic synthesis and material synthesis.     

Infrared and EPR Spectroscopic Studies of 2-C(2)H(2)F and 1-C(2)H(2)F Radicals Isolated in Solid Argon

2-fluorovinyl radicals were generated in solid argon by solid-state chemical reactions of mobile F atoms with acetylene and its deuterated analogues. Highly resolved EPR spectra of the stabilized radicals CHF&dbond;(*)CH, CDF&dbond;(*)CD, CHF&dbond;(*)CD, and CDF&dbond;(*)CH were obtained for the first time. The observed spectra were assigned to cis-2-fluorovinyl radical based on excellent agreement between the measured (a(F) = 6.50, a(betaH) = 3.86, a(alphaH) = 0.25 mT) hyperfine constants and those calculated using density functional (B3LYP) theory. Analogous experiments carried out using infrared spectroscopy yielded a complete assignment of the vibrational frequencies. An unusual reversible photochemical conversion is observed in which cis-2-fluorovinyl radicals can be partially converted to 1-fluorovinyl radicals by pulsed laser photolysis at 532 nm. Photolysis at 355 nm converts 1-fluorovinyl back to cis-2-fluorovinyl. High-resolution EPR and infrared spectra of 1-fluorovinyl were obtained for the first time. The measured hyperfine constants (a(F) = 13.71, a(H1) = 4.21, a(H2) = 1.16 mT) are in good agreement with calculated values. Copyright 2001 Academic Press.     

拉曼光谱方法研究1,1,3,3-四甲基脲的溶剂效应

测量了1,1,3,3-四甲基脲(TMU)在20种不同溶剂中的拉曼光谱,研究了TMU与溶剂之间的相互作用.将TMU羰基的拉曼频移分别与Kirkwood-Bauer-Magat(KBM)参数(ε-1)/(2ε+1)、溶剂受体数(acceptor number,AN)和线性溶剂自由能关系(linear solvation energy relationships,LSER)进行相关分析.结果表明,TMU的C=O键振动频移与KBM参数没有很好的线性关系,和受体数之间存在比较好的相关性,与LSER参数的线性关系最好.按受体数把溶剂分为质子性溶剂和非质子性溶剂,分别和羰基频移有好的相关性.通过对LSER参数的分析,可以很好地解释溶质和溶剂间的相互作用.     

Nitroxide radicals

The proton hyperfine constants have been measured, using N.M.R., for a series of sterically hindered alkyl aryl nitroxides. Computer reconstructions of the electron resonance spectra show that the coupling constants obtained from the pure radical are very similar to those obtained from the electron resonance spectra of dilute solutions. Comparison of methyl and proton splittings in identical positions suggests that the radicals are σ in character although the molecular-orbital calculations are unable to distinguish between σ or π radicals.     

Distribution and dimerization of some μ-imidodiphosphates

The distribution of chelate-forming reagents on a basis of arylimidodiphosphate between different organic solvents and 0.1 mol L⁻¹ HNO₃ has been studied by liquid-liquid extraction of ⁴⁶Sc. The values of dimerization constants K₂ in selected solvents, particularly on the chlorine and chlorine-fluorine hydrocarbon basis as well as distribution constants K_D(HA) have been determined.     

快速电子辐照F-40残存自由基热衰减的ESR研究

空气中辐照F-40样品中残存的链侧过氧自由基没有链端过氧自由基稳定.这些自由基的热衰减呈明显二级反应特征.反应表观活化能与指前因子这两个对反应速率常数起着相反影响的动力学参数之间存在着动力学补偿效应,表明自由基热衰减是同时在不同活性中心上进行的复杂反应.     

四种哌啶系氮氧自由基的ESR特性研究Ⅲ.pH效应和去氧效应

本文研究不同PH和去氧对数种哌啶系氮氧自由基在溶液中的ESR波谱的影响。其中两种水溶性自由基在PH为3-12的缓冲液内,有较高稳定性。对固态自由基,去氧并不影响其ESR特性,但在自由基溶液中,溶剂极性对如何显现去氧效应起着决定性的作用。在低极性溶液中,去氧使波谱显著变窄及增强,¹⁴N超精细三峰分裂的完善程度大大提高;而在水溶液中,去氧效应甚小。     

Vibrationally averaged spin densities on muons and protons in the hydroxyl and ethyl radicals

The observations of the residual isotope effect for muonated radicals are discussed using calculations on the hydroxyl and ethyl radicals. The origins are partly due to the non-linear increase of the hyperfine coupling constants with increase in bond length, although a significant contribution comes from the anharmonicity of the molecular vibrations.     

EPR studies of X-ray irradiated dithiophosphate metal complexes

Different metal O,O’-disubstituted dithiophosphates have been studied by Electron Paramagnetic Resonance (EPR) spectroscopy after X-ray irradiation at 77K. Formation of different radicals has been recorded depending on the central metal atom and on the substituents in the ligands. Phosphorus-centered radicals with hyperfine constants of about 800 G were recorded after radiolysis of zinc and cadmium complexes. Their shape and EPR parameters lead to their identification as phosphoranyl radicals (RO)₂P (S)S^?. In these compounds two different types of coordination of the ligands have been found by X-ray analysis. One dithiophosphate group is coordinated to the metal atom forming a chelate four-membered ring, and another one is a bridging group. The formation of phosphorus-centered radicals is related to the presence of a bridging, or interchelating group. In other metal dithiophosphates, with only chelating groups present, such radicals were not found. In platinum, palladium and nickel dithiophosphates similar EPR signals with small hyperfine coupling were observed. They have been identified as carbon-centered radicals with one phosphorus in γ-position.     

聚丙烯酸酯泡沫密度均匀性控制

低密度泡沫材料大多存在一定程度的密度不均匀性,这对其后续使用性能将带来不良影响。简述了ICF靶用聚丙烯酸酯泡沫的制备方法,对其密度不均匀性形成机理进行理论分析。在此基础上,改进其制备工艺,控制泡沫密度分布,并利用β射线检测技术,对制备工艺改进前后直径为mm量级的低密度聚丙烯酸酯泡沫柱进行密度分布表征。研究结果表明:改用聚四氟乙烯代替玻璃模具,有利于制得密度分布均匀的泡沫样品;选用最高的紫外光的能量,能促进自由基均匀分布,从而最终能够提高泡沫的密度均匀性。采用β射线对密度范围为10～100 mg.cm-3的TMPTA泡沫的密度均匀性进行表征,结果表明:随着泡沫密度的降低,密度均匀性越低,对制备工艺要求越高。