An improved Mclachlan type spin density calculation

A new method for the calculation of the spin density in π radicals is proposed within the McLachlan framework. Compared with previous methods, this new definition exhibits two main features: (i) for odd alternant neutral radicals, the computed spin density pattern is in better agreement with experiment (ii) for even alternant ionic radicals, the spin densities are identical in the cation and the anion, as required by the “ pairing theorem ”.     

The spin polarization model for hyperfine coupling constants

Summary The simple spin polarization model for calculation of the spin densities that determine hyperfine coupling constants in free radicals is examined. Spin-unrestricted approaches, both without and with removal of spin contamination, are discussed and compared with spin-restricted treatments. Basis set design and electron correlation effects are also considered. Calculations on small pi radicals are presented to illustrate the arguments. Explanations are advanced for why some of the simpler treatments seem to work better than might be expected.     

Two-photon chemistry in the laser jet: generation of radical intermidiates and their photochemical transformations

The novel laser jet technique provides sufficiently high photon densities to permit the observation of the photochemistry of photochemically generated radicals (two-photon chemistry) in the liquid phase. Four recent applications of this novel photochemically useful method are presented: these include the photochemistry of hydroxydiphenylmethyl, 9-hydroxyxanthenyl, diphenylmethyl, and benzoyl radicals under laser jet and normal photolysis conditions.

The regioselectivity of cross-coupling reactions of hydroxydiphenylmethyl or 9-hydroxyxanthenyl radicals with solvent-derived radicals changes when these species are electronically excited,i.e. under the high intensity conditions of the laser jet, cross-coupling at the para position (head-to-tail combination) is significantly enhanced relative to the normal coupling mode at the hydroxy-bearing radical site (head-to-head combination). Semiempirical calculations of the spin density distributions for the ground and first excited states of the radicals confirm the change in spin density from the hydroxy-bearing carbon atom to the conjugating benzene rings in these radical species on photoexcitation.

For the diphenylmethyl radical, two reaction pathways have been observed under the high photon densities of the laser jet: the electronically excited diphenylmethyl radical can either abstract a chlorine atom from carbon tetrachloride through an electron transfer process or can be photoionized on further photoexcitation (multiphoton chemistry). The resulting benzhydryl cation was trapped by methanol as the corresponding ether product, which unequivocally demonstrates that carbene formation by photoejection of a hydrogen atom does not take place under laser jet photolysis conditions.

An advantage of the high photon densities produced in laser jet photolysis is the high steady state concentration of short-lived transients that are generated, which enable unprecedented intermolecular reactions to be observed. Thus, about a millimolar concentration of tert-butoxy radicals can be obtained in the laser jet photocleavage of tert-butyl peroxide. When the tert-butoxy radicals are produced in the presence of benzaldehyde, the main product is tert-butyl benzoate. If carbon tetrachloride is also present, chlorobenzene can be detected. This is rationalized as the product derived from chlorine abstraction by phenyl radicals, which are presumably produced by the photodecarbonylation of benzoyl radicals.

An alternative method of obtaining benzoyl radicals is the two-photon cleavage of benzil. The laser jet photolysis of benzil in tert-butyl peroxide yields mainly tert-butyl benzoate, whereas in carbon tetrachloride, benzoyl chloride, chlorobenzene and ,,-trichloroacetophenone are observed. The first two products result from chlorine atom abstraction by the photochemically generated benzoyl and phenyl radicals, and the last product from in-cage cross-coupling between benzoyl and trichloromethyl radicals.

Such product studies provide detailed mechanistic information on the photochemical behaviour of electronically excited, short-lived transients which complements nicely the kinetic and spectral data of time-resolved laser flash studies. Consequently, the laser jet technique constitutes a valuable tool for determining the mechanism of two- photon reactions.     

Emploi d'une méthode de perturbation-variation pour l'étude de la polarisation de spin dans les radicaux libres aromatiques

A perturbation method has been used to deal with the problem of the interaction of configuration in the free aromatic radicals. We have considered only the mono-excitated configurations which are responsible for the specific effects due to the spin polarization; the corresponding wave functions are built up with the set of molecular orbitals LCAO SCF (occupied and virtual) of the ground-state configuration. We thus obtain a good distribution of spin densities on the rings of the studied radicals: the benzyl and the methylene-naphthyls radicals. The spin density on the extracyclic carbon remains too large as in the case of the SCF representation. This may be explained by the shape of the molecular orbital occupied by the unpaired electron in the SCF configuration, and the structure of the method used which disregards the excitated configurations involving this orbital.     

Production mechanisms of NH and NH2 radicals in N2-H2 plasmas

We measured the densities of NH and NH(2) radicals by cavity ring-down spectroscopy in N(2)-H(2) plasmas expanding from a remote thermal plasma source and in N(2) plasmas to which H(2) was added in the background. The NH radical was observed via transitions in the (0,0), (1,1), and (2,2) vibrational bands of the A(3)Pi <-- X(3)Sigma- electronic transition and the NH(2) radical via transitions in the (0,9,0) <-- (0,0,0) band of the A(2)A(1) <-- X(2)B(1) electronic transition. The measurements revealed typical densities of 5 x 10(18) m(-3) for the NH radical in both plasmas and up to 7 x 10(18) m(-3) for the NH(2) radical when N(2) and H(2) are both fed through the plasma source. In N(2) plasma with H(2) injected in the background, no NH(2) was detected, indicating that the density is below our detection limit of 3 x 1016 m-3. The error in the measured densities is estimated to be around 20%. From the trends of the NH(x) radicals as a function of the relative H(2) flow to the total N(2) and H(2) flow at several positions in the expanding plasma beam, the key reactions for the formation of NH and NH(2) have been determined. The NH radicals are mainly produced via the reaction of N atoms emitted by the plasma source with H(2) molecules with a minor contribution from the reaction of N+ with H(2). The NH(2) radicals are formed by reactions of NH(3) molecules, produced at the walls of the plasma reactor, and H atoms emitted by the plasma source. The NH radicals can also be produced by H abstraction of NH(2) radicals. The flux densities of the NH(x) radicals with respect to the atomic radicals are appreciable in the first part of the expansion. Further downstream the NH(x) radicals are dissociated, and their densities become smaller than those of the atomic radicals. It is concluded that the NH(x) radicals play an important role as precursors for the N and H atoms, which are key to the surface production of N(2), H(2), and NH(3) molecules.     

Electronic structures of the free radicals derived from ascorbic acid and α-hydroxytetronic acid

Calculations on several free radicals derived from ascorbic acid, and α-hydroxytetronic acid are reported. The calculations have been carried out both with the INDO method and the ab initio UHF method. The calculated spin densities are only consistent with the assignment of the structure of the predominant radical derived from these molecules to the anion radical.     

Numerical Study of HBr/He Discharges in Capacitive Coupled Plasma Reactor

Br-based plasmas potentially provide selective etching of Si. The characteristics of homogenous discharge in mixed gases of HBr and He are investigated numerically based on a self-consistent 2D fluid model. The model takes into account the primary processes like excitation and ionization. The reactions of radicals with radicals, neutrals with neutrals and radicals and neutrals are taken into account in HBr/He discharge and therefore can adequately represent discharge plasma. Based on simulation results of the self-consistent 2D fluid model, the dominant species for Si etching in HBr/He plasma discharge are Br, Br⁺, H and HBr⁺. The impact of frequency, voltage, electrode gap, and gas mixture ratio on the densities of these important species in HBr/He has been explored. Simulation results indicate that elevating high frequency electrode’s frequency and voltage, enhances etching species densities. Increasing the electrode gap, the densities of all plasma species decrease and vice versa. The addition of He to HBr plasma decreases Br and HBr⁺ densities while increases Br⁺ density. Densities of active species for Si etching and subsequently chemical etching versus physical sputtering in HBr/He plasma can be controlled by tuning input parameters and the desired etching can be achieved.     

碳前驱体CH3ArCH2NH2热解反应的热力学和动力学DFT研究

在实验研究基础上 ,通过量子化学理论计算对碳前驱体 CH3 Ar CH2 NH2 的热裂解机理作了进一步的研究 .利用 Gaussian98程序包中 AM1方法及 DFT UB3 LYP/3 -2 1 G*方法 ,对化合物 5种可能热裂解路径的热力学和动力学计算结果表明 ,CH3 Ar CH2 NH2 热裂解的主反应路径为生成自由基 CH3 Ar CH2 ·和 NH2 · ,其主反应路径 AM1计算的活化能 Ea=2 3 0 .78k J/mol,DFT计算的活化能 Ea=3 2 1 .1 8k J/mol;比较键焓计算的数据与相应的实验数据 ,发现 DFT计算结果与实验结果吻合得较好 ;通过分析优化的反应物及产物自由基的部分结构参数 ,了解了理论支持主反应的原因 ;计算的产物自由基的空间构型表明主反应路径生成的产物自由基相互间若进行稠环缩合反应 ,将获得分子平面取向性很好的稠环芳烃产物     

The UHF-CNDO/2 study of tetra-atomic radicals containing silicon and phosphorus

The UHF-CNDO/2 calculations have been performed for several radicals containing C, Si or P. The equilibrium geometries found by the minimization of the total energies and calculated spin densities are compared with results from ESR experiments and INDO calculations. The electronic structure of the radicals is discussed for their equilibrium geometries.     

ESR spectra of some new 2,6-di-t-butyl-4-arylphenoxyls

The proton HS and C¹³ splitting are considered for 2,4,6-tri-t-butyl-phenoxyl and various other radicals. The spin densities are determined and are compared with McLachlan MO densities. It is concluded that an aromatic substituent in the para position in phenoxyl withdraws some spin density, in particular reducing the density at position 4.     

自由基氧净电荷对过氧化物引发剂活性的影响

采用分子力学和半经验量子化学的方法 ,计算出了数十种有机过氧化物氧氧键均裂的键离解能 ,以及过氧化物生成的自由基的氧净电荷及其校正值 .数据的多重线性回归证明 ,影响有机过氧化物引发剂活性(常用 1 0h HLT表示 )的主要因素为自由基的电性和空间效应 ,这两者可以用自由基的氧净电荷及其校正值来表征 .在同种溶剂中 ,过氧化物生成的两个自由基的氧净电荷及其校正值 ,这四个因素与其半分解温度之间有较好的相关性 .这种相关性可以用来预测过氧化物的 1 0h HLT值 .     

Theoretical approach to fluorine substitution in X2NO and X2CN free radicals. Comparison between ab initio UHF and RHF + perturbation treatments

Non-empirical calculations have been performed to analyze the effects of fluorine substitution on the geometry and electronic properties of two series of π and σ radicals. Both UHF and RHF + perturbation methods have been used and the results are compared as a function of the basis set quality. As concerns geometry and spin-free electronic properties the results are independent of the UHF or RHF formalism, but highly sensitive to the basis set. The STO-3G basis is unable to reproduce even general trends. Polarization functions always play a relevant role and correlation effects seem not negligible at least for fluorine-containing radicals. The molecular shape of π radicals changes from a planar to a pyramidal geometry when increasing the electronegativity of the substituents. On the contrary, σ radicals always remain planar. Unprojected UHF spin densities are closer to the RHF + perturbation results for small spin contamination (X₂NO). On the contrary, it is the projected UHF spin density which is in better agreement with the RHF + perturbation value for large spin contamination (X₂CN). No simple correlation can be found between spin densities and gross atomic spin populations. For H₂NO the spin density at nitrogen is smaller than at the oxygen nucleus, but substitution may enhance or reverse this trend.     

The cyano nitronyl nitroxide radical: experimental and theoretical evidence for the fourth case of the McConnell-I mechanism

The nitronyl nitroxide 2-cyano-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (1) crystallises in the tetragonal P42(1)m space group with a=7.4050(7), c=8.649(1) A. In the crystal the molecules form layers parallel to the ab plane in which they are orthogonal to each other. In the layers there are close contacts, 2.953(2) A, between the NO groups and the bridging carbon atoms of the O-N-C-N-O fragment of neighbouring radicals. The calculated spin density shows a positive population mainly and equally localised on the NO groups and small but significant negative spin densities on the bridging carbon atom and the cyano nitrogen. Absorption spectra show temperature-dependent transitions related to the magnetic behaviour. The temperature dependence of the magnetic susceptibility in the range 2-300 K reveals that couplings between the radicals are antiferromagnetic, and is interpreted by considering a two-dimensional square array of spin S=1/2 antiferromagnetically coupled (J=-10 cm(-1) and g=2.01). This is interpreted as an exchange coupling through close contact between positive and negative spin densities in orthogonal orbitals on oxygen and carbon atoms, respectively.     

Structure of isolated and solvated peroxyl radicals

We have investigated the structure of HO₂ and a series of alkyl peroxyl radicals ROO using a variety of quantum mechanical methods. We first compute the geometries, vibrational frequencies, electronic charge distributions, and spin densities for the series of radicals considered in the gas phase. Significant differences with respect to previous calculations have been pointed out in a few cases. In particular, we show the fundamental importance of electronic correlation when computing net atomic charges and spin densities, which have generally been estimated in the litterature by means of Hartree–Fock SCF electronic densities. Solvation effects on the geometry and electronic structure have been estimated by carrying out self-consistent reaction field computations in a polarizable continuum environment with relative dielectric permittivity equal to that of liquid water. Large electronic polarization is predicted in such conditions. This may be important in order to understand reactive properties of the radicals in different media. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1039–1048, 1999     

ESR studies of vinyl polymerization. II. Initial reactions of vinyl esters with redox systems in aqueous media

ESR measurements of transient radicals during redox polymerization of various vinyl esters in aqueous solutions have been made by using the rapid-mixing flow method. The initiation was by means of hydroxyl and amino radicals from the systems titanous chloride-hydrogen peroxide and titanous chloride-hydroxylamine, respectively. The well resolved hyperfine structures obtained at monomer concentrations of about 0.05 mole/1. are unambiguously assigned to the monomer radicals formed by addition of initiator radicals to monomers. At higher monomer concentrations, additional weak signals attributed to the growing polymer radicals were observed. The effect of reaction conditions on the signal intensity has been studied in particular for vinyl acetate. The coupling constants of monomer radicals from various vinyl esters (acetate, propionate, butyrate, crotonate, and isopropenyl acetate) were obtained and the spin densities calculated. From the ESR spectra, the monomer radicals have a conformation with the substituent R (R = HO or NH₂) of R? CH₂? CH(OCOR′) locked in a position above or below the radical plane. This is tentatively interpreted as due to formation of intramolecular hydrogen bonds to ring structures or complexes with titanium ions. In addition, hydrogen abstraction reactions of some model compounds for poly(vinyl acetate) have been briefly studied in relation to chain transfer and grafting reactions.     

Applications of the group function method

The general formulae for wave functions based on separated groups, given in a previous paper, are applied in a CNDO calculation of the ground state energy of a number of simple hydrocarbons and hydrocarbon radicals. It is shown that this approach compares favourably with the usual MO-CNDO method both with regard to the ground state energy and the interpretation in terms of chemical bonding. The results for ethane appear to be in fundamental agreement with the discussion of the origin of the barrier of rotation given by Sovers, Kern, Pitzer and Karplus. Regarding spin densities it turns out that the experimental splitting constants can be reproduced to the same level of accuracy as in the usual MO-CNDO method without using an abnormally low coupling constant.     

The problem of conjugation through key heteroatoms in arene-2,1,3-X-diazoles (X=O,S, Se)

Potassium metal and an electrochemical method are used to prepare anion radicals from benz-2,1,3-oxa-, -thia-, and -selenadiazoles, and also from naphtho-1,2-furazane and naphtho-1,2-selenadiazoles. The EPR spectra of these anions are analyzed, and the results compared with the calculated distributions of spin densities in the anions. It is concluded that the key heteroatoms in the molecules of the compounds investigated participate in conjugation in a definite way. This conclusion is also based on analysis of the electronic absorption spectra of arene-2,1, 3-X-diazoles. Apparently the distribution of spin density in the heterocyclic ring does not change substantially from benzologs to naphthologs.     

Etudes photochimiques—XI: Reaction de photosubstitution de la quinoxaline en milieu neutre

Irradiated in ether, quinoxaline is substituted on C₂ according to scheme 3. Radical VIII, which is formed by a monophotonic process is the intermediate of the reaction. VIII is also formed in methanol and in ethanol. It has been caracterised by ESR by the second moment method. Spin densities of VIII and of similar radicals formed from 3-methyl quinoxaline and 2,3-dimethyl quinoxaline have been calculated and agree with experimental results.     

Investigation of the anthracene-nitroxide hybrid molecule as a probe for hydroxyl radicals.

A new method for the determination of hydroxyl radicals is proposed. The method is based on the use of a hybrid molecule consisting of a fluorescent chromophore, anthracene, and a nitroxide radical. In the hybrid molecule, the nitroxide quenches the fluorescence of anthracene strongly. The reaction of hydroxyl radicals with dimethyl sulfoxide generates quantitatively methyl radicals, which then combine with the nitroxide moiety of the hybrid molecules to result in an increase in the fluorescence intensity. The fluorescence increase is proportional to the concentration of hydroxyl radicals. The proposed method is capable of detecting hydroxyl radicals generated in the Fenton system. It is a simple and sensitive technique for the determination of hydroxyl radicals.     

Calculation of hfs coupling constants by the roothaan “Plus” configuration interaction method within the INDO approximation

A new set of parameters, relating hfs coupling constants to the spin densities calculated by the RHF+CI method, within the INDO approximation, is given for¹H, ¹³C, ¹⁴N, ¹⁷O, ¹⁹F. On the whole the results, obtained for a homogeneous sample of organic radicals, are in better agreement with experimental ones, as compared with UHF method.