Effects of the local magnetic fields of paramagnetic particles on chemical reactions

Effects on reactivity from local magnetic fields are considered for conjugated monomers and polymers containing paramagnetic centers. The local activation effect is discussed in phenomenological terms in relation to complexes formed with diamagnetic reagents; results are given for the low-temperature pyrolysis of anthracene, the thermal polymerization of pyridine and quinoline on zinc chloride, interactions with conjugated free-radical systems (including diphenylpicrylhydrazyl DPPH), inhibition of thermal oxidation, and thermal polymerization of tetracyanoethylene.     

Kinetic solvent effects on peroxyl radical reactions

Kinetic solvent effects on peroxyl radical reactions are easily determined using a new peroxyester-based radical clock method.     

The magnetic field effects on photochemical reactions in ionic liquids

The magnetic field effects (MFEs) on photoinduced hydrogen abstraction reactions of benzophenone (BP) with thiophenol (PhSH) in the ionic liquids (ILs) N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl) imide (TMPA TFSI), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl) imide (P13 TFSI), and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl) imide (PP13 TFSI) were investigated at 296 K by using a nanosecond laser flash photolysis technique under magnetic fields of 0-1.7 T. Large MFEs were observed for the first time in the ILs. In TMPA TFSI, the yield of the benzophenone ketyl radical gradually decreased with increasing magnetic field strength from 0 to 1.7 T, producing a 20% decrease at 1.7 T.     

The structure of the paramagnetic impurities in triglycine sulfate single crystals

ENDOR investigations of the bis(glycinato)-Cu(II) (NH₂CH₂COO)₂Cu and bis(glycinato)-oxo-V(IV) (NH₂CH₂COO)₂VO complexes in the ferroelectric triglycine sulfate (TGS) single crystals are reported. Magnetic hyperfine constants of the ligand nuclei are given. The ENDOR investigati confirm the conclusion derived from EPR studies that the copper and vanadyl ions occupy interstitial lattice sites forming bis(glycinato) complexes wit triclinic symmetry C₁. Molecular theoretical calculations of the EPR and ENDOR parameters of different paramagnetic impurities in TGS were performe using the self-consistent charge and configuration (SCCC) version of the extended Hückel theory (EHT). By using perturbation theory the parameters of the spin hamiltonian were determined.     

The influence of an electric and magnetic field in chemical reactions

We have shown that a simultaneous application of uniform and constant electric and magnetic fields cannot affect the equality of the equilibrium enantiomeric populations in a racemic reaction mixture. Hence, whatever the sources of the rotations reported, they cannot have had their origin in uniform applied fields if the reactions have gone to completion.     

The influence of monomer chemistry on radical formation and secondary reactions during electron-beam polymerization

The vast majority of industrial electron-beam (EB) polymerizations are initiated via radical mechanism. Because radicals drive EB polymerization, understanding their formation and secondary reactions can provide insight into polymerization kinetics and property development. Primary and initiating radicals were quantified by measuring G(R^•) and G(M^•) , respectively, for various acrylate and methacrylate monomers. Monomer chemistry was shown to impact primary radical formation; however, increased primary radical concentration did not necessarily correlate to increased conversion. Despite exhibiting high values of G(R^•) , methacrylates achieved very little conversion and had G(M^•) values near zero. Acrylates achieved much higher G(M^•) values and rates of polymerization compared to their methacrylate counterparts. Additionally, the efficiency of primary radicals converting to initiating radicals, f(M^•) , for each acrylate monomer was shown to be a good predictor of the amount of gel fraction formed during polymerization. Understanding radical formation and secondary radical reactions can help guide the structure/processing conditions/properties relationships that are currently underdeveloped for EB reactions.     

Methyl substituent effects in radical cation Diels-Alder reactions

The effect of methyl substitution on the Diels-Alder radical cation reaction was studied using B3LYP with a 6-31G* basis set. Five separate pathways, one concerted and four stepwise, were examined for each possible position of a methyl substituent. None of the concerted transition structures could be located without symmetry constraints, and all but one of the structures obtained under these conditions were destabilized by a second-order Jahn-Teller distortion. A concerted pathway with simultaneous bond formation at C1 and C4 is therefore excluded. Stepwise pathways that had the methyl group either on a carbon involved in the initial bond formation or in a position where it could not stabilize the radical/cation were a few kcal/mol above alternate pathways. High transition state energies for the formation of vinylcyclobutane derivatives cause it to be a minor product in general. The pathway that proceeds through an anti-intermediate is the most favored, while the pathways forming the gauche-out intermediate that converts to the anti-intermediate is also strongly represented. Both of the major pathways lead directly to the formation of the methylcyclohexene product.     

Density-functional calculations of relativistic spin-orbit effects on nuclear magnetic shielding in paramagnetic molecules

Terms arising from the relativistic spin-orbit effect on both hyperfine and Zeeman interactions are introduced to density-functional theory calculation of nuclear magnetic shielding in paramagnetic molecules. The theory is a generalization of the former nonrelativistic formulation for doublet systems and is consistent to O(alpha4), the fourth power of the fine structure constant, for the spin-orbit terms. The new temperature-dependent terms arise from the deviation of the electronic g tensor from the free-electron g value as well as spin-orbit corrections to hyperfine coupling tensor A, the latter introduced in the present work. In particular, the new contributions include a redefined isotropic pseudocontact contribution that consists of effects due to both the g tensor and spin-orbit corrections to hyperfine coupling. The implementation of the spin-orbit terms makes use of all-electron atomic mean-field operators and/or spin-orbit pseudopotentials. Sample results are given for group-9 metallocenes and a nitroxide radical. The new O(alpha4) corrections are found significant for the metallocene systems while they obtain small values for the nitroxide radical. For the isotropic shifts, none of the three beyond-leading-order hyperfine contributions are negligible.     

Kinetic effects in radical addition reactions to radicophiles

Beyond expectation from polar effects rate acceleration is observed when captodative (cd) substituted 1,1-diphenylethylenes 6 and styrenes 7 undergo addition of isobutyronitrile radicals. The effect can be rationalised in terms of frontier orbital interactions.     

Short lived paramagnetic centers in quartz containing germanium impurities

Journal of Structural Chemistry -     

云团中顺磁性粒子对三重态正电子素寿命的影响

Ferrell很早就提出了正电子素o-Ps的猝灭理论,指出介质中的顺磁性粒子是影响o-Ps寿命的重要因素.Lee等测出O_2猝灭o-Ps的速率常数为10~(10)～10~(11)mol·L~(-1)·s~(-1),得出了顺磁猝灭反应是扩散控制的结论.但前人没有对云团中瞬态活性粒子对o-Ps的影响加以探讨.本文通过o-Ps寿命与氧分压的关系来研究云团中瞬态活性粒子对o-Ps猝灭的影响.     

Influence of dipolar interactions on radical pair recombination reactions subject to weak magnetic fields

Monte Carlo simulations of the effects of weak magnetic fields on the recombination of interacting radical pairs undergoing free diffusion in solution have been performed, with the aim of determining the influence on the low field effect of the magnetic dipolar coupling between the radicals. The suppression of singlet-triplet interconversion in the radical pair by the dipolar interaction is found to be pronounced at magnetic field strengths comparable to the hyperfine interactions in the radicals, to the extent that the low field effect is completely abolished. The averaging of the dipolar coupling by the translational diffusion of the radicals around one another is relatively efficient in the presence of strong magnetic fields but becomes ineffective in weak applied fields where the strength of the dipolar interaction is independent of the orientation of the inter-radical vector. Low field effects are only likely to be observed if the motion of the radical pair is restricted in some way so as to increase the likelihood that, having separated to the large distance required for the dipolar interaction to have a negligible effect, the radicals subsequently encounter and have the opportunity to recombine.     

The reactivity of organoboranes in radical substitution reactions

The parameters characterizing substitution reactions of the types R^· + BR₃, RO^· + BR₃, ROO^· + BR₃, and RS^· + BR₃ were calculated from the experimental data using the parabolic model of bimolecular radical reaction. Along with the enthalpy of the reaction, the following factors affect the activation energy: triplet repulsion in the transition state, difference in electronegativities of the atoms forming the reaction center, -bonds in the -position to the reaction center, steric hindrances, and force constants of the reacting bonds. The change in the dissociation energy of the B--C bond in organoboranes, in which alkyl substituents were replaced by alkoxyl and thiyl substituents, was estimated from the kinetic data. The parameters obtained make it possible to calculate the activation energies of individual reactions of four types under study.     

Interrelationship between reactivity and bulk activation effects in radical substitution reactions

Conclusions Similar to other reactions of this type, the reaction for cleaving H atom from saturated hydrocarbons by Cl atom obeys a linear equation with a small slope coefficient, which relates the logarithms of the rate constants and V. This is explained by a difference in the structures of the transition state.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 478–480, February, 1977.     

Solvent effects on the oxidative free radical reactions of 2-amino-1,4-naphthoquinones

Solvent effects on the manganese (III) initiated oxidative free radical reactions of 2-amino-1,4-naphthoquinones are described. This free radical reaction provides a novel method for the synthesis of benzo[f]indole-4,9-diones, benzo[f]indole-2,4,9-triones, benzo[b]carbazole-6,11-diones and benzo[b]acridine-6,11-diones. High chemoselectivity was observed in different solvents.