Studies of Particle Acceleration by an Active Microwave Medium

The PASER is potentially a very attractive method for particle acceleration, in which energy from an active medium is transferred to a charged particle beam. The effect is similar to the action of a maser or laser with the stimulated emission of radiation being produced by the virtual photons in the electromagnetic field of the beam. We have been investigating the possibility of developing a demonstration PASER operating at X-band. The less restrictive beam transport and device dimensional tolerances required for working at X-band rather than optical frequencies as well as the widespread application of X-band hardware in accelerator technology all contribute to the attractiveness of performing a PASER demonstration experiment in this frequency range. Key to this approach is the availability of a new class of active materials that exhibit photoinduced electron spin polarization. We will report on the status of active material development and measurements, numerical simulations, and progress towards a planned microwave PASER acceleration experiment at the Argonne Wakefield Accelerator facility.Comment: 9 pages, 3 figures, submitted to Proceedings of the 2006 Advanced Accelerator Concepts Worksho

Effects of Spiro-Cyclohexane Substitution of Nitroxyl Biradicals on Dynamic Nuclear Polarization

Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin–spin exchange interaction parameter (J), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1′-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents

Imidazoline and imidazolidine nitroxides as controlling agents in nitroxide-mediated pseudoliving radical polymerization

WOS:000431973300002Controlled, or pseudoliving, radical polymerization provides unique opportunities for the synthesis of structurally diverse polymers with a narrow molecular -weight distribution. These reactions occur under relatively mild conditions with broad tolerance to functional groups in the monomers. The nitroxide-mediated pseudoliving radical polymerization is of particular interest for the synthesis of polymers for biomedical applications. This review briefly describes one of the mechanisms of controlled radical polymerization. The studies dealing with the use of imidazoline and imidazolidine nitroxides as controlling agents for nitroxide-mediated pseudoliving radical polymerization of various monomers are summarized and analyzed. The publications addressing the key steps of the controlled radical polymerization in the presence of imidazoline and imidazolidine nitroxides and new approaches to nitroxide-mediated polymerization based on protonation of both nitroxides a nd monomers are considered

Smart Control of Nitroxide-Mediated Polymerization Initiators’ Reactivity by pH, Complexation with Metals, and Chemical Transformations

Because alkoxyamines are employed in a number of important applications, such as nitroxide-mediated polymerization, radical chemistry, redox chemistry, and catalysis, research into their reactivity is especially important. Typically, the rate of alkoxyamine homolysis is strongly dependent on temperature. Nonetheless, thermal regulation of such reactions is not always optimal. This review describes various ways to reversibly change the rate of C⁻ON bond homolysis of alkoxyamines at constant temperature. The major methods influencing C⁻ON bond homolysis without alteration of temperature are protonation of functional groups in an alkoxyamine, formation of metal⁻alkoxyamine complexes, and chemical transformation of alkoxyamines. Depending on the structure of an alkoxyamine, these approaches can have a significant effect on the homolysis rate constant, by a factor of up to 30, and can shorten the half-lifetime from days to seconds. These methods open new prospects for the application of alkoxyamines in biology and increase the safety of (and control over) the nitroxide-mediated polymerization method

Zinc(II) Hexafluoroacetylacetonate Complexes of Alkoxyamines: NMR and Kinetic Investigations. First Step for a New Way to Prepare Hybrid Materials

International audienceA series of Zn(hfac)(2) alkoxyamines (diethyl(2,2-dimethyl-1-(tert-butyl-(1-pyridylethoxy)amino)propyl)phosphonate) coordinated at ortho, meta and para positions of their pyridyl moiety were prepared and their homolysis rate constants k(d) measured. Up to a 30-fold increase in k(d) was observed for the meta-(Zn-RR/SS) and para-(Zn-RR/SS) regioisomers. Moreover, the activation is also controlled by the amount of pyridine, used as a ligand competitor. XRD, H-1 and P-31 NMR spectra show very different structures for each complexes in solid state and in solution depending on the diastereosiomers and on the regioisomers

Alkoxyamine Re-Formation Reaction. Effects of the Nitroxide Fragment: A Multiparameter Analysis

International audienceA few years ago, Studer and co-workers (Macromolecules 2006, 39, 1347-1352) reported the dramatic effect of the reaction of re-formation of alkoxyamines on the fate of the nitroxide-mediated polymerization (NMP) of styrene. This prompted us to investigate more carefully the effects of the nitroxide structure on the re-formation rate constant k(c). Ten new values of k(c) were obtained for the reaction of imidozalidine nitroxide and the phenethyl radical. These values were combined with the 21 values of k(c) reported in the literature for a multiparameter analysis (log(k(c)/M-1 s(-1)) = (10.22 +/- 0.10) + (0.46 +/- 0.02)E-s + (0.41 +/- 0.17)sigma(1)) using the electrical Hammett constant sigma(1) to describe both the stabilization and polar effects as well as the modified Taft steric constant E-s of the nitroxide. The same analysis was performed for the k(c) values of the cross-coupling reaction of nitroxides with tert-butoxylcarbonyl-2-prop-2-yl radical (log(k(c)/M-1 s(-1)) = (11.10 +/- 0.25) + (0.57 +/- 0.05)E-s + (1.42 +/- 0.18)sigma(1)) and tert-butoxycarbonylethyl radical (log(k(c)/M-1 s(-1)) = (10.23 +/- 0.16) + (0.35 +/- 0.03)E-s + (0.93 +/- 0.25)sigma(1)). These correlations were applied for the analysis of the NMP of styrene controlled by 6 pi(.), 6 theta(.), and 6p(.) using a Fischer phase diagram

Dual-initiator alkoxyamines with an N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) nitroxide moiety for preparation of block co-polymers

International audienceDual initiators for controlled radical/ionic polymerization reactions attract much attention in terms of preparation of new materials. We studied the potential of dual-initiator alkoxyamines 3 [based both on para-substituted aromatic ring for external triggering or initiation of orthogonal polymerization and on N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) nitroxide (SG1)] for nitroxide-mediated polymerization (NMP) with various monomers such as styrene, styrene sulphonate, 2-vinyl pyridine or methylmethacrylate. Alkoxyamines 3 were found to be as efficient in the NMP process as N-(2-methylpropyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-O-(2-carboxylprop-2-yl)hydroxylamine (1). Furthermore, in sharp contrast to 1, alkoxyamines 3 were easier to functionalize

C-ON bond homolysis of alkoxyamines triggered by paramagnetic copper(II) salts

International audienceThe metal complexation reactions of bis(hexafluoroacetylacetonato)copper(II)(Cu(hfac)(2)) with alkoxyamines (diethyl(2,2-dimethyl-1-(tert-butyl-(1-(pyridine-4-yl)ethoxy)amino)propyl)phosphonate and diethyl (2,2-dimethyl-1-(tert-butyl-(1-(pyridine-2-yl)ethoxy)amino)propyl)phosphonate) were studied. According to X-ray analysis, the molecular and crystal structures of 1:1 complexes depend on the configuration of the free alkoxyamines, that is dimeric (RSSR) and chain-polymeric (RR/SS) structures for para-pyridyl-substituted alkoxyamines, and cyclic unimeric (RS/SR) structure for ortho-pyridyl derivative. The complex (2:1 ratio Cu(hfac)(2)/alkoxyamine) for ortho-pyridyl-substituted alkoxyamine is not resolved. Upon warming, ortho complexes decomposed into free alkoxyamines and only a weak activation was observed. Upon warming, para complexes decomposed into their corresponding unimers, and then, a 21-fold increase in the rate constant of the C-ON bond homolysis was observed compared to the corresponding free alkoxyamines. Tuning of the homolysis rate constant of the C-ON bond via addition of pyridine is also reported