Radicals 2a Research Articles

Enhanced Conductivity and Magnetic Ordering in Isostructural Heavy Atom Radicals

Synthetic methods have been developed to generate the complete series of resonance-stabilized heterocyclic thia/selenazyl radicals 1a-4a. X-ray crystallographic studies confirm that all four radicals are isostructural, belonging to the tetragonal space group P42(1)m. The crystal structures consist of slipped pi-stack arrays of undimerized radicals packed about 4 centers running along the z direction, an arrangement which gives rise to a complex lattice-wide network of close intermolecular E2---E2' contacts. Variable temperature conductivity (sigma) measurements reveal an increase in conductivity with increasing selenium content, particularly so when selenium occupies the E2 position, with sigma(300 K) reaching a maximum (for E1 = E2 = Se) of 3.0 x 10(-4) S cm(-1). Thermal activation energies E(act) follow a similar profile, decreasing with increasing selenium content along the series 1a (0.43 eV), 3a (0.31 eV), 2a (0.27 eV), 4a (0.19 eV). Variable temperature magnetic susceptibility measurements indicate that all four radicals exhibit S = 1/2 Curie-Weiss behavior over the temperature range 20-300 K. At lower temperatures, the three selenium-based radicals display magnetic ordering. Radical 3a, with selenium positioned at the E1 site, undergoes a phase transition at 14 K to a weakly spin-canted (phi = 0.010 degrees) antiferromagnetic state. By contrast, radicals 2a and 4a, which both possess selenium in the E2 position, order ferromagnetically, with Curie temperatures of T(c) = 12.8 and 17.0 K, respectively. The coercive fields H(c) at 2 K of 2a (250 Oe) and 4a (1370 Oe) are much larger than those seen in conventional light atom organic ferromagnets. The transport properties of the entire series 1a-4a are discussed in the light of Extended Hückel Theory band structure calculations.

Generation of Thymine Radicals from 6-Alkoxy-5-bromo-5,6-dihydrothymine Derivatives by Radiolytic Reduction and Photolytic Dehalogenation

The reduction of 6-alkoxy-5-bromo-5,6-dihydrothymine derivatives ( 1a, b) by hydrated electrons (e aq (-)) generated in the radiolysis of deoxygenated aqueous solution was investigated. As the major products, 1-(6'-alkoxy-5',6'-dihydrothymin-5'-yl)thymines ( 6a, b), 5-(hydroxymethyl)uracil ( 8), 6-alkoxy-5,6-dihydrothymines ( 9a, b), and thymine ( 10) were produced in sufficient yields. This product distribution is indicative of the generation of 6-alkoxy-5,6-dihydrothymin-5-yl radicals ( 2a, b) as primary intermediates that undergo elimination of alkoxide ions (RO (-)) into thymine radical cations ( 3) followed by deprotonation at the N1 to form N-centered thymine radicals ( 4). The transient absorption spectra of the 5-yl radicals 2a- c were observed by means of nanosecond laser flash photolysis of 1a, b and 5-bromo-6-ethoxy-5,6-dihydrothymidine ( 1c) in deoxygenated aqueous solution, in which homolytic C5-Br bond dissociation occurred. In contrast to the reaction characteristics in aqueous solutions, the dimeric products were not obtained in acetonitrile, probably because in-cage hydrogen abstraction from the C5 methyl group by bromine atom leads to formation of methide type intermediates 20.

Photoinduced C−N Bond Cleavage in 2-Azido-1,3-diphenyl-propan-1-one Derivatives: Photorelease of Hydrazoic Acid

Photolysis of 3-azido-1,3-diphenyl-propan-1-one (1a) in toluene yields 1,3-diphenyl-propen-1-one (2), whereas irradiation of 3-azido-2,2-dimethyl-1,3-diphenyl-propan-1-one (1b) results in the formation of mainly 2,2-dimethyl-1,3-diphenyl-propan-1-one. Laser flash photolysis (308 nm) of 1a,b in acetonitrile reveals a transient absorption (lambda max = approximately 310 nm) due to the formation of radicals 4a and 4b, respectively, which have lifetimes of approximately 14 micros at ambient temperature. TD-DFT calculations (B3LYP/6-31+G(d)) reveal that the first and second excited states of the triplet ketone (T1K (n,pi*) and T2K (pi,pi*)) in azide 1a are almost degenerate, at approximately 74 and 76 kcal/mol above the ground state (S0), respectively. Similarly, azide 1b has T1K and T2K 75 and 82 kcal/mol above S0, respectively. The calculated transition state for cleaving the C-N bond is located 71 and 74 kcal/mol above S0 in azides 1a and 1b, respectively. The calculated bond dissociation energies for breaking the C-N bond are 55 and 58 kcal/mol for azides 1a and 1b, respectively, making C-N bond breakage accessible from T1K in azides 1 at ambient temperature. In comparison, the irradiation of azides 1 in argon matrices at 14 K lead to the formation of the corresponding triplet alkyl nitrenes (1-n), via intramolecular energy transfer from T2K. The characterization of 1-n was supported by isotope labeling, IR spectroscopy, and molecular modeling.

Dissecting the Proline Effect: Dissociations of Proline Radicals Formed by Electron Transfer to Protonated Pro-Gly and Gly-Pro Dipeptides in the Gas Phase

We report a combined experimental and computational study of the proline effect in model dipeptides Pro-Gly and Gly-Pro. Gas-phase protonated peptide ions were discharged by glancing collisions with potassium or cesium atoms at 3 keV collision energies, and the peptide radical intermediates and their dissociation products were analyzed following collisional ionization to anions. The charge reversal (+CR-) mass spectra of (Pro-Gly + H)+(1a+) and (Gly-Pro + H)+ (2a+) showed dramatic differences and thus provided a sensitive probe of ion structure. Whereas 1a+ completely dissociated upon charge inversion, 2a+ gave a nondissociated anion as the most abundant product. Ab initio and density functional theory calculations provided structures and vertical recombination energies (REvert) for 1a+ and 2a+. The recombination energies, REvert = 3.07 and 3.36 eV for 1a+ and 2a+, respectively, were lower than the alkali metal ionization energies and indicated that the collisional electron transfer to the peptide ions was endoergic. Radical 1a* was found to exist in a very shallow local energy minimum, with transition state energies for loss and migration of H indicating very facile dissociation. In contrast, radical 2a* was calculated to spontaneously isomerize upon electron capture to a stable dihydroxycarbinyl isomer (2e*) that can undergo consecutive and competitive isomerizations by proline ring opening and intramolecular hydrogen atom transfers to yield stable radical isomers. Radical 2e* and its stable isomers were calculated to have substantial electron affinities and thus can form the stable anions that were observed in the +CR- mass spectra. The calculated TS energies and RRKM kinetic analysis indicated that peptide N-C alpha bond dissociations compete with pyrrolidine ring openings triggered by radical sites at both the N-terminal and C-terminal sides of the proline residue. Open-ring intermediates were found in which loss of an H atom was energetically preferred over backbone dissociations. This provided an explanation for the proline effect causing low incidence of electron capture dissociations of N-C alpha bonds adjacent to proline residues in tryptic peptides and also for some peculiar behavior of proline-containing protein cation-radicals.

Competition between α-Cleavage and Energy Transfer in α-Azidoacetophenones

Molecular modeling demonstrates that the first excited state of the triplet ketone (T1K) in azide 1b has a (pi,pi*) configuration with an energy that is 66 kcal/mol above its ground state and its second excited state (T2K) is 10 kcal/mol higher in energy and has a (n,pi*) configuration. In comparison, T1K and T2K of azide 1a are almost degenerate at 74 and 77 kcal/mol above the ground state with a (n,pi*) and (pi,pi*) configuration, respectively. Laser flash photolysis (308 nm) of azide 1b in methanol yields a transient absorption (lambdamax=450 nm) due to formation of T1K, which decays with a rate of 2.1 x 105 s-1 to form triplet alkylnitrene 2b (lambdamax=320 nm). The lifetime of nitrene 2b was measured to be 16 ms. In contrast, laser flash photolysis (308 nm) of azide 1a produced transient absorption spectra due to formation of nitrene 2a (lambdamax=320 nm) and benzoyl radical 3a (lambdamax=370 nm). The decay of 3a is 2 x 105 s-1 in methanol, whereas nitrene 2a decays with a rate of approximately 91 s-1. Thus, T1K (pi,pi*) in azide 1b leads to energy transfer to form nitrene 2b; however, alpha-cleavage is not observed since the energy of T2K (n,pi*) is 10 kcal/mol higher in energy than T1K, and therefore, T2K is not populated. In azide 1a both alpha-cleavage and energy transfer are observed from T1K (n,pi*) and T2K (pi,pi*), respectively, since these triplet states are almost degenerate. Photolysis of azide 1a yields mainly product 4, which must arise from recombination of benzoyl radicals 3a with nitrenes 2a. However, products studies for azide 1b also yield 4b as the major product, even though laser flash photolysis of azide 1b does not indicate formation of benzoyl radical 3b. Thus, we hypothesize that benzoyl radicals 3 can also be formed from nitrenes 2. More specifically, nitrene 2 does undergo alpha-photocleavage to form benzoyl radicals and iminyl radicals. The secondary photolysis of nitrenes 2 is further supported with molecular modeling and product studies.

TR-EPR of single and double spin labelled C60 derivatives: observation of quartet and quintet excited states in solution

Time-resolved EPR spectra recorded after pulsed laser photoexcitation of two fullerene nitroxide radicals 1a and 1b and two fullerene nitroxide biradicals 2a and 2b in solution have been investigated at room temperature. The TR-EPR spectra of all the radicals are interpreted as arising from spin-polarized ground and excited states, the latter being obtained by the spin coupling of the nitroxide unpaired spins with the fullerene excited triplet state. The TR-EPR spectra of the monoradicals are consistent with strong (1a) or weak (1b) spin coupling, giving rise to pure quartet/doublet excited states (1a) or mixed states (1b). In the latter, the spectral simulation allowed the determination of the magnitude of the exchange coupling constant J TR between the triplet and the radical. The spin polarization of the excited and ground states is attributed to CIDEP effect induced by the radical triplet pair mechanism (RTPM). The sign of J TR is inferred from the sign of the spin polarization. In the case of biradicals 2a and 2b, the TR-EPR spectra are assigned, on the basis of hyperfine pattern and g-values, to quintet states obtained by spin coupling of the fullerene triplet photoexcited state with the two radicals. The spin polarization of the ground state is attributed to the triplet–triplet annihilation process and the sign of the polarization allows the assignment of the radical–radical exchange constant J RR.

Generation and characterization of ionic and neutral chloro(hydroxy) phosphanyl [Cl–P–OH] +/[rad] and chloro(thiohydroxy) phosphanyl [Cl–P–SH] +/[rad] in the gas phase by tandem mass spectrometry and computational chemistry

Dissociative electron ionization (70 eV) of diethyl chlorothiophosphate ( I) and ethylphosphonodichloridate ( II) generates moderately abundant m/ z 83 ions of composition [P, Cl, O, H] +. From tandem mass spectrometry experiments and theoretical calculations at the B3LYP/6-31G(d, p), G2, and G2(MP2) levels it is concluded that the majority of the ions have the structure of ClPOH + ( 1a +) and it is separated by high-energy barriers from its isomers HP(O)Cl + ( 1b +), HPOCl + ( 1c +), and OP–ClH ( 1d +). Neutralization–reionization experiments confirm the theoretical prediction that radical 1a is a stable species in the gas phase. The 70 eV electron ionization of ethylphosphonothioic dichloride ( III) yields m/ z 99 ions, predominantly of the structure ClPSH + ( 2a +). This conclusion follows from tandem mass spectrometry experiments and theoretical calculations. The calculations predict that ( 2a +) is separated by high-energy barriers from its isomers HP(S)Cl + ( 2b +), HPSCl + ( 2c +), and SP–ClH ( 2d +). Neutralization–reionization experiments confirmed that 2a radical is a stable species in the rarefied gas phase, which is in agreement with theoretical calculations.

Rate Constants for Anilidyl Radical Cyclization Reactions

[reaction: see text] N-Aryl-5,5-diphenyl-4-pentenamidyl radicals (3) were produced by 266 nm laser-flash photolysis of the corresponding N-(phenylthio) derivatives, and the rate constants for the cyclizations of these radicals were measured directly. The 5-exo cyclization reactions were fast (k(c) > 2 x 10(5) s(-1)), and radicals 3 generally behaved as electrophilic reactants with a Hammett correlation of rho = 1.9 for five of the six radicals studied. However, the p-methoxyphenyl-substituted radical 3f cyclized much faster than expected from the Hammett analysis. Variable temperature studies of parent radical 3a (aryl = phenyl) gave an Arrhenius function with log k = 9.2 - 4.4/2.3RT (kcal/mol). The rate constant for the reaction of p-ethylphenyl-substituted anilidyl radical 3b with Bu(3)SnH at 65 degrees C was k(T) = 4 x 10(5) M(-1) s(-1).

Generation of α‐Phosphonovinyl Radicals and Development of a New Route to Highly Functionalized Vinylphosphonates and Vinylphosphonate‐Incorporated Carbocyclic or Heterocyclic Compounds via a Radical Trapping Sequence.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Quantum Chemical and Master Equation Simulations of the Oxidation and Isomerization of Vinoxy Radicals

The vinoxy radical, a common intermediate in gas-phase alkene ozonolysis, reacts with O2 to form a chemically activated alpha-oxoperoxy species. We report CBS-QB3 energetics for O2 addition to the parent (*CH2CHO, 1a), 1-methylvinoxy (*CH2COCH3, 1b), and 2-methylvinoxy (CH3*CHCHO, 1c) radicals. CBS-QB3 predictions for peroxy radical formation agree with experimental data, while the G2 method systematically overestimates peroxy radical stability. RRKM/master equation simulations based on CBS-QB3 data are used to estimate the competition between prompt isomerization and thermalization for the peroxy radicals derived from 1a, 1b, and 1c. The lowest energy isomerization pathway for radicals 4a and 4c (derived from 1a and 1c, respectively) is a 1,4-shift of the acyl hydrogen requiring 19-20 kcal/mol. The resulting hydroperoxyacyl radical decomposes quantitatively to form *OH. The lowest energy isomerization pathway for radical 4b (derived from 1b) is a 1,5-shift of a methyl hydrogen requiring 26 kcal/mol. About 25% of 4a, but only approximately 5% of 4c, isomerizes promptly at 1 atm pressure. Isomerization of 4b is negligible at all pressures studied.

S-Alkylation-Induced Redox Reactions Leading to Reversible Sulfur−Sulfur Coupling in a Pentamethylcyclopentadienyl Ruthenium(III) Thiolate-Thioether System

Alkylation of [Cp*RuIII(tpdt)] (2) (Cp* = η5-C5Me5, tpdt = η3-S(CH2CH2S-)2) with MeI or Me3OBF4 resulted in the formation of a trans μ-η1-η1-S2 coupled species, [{Cp*RuII}2{μ-η6-(S(CH2)2S(CH2)2SMe)2}]2+ (3) as the predominant product. With MeI the reaction also gave a small amount of [Cp*RuII{η3-S(CH2CH2SMe)2}]I, 4(I), but in the presence of acrylonitrile (AN), the products were 4(I) and [Cp*RuII{η3-S((CH2)2S)2(CH2CHCN)}]I, 5(I), of which the latter was the sole product from the reaction of 2 with AN in the presence of iodine. 2 was electrochemically easily oxidized in a one-electron process; its chemical oxidation with I2 led to the isolation of [{Cp*RuII}2{μ-η6-(S(CH2CH2S)2)2}](I3)2, 6(I3)2, containing a cyclic RuII(μ-S2)2RuII core. A combination of electrochemical, EPR, UV−vis, and NMR experiments indicated that the solution phase chemistry of 3 is governed by its facile reversible dissociation into the mononuclear cation radical (3A). Based on this phenomenon were rationalized the pathways leading to ...

Diverse Reactions of Nitroxide‐Radical Adducts of Silylene, Germylene, and Stannylene

AbstractFor Abstract see ChemInform Abstract in Full Text.

An accelerating-reporting group for studies of radical heterolysis reactions and its application in an acid-catalyzed fragmentation reaction of an alpha,beta-dimethoxy radical.

The 2,2-diphenylcyclopropyl group was employed to accelerate reactions of alpha-methoxy radicals containing beta-leaving groups, to trap the products of either migration or heterolysis of the leaving group, and to provide a useful chromophore for laser flash photolysis kinetic studies. The reporting group biases reactions in favor of heterolytic fragmentation and most likely intercepts radical cations in ion pairs. The 1-methoxy-1-methyl-2-(diethylphosphatoxy)-2-(2,2-diphenylcyclopropyl)ethyl radical (3a) reacted faster than the kinetic resolution of the instrument (k > 2 x 10(8) s(-1)) in all solvents studied, and the 2-acetoxy analogue (3b) reacted much faster than related radicals that do not contain the cyclopropyl group (e.g., k = 1.1 x 10(6) s(-1) in CH3CN at ambient temperature). The rate constants and Arrhenius parameters for reactions of 3b indicated that the rate-limiting step in the reaction was heterolytic cleavage. The 1,2-dimethoxy-1-methyl-2-(2,2-diphenylcyclopropyl)ethyl radical (26) reacted in a general acid-catalyzed heterolysis reaction, and rate constants for protonation of the beta-methoxy group by a series of carboxylic acids were determined. The results suggest that acid-catalyzed reactions of beta-alkoxy radicals might be employed in synthetic conversions.

Diverse reactions of nitroxide-radical adducts of silylene, germylene, and stannylene

The results of the thermolysis of 1:2 adducts of stable group-14 element divalent compounds [R 2M:, R 2=1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl; 1b, M=Ge; 1c, M=Sn] to TEMPO radical are discussed in detail. Whereas the thermal reactions of the 1:2 adducts [R 2M(OR ′) 2, R ′=2,2,6,6-tetramethylpiperidin- N-yl; 3b, M=Ge; 3c, M=Sn] are understood to proceed by the initial homolysis of an M–O bond to give the corresponding aminoxy-substituted group-14 element radicals [R 2(R ′O)M ; 2b, M=Ge; 2c, M=Sn] and TEMPO, the subsequent reactions of 2b and 2c were remarkably different to each other; 2b favors the N–O bond fission (path b) to give the corresponding germanone, while 2c prefers the M–O bond fission (path a) to give stannylene ( 1c). In combining with our previous results for aminoxysilyl radical ( 2a) [R 2(R ′O)Si ], the origin of the remarkable differences in the reactivity among group-14 element radicals 2a– 2c is discussed on the basis of the theoretical calculations for model reactions. Improved syntheses of the precursor dichlorogermane and dichlorostannane of germylene ( 1b) and stannylene ( 1c), respectively, are described in Section 3.

C‐N vs. C‐C Bond Photodissociation in N‐(tert‐butyldiphenylmethyl)aniline

The direct photolysis of the title aniline compound Ph2(Me3C) C‐NHPh in acetonitrile with 248 nm laser light results in the dissociation of the C‐N bond giving the corresponding (tert‐Bu)diphenylmethyl radical 1a• and the anilino radical PhNH• detectable by ns‐laser flash photolysis (LFP) and ESR spectroscopy. The same radical 1a• is also produced from the corresponding (tert‐Bu)diphenylmethyl chloride Ph2(Me3C)C‐Cl either photolytically in MeCN through C‐Cl bond homolysis or pulse radiolytically in THF; the (tert‐Bu)diphenylmethyl cation is also detected, however, at sorter time scales because of its high reactivity in the solvent MeCN. At higher laser pulse intensities the radical 1a• is reexcited by a second photon leading through electrocyclization to the 4a,4b‐dihydro‐9‐tert‐butylfluorenyl radical (DHBF•) and finally to fluorenyl products. Product analyses of steady state photolyses using GC and GC‐MS gave as principal products (tert‐Bu)diphenylmethane, aniline and various 9‐substituted fluorenyl derivatives, corroborating further the primary C‐N bond cleavage and the formation of the radicals 1a• and PhNH•. Other conceivable dissociation pathways leading to the C‐CMe3 or N‐H bond rupture were not observed.

Mechanism of 1,3-migration in allylperoxyl radicals: computational evidence for the formation of a loosely bound radical-dioxygen complex.

The three pathways postulated for 1,3-migration of the peroxyl group in the allylperoxyl radical (1a), a key reaction involved in the spontaneous autoxidation of unsaturated lipids of biological importance, have been investigated by means of quantum mechanical electronic structure calculations. According to the barrier heights calculated from RCCSD(T)/6-311+G(3df,2p) energies with optimized molecular geometries and harmonic vibrational frequencies determined at the UMP2/6-311+G(3df,2p) level, the allylperoxyl rearrangement proceeds by fragmentation of 1a through a transition structure (TS1) with a calculated DeltaH++(298 K) of 21.7 kcal/mol to give an allyl radical-triplet dioxygen loosely bound complex (CX). In a subsequent step, the triplet dioxygen moiety of CX recombines at either end of the allyl radical moiety to convert the complex to the rearranged peroxyl radical (1a') or to revert to the starting peroxyl radical 1a. CX shows an electron charge transfer of 0.026 e in the direction allyl --> O(2). The dominant attractive interactions holding in association the allyl radical-triplet dioxygen pair in CX are due chiefly to dispersion forces. The DeltaH(298 K) for dissociation of CX in its isolated partners, allyl radical and triplet dioxygen, is predicted to be at least 1 kcal/mol. The formation of CX prevents the diffusion of its partners and maintains the stereocontrol along the fragmentation-recombination processes. The concerted 1,3-migration in allylperoxyl radical is predicted to take place through a five-membered ring peroxide transition structure (TS2) showing two long C-O bonds. The DeltaH++(298 K) calculated for this pathway is less favorable than the fragmentation-recombination pathway by 1.9 kcal/mol. The cyclization of 1a to give a dioxolanyl radical intermediate (2a) is found to proceed through a five-membered ring transition structure (TS3) with a calculated DeltaH++(298 K) of 33.9 kcal/mol. Thus, the sequence of ring closure 1a --> 2a and ring opening 2a --> 1a' is unlikely to play any significant role in allylperoxyl rearrangement 1a --> 1a'. In the three pathways investigated, the energy of the transition structure is predicted to be somewhat lower in either heptane or aqueous solution than in the gas phase. Although the energy lowering calculated for TS1 is smaller than the calculated for TS2 and TS3, it is very unlikely that the solvent effects may reverse the predicted preference of the fragmentation-recombination pathway over the concerted and stepwise ring closure-ring opening mechanisms.

The reactions of cytidine and 2'-deoxycytidine with SO4.- revisited. Pulse radiolysis and product studies.

The reactions of SO4.- with 2'-deoxycytidine 1a and cytidine 1b lead to very different intermediates (base radicals with 1a, sugar radicals with 1b). The present study provides spectral and kinetic data for the various intermediates by pulse radiolysis as well as information on final product yields (free cytosine). Taking these and literature data into account allows us to substantiate but also modify in essential aspects the current mechanistic concept (H. Catterall, M. J. Davies and B. C. Gilbert, J. Chem. Soc., Perkin Trans. 2, 1992, 1379). SO4.- radicals have been generated radiolytically in the reaction of peroxodisulfate with the hydrated electron (and the H. atom). In the reaction of SO4.- with 1a (k = 1.6 x 10(9) dm3 mol-1 s-1), a transient (lambda max = 400 nm, shifted to 450 nm at pH 3) is observed. This absorption is due to two intermediates. The major component (lambda max approximately 385 nm) does not react with O2 and has been attributed to an N-centered radical 4a formed upon sulfate release and deprotonation at nitrogen. The minor component, rapidly wiped out by O2, must be due to C-centered OH-adduct radical(s) 6a and/or 7a suggested to be formed by a water-induced nucleophilic replacement. These radicals decay by second-order kinetics. Free cytosine is only formed in low yields (G = 0.14 x 10(-7) mol J-1 upon electron-beam irradiation). In contrast, 1b gives rise to an intermediate absorbing at lambda max = 530 nm (shifted to 600 nm in acid solution) which rapidly decays (k = 6 x 10(4) s-1). In the presence of O2, the decay is much faster (k approximately 1.3 x 10(9) dm3 mol-1 s-1) indicating that this species must be a C-centered radical. This has been attributed to the C(5)-yl radical 8 formed upon the reaction of the C(2')-OH group with the cytidine SO4(.-)-adduct radical 2b. This reaction competes very effectively with the corresponding reaction of water and the release of sulfate and a proton generating the N-centered radical. Upon the decay of 8, sugar radical 11 is formed with the release of cytosine. The latter is formed with a G value of 2.8 x 10(-7) mol J-1 (85% of primary SO4.-) at high dose rates (electron beam irradiation). At low dose rates (gamma-radiolysis) its yield is increased to 7 x 10(-7) mol J-1 due to a chain reaction involving peroxodisulfate and reducing free radicals. Phosphate buffer prevents the formation of the sugar radical at the SO4(.-)-adduct stage by enhancing the rate of sulfate release by deprotonation of 2b and also by speeding up the decay of the C(5)-yl radical into another (base) radical. Cytosine release in cytidine is mechanistically related to strand breakage in poly(C). Literature data on the effect of dioxygen on strand breakage yields in poly(C) induced by SO4.- (suppressed) and upon photoionisation (unaltered) lead us to conclude that in poly(C) and also in the present system free radical cations are not involved to a major extent. This conclusion modifies an essential aspect of the current mechanistic concept.

Gd(1.33)Pt(3)(Al,Si)(8) and Gd(0.67)Pt(2)(Al,Si)(5): two structures containing a disordered Gd/Al layer grown in liquid aluminum.

Gd(1.33)Pt(3)Al(8) was synthesized by the combination of Gd and Pt in excess liquid aluminum. Addition of silicon resulted in the incorporation of a small amount of this element into the material to form the isostructural Gd(1.33)Pt(3)Al(7)Si. Both compounds grow as rodlike crystals with hexagonal cross section. The structures were refined in the rhombohedral space group R(-)3m, with cell parameters a = 4.3359(6) A and c = 38.702(8) A for the ternary and a = 4.3280(8) A and c = 38.62(1) A for the quaternary compound. The structure is comprised of stuffed arsenic-like PtAl(2) layers and disordered Gd/Al layers. Analysis of the hk0 zone reflections indicate the presence of an a = radical 3a supercell, but the structure is not ordered along c, as revealed by the highly diffuse reflections in the 0kl zone photos. Therefore, the compounds are disordered variants of the Gd(4)Pt(9)Al(24) type. Magnetic susceptibility studies reveal antiferromagnetic transitions at 15 K for the ternary and 7 K for the quaternary compound. Variation of the reactant ratio produces a different structure comprised of the same structural blocks, including the disordered Gd/Al layer. Gd(0.67)Pt(2)Al(5) and its quaternary analogue Gd(0.67)Pt(2)Al(4)Si form in the hexagonal system P6(3)/mmc with cell parameters a = 4.2907(3) A and c = 16.388(2) A for the ternary and a = 4.2485(6) A and c = 16.156(3) A for the quaternary compound.

Poly(1,2-phenylenevinylene) Ferromagnetically 3,5-Bearing Phenoxyl Radicals

A π-conjugated but non-Kekule- and nondisjoint-type molecule, poly(1,2-phenylenevinylene) 3,5-disubstituted with pendant phenoxyl radicals (1a), was designed as a new high-spin organic polymer and synthesized by the head-to-tail coupling polymerization of 2-bromo-3,5-bis(3,5-di-tert-butyl-4-aceotoxyphenyl)styrene (3) using a palladium catalyst. The corresponding high-spin dimer model compound bearing four phenoxyl radicals (2a) was also synthesized. The stepwise radical formation in 1 and 2 was electrochemically studied. The ESR signal at ΔM s = ±2 indicated a triplet ground state for 1 and 2. However, the SQUID data revealed that the spin concentration and the average spin quantum number of la and 2a remained at ca. 0.5 spin/phenol unit and 2/2, respectively. A degradation at the vinylene bridge was suggested after the radical generation.

Ferro- and antiferromagnetic interactions of cyano-substituted thioaminyl radicals

Four kinds of thioaminyls, N-(arylthio)-2-ethoxycarbonyl- ( 1), N-(arylthio)-2-acetyl- ( 2), N-(arylthio)-2-cyano- ( 3), and N-(arylthio)-2-fluoro-4,6-diarylphenylaminyls ( 4) were prepared. Thioaminyls 3 were quite persistent and could be isolated as radical crystals. For the isolated three radicals ( 3a, 3b and 3d) the magnetic susceptibility measurements were carried out using a SQUID magnetometer in the temperature range 1.8–300 K. The magnetic interactions of 3b and 3d were antiferromagnetic, and that of 3a was ferromagnetic. Analysis of the χ mol T versus T plots of 3a with the one-dimensional regular Heisenberg model gave 2 J/ k B=+11.2 K.