Unpaired electron distribution in acylaminyls

Unpaired electron distribution in acylaminyls and acylalkoxyaminyls has been investigated by e.s.r. measurements of ¹⁷O labelled radicals. Delocalisation of free spin onto carbonyl oxygen has been established for the former and onto carbonyl and etherial oxygens for the latter.     

Stable free-radical complexing reagents in applications of electron spin resonance to the determination of metals

Metals can be determined by electron spin resonance (e.s.r.) by using stable free radicals in which the molecules include complexing groups. Such reagents form complexes with metal ions which retain the properties of the free radicals producing the e.s.r. signal. The complexes formed can be separated from the excess of reagent and the metal concentrations measured from the signal intensity. This approach markedly expands the potential of e.s.r. as an analytical technique, because it is not limited to paramagnetic metals. The relatively high sensitivity of free radical determination by the e.s.r. method is an additional advantage. The properties of the spin-labelled β-diketone, 4-acetoacetyl-2,2,5,5-tetramethyl-3-imidazoline-1-oxyl, are described. Dissociation constants are reported for the enol form (pK_a^T = 6.56 ± 0.02) and for the protonated (at the imidazolyl nitrogen) form (pK_a^T = 1.91 ± 0.02), as well as partition constants for the hexane—water (log k_D(C₆H₁₄) = 0.96 ± 0.02) and chloroform—water (log K_D(CHCl₃) = 3.36 ± 0.04) systems. The reagent extracts Cu, Pb, Cd, Hg and Er readily; in the presence of caproic acid iron(III) is also extracted. Copper is extracted as a CuA₂, complex (log K_D = 2.5 ± 0.5; log K_ex = -2.80 ± 0.13; log β₂ = 14.3 ± 1.5). CuA₂ does not give an e.s.r. signal. Iron is extracted as a mixed complex with β-diketone and caproic acid. An extraction—e.s.r. method is reported for the determination of mercury with a detection limit of 2 × 10^-7 M.     

α- and γ-Regiocontrol and Enantiospecificity in the Copper-Catalyzed Substitution Reaction of Propargylic Phosphates with Grignard Reagents

The regioselectivity (r.s.) and enantiospecificity (e.s.) of the substitution reactions of secondary propargylic alcohol derivatives using reagents derived from ArMgBr and Cu salts were studied. First, the picolinate, 3-methylpicolinate, and diethylphosphonate derivatives of Ph(CH₂)₂CH(OH)C≡CTMS were reacted with PhMgBr/CuCN in ratios of 2.5:2.7–2.5:0.25. The use of 2.5:0.25 ratio in THF/DME (6:1) at 0 °C for 1 h afforded the α-substitution product from the phosphate with ≥98 % r.s. and 99 % e.s. CuBr⋅Me₂S gave similar selectivity. The reaction system was then applied to phosphates derived from R¹CH(OH)C≡CR² and ArMgBr to obtain synthetically sufficient r.s. and e.s. values with R²=TMS, Ph, whereas iPr was borderline in terms of size as an R¹ substituent. The presence of a substituent at the o-position of Ar marginally affected the selectivity. We also found that the use of PhMgBr/Cu(acac)₂ in a 2:1 ratio in THF produced the γ-substitution products (allenes) with high r.s. and e.s.     

Complexes of 4-hydroxyphenylthiocarboxyhydrazide with iron(II), cobalt(II,III), nickel(II), copper(II) and zinc(II)

Summary The 4-hydroxyphenylthiocarboxyhydrazide (Hoth) ligand has been characterized by i.r.,¹H and¹³C spectral studies. Its metal complexes with Fe^II, Co^II,III, Ni^II, Cu^II and Zn^II have been prepared and characterized on the basis of elemental analyses, molar conductance, magnetic susceptibility. Mössbauer, visible, e.s.r., i.r.,¹H and¹³C n.m.r. spectral studies. The bonding and stereochemistry of the complexes are discussed. Hoth and its Cu^II complexes have been screened towards bacteria, viruses and fungi.     

Bis[N-(2,6-di-t-butyl-1-hydroxyphenyl)salicylideneaminato]palladium(II) complexes and their radical intermediates generated by PbO2 and PPh3

New bis[N-(2,6-di-t-butyl-1-hydroxyphenyl)salicylideneminato]palladium(II) [Pd(L ^x )₂] complexes bearing HO and MeO substituents on the salicyaldehyde moiety were prepared, and their spectroscopic properties, as well as redox reactivity towards PbO₂ and PPh₃, examined by e.s.r. and u.v. spectroscopy. The complexes display charge-transfer bands in the 670–692 nm range in polar solvents, which are assigned to the d(Pd) * (chelated quinoid) transition. One-electron oxidation of Pd(L ^x )₂ produces Pd^II-stabilized radicals in which the unpaired electrons are localized on the phenoxy fragments and do not couple with the two radical centers. The complexes are easily reduced with PPh₃ via intramolecular electron-transfer from ligand to metal to give various radical intermediates and Pd. All detected radical species have been characterized by e.s.r. spectroscopy.     

A comparison of different methods in determining energies and geometries of open-shell systems

Three methods of obtaining eigenvectors for open-shell systems, namely the Roothaan restricted open-shell method, the “half-electron” method, and the use of ground state orbitals, are compared with each other both on a formal basis, and by contrasting theab initio energies predicted by these schemes for a series of radicals and triplets. The r.m.s. improvement in the energy of eight radicals by use of the Roothaan method rather than the half-electron approximation amounts to 4.8 kcal mole^?1, whereas the r.m.s. improvement for nineteen triplet states is 8.2 kcal mole^?1. Optimum geometries predicted by these two methods do not differ appreciably. The use of closed-shell eigenvectors rather than those of the half-electron method leads to very large errors for triplet states whose electron density distributions differ appreciably from those of the ground state.     

Intermediacy of Proton-Bound Dimers and Ion/Dipole Complexes in the Unimolecular Decompositions of Dialkyl-Peroxide Radical Cations: Evidence for a coupled proton and hydrogen-atom transfer

The unimolecular fragmentation reactions of the radical cations of diethyl, diisopropyl, dipropyl, isopropyl propyl, and di(tert-butyl) peroxide have been investigated by mass spectrometric and isotopic labeling techniques. Two competing pathways for unimolecular decomposition in the μs time regime (metastable ions) are observed: i) A combination of an α-C? C bond cleavage and a H migration gives rise to proton-bound dimers of two ketone or aldehyde molecules. ii) Ion/dipole complexes of alkyl cations and alkylperoxy radicals are generated by C–O bond cleavage. These complexes either exhibit direct losses of alkylperoxy radicals, or they rearrange via a coupled proton and H-atom transfer, this sequence of unprecedented isomerizations is completed by losses of alkyl radicals. Collisional activation experiments confirm that the ionic products of the latter process correspond to RR′C?OOH⁺; these ions can be regarded as protonated carbonyl oxides. In addition, we observe the elimination of alkenes leading to hydroperoxide radical cations and the expulsion of HO radicals. The latter process implies a C? C bond formation step between the two alkyl fragments leading to higher alkyl cations.     

Electron spin resonance study of polyglycols

Polyethylene glycols and polypropylene glycols were irradiated with gamma-rays from a ⁶⁰Co source at 77°K. Electron spin resonance spectra of the free radicals produced were recorded. It was found that the rupture in the polymer chain takes place at *C? C and C? O bonds. The radicals identified were ? O? CH₂ in polyethylene glycols and mainly in polypropylene glycols. Thermal decay of these radicals followed a second-order law, and the decay constant calculated was k = 2.32 × 10⁵ cm.³ mole^?1 sec.^?1 A complete discussion of the results is presented.     

An E.S.R. study of spin-trapping by azodicarboxylates to give hydrazyl radicals

Addition of photochemically- or thermally-generated radicals (X·) to dialkyl azodicarboxylates gives hydrazyl radicals [RCO₂(X)N·CO₂R] which are readily detected by e.s.r. spectroscopy.     

On the reactivity of diethyl hydroxyl amine toward free radicals

Diethyl hydroxyl amine is an efficient trap for alkyl, alkoxy, and peroxy radicals. The specific rate constant for the reaction of ethyl radicals (gas phase, 25°C), tert-butoxy radicals (benzene solution, 115°C), and poly (peroxystyryl) peroxy radicals (styrene solution, 50°C) were evaluated as 7.2 × 10⁵, 7.7 × 10⁷, and 2.9 × 10⁵ M^?1·sec^?1, respectively. Several possible secondary reactions of the nitroxide radicals are discussed.     

The E.S.R. Spectrum of the tetra(1-adamantyl)cyclobutadiene radical cation

A revised interpretation is proposed for the ¹H and ¹³C hyperfine coupling in the e.s.r. spectrum of the tetra-1-adamantylcyclobutadiene radical cation, and assumes that free rotation occurs about the 1-adamantyl bond.     

Unimolecular ethyl loss from metastable 13C labelled [C5H10O]+˙ ions with the oxygen on the middle carbon: An example for incomplete energy randomization?

The investigation of ¹³C labelled 3-pentanone cation radicals generated via isomerization of the corresponding [C₅H₁₀O]^+˙ enol cation radicals demonstrates unambiguously that, in contrast to previous investigations, unimolecular ethyl loss from ¹³C labelled 3-pentanone cation radicals is preceded by complete energy randomization.     

Evidence for a dose rate effect in gamma-radiolysis—I. Methylmethoxyacetate in aliphatic linear hydrocarbons

Alkanes (C6, C16, C17) containing a captodative solute, methylmethoxyacetate (MMA), were irradiated with ⁶⁰Co. Dimeric products were analysed by capillary gas chromatography. G-values were measured as a function of the concentration of the captodative solute. By reaction with alkyl radicals, the captodative solute gives a free radical stabilized by the captodative effect. The combination reaction of this radical with itself or with alkyl radicals gives solute or mixed dimers which were also analysed. A large dose rate effect was seen, when the solute was not able to scavenge most of the alkyl radicals. Very good purification of the starting materials was needed to achieve reliable results. This may explain some discrepancies which appear in the literature.     

Radiation-induced postpolymerization of nitroethylene

Radiation-induced postpolymerization of nitroethylene in 2-methyltetrahydrofuran glass has been studied and discussed in reference to the results obtained from ESR measurements. No postpolymerization occurred at the temperature below ?150°C. In the temperature range between ?135°C and ?78°C, the polymer yield decreased with increasing postpolymerization temperature. The polymer yield increased linearly with the increase of the preirradiation dose in the range below 0.9 × 10⁶ r. The mean value for chain initiation was estimated to be about 1.3. The following correlations were observed between the results of the postpolymerization and ESR measurements. The postpolymerization started in the temperature range between ?140°C and ?135°C, where the ESR spectrum due to the anion radicals of nitroethylene disappeared. The polymer yield of the postpolymerization decreased with the photoirradiation at ?196°C before warming the samples in parallel with the photobleachability of the anion radicals observed in the glassy mixture by the ESR method. It was concluded from these results that the radiation-induced postpolymerization was initiated by the anion radicals of nitroethylene formed by the capture of electons.     

Bond dissociations in hypervalent ammonium radicals prepared by collisional neutralization of protonated six-membered nitrogen heterocycles

Hypervalent organic ammonium radicals were generated by collisional neutralization with dimethyl disulfide of protonated 1,4-diazabicyclo[2.2.2]octane (1H⁺), N,N′-dimethylpiperazine (2H⁺) and N-methylpiperazine (3H⁺). The radicals dissociated completely on the 5.1 μs time-scale. Radical 1H^• underwent competitive N−H and N−C bond dissociations producing 1,4-diazabicyclo[2.2.2]octane and small ring fragments. Dissociations of radical 2H^• proceeded by N−H bond dissociation and ring cleavage, whereas N−CH₃ bond cleavage was less frequent. Radical 3H^• underwent N−H, N−CH₃ and N−C_ring bond cleavages followed by post-reionization dissociations of the formed cations. The pattern of bond dissociations in the hypervalent ammonium radicals derived from six-membered nitrogen heterocycles is similar to those of aliphatic ammonium radicals. © 1997 John Wiley & Sons, Ltd.     

Kinetics and rate constants for the reaction of tri-n-butylgermanium hydride with methyl iodide and carbon tetrachloride. The combination of methyl and trichloromethyl radicals in solution.

The kinetics of the photoinitiated reductions of methyl iodide and carbon tetrachloride by tri-n-butylgermanium hydride in cyclohexane at 25°C have been studied and absolute rate constants have been measured. Rate constants for the combination of CH₃? and CCl₃? radicals are equal within experimental error and are also equal to the values found for the self-reactions of most non-polymeric radicals in low viscosity solvents, i.e. ～1–3 × 10⁹ M^?1 sec^?1. Rate constants for hydrogen atom abstraction by CH₃? and CCl₃? radicals are both ～1?2 × 10⁵ M^?1 sec^?1. Tri-n-butyltin hydride is about 10–20 times as good a hydrogen donor to alkyl radicals as is tri-n-butylgermanium hydride. The strength of the germanium–hydrogen bond, D(n-Bu₃Ge–H) is estimated to be approximately 84 kcal/mole.     

DNA damage induced via independent generation of the radical resulting from formal hydrogen atom abstraction from the C1′-position of a nucleotide

Background: Deoxyribonucleotide radicals resulting from formal C1′-hydrogen atom abstraction are important reactive intermediates in a variety of DNA-damage processes. The reactivity of these radicals can be affected by the agents that generate them and the environment in which they are produced. As an initial step in determining the factors that control the reactivity of these important radical species, we developed a mild method for their generation at a defined site within a biopolymer.Results: Irradiation of oligonucleotides containing a photolabile nucleotide produced C1'-DNA radicals. In the absence of potential reactants other than O₂, approximately 90% of the damage events involve formation of alkaline-labile lesions, with the remainder resulting in direct strand breaks. The ratio of alkaline-labile lesions to direct strand breaks (∼ 9:1) is independent of whether the radical is generated in single-stranded DNA or double-stranded DNA. Strand damage is almost completely quenched under anaerobic conditions in the presence of low thiol concentrations. Competition studies with 02 indicate that the trapping rate of C1′-DNA radicals by β-mercaptoethanol is ∼ 1.1 x 10⁷ M⁻¹s⁻¹Conclusions: The mild generation of the C1'-DNA radical in the absence of exogenous oxidants makes it possible to examine their intrinsic reactivity. In the absence of other reactants, the formation of direct strand breaks from C1′-radicals is, at most, a minor pathway. Competition studies between β-mercaptoethanol and 02 indicate that significantly higher thiol concentrations than those in vivo or some means of increasing the effective thiol concentration near DNA are needed for these reagents to prevent the formation of DNA lesions arising from the C1'-radical under aerobic conditions.     

Electron paramagnetic resonance study of nitroxides generated from nitric oxide by reaction with transient radicals

Photochemical or thermal decomposition of azo‐compounds (such as 2,2^′‐azobisisobutyronitrile, 2,2^′‐azobis(2‐methylpropionamidine) dihydrochloride, dialkyl peroxides (such as tert‐butyl peroxide and diacyl peroxides (such as benzoyl peroxide) in anaerobic nitric oxide (NO)‐saturated dimethylsulfoxide (DMSO) or aqueous solutions yielded nitroxides. Well‐characterized electron paramagnetic resonance spectra of nitroxides revealed that NO was favorable for reacting with carbon‐centered and less stereo‐inhibited transient alkyl radicals, giving kinds of nitrosoalkane, typically nitrosomethane, which act sequentially as C‐nitroso compounds to trap transient radicals present in solution, yielding spin‐trapping adducts, i.e. nitroxides. Radicals, including sulfinyl radicals from UV‐irradiated DMSO, were trapped by the in situ formed CH₃NO. O‐centered radicals could not add to the freshly formed C‐nitroso compounds. Possible mechanisms are suggested. Copyright © 2002 John Wiley & Sons, Ltd.     

Reduced parabolic model for radical addition reactions

The transition state of addition of free radicals and atoms to multiple bonds is considered as a result of intersecting of two parabolic potential curves. One of them characterizes the stretching vibration of the attacked multiple bond, and another curve characterizes the stretching vibration of the bond formed in the transition state. The force constant of the latter is calculated by an empirical equation that correlates the force constant with the bond dissociation energy. In the framework of this model, the thermally neutral activation energy (E _e0) and the elongation of the attacked and formed bonds (r _e) in the transition state were calculated from the experimental data (activation energy (E _e) and enthalpy of reaction (H _e)) for the addition of an H atom and methyl, alkoxyl, aminyl, triethylsilyl, and peroxyl radicals to the C=C bond and the addition of H and CH₃ to the C=O and CC bonds. Analysis of the data obtained showed that E _e0 depends linearly on the |H _e| + E_e sum, i.e., E_e0/kJ mol^–1 = 14.2 + 0.61 · (E_e – H _e), and the bond elongation in the transition state for addition of the most part of radicals to ethylene and acetylene vary within (0.65–0.87)·10_–10 m. The factors affecting the activation energy of the radical addition reactions are discussed.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1542–August, 2004.     

Efficient Hyperpolarization of U‐13C‐Glucose Using Narrow‐Line UV‐Generated Labile Free Radicals

Free radicals generated by UV‐light irradiation of a frozen solution containing a fraction of pyruvic acid (PA) have demonstrated their dissolution dynamic nuclear polarization (dDNP) potential, providing up to 30 % [1‐¹³C]PA liquid‐state polarization. Moreover, their labile nature has proven to pave a way to nuclear polarization storage and transport. Herein, differently from the case of PA, the issue of providing dDNP UV‐radical precursors (trimethylpyruvic acid and its methyl‐deuterated form) not involved in any metabolic pathway was investigated. The ¹³C dDNP performance was evaluated for hyperpolarization of [U‐¹³C₆,1,2,3,4,5,6,6‐d₇]‐d ‐glucose. The generated UV‐radicals proved to be versatile and highly efficient polarizing agents, providing, after dissolution and transfer (10 s), a ¹³C liquid‐state polarization of up to 32 %.