ESR studies of methallyl monomers with redox systems

The effects of three types of free radical initiators (HO·, H₂N·, and H₃C·) from redox systems, have been studied for four types of methallyl monomers, by use of ESR with a flow system. The structure, the relative concentrations, and the steric conformations of the monomer radical intermediates have been derived from the ESR spectra. In the case of H₂N · and HO · addition to methallyl alcohol (MAA), methallyl amine (MAAm), and sodium methallyl sulfonate (SMAS), the ESR spectra of the reacting species are interpreted as monomer head radicals only (H₂N · and HO · are added to the monomer tail). Methallyl acetate (MAAc) with HO ·, is an exception, giving hydrogen abstraction to form an allyl type radical. This reaction may influence the polymerization behavior of MAAc. The methallyl monomers behave differently from the allyl monomers, where appreciable amounts of monomer tail radicals were found in addition to the head radicals which were the main species. For methallyl monomers, this may be due to steric hindrance caused by the two substituents on the α carbon. The CH₃ radicals add only to positively polarized reactive double bonds, i.e., in SMAS in this study, and allyl alcohol in a previous study. The coupling constants of β CH₂ protons vary considerably with the substituents. For β₁ protons, the coupling constants decrease in the order OH > CH₃ > NH₂. For β₂ protons (allyl hydrogen), the coupling constants decrease in the order CH₂OH > CH₂NH₂ > CH₂OCOCH₃Na, i.e., the constants decrease in the order of increased bulkiness of the groups. Some exceptions are interpreted as due to complex formation with Ti⁴⁺. The effects of pH of the reaction medium are largely those expected.     

NMR characterization of α- and β-mannopyranurono-2,6-lactones

Determining the stereochemistry at the anomeric position of glycosides is imperative in chemical synthesis. As for 1,2-cis-β-glycosides such as β-mannosides, ¹³C-¹H coupling constants, ¹J_CHs, are conventionally used when glycosides are in an ordinary chair conformation. In this work, we searched for appropriate criteria to determine the stereochemistry of products in a recently developed glycosylation reaction using 2,6-lactones. As a result, we found that α-glycosides exhibited vicinal coupling constants of around 3.0?Hz?at the anomeric proton, as well as a long-range coupling between protons at C-1 and C-5, whereas β-glycosides had coupling constants of 1.2?Hz?at most. These figures are expected to be useful for future assignments. In addition, the observations obtained from this study revealed the conformation of glycosides with a 2,6-lactone moiety.     

ESR study of γ-irradiated 1,3,5-trithiane and its derivatives

Radicals formed in γ-irradiated 1,3,5-trithiane and its derivatives at room temperature have been studied by ESR spectroscopy. ESR evidence establishes that 1,3,5-trithiane as well as α- and β-2,4,6-trimethyl-1,3,5-trithiane yield radicals mainly by loss of the hydrogen atom, whereas α- and β-2,4,6-triphenyl-1,3,5-trithiane and 2,4,6-trimethyl-2,4,6-triphenyl-1,3,5-trithiane produce radicals that are parent radical cations, which are also present in small amounts in aliphatic trithianes. Furthermore, in all the spectra examined R–S^.-type radicals are observed. The probable initiation process of radiation-induced 1,3,5-trithiane polymerization is discussed briefly.     

ESR studies on hydroxyl radical reactions with glycine

Radicals H₂N? ?H? COOH and H₂N? ?H? COO^? which have different ESR. spectra are observed during the reaction of hydroxyl radicals with glycine in aqueous solution. Rapid and reversible exchange between the different dissociated radicals is induced by addition of phosphoric acid. The pH dependence of the ESR. spectra yields the pK value and rate constants for proton transfer reactions between the radicals and phosphoric acid.     

Studies on the tacticity of polyacrylonitrile. III. NMR spectra of stereoisomers of 2,4,6-tricyanoheptane as model compounds of polyacrylonitrile

The three stereoisomers of 2,4,6-tricyanoheptane were separated and their NMR spectra were studied as the three-unit model compounds of polyacrylonitrile. The chemical shifts and coupling constants obtained from the NMR spectra were compared to those of the two-units model compounds. The geminal protons for the isotactic methylene are not equivalent, but the difference of the chemical shifts is smaller than that in the meso two-unit model. The racemic methylene signals appear at higher field than the meso methylene in the heterotactic three-unit model as in the case of the racemic and the meso two-unit models. The signals of the methinic protons in triads appear in the order: isotactic CH, heterotactic CH, and syndiotactic CH from the high field. From the observed values of the vicinal coupling constants, the chain conformations of the model compounds are also discussed.     

ESR study of primary radical formation during mechanical degradation of poly(2,6-dimethyl-p-phenylene oxide) solid

Radical formation during mechanical degradation of solid poly(2,6-dimethyl-p-phenylene oxide) (PPO) was investigated by electron spin resonance (ESR). The ESR spectrum of PPO fractured at room temperature in air consisted of eight lines with a separation of about 5.5 gauss with g = 2.0043, indicating a small asymmetry. For PPO fractured in liquid nitrogen, a similar spectrum was observed at ?196°C in air or in vacuo. These spectra have been identified as belonging to a 2,6-dimethyl-substituted phenoxy radical and thus indicate the occurrence of main-chain rupture. The phenyl radical which was expected to be formed together with a 2,6-dimethyl-substituted phenoxy radical could not be detected, but at temperatures below ?46°C a small hump was observed at g = 2.034. By subtracting the spectrum observed after decay of this hump from the original one, the resulting curve was the characteristic asymmetric spectrum of a peroxy radical, which was presumably formed by the reaction between a phenyl radical and oxygen. The radical decay curve showed two stepwise-decaying regions; one located in the temperature region between about ?120°C and ?80°C where only a small number of radicals decayed, another located in the temperature region from about ?30°C to 100°C where almost all mechanically formed radicals decayed. The latter radical decay, which occurred considerably below the glass-transition temperature of PPO, was attributed to the molecular motions associated with the mechanical β^* relaxation on the basis of the activation energy and the temperature region.     

Initial stage in radiation polymerization of hydroxyethyl methacrylate-water system at low temperatures

The initial stage in the radiation polymerization of the hydroxyethyl methacrylate water system at low temperatures was studied. The polymerization was accelerated by the presence of water; the effect increased with rising temperature above T_g. The polymerization rate had a maximum near ?50°. The initiating and propagating radicals were identified by studies with ESR. Irradiated hydroxyethyl methacrylate at low temperatures gave a 7-line spectrum, which was assigned to the initiating radical having equivalent protons. This spectrum was changed to a 9-line spectrum at ?120 to ?100°; it was assigned to the propagating radical. The temperature dependence of the ESR spectrum of irradiated hydroxyethyl methacrylate-water systems was studied to examine the effect of water on the propagating radical.     

ESR study on radical polymerization and its application to microemulsion systems

Well-resolved electron spin resonance (ESR) spectra of propagating radicals of vinyl and diene compounds were observed in a single scan by a conventional CW-ESR spectrometry without the aid of computer accumulation and the specially designed cavity and cells. Although solvents which could be used for ESR measurements were restricted to nonpolar solvents, such as benzene, toluene, and hexane, new information on dynamic behavior and reactivity of the propagating radicals in the radical polymerization of vinyl and diene compounds were obtained. Thus, values of propagation rate constants (k_p) for vinyl and diene compounds were determined by an ESR method. Some of the k_p values were in a fair agreement with those obtained by a pulsed laser polymerization (PLP) method. Furthermore, polymer chain effect on apparent k_p was clearly observed in the radical polymerization of macromonomers and in the microemulsion polymerization. In ESR measurement on inclusion polymerization system, important information on the origin of the 9-line spectrum observed in the radical polymerization of methacrylate propagating radicals was obtained.     

ESR studies on radical polymerization of vinyl ethers

ESR studies on the radical polymerization of vinyl ethers were performed from ?50°C to room temperature using di-tert-butylperoxide as a photoinitiator. Well resolved ESR spectra were assigned to propagating radicals of vinyl ethers. Their hyperfine splitting constants due to α-proton were about 16 G, being smaller than those of ethyl acrylate and vinyl acetate. The smaller constants is ascribed to a deviation of the propagating radicals from sp² hybrid structure. The reason why high polymers are not obtained from vinyl ethers by radical polymerization is discussed on the basis of information from the ESR studies.     

An NMR,ESR, ENDOR and TRIPLE resonance investigation of a cation radical formed from 1,2,4,5-tetramethoxybenzene

Several methods have been established for preparing cation radicals from 1,2,4,5-tetramethoxybenzene that allow highly resolved ESR spectra to be recorded. Precise values of the hyperfine coupling constants for the aromatic and methoxy protons have been obtained; the values are 0.2268±0.0004 and 0.0863±0.0002 mT, respectively, with dichloromethane as solvent. No temperature dependence is evident. TRIPLE resonance experiments showed that both coupling constants have the same sign. NMR experiments provided contact shift and line broadening measurements; these proved that both the above constants are positive and led to a value of 3.1 (±0.3)×10⁸M^?1 s^?1 at 23°C for the rate constant for electron exchange between the cation radical and the parent compound.     

Change with temperature of the ESR spectra of methacrylic acid radicals

In order to elucidate the structure of methacrylic acid radicals, the change with observation temperature of the ESR spectrum of free radicals trapped in solid methacrylic acid γ-irradiated at ?196°C was studied. Below ?80°C, we found a 9-line spectrum, which is similar to the ordinary 9-line spectrum observed in irradiated poly(methacrylic acid) or poly(methyl methacrylate), but which differs in the stronger intensity of the so-called 4-line component. Our 9-line spectrum changes reversibly into a 13-line spectrum above ?80°C. With broad-line NMR measurements of methacrylic acid, it was found that there is such an unusual crystalline transition around ?30°C that the line width is narrower in the lower-temperature region (phase II) than that in the higher-temperature region (phase I). The change of the ESR spectrum can be interpreted in terms of the exchange of the two β-protons due to the hindered oscillation around the C_α? C_β bond of the single radical ···C_βH₂C_α(CH₃)COOH if one assumes the gradual change of the hindering potential barrier caused by the crystalline transition and the lower barrier in phase II. The modified Bloch treatment gave the hindering potential barrier to be 7.2 kcal/mole in phase I and 1.5 kcal/mole in phase II. The difference between our 9-line spectrum and the ordinary one with the very weak 4-line component comes from the difference of the surrounding matrix.     

Electron spin resonance study of γ-irradiated poly(acrylic acid)

Electron spin resonance (ESR) spectra of poly(acrylic acid) (PAA) γ-irradiated in air at room temperature and recorded at room temperature and at liquid-nitrogen temperature have been studied to identify the radiation products. The ESR spectra are composed of eight lines with hyperfine splittings of 23 ± 1 G and 11 ± 1 G. The method of least-squares total curve fitting, employing the Lorentzian line shape function, to the observed spectra enabled the assignment of the spectra. Computed spectra obtained by the superposition of a singlet and the spectra due to chain radicals are considered to give the best fits to the observed ESR spectra. The singlet is assigned to the radicals COOH, and the component 10-line spectra are assigned to the chain radicals CH₃? CH? CH₂ ～ and/or ～ CH₂? CH? CH₂ ～. The observed change in line shape with temperature of the ESR spectra is attributed to the hindered oscillations of the methyl groups about the C_α? C_β bond axis of the chain radicals. The existence of the methyl groups is confirmed by the measurement of infrared absorption.     

N-tert-Butoxy-1-aminopyrenyl Radicals. Isolation,Electronic Structure,and Magnetic Characterization

N-tert-Butoxy-2,7-di-tert-butyl-1-pyrenylaminyl (4), N-tert-butoxy-2-tert-butyl-1-pyrenylaminyl (5), and N-tert-butoxy-7-tert-butyl-1-pyrenylaminyl (6) free radicals were generated by the reaction of the lithium amides of the corresponding 1-aminopyrenes with tert-butyl peroxybenzoate in THF at -78 degrees C. Although 6 could not be isolated due to the gradual decomposition in solution, 4 and 5 were quite persistent and could be isolated as monomeric radical crystals. The X-ray crystallographic analyses for the isolated free radicals were successfully carried out, indicating that the N and O atoms are almost coplanar with the pyrene ring. The ESR spectra of 4 and 5 were very complex due to the presence of many magnetically unequivalent protons. Therefore, the proton hyperfine coupling (hfc) constants were determined by (1)H ENDOR/TRIPLE resonance spectroscopy. To assign the hfc constants for the pyrene ring protons, a partially deuterated radical, 4-d(4), was prepared and the ENDOR and ESR spectra were measured. To discuss the spin density distribution for 4 and 5 ab initio molecular orbital calculations were performed by the DFT UBecke 3LYP method, using the STO 6-31G basis set. Magnetic susceptibility measurements were carried out for 4 and 5 with a SQUID magnetometer. For 4 a weak antiferromagnetic interaction was observed, and for 5 a very strong antiferromagnetic interaction was observed. The antiferromagnetic interactions were explained by their crystal structures.     

EPR spectra of halo-substituted 2,4,6-triphenylpyranyl radicals

The hyperfine structure of three halo-substituted pyranyl radicals, 2,6-bis(p-chlorphenyl)-4-phenylpyranyl, 2,6-bis(perfluorophenyl)-4-phenylpyranyl, and 2,6-bis(perfluorophenyl)-4-(o,p-dichlorophenyl)pyranyl, was investigated by EPR. The EPR spectra contained splitting from the protons of the aromatic rings and magnetic fluorine nuclei. An ortho-effect phenomenon was detected, the ratio of the constants of splitting on the ortho and para fluorine atoms a _F ⁿ /a _F ^o =2, unlike in the case of unsubstituted pyranyl radicals, where a _H ⁿ a _H ^O .Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 6, pp. 741–743, November–December, 1986.     

Hyperconjugation and homoconjugation in alkyl radicals bearing β-phosphorus substituents

The isotropic ³¹P hyperfine splitting constants derived from the ESR spectra of a series of β-phosphorus-substituted alkyl radicals are in the order expected for hyperconjugative spin transmission.     

Stereochemical characterization of some mono- and diaryl substituted ethylene oxides by NMR spectroscopy

NMR spectra of several styrene, stilbene and stilbazole oxides have been determined, and chemical shifts and coupling constants have been correlated with cis-and trans-configurations. Assignments have been made for all protons, and double resonance technique and ¹³C? H coupling constants have been used in some particular cases. An explanation is proposed for the observation that chemical shifts of oxirane protons are higher for cis than for trans isomers.     

Estimates of magnetic resonance parameters in semi-empirical quantum chemistry

The possibility to reliably estimate the magnetic resonance parameters (MRP) can significantly increase the information content of experimentally recorded spectra. Here we consider in detail semi-empirical estimates of the ESR spectra parameters: isotropic hyperfine coupling constants, anisotropic hyperfine coupling tensors and g-tensors. The results show that the semi-empirical procedures give estimates of MRP comparable in quality with those of complicated ab initio and DFT schemes. It was underlined that a special attention should be given to geometric parameters of free radicals. The automatic procedure to determine molecular geometries of free radicals on the ground of their spectral characteristics was discussed.     

1H-NMR Spectra of Cyclohexa-1,4-dienes and Cyclohexenes Annellated to Bicyclo[2.2.1]hept-2-enes. The Inter-Ring Homoallylic H,H Coupling Constants as Stereochemical Probes

The 360-MHz-¹H-NMR spectra of cyclohexa-1,4-dienes and cyclohexenes annellated to bicyclo[2.2.1]hept-2-enes and 7-oxabicyclo[2.2.1]hept-2-enes show inter-ring homoallylic coupling constants between the bridgehead protons of the bicyclo[2.2.1]heptenes and the exo-protons of the allylic methylene groups (0.8 ± 0.15 Hz for bicyclo[2.2.1]hept-2-enes; 0.8–1.4 Hz for 7-oxabicyclo[2.2.1]hept-2-enes). Contrastingly, the corresponding coupling between the bridgehead protons and the endo-protons is absent. The observed values are compared with those calculated by the INDO and CNDO/2 methods and discussed in the light of the bicyclo[2.2.1]hept-2-ene bond π-anisotropy. Vicinal as well as intra-ring homoallylic coupling constants are consistent with a small puckering of the cyclohexa-1,4-diene rings toward the endo-face. The allylic exo-methylene protons are more deshielded than the endo-protons independent of the nature of the substituents, the nature of the bridges, and the degree of unsaturation of the annellated systems. These results constitute a probe for the configuration of cyclohexa-1,4-dienes and cyclohexenes annellated to these bicyclic skeletons.     

Paramagnetische organothallium(III)-verbindungen in lösung : II. Esr-untersuchungen paramagnetischer diphenylthallium(III)-komplexemit o-aminophenolen und o-phenylendiaminen

Diphenylthalliumhydroxide reacts in organic solvents with o-aminophenols and o-phenylenediamines to form paramagnetic species, the ESR spectra of which have been investigated. The reaction has been run using 32 ligands in numerous solvents, therefore the general application of the reaction is proved.The ESR spectra of the solutions show typical hydrogen nitrogen hyperfine structures indicating a NH-analogue semiquinone structure for the ligands. Furthermore a very large splitting due to magnetic coupling of the free electron with ²⁰³Tl- and ²⁰⁵Tl-nuclei is observed. In contrast to the paramagnetic diphenylthallium—semiquinone complexes [1a], the different coupling of ²⁰³Tl- and ²⁰⁵Tl-nuclei could be resolved in several cases.The thallium splitting observed increases from the semiquinone complexes (~10 G) to the aminophenol radicals (~40 G) and reaches a maximum value for the o-phenylenediamine derivatives (~90 G). The corresponding g-values decrease strongly in this direction. Thus a clear distinction of these classes of ligands becomes possible.The experimental thallium coupling constants and the g-factors depend largely on the substituent of the ligand, the solvent and the temperature. These effects are interpreted in terms of solvation phenomena.Based on the sum of the observed effects we assume that in all cases the radicals investigated are ion pairs in which the diphenylthallium cation interacts with a new type of ligand, namely NH-analogue semiquinones.     

Electron Spin Resonance of α-(Alkoxycarbonyl)alkyl Radicals in Solution

High-resolution ESR. spectra of the radicals CH₂COOR, CH₃CHCOOR and (CH₃)₂CCOOR with R?CH₃, CH₂CH₃, CH(CH₃)₂ and C(CH₃)₃ in liquid solution confirm planar energy-minimum structures with substantial barriers to internal rotation about the ?, CO-bonds (?40 kJ/mol) and partial π-electron delocalization. The assignments of coupling constants to protons in isomeric positions and the conclusions on radical structures are supported by INDO-calculations.