Electron spin resonance spectra of α-sulfonyl iminoxy radicals

Syn- andanti-α-sulfonyl iminoxy radicals have been generatedin situ by lead tetra-acetate oxidation of α-oximino sulfones. The ESR spectra of the geometrical isomers have been assigned and the¹⁴N and¹H hyperfine splitting constants are discussed in terms of the proposed structures.     

Ab initio calculations of isotropic hyperfine coupling constants in β-ketoenolyl radicals

Ab initio unrestricted Hartree–Fock (UHF ), unrestricted second-order Møller–Plesset (UMP 2) perturbation, unrestricted coupled cluster (UCCD ), and unrestricted quadratic configuration interaction (UQCISD ) calculations have been performed on the organic radicals CH₃, CH₃CH₂, CH₂CHCH₂, CH₃CHCOO^?, HCOCHCOH, CH₃COCHCOH, CH₃COCHCOCH₃, and CH₃COC(CH₃)COCH₃, using double-zeta and split-valence-plus-polarization basis sets. These radicals are derived from common organic ligands and have been observed in recent experimental work on tris(β-ketoenolato)cobalt(III) complexes. Their geometry has been optimized at the UHF level using the two mentioned basis sets. From these calcuations, values for the isotropic hyperfine coupling constants at the hydrogen atoms are predicted and compared with the experimental results. The usefulness of semiempirical extrapolations based on limited basis sets and treatment of electron correlation effects is carefully analyzed in the examples considered. © 1994 John Wiley & Sons, Inc.     

Voltammetric studies on the α-tocopherol anion and α-tocopheroxyl (Vitamin E) radical in acetonitrile

The α-tocopheroxyl radical was generated voltammetrically by one-electron oxidation of the α-tocopherol anion (E ^r_1/2=−0.73 V versus Ag|Ag⁺) that was prepared by reacting α-tocopherol with Et₄NOH in acetonitrile (with Bu₄NPF₆ as the supporting electrolyte). Cyclic voltammograms recorded at variable scan rates (0.05–10 V s⁻¹), temperatures (−20 to 20°C) and concentrations (0.5–10 mM) were modelled using digital simulation techniques to determine the rate of bimolecular self-reaction of α-tocopheroxyl radicals. The k values were calculated to be 3×10³ l mol⁻¹ s⁻¹ at 20°C, 2×10³ l mol⁻¹ s⁻¹ at 0°C and 1.2×10³ l mol⁻¹ s⁻¹ at −20°C. In situ electrochemical-EPR experiments performed at a channel electrode confirmed the existence of the α-tocopheroxyl radical.     

Electron paramagnetic resonance spectroscopy of γ-irradiated cellulose and related compounds

EPR studies were carried out on different irradiated cellulosic materials. The shape of the EPR spectrum was found to depend upon the fine structure of cellulose. The peak-to-peak distances of the large peaks on the differential curves were found to be 23.3 and 24.8 gauss, respectively, for the amorphous and recrystallized cellulose. The number of free radicals present per unit weight in the recrystallized cellulose and in the amorphous cellulose were found to be in the ratio of 2.1:1. The EPR spectrum of a sample of irradiated cotton crystallites which was soaked in acetone before being dried was markedly symmetrical, in contrast to the EPR spectra of samples of cotton linters and of cotton crystallites dried from water, and was considered to be a reflection of the lower intermolecular bonding in the former. For corresponding doses, the free radicals on cotton crystallites have the highest g value, followed by cotton linters, followed by regenerated cellulose I. The g value decreased as the irradiation dose was increased.     

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.     

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

Low temperature relaxations in poly(methacrylic acid) (PMAA) have been studied by electron spin resonance (ESR) spectroscopy. The observed 8 line ESR spectra of irradiated PMAA in the temperature range 77-300K (LNT-RT) is attributed to the free radicals of the type ～ CH₂? CH? CH₃. Assignment of ESR spectra to free radicals has been made on the basis of magnetic parameters employed to simulate ESR spectra at different temperatures. Further, ESR spectra below LNT have been simulated, using the set of parameters employed to simulate the experimental spectrum at LNT. Magnetic parameters of the ESR spectra at LNT and below LNT indicate γ- and δ-relaxations of PMMA chains. © 1994 John Wiley & Sons, Inc.     

Electron spin resonance studies of polycarbonate irradiated by γ-rays and ultraviolet light

Paramagnetic species produced in polycarbonate (PC) by γ- or ultraviolet irradiation were investigated by ESR. In γ-irradiation, scissions of carbonate groups in the main chain occur. ESR spectra (g = 2.003₄) composed of a sharp singlet, some broad singlets, and a small signal with hyperfine structure are obtained, and they are assigned to trapped electrons, positive radical ions, phenoxy-type free radicals, phenyl radicals, and ? O? C₆H₄? C(CH₃)₂ radicals. The G value for total yields of paramagnetic species at 77°K is 1.8. The percentage of CO and CO₂, the dominant gases evolved, is 65.4 and 33.8%, respectively. In ultraviolet irradiation, energy is absorbed selectively at the surface region. The surface region becomes insoluble in methylene chloride because of crosslinking of phenyl groups. The ESR spectrum obtained at 77°K is a broad singlet and assigned to phenoxy-type free radicals, phenyl radicals, and polyenyl-type free radicals. Some differences in effects of γ- and ultraviolet irradiation of PC are discussed.     

Isomerization of unsaturated radicals. V. Unimolecular isomerization of hot α,α- and α,β-dimethallyl radicals

The 147 nm photolysis of 3,3 dimethylbut-1-ene leads mainly to the formation of very hot (?375 kJ/mol) α,α-dimethallyl radicals. On the other hand, that of 3-methyl-cis-and trans-pentene-2, as well as that of 2,3-dimethylbut-1-ene is a source of very hot α,β-dimethallyl radicals. These allylic radicals are coolled down using pressure and are allowed to combine with available methyl radicals. From the formation of various C₆H₁₂ products, it is concluded that the very hot α,α- radical isomerizes towards the α,β-structure at low pressures and vice versa. The equilibrium constant of the following process has been evaluated to be 1.72 ± 0.30.      

DFT calculations of hyperfine coupling constants of organic π radicals and comparison with empirical equations and experiment

Density functional theory calculations (UB3LYP/EPR‐III) for a series of radicals and radical ions were performed to check the internal consistency of the method and its implications to the theoretical concepts of electron paramagnetic resonance such as π–σ spin polarization, hyperconjugation and phenyl hyperconjugation. In the second part, experimental data for seven radicals (43 hyperfine coupling constants) are compared with calculations, yielding a correlation of r² = 0.97. Copyright © 2016 John Wiley & Sons, Ltd.     

Mass spectrometric comparison of ordinary and fully deuterated α- and β-carotene

The individual, deuterated, isomeric α- and β-carotenes were isolated from the green alga, Scenedesmus obliquus, cultivated in D₂O containing 99·7 to 99·8 atom percent deuterium. Mass spectroscopy showed that both the α- and β-deuterio-carotene preparations contained principally the fully deuterated pigment molecules (C₄₀D₅₆), small quantities of deuterated molecules with one proton (C₄₀D₅₅H) and yet smaller quantities of deuterated molecules with two protons (C₄₀D₅₄H₂). From statistical calculations the deuterio-carotene preparations also contained one to several isotopically-substituted deuterio-carotenes of each mass in the mass range 585 to 599 because of variation of the number of ¹³C and H atoms per molecule. The mass fragmentation of the deuterated pigments was analogous to that of the respective ordinary α- and β-carotene. It indicated that the protons in the C₄₀D₅₅H and C₄₀D₅₄H₂ molecules were distributed approximately randomly in various parts of the structure as in the terminal rings and in the ends and central portions of the polyene chain.     

Formation of [M − H]+ ions in electron impact mass spectra of nitrosocarbonyl compounds α- and γ-nitrosocarbonyl compounds

The presence of the [M + H]⁺ ions and the absence of the monomer molecular ions M^+· in the mass spectra of some tertiary α- and γ-nitrosocarbonyl compounds is reported. This effect is caused by the rearrangement of the mobile hydrogen in the α-carbonyl position in the fragmentation pattern of the dimer molecular ions 2M^+·.     

Electron impact studies. CVII—Negative ion mass spectra of the oxime group. Monoximes derived from α-dicarbonyl systems

Specific losses of water are observed from the molecular anions of monoximes of α-diketones. Labelling studies, kinetic energy release values and the +E spectra of ions have been used to aid in the elucidation of the fragmentation pathways. It is proposed that the majority of ions have α-keto nitrile structures.     

Mass spectra of esters of α-hydroxy and α-methoxy acids

The most abundant fragment produced by electron bombardment of esters of the type R¹R²C(OR³)CO₂R⁴ is the R¹R²C = \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm O}\limits^{{\rm + } \cdot } $\end{document}R³ ion. Methyl glycollate (R¹ = R² = R³ = H, R⁴ = Me) eliminates the HCO˙ radical by a complex rearrangement involving the methylenic hydrogen atoms. The methyl and ethyl esters of methoxyacetic acid (R¹ = R² = H, R³ = Me, R⁴ = Me or Et) eliminate formaldehyde by the McLafferty rearrangement.     

Dimers from α-alkoxybenzyl radicals. An NMR study

meso and dl Dimers (ArCHOR)₂ where R is Me, Et, ⁱPr, ^tBu, cyclohexyl and 1-adamantyl may readily be differentiated by their NMR spectra; the benzylic protons of the meso isomer always absorb at a slightly higher field than those of the dl isomer in each of the solvents used. Differences in chemical shift are discussed in terms of preferences in conformer distribution. The formation of equal amounts of both dimers from the corresponding radical ArCHOR shows that steric and polar factors are not important in influencing the dimerization. Magnetic non-equivalence due to the presence of asymmetric centres was found in some of the compounds discussed above.     

The high resolution mass spectra of α,β-unsaturated and α-chloromercuri-α,β-unsaturated steroidal ketones

The high resolution mass spectra (500 eV) of some α,β-unsaturated steroidal ketones have been studied and compared with the spectra of the corresponding α-chloromercuri ketones. In the latter, the carbon-mercury bond frequently remains intact at the expense of the fission of two carbon-carbon bonds. The abundance of mercury-containing ions allows the use of the mercury atom fingerprint in confirming ring B fragmentation of the steroid nucleus at C(6)–C(7) and C(9)–C(10) for 5α-androst-1-ene-3,17-dione, 1,4-androstadiene-3,17-dione and their 2-chloromercuri derivatives; and at C(7)–C(8) and C(9)–C(10) for 1,4,6-androstatriene-3,17-dione, 1,4,6-androstarien-17 β-ol-3-one and their 2-chloromercuri derivatives. 2-Chloromercuri-1,4,6-androstatriene-3,17-dione and 2-chloromercuri-1,4,6-androstatrien-17 β-ol-3-one also give an abundant ion as the result of ring C fragmentation at C(8)–C(14) and C(11)–C(12), the chloromercuri group being replaced by a hydrogen atom. This ring C cleavage gives the only recognizable distinctive fragmentation ion for 1,4,6-pregnatriene-3,20-dione and 2-chloromercuri-1,4,6-pregnatriene-3,20-dione. For most of the mercurated steroids, the low resolution mass spectra (70 eV) are reported. In these spectra, the fragmentation patterns are similar to those obtained using the higher ionization energy employed for the high resolution spectra.