Cis Radical Research Articles

DFT and ENDOR Study of Bixin Radical Cations and Neutral Radicals on Silica-Alumina.

Bixin, a carotenoid found in annatto (Bixa orellana), is unique among natural carotenoids by being water-soluble. We stabilized free radicals from bixin on the surface of silica-alumina (Si-Al) and characterized them by pulsed electron-nuclear double resonance (ENDOR). DFT calculations of unpaired electron spin distribution for various bixin radicals predict the EPR hyperfine couplings. Least-square fitting of experimental ENDOR spectra by spectra calculated from DFT hyperfine couplings characterized the radicals trapped on Si-Al. DFT predicts that the trans bixin radical cation is more stable than the cis bixin radical cation by 1.26 kcal/mol. This small energy difference is consistent with the 26% trans and 23% cis radical cations in the ENDOR spectrum. The remainder of the ENDOR spectrum is due to several neutral radicals formed by loss of a H(+) ion from the 9, 9', 13, or 13' methyl group, a common occurrence in all water-insoluble carotenoids previously studied. Although carboxyl groups of bixin strongly affect its solubility relative to other natural carotenoids, they do not alter properties of its free radicals based on DFT calculations and EPR measurements which remain similar to typical water-insoluble carotenoids.

Generation of polyacetylene sulfoxide radicals through spin migration from the main-chain to the sulfoxide moiety in the side chain of poly[p-(n-butylsulfoxide)phenylacetylene] prepared with a [Rh(norbornadiene)Cl]2 catalyst

A phenylacetylene bearing an n-butylsulfoxide group, i.e., p-(n-butylsulfoxide)phenylacetylene (1) was prepared in high yields using the [Rh(norbornadiene)Cl]2–NEt3 catalyst in the presence of various solvents under mild conditions. The resulting polymer, poly[p-(n-butylsulfoxide)phenylacetylene] (poly(1)), was characterized in detail by 1H NMR, ESR, laser Raman, and diffuse reflective UV–vis methods. The data clearly showed that cis-to-trans isomerization of the polymer can be induced when pressure is imposed to the polymer at room temperature, rotationally breaking the cis CC bonds to generate the cis and trans radicals. Further, the spin density in the cis radical was migrated from the main-chain to the sulfoxide moiety as the side chain of the phenyl ring to magnetically interact with the first two methylene protons in the n-butyl group giving a triplet line ESR spectrum with an extremely large g value, g=2.0081.

Poly[(para‐halogenated)phenylacetylene]s Prepared with a [Rh(norbornadiene)Cl]2 Catalyst: Syntheses and Structure Elucidation

AbstractSummary: pXPAs were stereospecifically polymerized using [Rh(nbd)Cl]2 as catalyst in the presence of a mixed solvent, i.e., ethanol and triethylamine, at 20 °C. The resulting PpXPAs were characterized in detail in order to predict geometrical structures concerning cis and trans configurations in the main‐chain CC bonds using 1H NMR, laser Raman, XRD, and ESR methods. The data showed that the PpXPAs have an extremely high content of the cis‐transoid form. Immobile unpaired electrons, ca. 1.2–3.1 × 1017 spins · g−1, called the cis radicals, were found to be stabilized even in the pristine cis PpXPAs. The observed g values of the pristine PpXPAs could be correlated with the spin‐orbit coupling constant ζ of the halogen atom at the para position of the phenyl.Second derivative ESR spectra of PpIPA observed (a) before and (b) after compression, and the suggested respective structures.magnified imageSecond derivative ESR spectra of PpIPA observed (a) before and (b) after compression, and the suggested respective structures.

Origin of the color of π‐conjugated columnar polymers: Poly(p‐n‐octylthiophenyl)acetylene prepared using a [Rh(norbornadiene)Cl]2 catalyst

AbstractHighly stereospecific polymerization of a novel sulfur containing aromatic acetylenes, that is, (p‐n‐octylthiophenyl)acetylene (pOctSPA), was successfully performed using the Rh complex, [Rh(norbornadiene)Cl]2‐TEA, catalyst in the presence of various solvents under mild conditions. The resulting polymers were characterized in detail by 1H NMR, ESR, laser Raman, diffuse reflective UV‐Vis (DRUV‐Vis), and wide angle X‐ray diffraction methods. The data showed that the resulting polymers bear cis‐transoid form, which can induce the cis‐to‐trans isomerization when the cis polymers are subjected to pressure at room temperature under vacuum, breaking rotationally the cis CC bonds in the main‐chain giving two kinds of π‐radicals, the so‐called cis radical and trans radical as the origin of a polymer magnet like a novel spin glass material. Further, the resulting cis poly(acetylene)s were found to have a helical main‐chain, which is packed in pseudohexagonal crystal called π‐conjugated columnar or nano π‐conjugated columnar as a novel color controllable material. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2836–2850, 2005

Copolymers of phenylacetylene and para‐nitrophenylacetylene with nonlinear optical properties: Further insight on the conformational structure

AbstractCopolymers of phenylacetylene (PA) and para‐nitrophenylacetylene (pNPA), named poly(PA‐co‐pNPA), were obtained in different PA/pNPA ratios and different reaction conditions with Rh(I) catalysts. The structure of the copolymers was investigated with IR, laser Raman, 1H NMR, electron spin resonance (ESR), and diffuse reflective ultraviolet–visible (DRUV) light spectroscopies. The pristine polymers had a cis–transoidal structure as the predominant conformation with some trans sequences. Detailed ESR studies supported by computer simulation and conformation analysis have suggested that the trans sequences were due to pNPA sequences and that the cis‐CC bond sequences of pNPA were associated with a stabilized cis radical formed by four to five of pNPA monomers. This particular stabilization was probably the reason for the higher reactivity of pNPA as compared with PA. These cis sequences were preferentially cleaved to generate π radicals. The compression and, to a minor extent, thermal treatment of poly(PA‐co‐pNPA) samples induced a cis‐to‐trans isomerization, leading to a trans–transoidal form with a planar zigzag structure and with a conjugation length up to n = 24 repeat units, determined by DRUV and Raman experiments. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2365–2376, 2004

Mn(III)-based oxidative free-radical cyclizations of substituted allyl alpha-methyl-beta-ketoesters: syntheses, DFT calculations, and mechanistic studies.

The cyclizations of the substituted allyl alpha-methyl-beta-ketoester radicals 11, 14, and 18 were studied by the DFT method at the UB3LYP/6-31G level; the results show that the cis cyclization is easier than the corresponding trans cyclization, but the generated cis radicals are not necessarily more stable than the corresponding generated trans radicals after the cyclizations. The free-radical cyclizations of 11, 14, and 18 in the presence of Mn(OAc)(3) in acetic acid or acetonitrile are all reversible and operate under thermodynamic control, and stereoselectivity of the cyclizations depends on relative stability of the cyclization-generated radicals. Therefore, the oxidative free-radical cyclization of allyl alpha-methyl-beta-ketoester 5a with Mn(OAc)(3) gives a cis product as a major product, while the same oxidative free-radical cyclizations of substituted allyl alpha-methyl-beta-ketoesters 5b and 5c with Mn(OAc)(3) produce trans products as major products.

ChemInform Abstract: Charge Transfer and Cis‐Trans Photoisomerization.

The photophysical and photochemical properties of cis- and trans-1,2-bis(1-methyl-4-pyridinio)ethylene dication in acetonitrile are examined. These properties are significantly altered by the charge-transfer complexation in the ground state. The free olefins undergo cis-trans photoisomerization with a quantum yield of ca. 0.3. The cis isomer does not fluoresce unlike the trans isomer ({phi}{sub F} = 0.01, {tau}{sub F} = 5 {times} 10{sup {minus}11} s). The fluorescence of the trans isomer is quenched by complexation with iodide ions in the ground state. In addition, static quenching occurs within a 16-{angstrom} radius. The constants of the CT complexation with the iodide ion (in the ground state) are 1.6 M{sup {minus}1} for cis and 4.6 M{sup {minus}1} for trans. Upon excitation in the charge-transfer absorption band, the trans iodide complex gives the trans radical with a quantum yield of 0.06. The cis iodide complex leads not only to the formation of the cis radical isomer with a quantum yield of 0.02 but also to the trans isomer dication (quantum yield 0.05). The latter reaction can be interpreted either as a germinate pair catalyzed event or as isomerization in the excited charge-transfer state from the results of the nanosecond and picosecond laser flash photolysis experiments.

Charge transfer and cis-trans photoisomerization

The photophysical and photochemical properties of cis- and trans-1,2-bis(1-methyl-4-pyridinio)ethylene dication in acetonitrile are examined. These properties are significantly altered by the charge-transfer complexation in the ground state. The free olefins undergo cis-trans photoisomerization with a quantum yield of ca. 0.3. The cis isomer does not fluoresce unlike the trans isomer ({phi}{sub F} = 0.01, {tau}{sub F} = 5 {times} 10{sup {minus}11} s). The fluorescence of the trans isomer is quenched by complexation with iodide ions in the ground state. In addition, static quenching occurs within a 16-{angstrom} radius. The constants of the CT complexation with the iodide ion (in the ground state) are 1.6 M{sup {minus}1} for cis and 4.6 M{sup {minus}1} for trans. Upon excitation in the charge-transfer absorption band, the trans iodide complex gives the trans radical with a quantum yield of 0.06. The cis iodide complex leads not only to the formation of the cis radical isomer with a quantum yield of 0.02 but also to the trans isomer dication (quantum yield 0.05). The latter reaction can be interpreted either as a germinate pair catalyzed event or as isomerization in the excited charge-transfer state from the results of the nanosecond and picosecond laser flash photolysis experiments.

Cis and trans olefin radicals: equilibrium and photoisomerization

Electrochemical reduction of 1,2-bis(1-methyl-4-pyridino)ethylene dication gives the same reduction potentials (-0.53 and -0.72 V vs SCE in CH/sub 3/CN) for both cis and trans isomers. Only the absorbance spectrum of the trans radical (epsilon(max) 57,000 M/sup -1/ cm/sup -1/ at 517 nm) is observed. However, by using pulse radiolysis the radicals of both cis and trans isomers are observed in aqueous solution. Although their absorbance spectra are similar, the extinction coefficients are very different. The cis radical is intrinsically stable but undergoes rapid electron exchange with the trans dication isomer. Cis and trans radicals equilibrate through electron exchange, the limiting rate constants in both directions of the equilibrium being 2 x 10/sup 9/ M/sup -1/ s/sup -1/ (cis to trans) and 3 x 10/sup 8/ M/sup -1/ s/sup -1/ (trans to cis). The photoisomerization of the olefin radicals is essentially one-ray: the quantum yield for the cis radical is 0.2, while that of the trans is less than 0.01.

Stereospecific cis radical addition. Convincing evidence for a 1,3-radical transfer reaction

The addition of ethyl NN-dichlorocarbamate or NN-dichlorophosphoramidates to five-membered cyclic alkenes at 0 °C is stereospecific and cis; these results are conveniently explained by a 1,3-atom shift mechanism.

A study of the isomerization of cis-azobenzene anion radical in dimethylformamide

The electrochemical reduction of cis-azobenzene in dimethylformamide has been studied by polarography, controlled potential coulometry and cyclic voltammetry. The cis-azobenzene radical anion obtained in the first 1e stage isomerizes rapidly to the trans anion. Its lifetime is of the order of 0.1 ms at room temperature. A comparison is made with analogous ethylenic cis radical anions.

A study of the isomerization of cis-azobenzene anion radical in dimethylformamide

The electrochemical reduction of cis-azobenzene in dimethylformamide has been studied by polarography, controlled potential coulometry and cyclic voltammetry. The cis-azobenzene radical anion obtained in the first 1 e stage isomerizes rapidly to the trans anion. Its lifetime is of the order of 0.1 ms at room temperature. A comparison is made with analogous ethylenic cis radical anions.

The electroreduction of cis- and trans-thioindigo in N,N-dimethylformamide solutions

The reduction of the cis- and trans-isomers of thioindigo (TI) at a platinum electrode was studied by cyclic voltammetry, controlled potential coulometry, rotating ring—disk electrode voltammetry, and e.s.r. spectroscopy. The trans-isomer produces predominantly the radical anion upon one-electron reduction while the cis radical anion isomerizes to the trans form and dimerizes much more rapidly than trans-TI. Reduction of both forms at the second reduction wave or in the presence of benzoic acid proceeds by a two-electron reaction probably to a protonated dianion species. Both the dimeric dianion and the protonated dianion can be oxidized back to trans-TI.