Trans Radical Research Articles

Configuration and conformation of poly(3-carbazolylacetylene) including cis and trans radicals revealed by ESR spectroscopy

π-Conjugated cis and trans radicals of poly(N-isobutyl-3-carbazolylacetylene) were stereospecifically prepared using the [Rh(norbornadiene)Cl]2-NEt3 catalyst, and their geometric structures were determined using the electron spin resonance (ESR) method.

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.

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

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.

Direct and sensitized (energy and electron transfer) geometric isomerization of stilbene within zeolites: a comparison between solution and zeolite as reaction media

The trans- and cis-stilbenes upon inclusion in NaY zeolite are thermally stable. Direct excitation and triplet sensitization results in geometric isomerization and the excited state behavior under these conditions are similar to that in solution. Upon direct excitation, a photostationary state consisting of 65% cis and 35% trans isomers is established. Triplet sensitization with 2-acetonaphthone gave a photostationary state consisting of 63% cis and 37% trans isomers. These numbers are similar to the ones obtained in solution. Thus, the presence of cations and the confined space within the zeolite have very little influence on the overall chemistry during direct and triplet sensitization. However, upon electron transfer sensitization with N-methylacridinium (NMA) as the sensitizer within NaY, isomerization from cis-stilbene radical cation to trans-stilbene occurs and the recombination of radical ions results in triplet stilbene. Prolonged irradiation gave a photostationary state (65% cis and 35% trans) similar to triplet sensitization. This behavior is unique to the zeolite and does not take place in solution. Steady state fluorescence measurements showed that the majority of stilbene molecules are close to the N-methylacridinium sensitizer. Diffuse reflectance flash photolysis studies established that independent of the isomer being sensitized only trans radical cation is formed. Triplet stilbene is believed to be generated via recombination of stilbene radical cation and sensitizer radical anion. One should be careful in using acidic HY zeolite as a medium for photoisomerization of stilbenes. In our hands, in these acidic zeolites isomerization dominated the photoisomerization.

Reductive Dechlorination of Trichloroethylene: A Computational Study

Vitamin B12 catalyzes the reductive dechlorination of several ubiquitous pollutants including the conversion of trichloroethylene (TCE) to ∼95% cis-1,2-dichloroethylene (DCE) and small amounts of trans-DCE and 1,1-DCE. The origins of this unexpected selectivity were investigated using density functional and coupled-cluster theory. At all levels of theory considered, the initially formed trichloroethylene radical anion is an unstable species. Breakage of one of the three C−Cl bonds during the dissociative process gives the most stable ion complex when the two remaining chlorines occupy a cis geometry. Once formed, the cis-1,2-dichloroethen-1-yl radical is about 6 kJ/mol more stable than the corresponding trans radical and 21 kJ/mol more stable than the 1,1-dichloroethen-2-yl radical. The calculated relative energies can be rationalized by delocalization of the unpaired electron over the nonbonding orbitals of the α-chlorine. The computed geometries of the radicals suggest significant interactions between t...

Insertion of the Al Atom into Alkyl Ethers: Semiempirical SCF MO and Matrix Isolation ESR Study

The insertion process of the Al atom into trimethylene oxide (TMO) and that into dimethyl ether (DME) were examined by a semiempirical SCF molecular orbital method (AM1) and by matrix isolation ESR spectroscopy. The MO study revealed the three step process: (1) an approach of the Al atom toward the ethereal oxygen along the C2v axis of the molecule driven by a favorable overlap between the aluminum SOMO (the 3pz orbital) and the LUMO of the molecule of A1 symmetry, (2) formation of the Al−O dative bond with concurrent scission of one of the C−O bonds, and (3) completion of the process by the three-electron bonding scheme between the unpaired electron on the alkyl terminus and the lone pair electrons on Al. The MO study also revealed that the process would be spontaneous for the Al/TMO system, but for the Al/DME system, there existed a barrier of ∼8 kcal/mol for the onset of the methyl group cleavage from the dative complex Al:O(CH3)2. The MO study also revealed that, in the Al/DME case, the insertion product CH3−Al−O−CH3 of the cis conformation was initially formed but it might readily convert to the more stable trans form. The ESR study revealed that the Al atom insertion into TMO occurred spontaneously. Well-resolved spectra of the expected insertion radical were observed. In the case of DME, the ESR study revealed the presence of both the intermediate dative complex and the final insertion product in the as-prepared matrix. When the matrix was subsequently irradiated with red light (λ = 700 ± 50 nm), the dative complex resumed the process and converted to the insertion product. The insertion radicals of both the cis and trans forms were observed in the argon system, but only the more stable trans radical was observed in the neon system.

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.

Electron spin resonance spectra of radical cations derived from stilbenes and phenanthrenes

The same e.s.r. spectrum is observed when either cis- or trans-stilbene is photolysed in a solution of mercury(II) trifluoroacetate in trifluoroacetic acid. This spectrum is ascribed to the near-planar trans radical cation in which rotation about the Ph–C bonds is fast on the e.s.r. time scale, with a(2H olefinic) and a(2H para) 4.53, a(4H ortho) 2.78,a(4H meta) 0.715 G. In contrast, the trans radical anion shows restriction of rotation about the Ph–C bonds. The spectra of the radical cations derived from trans-4,4′-dimethyl- and trans-4,4′-di-t-butyl-stilbene are compatible with this interpretation. These stilbene radical cations do not cyclise to phenanthrenes: solutions of phenanthrene and dimethylphenanthrene under the same conditions show the different spectra of the corresponding phenanthrene radical cations. A satisfactory simulation of the spectrum of the former compound can be obtained with the hyperfine coupling constants a[H(1,8)]2.94, a[H(2,7)] 1.34, a[H(3,6)] 2.94, a[H(4,5)] 1.34, a[H(9,10)] 4.91 G.

An electron spin resonance investigation and molecular orbital calculation of the anion radical intermediate in the enzymatic cis-trans isomerization of furylfuramide, a nitrofuran derivative of ethylene

The enzymatic cis-trans isomerization of nitrofuran derivatives has been proposed to occur via the formation of a radical anion intermediate. ESR investigations, in conjunction with intermediate neglect of differential overlap (INDO) molecular orbital calculations, support this concept by demonstrating the enzymatic generation of cis and trans radical anions of 3-(5-nitro-2-furyl)-2-(2-furyl) acrylamide. The INDO calculations further indicate that the rotational barrier between the cis and trans anion radicals of this compound is only 5–10 kcal/mol, whereas a 70 kcal/mol barrier exists for the parent geometric isomers. Hyperfine splitting constants for the cis-trans conformers have been assigned on the basis of INDO calculations. Surprisingly, only the nitrogen hyperfine splitting of the nitro group is distinguishably different in the two conformers, a result which is not inconsistent with the INDO calculations.