Thermal and photo-chemical oxidation of poly(vinyl chloride): Implication of α,β-unsaturated carbonyl groups

The thermal and photo-chemical oxidation of poly(vinyl chloride) film has been studied under the same conditions using second-order derivative uv and fluorescence spectroscopy and hydroperoxide analysis. Four absorption bands are observed to undergo changes. The first, at 210 nm, associated with a single vinyl group, is unstable and disappears rapidly on thermal treatment. The second, at 230 nm, increases rapidly, as does a fourth small band in the near uv at 325 nm, and is concurrent with the growth in a new fluorescent product which is attributed to an α,β-unsaturated carbonyl group. The third, at 275 nm, increases at a low linear rate during thermal oxidation but at a catalytic rate during photo-chemical oxidation, and is associated with aliphatic ketonic/aldehydic groups. Hydroperoxides are also produced at a linear rate during thermal oxidation but do not match the carbonyl growth photo-chemically. Only the fluorescent oxidation product appears to match closely the autocatalytic rate of carbonyl production photo-chemically. However, even here, a contribution from hydroperoxide initiation is evident and this is probably the major initiator in mildly processed material.     

Photo- and thermal-oxidation of polyethylene: Comparison of mechanisms and influence of unsaturation content

The behaviour of polyethylene with different contents in vinyl and t-vinylene groups have been studied by photooxidation with λ ≥ 300 nm light or by thermooxidation at a temperature of 100 °C. The oxidation was studied by infrared spectroscopy and it was shown that the same oxidation products were obtained, but with different relative concentrations depending on the conditions of ageing, i.e. photochemical or thermal conditions. The mechanisms by which the oxidation products are formed were recalled. The differences between photo- and thermo-oxidation were evidenced on the basis of the stability of ketones that do not accumulate in photochemical conditions, as a result of Norrish reactions. The influence of the initial amount of unsaturated groups on the rates of oxidation was characterized. It was shown that the concentration of unsaturations had no effect on the rate of photooxidation but dramatically influenced the stability in thermooxidative conditions.     

Thermal and photochemical solvolysis of (E)- and (Z)-2-phenyl-1-propenyl(phenyl)iodonium tetrafluoroborate: benzenium and primary vinylic cation intermediates

The thermal and photochemical solvolysis of the two stereoisomeric 2-phenyl-1-propenyl(phenyl)iodonium tetrafluoroborates has been investigated in alcoholic solvents of varying nucleophilicity. The product profiles and rates of product formation in the thermal reaction are all compatible with a mechanism involving cleavage of the vinylic C-I bond assisted by the group in the trans position (methyl or phenyl), always leading to rearranged products. Depending on the nucleophilicity of the solvent, the primarily formed cations may or may not further rearrange to more stable isomers. The less reactive Z compound also yields some unrearranged vinyl ether product in the more nucleophilic solvents via an in-plane S(N)2 mechanism. The mechanism of the photolysis involves direct, unassisted cleavage of the vinylic, and aromatic, C-I bond in an S(N)1 mechanism. This produces a primary vinyl cation, which is partially trapped prior to rearrangement in methanol. The unrearranged vinyl ethers are mainly formed with retention of configuration via a lambda3-iodonium/solvent complex in an S(N)i mechanism. Thermal and photochemical solvolyses of iodonium salts are complementary techniques for the generation of different cation intermediates from the same substrate.     

Radical-induced generation of small silver particles in SPEEK/PVA polymer films and solutions: UV-Vis, EPR, and FT-IR studies

The present study is centered on the processes involved in the photochemical generation of nanometer-sized Ag particles via illumination at 350 nm of aqueous solutions and cross linked films containing sulfonated poly(ether ether ketone) and poly(vinyl alcohol). Optical and electron paramagnetic resonance experiments, including electron nuclear double resonance data, proved conclusively that the photogenerated chromophore exhibiting a band with lambda(max) = 565 nm is an alpha-hydroxy aromatic (ketyl) radical of the polymeric ketone. This reducing species was produced by illumination of either solutions or films, but the radical lifetime extended from minutes in the fluid phase to hours in the solid. Direct evidence is presented that this long-lived chromophore reduces Ag(I), Cu(II), and Au(III) ions in solution. A rate constant of k = 1.4 x 10(3) M(-)(1) s(-)(1) was obtained for the reduction of Ag(+) by the ketyl radical from the post-irradiation formation of Ag crystallites. FTIR results confirmed that the photoprocess yielding polymeric ketyl radicals involves a reaction between the macromolecules. The photochemical oxidation of the polymeric alcohol, as well as the formation of light-absorbing macromolecular products and polyols, indicates that the sulfonated polyketone experienced transformations similar to those encountered during illumination of the benzophenone/2-propanol system.     

High-performance liquid chromatography method for the separation of isomers of 9 beta-methylcarbacyclin (ciprostene) and the analysis of pharmaceutical samples

An high-performance liquid chromatography method was developed that separates (5Z)-9 beta-methyl-6 alpha-carbaprostaglandin I2 (9 beta-methylcarbacyclin; ciprostene) from several closely related isomers: (i) a double bond isomer at C-5, (ii) an epimer at C-15, and (iii) combinations of i and ii. The method uses a Zorbax ODS column with a mobile phase consisting of acetonitrile-buffer pH 2.5 at a ratio of 45:55 for determination of purity and 40:60 for optimum separation of isomers. Detection is measurement of absorbance at 200 nm. Peak heights are measured relative to butylparaben which is used as an internal standard. The method is linear, free from interferences from degradation of ciprostene and has a relative standard deviation of 0.5-1% for solutions at 60 micrograms/ml. This assay has been applied to bulk powders and solutions of the drug. Stressed chemical stability studies have shown that the C-15 allylic secondary alcohol group undergoes epimerization under acidic conditions in solution and oxidation in the solid and solution state under thermal stress or in the presence of oxidants.     

ON THE EMERSON ENHANCEMENT EFFECT IN THE FERRICYANIDE HILL REACTION IN CHLOROPLAST FRAGMENTS*

Abstract— The Emerson effect is demonstrated in the ferricyanide Hill reaction when the rates of steady-state oxygen evolution are measured in spinach chlorplast fragments exposed to red (650 nm) and far-red (700 nm) light of high but not saturating intensity. However, at very low light intensity, the Emerson effect could not be observed. These experiments suggest that ferricyanide can be reduced at two sites. At low light intensity, the rate at one site predominates and at this site one photochemical system is active. At high light intensity, however, the action at a site that is dependent on the cooperation of two photochemical systems predominates. The action spectra of the ferricyanide Hill reaction measured in the presence of an excess of 650 nm or in the excess of 700 nm light show two peaks: one at 650 nm due to chlorophyll b and the other around 675 nm due to chlorophyll a. The ratio of chlorophyll a to chlorophyll b peaks is about 1.4 when 650 nm background light is used; the same ratio is about 0.7 with 700 nm background light. The two pigment systems seem to contain both chlorophyll a and chlorophyll b but in different proportions.     

Oxidation reactions of some natural volatile aromatic compounds: anethole and eugenol

trans-Anethole [1-methoxy-4-(trans-prop-1-en-1-yl)benzene] was isolated from anise seed oil (Pimpinella anisum). Its photochemical oxidation with hydrogen peroxide gave the corresponding epoxy derivative together with 4-methoxybenzaldehyde. The thermal oxidation of trans-anethole with 3-chloroperoxybenzoic acid at room temperature resulted in the formation of dimeric epoxide, 2,5-bis(4-methoxyphenyl)-3,6-dimethyl-1,4-dioxane, as the only product. Photochemical oxygenation of trans-anethole in the presence of tetraphenylporphyrin, Rose Bengal, or chlorophyll as sensitizer led to a mixture of 1-(4-methoxyphenyl)prop-2-en-1-yl hydroperoxide and 4-methoxybenzaldehyde. Eugenol was isolated from clove oil [Eugenia caryophyllus (Spreng.)]. It was converted into 2-methoxy-4-(prop-2-en-1-yl)phenyl hydroperoxide by oxidation with hydrogen peroxide under irradiation. Thermal oxidation of eugenol with 3-chloroperoxypenzoic acid at room temperature produced 2-methoxy-4-(oxiran-2-ylmethyl)phenol, while sensitized photochemical oxygenation (in the presence of Rose Bengal or chlorophyll) gave 4-hydroperoxy-2-methoxy-4-(prop-2-en-1-yl)cyclohexa-2,5-dien-1-one. Published in Russian in Zhurnal Organicheskoi Khimii, 2008, Vol. 44, No. 6, pp. 834–841. The text was submitted by the authors in English.     

ELECTRIC LINEAR DICHROISM OF P700 CHLOROPHYLL U-PROTEIN COMPLEXES

Abstract— The P₇₀₀ chlorophyll a -protein complex (CPI) isolated from green plants was oriented in aqueous solutions using pulsed electric fields of up to 6700 V cm^-1. The electric linear dichroism spectrum is reported in the range of 400–720nm. Positive peaks in the linear dichroism Δ A = A _I - A ₁ (where A_I and A₁ are the absorbance components in which the polarizer orientation is parallel and perpendicular with respect to the electric field. respectively) are observed at 443 and 686 nm. The ΔA signal at 686 nm is discussed in terms of either a specialized chlorophyll form absorbing at 686 nm. or due to an exciton component absorbing at the same wavelength.     

DODECYLPYRIDINIUM ALKANOATES STABILIZE DISPERSED CHLOROPHYLL

In the presence of a surfactant that does not ligate Mg, chlorophyll is adsorbed to polyethylene particles swollen with tetradecane principally as the infrared-absorbing, highly polymeric species Chl 740. Examples of such surfactants are quaternary ammonium salts and soaps. However, when surfactants of opposite charge are present together, in this case dodecylpyridinium iodide and Na butyrate or myristate, chlorophyll may exist entirely in a dispersed form absorbing around 666 nm. Absorption and fluorescence spectral data show that much of the dispersed pigment is still associated, but as dimeric and perhaps short oligomeric species. It is concluded from fluorescence lifetime analysis that most of the observed fluorescence comes from a small population of chlorophyll that is probably monomeric and isolated; fluorescence of more highly associated species decays with a wide range of lifetimes. The capacity for photochemical sensitization of these particles with dispersed chlorophyll is similar to that of particles with ligating surfactants examined earlier. Structures are suggested for chlorophyll association in which Mg is ligated by water hydrogen-bonded to a carboxylate group, while the dodecylpyridinium counterion lies near the chlorophyll ring. The effect of the two surfactants is synergistic. Overall, spectra of dispersed chlorophyll adsorbates resemble most closely those of colloidal chlorophyll suspensions prepared by dilution of solutions in polar organic solvents with water.     

1,1-Dicyano-2,2-diphenyl-1,2-dihydronaphthalene: photochromism and evidence for photochemically induced C[bond]CN bond cleavage

The photoreactions of the 1,1-dicyano-2,2-diphenyl-1,2-dihydronaphthalene (DHN-1) have been studied by photochemical techniques under various conditions at room temperature. A transient species, T(C), with a major maximum at 545 nm in the UV-visible spectrum, detected in small yield in polar aprotic solvents and in large yield in trifluoroethanol (TFE) and hexafluoropropan-2-ol (HFP), is assigned as a carbocation (1-cyano-2,2-diphenylnaphthalenium), which is generated photochemically by elimination of CN(-). The decay of T(C) was found to be a first-order process in HFP, in which the longest lifetime of 0.4 s was observed; in TFE, the carbocation decays on the ms timescale, while the shortest lifetime of 1 micros was found in ethanol. The yield of T(C) increases strongly upon addition of water to alcohols or acetonitrile, and remains substantial in the presence of large amounts of water (1-20 M). On addition of water to TFE or HFP, the lifetime of the carbocation becomes much shorter. This is supported by pulse-induced charge formation due to the carbocation and release of CN(-); the conductivity decay is related to the lifetime of the carbocation under selected conditions. In addition, a major irreversible and a minor thermally reversible photoprocess were spectroscopically observed in solvents of low as well as high polarity. The former photoproduct, absorbing below 300 nm, is tentatively ascribed to benzobicyclohexenes A produced by phenyl-vinylmethane rearrangement. In the latter process, a longer-lived benzoquinodimethane derivative B, having a maximum at 400-430 nm, is formed; this is related to ring opening and closure and represents a new example of photochromic ten-electron electrocyclisation. The distinct differences between the photochromism of DHN and that of dihydroazulene (DHA) stem mainly from stereoelectronic effects. This study, however, has also revealed that photochemically induced bond heterolysis and photochemical/thermal electrocyclisation, representing two basic processes of photochromism, may occur in one molecular unit.     

NONLINEAR LASERSPECTROSCOPIC INVESTIGATIONS OF THE PIGMENT-PIGMENT INTERACTION WITHIN THE LIGHT-HARVESTING COMPLEX OF PHOTOSYSTEM II

Using a pump and test beam technique in the frequency domain with pump pulses in the nanosecond time range, the nonlinear transmission properties were investigated at room temperature in photosystem (PS) II membrane fragments and isolated light-harvesting chlorophyll a/b-protein preparations (LHC II preparations). In LHC II preparations and PS II membrane fragments, respectively, pump pulses of 620 nm and 647 nm cause a transmission decrease limited to a wavelength region in the nearest vicinity of the pump pulse wavelength (full width at half maximum ' 0.24 nm). In contrast, at 670 nm neither a transmission decrease nor a narrow band feature were observed. The data obtained for PS II membrane fragments and LHC II preparations at shorter wavelengths (620 nm, 647 nm) were interpreted in terms of excited state absorption of whole pigment-protein clusters within the light-harvesting antenna of photosystem II. The interpretation of the small transmission changes as homogeneously broadened lines led to a transversal relaxation time for chlorophyll in the clusters of about 4 ps.     

FLUORESCENCE SPECTROSCOPY OF A P700-CHLOROPHYLL-PROTEIN COMPLEX*

Abstract. Chlorophyll-protein complexes enriched in the Photosystem I reaction center chlorophyll (P700) exhibit a fluorescence emission maximum at 696 nm at - 196°C The height of this 696 nm emission relative to the emission at 683 nm from antenna chlorophyll a increases proportionally with the P700 concentration while the total fluorescence yield of the complex decreases. The 696 nm emission could possibly be from an absorbing form of antenna chlorophyll a that may be somewhat enriched along with P700 in Photosystem I fractions. However, evidence resulting from glycerol treatment which appears to decrease the rate of resonance energy transfer between antenna chlorophyll and P700 favors the hypothesis that the emission comes from a photooxidized P700 dimer (Chl⁺-Chl) absorbing near 690 nm. In turn, this fluorescence evidence provides additional support for the model of a P700 dimer involving exciton interaction. Absorption in the wavelength region of 450 nm specifically excites emission at 696 nm from the P700-chlorophyll complex.     

New oxidative tools for the functionalization of the cephalostatin north 1 hemisphere

Dimethyldioxirane (DMDO) C-H oxidation of ketone 17 to hemiketal 18 (82%), bis-dehydration to vinyl ether 21 (77%), and DMDO again provides C-23 axial alcohol 23 (99%). Routine processing, including a double-stereoselective Sharpless AD reaction (de >98%), gives alcohols 7 and 32. C-23 deoxy substrate 7 undergoes Suarez hypoiodite oxidative cyclization to (natural) beta spiroketal 34, but compound 32, bearing a C-23 silyl ether, generates unnatural spiroketal 33. [reaction: see text]     

Oxidation of acyclic alkenes and allyl and benzyl ethers with DIB/t-BuOOH/Mg(OAc)2

Oxidation of (11Z)-1′,2′-didehydrostemofoline with DIB/TBHP/Mg(OAc)₂·4H₂O resulted in oxidative cleavage of the C-11–C-12 double bond instead of the desired allylic oxidation of the 1-butenyl side chain. Stemofoline gave a similar result. The oxidation of more simple terminal alkenes was regioselective and gave vinyl ketones while allyl and benzyl ethers gave acrylate and benzoate esters, respectively. Allyl and benzyl ethers could be chemoselectively oxidized in the presence of a terminal alkene or benzyl group. Oxidation of an internal alkene was poorly regioselective, in contrast to the oxidation of 1-substituted cyclohexenes.     

AM/AMPS共聚物光化学改性

水溶性高分子AM/AMPS系由丙烯酰胺(AM)和2-丙烯酰胺基-2-甲基丙磺酸(AMPS)组成.本文应用光化学方法对AM/AMPS共聚物进行改性,得到活性可聚合产物.应用这种方法,在高分子链上生成的过氧化物含量随UV光照时间有明显变化,照射5-10 min过氧化物含量最高.值得指出,较高的过氧化物生成量仅在采用二苯酮作为光敏剂和保持低温的条件下才能得到.这种活性改性AM/AMPS共聚物,可用光聚合或热聚合方法引发不同烯类单体共聚合,得到不同用途的接枝/交联共聚物产品.     

A facile route to vinyl- and arylphosphonates by vinyl and aryl radical trapping with (MeO)3P

The generation of vinyl or aryl radicals under classical, thermal AIBN/n-Bu(3)SnH conditions at 80 degrees C in the presence of an excess of (MeO)(3)P gives rise to the corresponding vinyl- or arylphosphonates in good yields. This approach complements the photochemical reactions of the same systems previously used. Reactions with the individual stereoisomers of MeCH=CHMeBr (thermal AIBN/n-Bu(3)SnH conditions) afford a radical-equilibrated 96/4 E/Z ratio of vinylphosphonates. Substitution of (TMS)(3)SiH for n-Bu(3)SnH yields an approximately 1/1 ratio of separable E and Z vinylphosphonate diastereomers.     

Mechanistic interpretation of photochemical thallimetric oxidations catalysed by chloride and bromide ions

The thallimetric oxidation of carboxylic acids appears to proceed through free radical and intermediate activated complex mechanisms. The thermal and photochemical uncatalysed oxidation reactions appear to proceed through the formation of an intermediate metal-substrate complex that eventually decomposes to give the products. However, photochemical oxidation in the presence of chloride and bromide ions appears to proceed through a two-electron step via a halo bridge mechanism. In the presence of bromide at 2–3 times the concentration of thallium(III), the photochemical reduction mainly proceeds through a free radical mechanism involving a one-electron step via the formation of thallium(II) species. The nature and concentration of halide ions appear to be critical in deciding the path of the reaction.     

Synthesis and characterization of poly(vinyl pyrrolidone)-capped bismuth nanospheres

Poly(vinyl pyrrolidone)-capped bismuth nanospheres were synthesized by a simple and convenient wet chemical method. In the process, bismuth nitrate was reduced by ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP) at 185 °C in air. PVP was used as a protecting agent to prevent oxidation of the sperical bismuth particles. PVP molecules were absorbed on the surface of bismuth nanospheres through the interaction of O–Bi bond which was confirmed by Fourier transform infrared (FT-IR) measurement. The thermal analysis shows the samples contained about 73 wt.% metallic bismuth. The optical absorption spectrum of poly(vinyl pyrrolidone)-capped bismuth nanosphere shows a strong absorption band at 275 nm.     

Photochemical functionalization of hydrogen-terminated diamond surfaces: a structural and mechanistic study

Hydrogen-terminated diamond surfaces can be covalently modified with molecules bearing a terminal vinyl (C=C) group via a photochemical process using sub-band-gap light at 254 nm. We have investigated the photochemical modification of hydrogen-terminated surfaces of nanocrystalline and single-crystal diamond (111) to help understand the structure of the films and the underlying mechanism of photochemical functionalization. A comparison of the rates of photochemical modification of single-crystal diamond and nanocrystalline diamond films shows no significant difference in reactivity, demonstrating that the modification process is not controlled by grain boundaries or other structures unique to polycrystalline films. We find that both single-crystal and polycrystalline hydrogen-terminated diamond samples exhibit negative electron affinity and are functionalized at comparable rates, while oxidized surfaces with positive electron affinity undergo no detectable reaction. Gas chromatography-mass spectrometry (GC-MS) analysis shows the formation of new chemical products in the liquid phase that are formed only when the alkenes are illuminated in direct contact with H-terminated diamond, while control experiments with other surfaces and in the dark show no reaction. Our results show that the functionalization is a surface-mediated photochemical reaction and suggest that modification is initiated by the photoejection of electrons from the diamond surfaces into the liquid phase.     

Synthetic chlorins bearing auxochromes at the 3- and 13-positions

Synthetic chlorins bearing diverse auxochromes at the 3- and 13-positions of the macrocycle are valuable targets given their resemblance to chlorophylls a and b, which bear 3-vinyl and 13-keto groups. A de novo route has been exploited to construct nine zinc chlorins bearing substituents at the 3- and 13-positions and two benchmark zinc chlorins lacking such substituents. The chlorins are sterically uncongested and bear (1) a geminal dimethyl group in the reduced pyrroline ring, (2) a H, an acetyl, a triisopropylsilylethynyl (TIPS-ethynyl), or a vinyl at the 3-position, (3) a H, an acetyl, or TIPS-ethynyl at the 13-position, and (4) a H or a mesityl at the 10-position. The synthesis of the 13-substituted chlorins relied on p-TsOH x H2O-catalyzed condensation of an 8,9-dibromo-1-formyldipyrromethane (eastern half) and 2,3,4,5-tetrahydro-1,3,3-trimethyldipyrrin (western half), followed by metal-mediated oxidative cyclization, affording the 13-bromochlorin. Similar use of a bromo- or TIPS-ethynyl-substituted western half provided access to 3-substituted chlorins. A 3-bromo, 13-bromo, or 3,13-dibromochlorin was further transformed by Pd-coupling to introduce the vinyl group (via tributylvinyltin), TIPS-ethynyl group (via TIPS-acetylene), or acetyl group (via tributyl(1-ethoxyvinyl)tin, followed by acidic hydrolysis). In the 10-mesityl-substituted zinc chlorins, the series of substituents, 3-vinyl, 13-TIPS-ethynyl, 3-TIPS-ethynyl, 13-acetyl, 3,13-bis(TIPS-ethynyl), 3-TIPS-ethynyl-13-acetyl, or 3,13-diacetyl, progressively causes (1) a redshift in the absorption maximum of the B band (405-436 nm) and the Q(y) band (606-662 nm), (2) a relative increase in the intensity of the Q(y) band (I(B)/I(Q) = 4.2-1.5), and (3) an increase in the fluorescence quantum yield phi(f) (0.059-0.29). The zinc chlorins bearing a 3-TIPS-ethynyl-13-acetyl or a 3,13-diacetyl group exhibit a number of spectral properties resembling those of chlorophyll a or its zinc analogue. Taken together, this study provides access to finely tuned chlorins for spectroscopic studies and diverse applications.