Crown ether bridged homo- and heterodinuclear copper(II) and nickel(II) cyclidene complexes: Interaction with anions

Homodinuclear (3CuCu, 3NiNi) and heterdionuclear (3CuNi) cyclidene complexes linked by two 1,10-diaza-18-crown-6 ethers were synthesized and their properties characterized. For the 3CuCu in the negative potential range the mixed-valence state Cu^I-Cu^II was observed and the comproportionation constants were determined. The redox potentials of M^II/III showed a good correlation with DN of the solvent. In acetonitrile solution containing 0.1 mol dm⁻³ Bu₄NPF₆ reversible M^II/III redox processes were observed. However, in the presence of stronger ion pairing anions (e.g. BF₄⁻ and ClO₄⁻) the “potential inversion” occurred. Redox potentials of dinuclear complexes were compared with mononuclear cyclidene complexes. The influence of selected anions (Cl⁻ and NO₃⁻) on the redox process M^II/III was studied. The effect of Cl⁻ anion was different for 3CuCu and 3NiNi. In 3CuCu coordination of two chloride anions took place after oxidation of copper centers. In the 3NiNi complex two Cl⁻ anions were coordinated to one of the nickel(II) centers facilitating oxidation, at different potentials, of both nickel(II) cations. The behaviour of heteronuclear complex with Cl⁻ anions was similar to 3NiNi. All dinuclear complexes interacted with NO₃⁻ anions and the observed potential shifts were larger for nickel(II) than for copper(II) cations.     

Synthesis and electrochemical characterisation of benzylmercapto and dodecylmercapto tetra substituted cobalt, iron, and zinc phthalocyanines complexes

The work reports on cyclic voltammetry (CV), square wave voltammetry and spectroelectrochemistry of the following complexes: tetrakis (benzylmercapto) phthalocyanine complexes of Zn(II) (ZnTBMPc, 4a), Co(II) (CoTBMPc, 5a), and Fe(II) (FeTBMPc 6a); tetrakis (dodecylmercapto) phthalocyanine complexes of Zn(II) (ZnTDMPc, 4b), Co(II) (CoTDMPc, 5b), and Fe(II) (FeTDMPc, 6b). More reversible CV couples were observed for complexes 4a, 5a, and 6a containing thiol phenyl ring substituents. Complexes 4b, 5b, and 6b containing long chain thiol substituents showed less reversible couples. Complexes 6a and 6b showed a relatively large number of redox processes (5 for 6a and 6 for 6b) within the potential window employed in this work. The processes for FePc derivatives (6a) are assigned to Fe^IIIPc⁻¹/Fe^IIIPc⁻², Fe^IIIPc⁻²/Fe^IIPc⁻², Fe^IIPc⁻²/Fe^IPc⁻², Fe^IPc⁻²/Fe^IPc⁻³, and Fe^IPc⁻³/Fe^IPc⁻⁴ and for the CoPc derivative (5a) to Co^IIIPc⁻¹/Co^IIIPc⁻², Co^IIIPc⁻²/Co^IIPc⁻², Co^IIPc⁻²/Co^IPc⁻², and Co^IPc⁻²/Co^IPc⁻³.     

Crown ethers appended copper tetraazamacrocyclic complexes as receptors for cationic guests

Tetraazamacrocyclic copper(II) complexes linked by the system of conjugated double bonds with one or two benzo-15-crown-5 ether moieties (4Cu and 5Cu) as well as model compounds were synthesized and characterized. The CV of these complexes in acetonitrile (ACN) solution exhibited reversible Cu^II/I redox process. Complexation of Na⁺ and Mg²⁺ cations with 5Cu in the gas phase was confirmed by electrospray mass spectrometry. In solution complexation of Li⁺ and Na⁺ with 5Cu was weak. Complexation of Cl⁻ anion to coordinated copper in 5Cu resulted in an appreciable shift in the Cu^II/I redox potential after addition of studied cations. The addition of increasing amounts of Mg²⁺ cation to the solution of 5Cu resulted in a significant shift of the Cu^II/I reduction process accompanied by further reduction to Cu⁰ followed by replacement of copper by Mg²⁺ cation. Barium(II) cation interacted only with reduced (Cu⁺) form of 5Cu.     

Electrochemical characterisation of tetra- and octa-substituted oxo(phthalocyaninato)titanium(IV) complexes

The synthesis and electrochemical characterisation of the following oxotitanium tetra-substituted phthalocyanines are reported: 1,(4)-(tetrabenzyloxyphthalocyaninato)titanium(IV) oxide (5a); 1,(4)-{tetrakis[4-(benzyloxy)phenoxy]phthalocyaninato}titanium(IV) oxide (5b); 2,(3)-(tetrabenzyloxyphthalocyaninato)titanium(IV) oxide (6a) and 2,(3)-{tetrakis[4-(benzyloxy)phenoxy]phthalocyaninato}titanium(IV) oxide (6b). The electrochemical characterisation of complexes octa-substituted with 4-(benzyloxy)phenoxy (9b), phenoxy (9c) and tert-butylphenoxy (9d) groups is also reported. The cyclic voltammograms of the complexes exhibit reversible couples I-III and couple IV is quasi-reversible for complexes 5a, 5b, 6a and 6b. The first two reductions are metal-based processes, confirmed by spectroelectrochemistry to be due to Ti^IVPc²⁻/Ti^IIIPc²⁻ and Ti^IIIPc²⁻/Ti^IIPc²⁻ redox processes and the last two reductions are ring-based processes due to Ti^IIPc²⁻/Ti^IIPc³⁻ and Ti^IIPc³⁻/Ti^IIPc⁴⁻. Chronocoulometry confirmed a one-electron transfer at each reduction step. The electrochemistry of the above complexes is also compared to the previously reported 5c, 5d, 6c and 6d.     

With different structure ligands heterogeneous Ziegler-Natta catalysts for the preparation of copolymer of ethylene and 1-octene with high comonomer incorporation

Comparison with the conventional Ziegler-Natta catalyst TiCl₄/MgCl₂ (I), the modified supported Ziegler-Natta catalysts (iso-PentylO)TiCl₃/MgCl₂ (II) and (BzO)TiCl₃/MgCl₂ (III) were prepared as efficient catalysts for copolymerization of ethylene with 1-octene. The complexes (II) and (III) were desirable for the production of random ethylene/1-octene copolymers coupled with higher molecular weight, higher comonomer incorporation within copolymer chain and good yield even at high temperature 80 ^°C and fairly low Al/Ti molar ratio of 100. The effects of catalysts ligands, Al/Ti molar ratio, polymerization temperature, as well as concentration of 1-octene on the catalytic activity, molecular weight and microstructure of the copolymer were investigated in detail. The structure and properties of the copolymers were characterized with ¹³C NMR, GPC, DSC and WAXD. The kinetic results also indicate that these catalysts (II) and (III) show higher catalytic activity and the produced polymers feature higher molecular weight, because of lower ratio of K_trm/K_p and K_tra/K_p, and higher ratio of K_tra/K_trm which indicates that chain transfer to cocatalyst is predominant.     

Electrocatalytic four-electron reductions of O2 to H2O with cytochrome c oxidase model compounds

Three different complexes, heme-Cu ([(⁶L)Fe^IICu^I]⁺ (1), ⁶L=a binucleating ligand having a heme and covalently tethered copper binding tris(2-pyridyl)methylamine tetradentate moiety), heme complex ((⁶L)Fe^II (4), (with “empty” tethered chelate)), and the “parent” iron-porphyrinate ((F₈TPP)Fe^II (5), F₈TPP=tetrakis(2,6-difluorophenyl)porphyrinate) were employed for the electrocatalytic reduction of O₂. Complexes 1 and 4 reduce O₂ to water (four-electron reduction) with good efficiency (74 and 59%, respectively), but complex 5 exhibited only an ∼20% efficiency, thus primarily the two-electron reduction to hydrogen peroxide occurred. The results of the present electrochemical O₂-reduction studies and the previous studies have elucidated the O₂-binding nature of these three complexes, indicating the formation of quite stable Fe^III(O₂²⁻)Cu^II (peroxo) or Fe^IIIO₂⁻ (superoxo) species. In line with the thinking of other researchers, the fact that both 1 and 4 can well stabilize Fe^IIIO₂⁻ superoxo species may suggest that the formation of the latter is a key to the pathway favoring four-electron reduction of dioxygen to water.     

Colorless and high organosoluble polyimides from 2,5-bis(3,4-dicarboxyphenoxy)-t-butylbenzene dianhydride and aromatic bis(ether amine)s bearing pendent trifluoromethyl groups

A series of polyimides III_a-h characterized by colorlessness, high transparency, high solubility, and good mechanical property, was synthesized from the aromatic dianhydride, 2,5-bis(3,4-dicarboxyphenoxy)-t-butylbenzene dianhydride (I), and various aromatic diamines (II_a-h) with pendent trifluoromethyl group via polyaddition, chemical imidization, and direct cast films. The III series showed more colorless than the polyimides (V and VI series) of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) contained, the VI series was synthesized from the II with 6FDA. These films III had cut-off wavelengths between 371 and 376 nm, as well as b* value (a yellowness index) ranging from 3.0 to 4.7. In fact, it is so far the most colorless aromatic polyimide in our systematical researches. The III series had inherent viscosity ranging from 0.72 to 1.33 dL/g and showed excellent solubility in a variety of organic solvents. They were soluble in a concentration of 5-10% in the amide polar solvent, ether solvent, and chlorinated solvent. These films showed strength tensile of 97-123 MPa, dielectric constants of 2.78-3.28 (1 MHz), and moisture absorptions of 0.11-0.36 wt%. The glass transition temperature of the III series was recorded at 214-259 °C, the 10% weight loss temperature was over 468 °C, and the residue was more than 47% at 800 °C in nitrogen.     

Synthesis of 1,9-bis[glycidyloxypropyl]penta(1′H,1′H,2′H,2′H-perfluoroalkylmethylsiloxane)s and copolymerization with piperazine

A series of 1,9-bis[glycidyloxypropyl]pentasiloxanes (IV-VI) were prepared by the platinum catalyzed hydrosilylation of 1,9-dihydridodecamethylpentasiloxane (I), 1,9-dihydrido-3,5,7-tris(3′,3′,3′-trifluoropropyl)heptamethylpentasiloxane (II), and 1,9-dihydrido-3,5,7-tris(1′H,1′H,2′H,2′H-perfluorooctyl)heptamethylpentasiloxane (III) with allyl glycidyl ether. Subsequently, IV-VI were copolymerized with piperazine to form high molecular weight copoly(carbosiloxane)s (VII-IX). The structures of the 1,9-bis[glycidyloxypropyl]penta-siloxanes (IV-VI) and copoly(carbosiloxane)s (VII-IX) were determined by ¹H, ¹³C, ²⁹Si, and ¹⁹F NMR as well as IR spectroscopy. The molecular weight distributions (M_w/M_n) of VII-IX have been characterized by gel permeation chromatography and their thermal properties measured by differential scanning calorimetry and thermal gravimetric analysis.     

Copolymerization of propene with low amounts of ethene in propene bulk phase

Propene homopolymers and propene-ethene copolymers with small amount of ethene were synthesized using three catalyst systems [p-CH₃OPh₂C(2,7-di-^tertBuFlu)(Cp)]ZrCl₂/Me₂HNPh][B(C₆F₅)₄] (Cat I), [p-CH₃OPh₂C(2,7-di-^tertBuFlu)(Cp)]ZrCl₂/[Ph₃C][B(C₆F₅)₄] (Cat II) and [p-CH₃OPh₂C(2,7-di-^tertBuFlu)(Cp)]ZrCl₂/MAO (Cat III) in propene bulk phase. The activity of the catalyst was dependent on the formed ion pair: Cat I showed the highest activity, up to 96,000 kg_pol/mol_Zr h, at the used polymerization conditions. The produced homo- and copolymers have high molecular weights, between 400 and 600 kg/mol. The ethene incorporation rate was nearly the same with the different catalyst systems; the amount of ethene in the copolymer was increasing linearly with the amount of ethene in feed. The syndiotacticity of the homopolymer was highest, [rrrr]>90%, with Cat I when it was 81.4 with Cat II and 80.8 with Cat III. The small amount of ethene in the copolymer allows the control of the melting and crystallization behavior. Also the crystallization temperatures of the copolymers were found to depend on the used catalyst system.     

Synthesis, characterization and properties of tetramethyl stilbene-based epoxy resins for electronic encapsulation

Two novel tetramethyl stilbene-based novolac (II and IV) were synthesized from 2,6-dimethyl phenol and chloroacetaldehyde dimethylacetal or chloroacetone, and then the resulted novolacs were epoxidized to tetramethyl stilbene-based epoxy resins (III and V). The proposed structures were confirmed by FTIR, elemental analysis, mass spectra, NMR spectra and epoxy equivalent weight titration. The synthesized tetramethyl stilbene-based epoxy resins were cured with 4,4-diaminodiphenyl methane (DDM) and 4,4-diaminodiphenyl sulfone (DDS). Thermal properties of cured epoxy resins were studied using dynamic mechanical analyzer, differential scanning calorimeter, thermal expansion analyzer and thermal gravimetric analyzer (TGA). These data were compared with that of the commercial tetramethyl biphenol (TMBP) epoxy system. According to the experimental data, the order of T_g for cured epoxy system is III>TMBP>V. The order of moisture absorption for cured epoxy system is V<III<TMBP. According to TGA, the 5% degradation temperatures in nitrogen atmosphere were in the range 370-377 and 397-412 °C for DDM and DDS curing systems, respectively. In air atmosphere, the 5% degradation temperatures were in the range 372-385 and 410-411 °C for DDM and DDS curing systems, respectively. The CTE is in inverse order with T_g, therefore, III/DDS<TMBP/DDS<V/DDS.     

Properties of organosoluble aromatic polyimides from 3′-trifluoromethyl-3,4′-oxydianiline

A colorless fluorinated diamine, 3′-trifluoromethyl-3,4′-oxydianiline (3′-CF₃-3,4′-ODA) (II) was prepared through the nucleophilic substitution reaction of 3-nitrophenol and 2-chloro-5-nitrobenzotrifluoride by catalytic reduction with hydrazine and Pd/C. A series of Polyimides V were synthesized from the diamine II with various aromatic dianhydrides III_a-f via thermal and chemical imidization. These polyimides had inherent viscosities ranging from 0.88 to 1.12 dl/g. A comparison of V, VI to analogous polyimides VII, VIII. VI, VII and VIII was based on 3′4-ODA, 3-CF₃-4,4′-ODA, 4,4′-ODA, respectively. In terms of the color of PI revealed that the color intensity of phenoxy-containing amine of the meta-structure and the para-structure with the CF₃ group would fell off color intensity. The color intensity of the four polyimide series was lessened in the following order: V>VII>VI>VIII. The solubility of V is better than VI, VII and VII. The polyimide V films had a tensile strength ranging from 124 to 147 MPa, elongation at break from 9 to 65%, and initial modulus from 2.3 to 2.8 GPa. The glass transition temperature of polymers was recorded at 234-313 °C. They had 10% weight loss at a temperature above 515 °C and left more than 50% residue even at 800 °C in nitrogen. Compared with polyimides VI, V showed the lower dielectric constants of 2.80-3.50 (40 MHz), and moisture absorptions in the range of 0.44-1.02 wt%.     

Synthesis, electrochemical and spectroelectrochemical characterization of novel soluble phthalocyanines bearing chloro and quaternizable bulky substituents on peripheral positions

A new phthalonitrile derivative (2), bearing diethylaminophenoxy - and chloro-substituents at peripheral positions was synthesized in this work. Cyclotetramerization of (2) in hexanol gave the desired metal-free (4) and metallophthalocyanines (5-8). These new phthalocyanines (4-8) were converted into water-soluble quaternized products by the reaction with methyl iodide (9-11). The novel compounds have been characterized by using elemental analysis, UV-Vis, FT-IR, ¹H NMR and MS spectroscopic data. The aggregation behaviors of the phthalocyanine complexes were studied in different solvents and concentrations. Electrochemical and spectroelectrochemical characterization of the complexes were also performed in solution. Cobalt phthalocyanine gives both metal-based and ring-based reduction processes in comparison to the complexes having 2H⁺, Zn²⁺, Ni²⁺ and Cu²⁺ metal center which give only ring-based reduction processes. Electrochemical and spectroelectrochemical measurements exhibit that all complexes oxidatively electro-polymerize on the Pt working electrode during repetitive cyclic voltammetry measurements. An in-situ electrocolorimetric method was applied to investigate the color of the electrogenerated anionic and cationic forms of the complexes for possible electrochromatic applications.     

Colorless polyimides from 2,3,3′,4′-biphenyltetracarboxylic dianhydride (α-BPDA) and various aromatic bis(ether amine)s bearing pendent trifluoromethyl groups

A series of organosoluble and light-colored polyimides (III) was prepared from 2,3,3′,4′-biphenyltetracarboxylic dianhydride (α-BPDA) with various fluorinated aromatic bis(ether amine)s via two-step method with thermal or chemical imidization of poly(amic acid)s yielded polyimides. The III series had inherent viscosity of 0.74-1.01 dl/g and showed excellent solubility in a variety of organic solvents. They were soluble in the amide polar solvent, ether-type solvent, and chlorinated solvent. These polyimide films also showed a high optical transparency and less color intensity, with an ultraviolet-visible absorption edge of 369-382 nm and low b^* values (a yellowness index) of 5.0-11.7. Glass-transition temperature of the III series was recorded at 244-319 °C and higher than the isomeric polyimides V series. Compared with the nonfluorinated polyimides IV, the III series showed lighter-colored and lower dielectric constants and moisture absorptions. The good tensile properties and excellent thermal properties of the III series were also observed.     

Highly active and trans-1,4 specific polymerization of 1,3-butadiene catalyzed by 2-pyrazolyl substituted 1,10-phenanthroline ligated iron (II) complexes

A new family of iron (II) complexes (2a–h) bearing tridentate 2-pyrazolyl substituted 1,10-phenanthroline ligands were successfully prepared and characterized by IR spectroscopy, elemental analysis, and mass spectra. Complexes 2a–f and 2h were further confirmed by the X-ray crystallographic analysis. 2a, 2b, 2e, and 2f adopted distorted trigonal bipyramidal configuration. 2c displayed a distorted octahedron formed by six coordinated nitrogen atoms of the two ligands. Linked by two bridged chloride atoms, complex 2d was a centrosymmetric dimmer, and complex 2h adopted a six-coordinate distorted octahedral geometry due to the coordination of two solvent molecules. These complexes activated by alkylaluminum were examined in butadiene polymerization. In combination with AlⁱBu₃, complexes 2a–c exhibited high catalytic activity (73.5%–94.3%) at 20 °C, whereas other complexes exhibited much lower activity. Interestingly, the activity and selectivity of the complexes increased as increasing polymerization temperature. In particular, 2b and 2c displayed both high activity (99% and 80%, respectively) and trans-1,4 selectivity (95.6% and 96.2%, respectively) at 60 °C. The trans-1,4 selectivity of 2b varied as alkylaluminum used as a cocatalyst, in the following order: AlⁱBu₃ > AlOct₃ > AlEt₃ > AlMe₃, whereas much lower trans-1,4 selectivity was observed in the cases of using MAO and MMAO.     

Synthesis and properties of organosoluble polyimides based on 1,1-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]cyclohexane

New aromatic diamine with cyclohexane cardo group substituted with trifluoromethyl group in the side chain, 1,1-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]cyclohexane (II), was prepared through the nucleophilic substitution reaction of 1,1-bis(4-hydroxyphenyl)cyclohexane and 2-chloro-5-nitrobenzotrifluoride in the presence of potassium carbonate, to yield the intermediate dinitro compound I, followed by catalytic reduction with hydrazine and Pd/C to afford the diamine II. Fluorinated polyimides (IVa-g) were prepared from the II with various aromatic dianhydrides via thermal or chemical imidization of poly(amic acid). These polyimides had inherent viscosity ranging from 0.72 to 1.16 dl/g and showed excellent solubility in a variety of organic solvents. They were soluble in a concentration of 10% in the amide polar solvent, and 1-5% in the other testing solvents. IV films showed good mechanical properties, excellent thermal stability. The 10% weight loss temperature was above 476 °C in nitrogen or air, and the glass transition temperature was recorded at 214-278 °C. In comparison of the IV series with the analogous nonfluorinated polyimides (V series) based on 1,1-bis[4-(4-aminophenoxy)phenyl]cyclohexane (II′), IV series revealed better solubility, lighter-colored and lower dielectric constants and moisture absorptions. Their films had cut-off wavelengths in the range of 364-414 nm, b^* value (a yellowness index) ranging from 3.3 to 66.3, dielectric constants of 3.02-3.55 (1 MHz), with moisture absorption of 0.16-0.36 wt%.     

Paired electrochemical synthesis of new organosulfone derivatives

Electrochemical oxidation of “cathodically generated 4-aminocatechol (2)” has been studied in the presence of 4-toluenesulfinic acid (4a) and benzenesulfinic acid (4b) as nucleophiles in aqueous solutions, using cyclic voltammetry and controlled-potential coulometry. The results indicate that the o-benzoquinone derived from 4-aminocatechol (2) participates in Michael addition reaction with 4a or 4b to form the corresponding new organosulfone derivatives (5a and 5b). In this work we have proposed a mechanism for the electrode process. A Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox mediator was used for the anodic oxidation of 4-aminocatechol (2) to the corresponding o-quinone 3. The indirect electrochemical process consists of a multi-step such as (a) cathodic reduction of 4-nitrocatechol (1) to 4-aminocatechol (2), (b) chemical oxidation of 4-aminocatechol (2) to 4-aminoquinone (3) with the resulting Fe(CN)₆³⁻, (c) the chemical reaction of 4-aminoquinone (3) with 4-toluenesulfinic acid (4a) or benzenesulfinic acid (4b), and (d) the anodic regeneration of Fe(CN)₆³⁻. The paired electrochemical synthesis of organosulfone derivatives (5a and 5b) has been successfully performed in an one-pot process at carbon rod electrode as a working and platinum as a counter electrode in an undivided cell.     

Novel coumarin fluorescent dyes: Synthesis, structural characterization and recognition behavior towards Cu(II) and Ni(II)

A series of novel coumarin dyes (3-12) were designed and synthesized. The structures of the dyes were characterized by IR, ¹H NMR, ¹³C NMR, ¹⁹F NMR, MS and single crystal X-ray diffraction. 3-(2-Benzoylhydrazonotrifluoroethyl)-7-(N,N-diethylamino)coumarin (11) could recognize Cu²⁺ and Ni²⁺ selectively in aqueous solution. Upon addition of Cu²⁺ or Ni²⁺ to 11 a blue shift or a bathochromatic shift of the absorption band was observed while the emission band blue-shifted with decrease in the fluorescence intensity. Upon addition of Cu²⁺ the color of the solution of 11 changed from orange to red. The results showed that 11 could be used as an optical chemosensor of Cu²⁺ and Ni²⁺.     

Metallocene-catalyzed synthesis of polyethylenes with side-chain triarylamines: Effects of catalyst structure and triarylamine functionality

The copolymerization of ethylene with 8-triarylamine (TAA) substituted 1-octene monomers (TAA = triphenylamine (M1), N,N-diphenyl-m-tolylamine (M2), N,N-diphenyl-1-naphthylamine (M3)) using various types of group 4 single-site catalytic systems (Cp₂ZrCl₂ (C1), rac-EBIZrCl₂ (C2), rac-SBIZrCl₂ (C3), i-PrCpFluZrCl₂ (C4), Me₂Si(η⁵-C₅Me₄)(η¹-N-^tBu)TiCl₂ (C5)) was investigated to prepare functionalized polyethylene with side-chain TAA groups. The metallocene/methylaluminoxane (MAO) catalytic systems (C1-C4) efficiently lead to the production of high-molecular-weight poly(ethylene-co-M1). While the C4/MAO catalytic system shows the highest comonomer response, the C5/MAO system exhibits the poor compatibility with the M1 comonomer. Copolymerization results of ethylene with M1-M3 using C4/MAO indicate that M1-M3 are well tolerated by both the cationic active species of C4 and MAO cocatalyst, giving rise to the copolymers with high levels of activity and molecular weight. Inspection of the aliphatic region of the ¹³C NMR spectra of the copolymers (P1-P3) having ca. 11 mol% of M1-M3, respectively, reveals the presence of isolated comonomer units with prevailing [EEMEE] monomer sequences in the polymer chain. UV-vis absorption and PL spectra exhibit an apparent low-energy band broadening for P1 and P2 indicative of intrachain aggregate formation. Whereas P2 and P3 undergo completely reversible one-electron oxidation process, P1 shows relatively poor oxidational stability.     

Two-dimensional spin-exchange solid-state NMR study of the crystal structure of cellulose II

A two-dimensional (2D) ¹³C-¹³C spin-exchange NMR experiment was applied to the uniformly ¹³C-enriched cellulose II in order to obtain interatomic distance information in the cellulose II crystal. A radio frequency-driven recoupling (RFDR) rotor-synchronized π pulse sequence was incorporated during the mixing time of the 2D experiment. The 2D spin-exchange NMR recorded with a short mixing time (0.80-2.58 ms) provided the correlations between a pair of strongly coupled ¹³C spins such as neighbor carbon nuclei. This spectrum enabled us to assign all ¹³C resonance lines of two kinds of anhydroglucose residues A and B in the structure of cellulose II. On the basis of the ¹³C resonance assignment of residues A and B, the interatomic distances from each C1 to the other carbon nuclei were compared by measuring the 2D spectra recorded with longer mixing times (5.12-20.48 ms). As a result, it was revealed that the respective residues A and B are composed of independent chains (-A-A- and -B-B- repeating units) and that there are no -A-B- repeating units in the chain. This experimental technique is expected to be applicable to conformation analysis of polysaccharides as well as the other cellulose polymorphs.