Cellulose polymorphism study with sum-frequency-generation (SFG) vibration spectroscopy: identification of exocyclic CH2OH conformation and chain orientation

Sum-frequency-generation (SFG) vibration spectroscopy is a technique only sensitive to functional groups arranged without centrosymmetry. For crystalline cellulose, SFG can detect the C6H₂ and intra-chain hydrogen-bonded OH groups in the crystal. The geometries of these groups are sensitive to the hydrogen bonding network that stabilizes each cellulose polymorph. Therefore, SFG can distinguish cellulose polymorphs (I_β, II, III_I and III_II) which have different conformations of the exocyclic hydroxymethylene group or directionalities of glucan chains. The C6H₂ asymmetric stretching peaks at 2,944 cm^?1 for cellulose I_β and 2,960 cm^?1 for cellulose II, III_I and III_II corresponds to the trans-gauche (tg) and gauche-trans (gt) conformation, respectively. The SFG intensity of the stretch peak of intra-chain hydrogen-bonded O–H group implies that the chain arrangement in cellulose crystal is parallel in I_β and III_I, and antiparallel in II and III_II.     

Lateral thermal expansion of cellulose Iβ and IIII polymorphs

Measurements of the thermal expansion coefficients (TECs) of cellulose crystals in the lateral direction are reported. Oriented films of highly crystalline cellulose I_β and III_I were prepared and then investigated with X‐ray diffraction at specific temperatures from room temperature to 250 °C during the heating process. Cellulose I_β underwent a transition into the high‐temperature phase with the temperature increasing above 220–230 °C; cellulose III_I was transformed into cellulose I_β when the sample was heated above 200 °C. Therefore, the TECs of I_β and III_I below 200 °C were measured. For cellulose I_β, the TEC of the a axis increased linearly from room temperature at α_a = 4.3 × 10^?5 °C^?1 to 200 °C at α_a = 17.0 × 10^?5 °C^?1, but the TEC of the b axis was constant at α_b = 0.5 × 10^?5 °C^?1. Like cellulose I_β, cellulose III_I also showed an anisotropic thermal expansion in the lateral direction. The TECs of the a and b axes were α_a = 7.6 × 10^?5 °C^?1 and α_b = 0.8 × 10^?5 °C^?1. The anisotropic thermal expansion behaviors in the lateral direction for I_β and III_I were closely related to the intermolecular hydrogen‐bonding systems. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1095–1102, 2002     

Molecular dynamics simulation of dissociation behavior of various crystalline celluloses treated with hot-compressed water

The dissociation behavior of the crystalline cellulose polymorphs I_β, II, III_I, and IV_I (Cell I_β, etc.) at 503 K and 100 bar was studied by molecular dynamics simulation, and the mechanism of the experimental liquefaction during treatment with hot-compressed water was elucidated. The results showed that the mini-crystals of Cell I_β and Cell IV_I exhibited similar resistance to dissociation, which implies the occurrence of crystal transformation from Cell IV_I to Cell I. On the other hand, the mini-crystal of Cell II gradually dissociated into the water environment with the progress of time in the simulation. The water molecules gradually penetrated the Cell II crystal, especially along the (1 \(\overline{1}\) 0) crystal plane. In contrast, the dissolution behavior differed for the surface and the core areas of the mini-crystal of Cell III_I. The cellulose chains on the surface were dissociated into the water environment, whereas the ordered structure of the chains in the core region was maintained for the entire simulation period. The detailed investigation showed that the core part of Cell III_I was transformed into Cell I at an early stage of the simulation: Cell I is resistant to dissociation of the structure even in the hot-compressed water environment. It can be confirmed that the stability of these four crystals under high temperature and pressure conditions follows the order: Cell II < III_I < IV_I ≈ I_β.     

(E)‐(4‐Hydroxyphenyl)(4‐nitrophenyl)diazene, (E)‐(4‐methoxyphenyl)(4‐nitrophenyl)diazene and (E)‐[4‐(6‐bromohexyloxy)phenyl](4‐cyanophenyl)diazene

Syntheses and X‐ray structural investigations have been carried out for (E)‐(4‐hydroxy­phenyl)(4‐nitro­phenyl)­diazene, C₁₂H₉N₃O₃, (Ia), (E)‐(4‐methoxy­phenyl)(4‐nitro­phenyl)­diazene, C₁₃H₁₁N₃O₃, (IIIa), and (E)‐[4‐(6‐bromo­hexyl­oxy)­phenyl](4‐cyano­phenyl)­diazene, C₁₉H₂₀BrN₃O, (IIIc). In all of these compounds, the mol­ecules are almost planar and the azo­benzene core has a trans geometry. Compound (Ia) contains four and compound (IIIc) contains two independent mol­ecules in the asymmetric unit, both in space group P (No. 2). In compound (Ia), the independent mol­ecules are almost identical, whereas in crystal (IIIc), the two independent mol­ecules differ significantly due to different conformations of the alkyl tails. In the crystals of (Ia) and (IIIa), the mol­ecules are arranged in almost planar sheets. In the crystal of (IIIc), the mol­ecules are packed with a marked separation of the azo­benzene cores and alkyl tails, which is common for the solid crystalline precursors of mesogens.     

IR and Raman spectral and X-ray structural studies of polymorphic forms of sulfamethoxazole

The crystal structure of the polymorphic form III (hemihydrate) of sulfamethoxazole (SMZ) was determined to exist in both the E- and Z-forms by X-ray analysis and was compared with the polymorphic forms I and II which are known to exist in the E-form. IR spectra of I–III and their corresponding forms I_D–III_D which contain the deuterated amino and amido groups and D₂O and Raman spectra of I–III have been measured. For I–III, assignment of the stretching vibration [ν(NH) and ν(CH)] bands of amino and amido groups and the CH bond of the isoxazole ring involved in the inter-molecular hydrogen bonds has been proposed based on consideration of the IR and Raman spectra and the results of X-ray analysis. A relationship was established between the relative intensity and wavenumbers for the ν(CH) band in the inter-molecular C(sp²)H⋯X hydrogen bond of the E-form.     

Influence of the Axial Alkyl Ligand on the Reduction Potential of Alkylcob(III)alamins and Alkylcob(III)yrinates

The irreversible-reduction potentials of 26 alkylcob(III)alamins (RCbl^III 1a – z ) and 26 alkylcob(III)yrinates (R‘Cby’^III; 2a – z ) (E_p 1a – z and E_p 2a – z , resp.) have been measured in situ by single-scan voltammetry of hydroxocob(III)alamin hydrochloride (vitamin B_12b- HCl; 1 ) or heptamethyl cob(II)yrinate perchlorate (ClO₄‘Cby’^II; 2 ) in presence of the corresponding alkyl halides (RX; 3a – z ) in DMF. The reduction potentials of alkylcobalt complexes exhibiting half-life times as short as a few seconds become measurable by this technique. Thermodynamic cycles prove that the observed reduction potentials are closely related to the standard reduction potentials E°(R? Co^III + e^??R? + Co^I). Electron-withdrawing groups and/or an increased degree of substitution at the Co-bound C-atom in RCbl^III and, R‘Cby’^III shift E_p( 1a – z ) and E_p ( 2a – z ) towards positive potentials. Linear correlations have been found between E_p( 1a – z ) (E_p( 2a – z )) of RCbl^III (R‘Cby’^III) and the pK_a of RH (or the Taft σ*- or the Hammett σ-values of R) within each class of R, i. e. MeCbl^III (Me‘Cby’^III), primary RCbl^III (R‘Cby’^III) and secondary RCbl^III (R‘Cby’^III). The correlations allow to distinguish between electronic effects of the Co-bound alkyl residues and their steric interactions with the corrin side chains. The correlations have further been used to visualize the light-induced formal insertion of an olefin into the Co, C-bond of an alkylcobalamin (Scheme 2, 1a → 1u ), a key step in the vitamin-B₁₂-catalized C, C-bond formation.     

Exceptionally Long‐Lived Luminescence Emitted from TbIII Ion Caged in an AgI–TbIII–Thiacalix[4]arene Supramolecular Complex in Water

The compositions and photophysical properties of luminescent ternary complexes of thiacalix[4]arene‐p‐sulfonate (TCAS), Tb^III, and Ag^I ions were determined. At pH 6, Ag^I₂?Tb^III₂?TCAS₂ formed. Moreover, at pH 10, in the presence of a 20‐fold excess of Ag^I and a 50‐fold excess of TCAS with respect to Tb^III, Ag^I₂?Tb^III?TCAS₂ formed as the main luminescent species. The structure of these complexes was proposed: two TCAS ligands are linked by two S–Ag^I–S linkages to adopt a double‐cone supramolecular structure. Furthermore, each Tb^III ion in the former complex accepts O^?, S, O^? donation, whereas in the latter, the Tb^III center accepts eightfold O^? donation. The luminescence quantum yield (Φ) of Ag^I₂?Tb^III₂?TCAS₂ (0.16) was almost equal to that of Tb^III?TCAS, but the luminescence lifetime τ of the former (=1.09 ms) was larger than that of the latter. For Ag^I₂?Tb^III?TCAS₂, the yield Φ (=0.11) was small, which is attributed to the low efficiency of photosensitization (η=0.11). However, the τ value (4.61 ms) was exceptionally large and almost equal to the natural luminescence lifetime of Tb^III (4.7 ms), which is due to the absence of coordinating water molecules (q=0.1). This is compatible with the proposed structure in which the Tb^III ion is shielded by a supramolecular cage that expels coordinated water molecules responsible for luminescence quenching.     

Stoichiometry and stability of cellulose-hydrazine complexes

Highly crystalline cellulose samples from green algae (cellulose I) and mercerized ramie (cellulose II) were treated with anhydrous hydrazine and the resulting complexes were analyzed by synchrotron X-ray diffraction and thermogravimetry. Cellulose I-hydrazine complex could be fully described by a two-chain monoclinic unit cell, a = 0.879 nm, b = 1.076 nm, c = 1.038 nm, and γ = 122.0°, with space group P2₁. Cellulose II-hydrazine complex prepared from mercerized ramie gave a different two-chain monoclinic unit cell, a = 1.042 nm, b = 1.046 nm, c = 1.038 nm, γ = 129.7°, also with space group P2₁. Though having different crystal structures, the number of hydrazine molecules per glucopyranoside residue was 0.82 for cellulose I-complex and 0.93 for cellulose II-complex, probable stoichiometric value of 1.0. Hydrazine could be extracted from the complexes by organic solvents retaining the crystalline orders, resulting in the allomorphic conversion to cellulose III_I and cellulose III_II, both having non-staggered chain arrangements. These features are similar to those of cellulose-ethylenediamine complexes.     

Elastic modulus of the crystalline regions of polyimide derived from poly(amic acid)–biphtalic dianhydride and p-phenylene diamine

The elastic modulus E_l of the crystalline regions in the direction parallel to the chain axis was measured by X-ray diffraction for polyimide derived from polyamic acid (biphthalic dianhydride and p-phenylene diamine). These specimens were cured by two different routes: curing at 200 °C, and at 400 °C for 1 h, respectively (2STEPS), and curing from 80 °C to 400 °C stepwise (nine steps) for 1 h at each step (STEPWISE). The E_l values of 54–169 GPa were obtained for the STEPWISE specimen and 80–178 GPa for the 2STEPS specimen, depending on the meridional reflection employed for measurement of the E_l value. A linear relationship between the E_l value and the fiber identity period was obtained from each meridional diffraction, such that the E_l value increased with an increase in the fiber identity period. The E_l value of the fully extended structure was estimated to be 210 GPa. These are considered to be due to the coexistence of polymorphs with different skeletal structures. The crystalline regions of the 2STEPS specimen seems to consist of a more extended skeleton than those of the STEPWISE specimen. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3294–3301, 1999     

Kinetics of the gas-phase thermal elimination of N-alkyl-pyrazoles

The kinetic study of the gas-phase thermal elimination reactions of N-ethyl-3,5 dimethyl-pyrazole (I), N-ethyl-pyrazole (II), N-sec-butyl-pyrazole (III), and N-tert-butyl-pyrazole (IV) using a flow system is reported. After obtaining activation parameters for I we carried out competitive reactions with II, III and IV using I as internal standard to obtain their E_a. The values of Δ(ΔH) calculated for II, III and IV agree with the little differences in E_a experimentally found.     

Partial crystalline transformation of solvated cellulose IIII crystals, reproduced by theoretical calculations

In hot-water molecular dynamics simulation at 370 K, four cellulose III_I crystal models, with different lattice planes and dimensions, exhibited partial crystalline transformations of (1 ?1 0) chain sheets, in which hydroxymethyl groups were irreversibly rotated from gt into tg conformations, accompanied by hydrogen-bond exchange from the original O3–O6 to cellulose-I-like O2–O6 bonds. The final hydrogen-bond exchange ratio was about 95 % for some of the crystal models after 50 ns simulation. The corrugated (1 ?1 0) chain sheet was converted to a cellulose-I-like flat chain sheet with a slightly right-handed twist. The 3D structures of the three types of isolated chain sheet models were optimized using density functional theory calculations to compare their stabilities without crystal packing forces. The cellulose Iβ (1 0 0) models were more stable than the cellulose III_I (1 ?1 0) models. The optimized structure of cellulose III_I (1 0 0) models deviated largely from the initial sheet form. It was proposed to the crystalline transformation from cellulose III_I to Iβ that conversion of the chain sheet structure first take place, followed by sliding of the chain sheet along the fiber axis.     

The structure of the complex of cellulose I with ethylenediamine by X-ray crystallography and cross-polarization/magic angle spinning 13C nuclear magnetic resonance

X-ray crystallographic and cross-polarization/magic angle spinning ¹³C nuclear magnetic resonance techniques have been used to study an ethylenediamine (EDA)-cellulose I complex, a transient structure in the cellulose I to cellulose III_I conversion. The crystal structure (space group P2 ₁ ; a = 4.546 Å, b = 11.330 Å, c = 10.368 Å and γ = 94.017°) corresponds to a one-chain unit cell with one glucosyl residue in the asymmetric unit, a gt conformation for the hydroxymethyl group, and one EDA molecule per glucosyl residue. Unusually, there are no O–H···O hydrogen bonds between the cellulose chains; the chains are arranged in hydrophobic stacks, stabilized by hydrogen bonds to the amine groups of bridging EDA molecules. This new structure is an example of a complex in which the cellulose chains are isolated from each other, and provides a number of insights into the structural pathway followed during the conversion of cellulose I to cellulose III_I through EDA treatment.     

Hydrolysis behavior of various crystalline celluloses treated by cellulase of Tricoderma viride

Cellobiose and glucose are valuable products that can be obtained from enzymatic hydrolysis of cellulose. This study discusses changes in the crystalline form of celluloses to enhance the production of sugars and examines the effect on structural properties during enzymatic hydrolysis. Various crystalline celluloses consisting of group I (cell I, cell III_I, cell IV_I) and group II (cell II, cell III_II, cell IV_II) of similar DPs were prepared as starting materials. The similar DP values allowed a more direct comparison of the hydrolysis yields. The outcomes were analyzed and evaluated based on the residues and supernatants obtained from the treatment. As a result: (1) action of the cellulase of Trichoderma viride decreased both DP and crystallinity, with greater changes in group II celluloses, (2) the polymorphic interconversion process that occurred for cell III_I, cell IV_I, cell III_II and cell IV_II during the treatment was independent of the enzymatic hydrolysis, thus, the hydrolysis behaviors depended on the starting material of the celluloses, and (3) higher sugar production was obtained from cell III_I and group II. Therefore, the hydrolysis behavior of the various crystalline celluloses depended on the particular polymorph of the starting material.     

A Pedagogical Illustration of the Determination of the Nature and Strength of Bonds in Crystalline Compounds from X-ray Diffraction and Infrared Spectroscopy Studies

This article describes a method used to teach students how X-ray crystallography and infrared spectroscopy analysis can be used to obtain information about the nature and strength of the bonding in the crystalline compounds M^IM^III(SO₄)₂ (with M^I = K⁺, Rb⁺, Cs⁺ and M^III = Al³⁺, Cr³⁺, Fe³⁺). These sulfates form an isomorphic series. The influences of specific M^IM^III ions on the variation of the a and c parameters and on the position of IR absorption bands are described. Additionally, X-ray crystallography and infrared spectroscopy studies of the double sulfates M^IM^III(SO₄)₂ show students the existence of [SO₄-M^III-SO₄]^– layers in the crystallized products; the covalent character of M^III-O attractions, which give cohesion in these layers; the existence of M^I layers between [SO₄-M^III-SO₄]^– layers, and the electrovalent character of M^I-O interactions.     

Elastic modulus of the crystalline regions of chitin and chitosan

The elastic moduli E_l of the crystalline regions of α‐chitin and chitosan in the direction parallel to the chain axis were measured by X‐ray diffraction. The E_l values were 41 GPa for α‐chitin, and 65 GPa for chitosan, respectively, at 20°C. The contracted skeletons of α‐chitin and chitosan are the key factor for the low E_l values compared with that (138 GPa) of cellulose I. The E_l value of α‐chitin was constant at 41 GPa both at −190°C and 150°C, which indicates that the molecular chain of α‐chitin is stable against heat within the temperature and stress range studied. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1191–1196, 1999     

Electrochemical Behavior of a Glassy Carbon Electrode Chemically Modified with Nickel Pentacyanonitrosylferrate in Presence of Sulfur Compounds

The glassy carbon electrode was modified with a nickel pentacyanonitrosylferrate film by electrodeposition of Ni and subsequent derivatization with NaPCNF. The film was characterized by XPS and electrochemical methods. Cyclic voltammetry of the NiPCNF onto the GC shows a redox couple (Fe^III/Fe^II) with E°′ of 538 mV (I_pa/I_pc around 1) and ΔE_p of 93 mV in 0.5 mol L^?1 KNO₃, with a diffusion‐controlled process. There was a decrease of anodic peak currents of the film in the presence of sulfide and 2‐propanethiol due to a precipitation reaction on the film surface by nucleophilic attack.     

TL glow-curve deconvolution functions for various kinetic orders and continuous trap distribution: Acceptance criteria for E and s values

The glow curve deconvolution (GCD) analysis of a composite thermoluminescence (TL) glow curve into its individual glow-peaks needs appropriate equations describing a single glow peak. In the present work, new single glow peak equations are presented, which are produced by transformation of the I(n ₀,E,s,T) and I(n ₀,E,s,b,T) single glow-peak equations into I(I _m,T _m,E,T) and I(I _m,T _m,E,b,T), respectively. Moreover, equations of the forms I(I _m,T _m,w,b,T) are also introduced. The proposed equations have two basic advantages: (1) they use parameters, which are directly obtained from the experimental glow peaks and (2) their accuracy is equal to that of the original thermoluminescence single glow-peak equations.     

A TRICHLORO-BRIDGED DIRUTHENIUM (II,III) COMPLEX: PREPARATION,PROPERTIES AND X-RAY STRUCTURE OF TRI(-μ-CHLORO) DICHLOROCARBONYLTRIS (TRIPHENYLPHOSPHINE)DIRUTHENIUM(II,III)

Abstract

The triply chloro-bridged binuclear complex [Ru₂Cl₅(CO)(PPh₃)₃]·CH₂Cl₂, (PPh₃ = triphenylphosphine), M_r = 1279.23, prepared from the precursor compound [RuCl₃(PPh₃)₂DMA]·DMA (DMA = N,N′-dimethylacetamide) and crystallizes in the monoclinic space group P2₁/c. The structure was solved from 6994 independent reflections for which I > 3σ(I) by Patterson and difference Fourier techniques and refined to a final R = 0.042. The complex is formed by two Ru atoms bridged through three chloride anions. One Ru atom is further coordinated to two non-bridging Cl atoms and a triphenylphosphine ligand, whereas the other is bonded to two PPh₃ ligands and a carbon monoxide molecule. The presence of Ru^III was confirmed by EPR data. The absence of an intervalence charge-transfer transition (IT) in the near-infrared spectrum suggests that the binuclear complex is of a localized valence type. The IR spectrum shows a ν_CO band at 1964cm^? and ν_Ru-Cl bands at 328, 280 cm^?1, corresponding to chlorides at terminal positions and 250, 225 cm^?1 for the bridged ones. Two redox processes, Ru^II/Ru^II (E_1/2 = -0.29 V) ← Ru^II/Ru^III ← (E_1/2 = 1.19 V) Ru^III/Ru^III, were observed by cyclic voltammetry.     

Two polymorphs of 4‐propyl‐3,4‐dihydro‐2H‐naphtho[1,2‐b]pyran‐5,6‐dione

Two polymorphs of the title compound, C₁₆H₁₆O₃, have been obtained from the same solution. One polymorph, (I_m), crystallizes in the monoclinic space group P2₁, while the other, (I_o), crystallizes in the orthorhombic space group P2₁2₁2₁. The cell constants of the two polymorphs are surprisingly similar. Whereas the a and b axes are equal in the two structures, the c axis in (I_o) is twice as long as that in (I_m). The monoclinic angle β is 95.084 (9)° compared with 90° in the orthorhombic crystal system. The cell volume of (I_m) is almost exactly half of the cell volume of (I_o). The packing motifs are also very similar in the two structures. However, whereas the molecules in (I_m) are related by a twofold screw axis just in the direction of the b axis, in (I_o) there are twofold screw axes along all three directions of the unit cell.     

Radiation-Induced Polymerization and Pressure-Volume Behavior of Acrylonitrile at High Pressure

Radiation-induced polymerization and pressure-volume (P-V) measurements of acrylonitrile (AN) were studied up to 8000 kg/cm² in the temperature range of 6–72°C. P-V isotherms of AN have several small breaks, A phase diagram of AN was obtained from the breaking pressures and temperatures. Liquid phases were named L_I, L_II, and L_III, from low to high pressure. The polymerization behavior and volume contraction on polymerization changed in L_I, L_II, and L_III. The difference in entropy between original and activated states decreased with increasing pressure at the same phase, but increased with phase change in L_I to L_II and L_II to L_III. It was concluded from these results and from IR data on PAN that molecular packing of AN in liquid changed in L_I, L_II, and L_III. In L_II and L_III, AN molecules aligned in a less suitable geometry for polymerization than in L_I.