Photodegradation of cellulose acetate fibers

The photodegradation of cellulose acetate fibers by ultraviolet light in vacuo at 77°K and at ambient temperature was studied. Three kinds of light sources with different wavelengths between 2353 and 6000 Å were employed. ESR studies at 77°K show that several kinds of free radicals are produced from cellulose diacetate (CDA) and cellulose triacetate (CTA) fibers when irradiated with light of wavelength shorter than 2800 Å. Among these methyl radicals formed decayed within 210 min at 77°K. When the temperature was raised above 77°K, radical transformation occurred at 87°K and most of the free radicals decayed at 193°K, whereas the cellulosic radicals were stable at this and even at higher temperatures. Ultraviolet spectroscopy studies revealed that the main chromophores are the carbonyl function of the acetyl group and acetal groups in the polymer. The photodegradation of the polymers at ambient temperature resulted in the formation of gaseous products (mainly CO, CO₂, and CH₄), together with the loss of bound acetic acid content and sample weight. Decreases in viscosity and reduction of tensile strength and elongation were also observed in the irradiated samples, revealing that the overt effects of ultraviolet light on cellulose acetate fibers are interpreted in terms of free-radical reactions ultimately leading to main-chain and side-group scissions, unsaturation, and the formation of small molecule fragments. Among these, main-chain scission took place predominantly in CDA fiber and side-group scission in CTA fiber. The mechanism of the fundamental photochemical degradation processes of cellulose acetate fibers is elucidated.     

Bond length of perchlorate at different temperatures: X‐ray and neutron comparison

The averages (average deviations from the mean are given in square brackets) of uncorrected Cl—O bond distances in a perchlorate anion from an X‐ray diffraction analysis of (N‐{2‐[bis(pyridin‐2‐ylmethyl)amino]ethyl}pyridine‐2‐carboxamidato)(nitric oxide)manganese perchlorate acetonitrile disolvate, [Mn(C₂₀H₂₀N₅O)(NO)]ClO₄·2CH₃CN or [Mn(PaPy₃)(NO)]ClO₄·2CH₃CN, decrease from 1.447 [4] Å at 10 K to 1.428 [4] Å at 170 K. The 10 K value is close to the neutron value (1.441 [1] Å) at 18 K. Comparisons are made with a second X‐ray study at 30 K [1.444 (8) Å] and to libration‐corrected, density functional theory (DFT), and Cambridge Structural Database (CSD) values.     

ESR single-crystal study of radical and radical-pair formation in some γ-irradiated vinyl monomers

Single crystals of methyl methacrylate (MMA), methyl acrylate (MA), and acrolein (A) have been prepared by a low-temperature technique. After irradiation with γ-rays at 77°K the paramagnetic species were identified by ESR spectroscopy. MMA gave a seven-line single spectrum from radicals formed by hydrogen addition. The hyperfine coupling constants are slightly anisotropic with a mean value of 22 G. Radical pairs were observed as ΔM_s = 1 and ΔM_s = 2 transitions; the hyperfine coupling was 11 G. From the strongly anisotropic dipolar interaction, upper limits for the distances between the pair components were calculated to be 5.45 Å and 6.3 Å. MA gave a five-line main spectrum with the same hyperfine coupling values and two radical pairs, one with a distance 5.9 Å between the components. In a there was also a strongly anisotropic interaction. The hyperfine coupling of the ΔM_s = 2 transition was 9.8 G. The number of radical pairs compared to the total number of radicals increases only slightly with the radiation dose. This makes it likely that pair formation occurs in the spurs and blobs formed by the γ-radiation. At an increased temperature the radical pairs disappeared; the spectrum of MMA changed to that characteristic of propagating polymer radicals.     

Formation of free radicals in photoirradiated cellulose. I. Effect of wavelength

Formation of free radicals in photoirradiated cellulose has been studied by means of ESR spectroscopy at 77°K. Three kinds of light sources with different wavelengths between 2500 and 4000 ÅR were employed. No radicals detectable by ESR were recorded when cellulose was irradiated with light of wavelength longer than 3300–3400 ÅR. Hydrogen atoms that generated a doublet spectrum (ΔH = 508 G) were observed when cellulose was irradiated with light longer than 2800 ÅR. Hydrogen atoms and formly radicals that generated doublet spectra with splitting constants of 508 and 129 G, respectively, were observed when irradiated with light shorter than 2800 ÅR. The scission of the polymer chain in cellulose is evident from decrease of the degree of polymerization, and the results of mass spectrometric analysis indicated H₂, CO, CO₂, and H₂O to be the main volatile products of cellulose upon photoirradiation.     

Formation of free radicals in photoirradiated cellulose. VII. Radical decay

The ESR spectra of untreated and photosensitized celluloses irradiated with three different ultraviolet light sources, i.e., λ > 2537 Å, λ > 2800 Å, λ > 3400 Å, at 77°K under vacuum were studied. Based on the warm-up process, that is, warming the sample from 77°K to 273°K for a certain time and recorded at 77°K, the decay behavior of free radicals of celluloses was examined for changes of the pattern and the intensities of ESR spectra. For the untreated samples irradiated with light of λ > 2537 Å and λ > 2800 Å, beside the two doublet spectra originating from hydrogen atoms (508 gauss splitting) and formyl radicals (129 gauss splitting), the observed sevenline spectrum was resolved to be a superposition of a singlet (ΔH_msl = 16 gauss), a doublet (24 gauss splitting), a triplet (34 gauss splitting), and a quartet (overall width, 88 gauss) spectrum. For the photosensitized samples irradiated with light of λ > 3400 Å, the 1:1:1 three-line spectrum was resolved to be a superposition of a singlet (ΔH_msl = 27 gauss), a doublet (43 gauss splitting), and a triplet (34 gauss splitting) spectrum. The five-line spectra of the photosensitized samples irradiated with light of λ > 2537 Å and λ > 2800 Å were resolved to be a superposition of a singlet (ΔH_msl = 27 gauss), a doublet (43 gauss splitting), and a triplet (34 gauss splitting) spectrum. Based on these findings, the conclusion was drawn that at least six kinds of spectra, generated from six kinds of radical species, were formed in cellulose irradiated with ultraviolet light under appropriate experimental conditions.     

γ-radiolysis of acetylated cotton cellulose: An ESR study

The nature and behavior of free radicals induced in acetylated cotton celluloses irradiated with γ-rays have been studied by electron spin resonance (ESR) spectroscopy. Dehydrogenation and deacetylation appear to be responsible for the free radicals observed from samples irradiated at 77°K. The degree of substitution enhanced the yield of acetyl radicals when the samples were irradiated at 77°K and adversely affected the overall radical concentration when irradiation was done at 300°K. In addition, the ESR spectra of samples irradiated under vacuum at 300°K were more intense than those obtained from samples irradiated in air. The nature, yield, and post-irradiation behavior of the primary radicals are discussed in the light of the ultimate chemical effects observed.     

Resonance Raman and luminescence studies on the quasi-one dimensional crystals of [Pt(en)z][Pt(en)2Cl2](ClO4)4

Resonance Raman scattering, infrared luminescence and absorption spectra of [Pt(en)₂][Pt(en)₂Cl₂](ClO₄)₄ single crystals have been measured for polarized light at 2 K and room temperature. Contrary to the previous works on polycrystals, the incident photon energy dependence of the Raman cross section due to the symmetric ClPt^IVCl stretching mode has a resonance peak at the charge-transfer absorption edge, which is well explained with the two-band model in a one-dimensional system. Two characteristic structures observed below the absorption edge are concluded not to be related to the charge-transfer transition by measurements of the excitation spectra both of the Raman scattering and of the luminescence. The origin of this luminescence is also discussed.     

Electron spin resonance study of γ-irradiated poly(methacrylic acid)

Low temperature relaxations in poly(methacrylic acid) (PMAA) have been studied by electron spin resonance (ESR) spectroscopy. The observed 8 line ESR spectra of irradiated PMAA in the temperature range 77-300K (LNT-RT) is attributed to the free radicals of the type ～ CH₂? CH? CH₃. Assignment of ESR spectra to free radicals has been made on the basis of magnetic parameters employed to simulate ESR spectra at different temperatures. Further, ESR spectra below LNT have been simulated, using the set of parameters employed to simulate the experimental spectrum at LNT. Magnetic parameters of the ESR spectra at LNT and below LNT indicate γ- and δ-relaxations of PMMA chains. © 1994 John Wiley & Sons, Inc.     

Electron spin resonance studies of polycarbonate irradiated by γ-rays and ultraviolet light

Paramagnetic species produced in polycarbonate (PC) by γ- or ultraviolet irradiation were investigated by ESR. In γ-irradiation, scissions of carbonate groups in the main chain occur. ESR spectra (g = 2.003₄) composed of a sharp singlet, some broad singlets, and a small signal with hyperfine structure are obtained, and they are assigned to trapped electrons, positive radical ions, phenoxy-type free radicals, phenyl radicals, and ? O? C₆H₄? C(CH₃)₂ radicals. The G value for total yields of paramagnetic species at 77°K is 1.8. The percentage of CO and CO₂, the dominant gases evolved, is 65.4 and 33.8%, respectively. In ultraviolet irradiation, energy is absorbed selectively at the surface region. The surface region becomes insoluble in methylene chloride because of crosslinking of phenyl groups. The ESR spectrum obtained at 77°K is a broad singlet and assigned to phenoxy-type free radicals, phenyl radicals, and polyenyl-type free radicals. Some differences in effects of γ- and ultraviolet irradiation of PC are discussed.     

[Bis(2‐diphenylphosphinoethyl) sulfide‐κ3P,S,P′](triphenylphosphine‐κP)platinum(II) diperchlorate acetone solvate

In the title compound, [Pt(C₁₈H₁₅P)(C₂₈H₂₈P₂S)]­(ClO₄)₂·­C₃H₆O or [Pt(PPh₃)(PSP)](ClO₄)₂·CH₃COCH₃, where PSP is the potentially tridentate chelate ligand bis(2‐di­phenyl­phosphinoethyl) sulfide, all three donor groups of the PSP ligand are coordinated to the central Pt atom, with Pt—P = 2.310 (1) Å and Pt—S = 2.343 (1) Å. The fourth coordination site is occupied by the P donor of the tri­phenyl­phosphine ligand [Pt—P = 2.289 (1) Å]. The complex cation has exact mirror symmetry, with the S atom, the Pt atom and the P atom of the PPh₃ ligand in the mirror plane. The Pt atom has a distorted square‐planar coordination geometry. A π–π interaction is present between the phenyl rings of the PPh₃ ligand and the terminal –PPh₂ group of the PSP chelate.     

A low temperature phase transition in NaOD near 150 K

A phase transition in NaOD has been observed calorimetrically near 150 K. The estimated enthalpy of the transition is 200(75) $\text{Joules}\text{mole}$. This phase transition has no corresponding analog in the protonated material, NaOH, down to 10 K. Other physical techniques also reveal the phase transition but give slightly varying values for the transition temperature. These techniques include ²³Na nuclear quadrupole resonance, ²³Na nuclear magnetic resonance and ²D magnetic resonance. The resonance experiments indicate that the phase transition is a continuous one and that in the lower temperature phase there is one sodium and deuterium crystal Iographic site per cell. X-ray powder diffraction of the low temperature phase indicates that it is probably monoclinic with a_m = 3.388(1)A, b_m = 11.34(1)A, c_m = 3.418(1)A,β = 90.52° and V = 131 .4ų. The volume of the unit cell is greater at the lower temperature than at the higher temperature. The ²³Na pure quadrupole resonance frequency at 77 K is 1.784(2) MHz with an asymmetry parameter of 0.82(5).     

ESR study of the structure of intercalation compounds of fluorinated graphite and processes occurring therein using No2 and ClO2 radicals as spin probes

For intercalation compounds of fluorinated graphite at 77–300 K, the parameters of the spin Hamiltonian are determined from the angular dependence of the ESR spectra, the NO₂ and ClO₂ radicals are located, and their mobility at low temperatures is studied. At 77–200 K, vibrational and rotational motions around the oxygen-oxygen axis are defreezed. This is attributed to donor-acceptor interactions between the negatively charged oxygen atoms of NO₂ and ClO₂ radicals and the positively charged carbon atoms of fluorinated graphite. The calculated activation energies (0.35 kcal/mole for ClO₂ and 0.6 kcal/mole for NO₂) for defreezing the rotational-vibrational motions of these two radicals are close. Using NO₂ radicals as spin probes allowed us to find two types of regions in ICFG, which differ in the rate of NO₂ diffusion. These regions are supposed to be the filled and unfilled parts of graphite fluoride layers. It is attempted to explain the low pressure of the saturated vapor of the intercalate over ICFG by the presence of unfilled layers near the boundaries of ICFG flakes. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 2, pp. 253–260, March–April, 1998.     

A spectroscopic and ab initio study on Bi(III) complex formation with 3-mercaptopropanesulfonic acid

This article deals with complex formation of Bi(III) with 3-mercaptopropanesulfonic acid (H₂MPS) in aqueous perchloric acid solutions, with synthesis and characterization of a solid 3-mercaptopropanesulfonate complex of bismuth(III). The stoichiometry and structures of Bi-MPS species in aqueous solution and of a solid complex have been studied by UV–Vis, ¹H-NMR, ICP-AES, Raman, and EXAFS spectroscopic methods; the structures have also been simulated with DFT/PBE0 calculations. The Bi(III) L_III-edge EXAFS oscillation for a solid compound with the empirical formula [Bi(HMPS)₂(ClO₄)]⁰ was simulated with two Bi–S interatomic distances at 2.50 ± 0.01 Å, two Bi–O distances at 2.56 ± 0.02 Å and two Bi–O distances at 2.75 ± 0.02 Å. Implementation of the same approach for aqueous solutions on the assumption of S₃BiO₃ coordination at the H₂MPS?:?Bi(III) mole ratio ≥ 3.0 revealed three Bi–S bonds at 2.53 ± 0.02 Å and three Bi–O bonds at 2.68 ± 0.02 Å, respectively. Optimized geometries, electronic structures of Bi(HMPS)₃ and [Bi(HMPS)₂ClO₄]⁰, vibrational properties of [Bi(HMPS)₂ClO₄]⁰, and electronic absorption spectrum of Bi(HMPS)₃ species obtained by DFT and TD–DFT modeling are consistent with empirical parameters. In the UV–Vis spectrum of Bi(HMPS)₃ the LMCT and MLCT S^2? ? Bi³⁺ band appears at 268 nm.     

The 3[ndσ*(n+1)pσ] Emissions of Linear Silver(I) and Gold(I) Chains with Bridging Phosphine Ligands

The complexes [Au₃(dcmp)₂][X]₃ {dcmp=bis(dicyclohexylphosphinomethyl)cyclohexylphosphine; X=Cl^? ( 1 ), ClO₄^? ( 2 ), OTf^? ( 3 ), PF₆^? ( 4 ), SCN^?( 5 )}, [Ag₃(dcmp)₂][ClO₄]₃ ( 6 ), and [Ag₃(dcmp)₂Cl₂][ClO₄] ( 7 ) were prepared and their structures were determined by X‐ray crystallography. Complexes 2 – 4 display a high‐energy emission band with λ_max at 442–452 nm, whereas 1 and 5 display a low‐energy emission with λ_max at 558–634 nm in both solid state and in dichloromethane at 298 K. The former is assigned to the ³[5dσ*6pσ] excited state of [Au₃(dcmp)₂]³⁺, whereas the latter is attributed to an exciplex formed between the ³[5dσ*6pσ] excited state of [Au₃(dcmp)₂]³⁺ and the counterions. In solid state, complex [Ag₃(dcmp)₂][ClO₄]₃ ( 6 ) displays an intense emission band at 375 nm with a Stokes shift of ≈7200 cm^?1 from the ¹[4dσ*→5pσ] absorption band at 295 nm. The 375 nm emission band is assigned to the emission directly from the ³[4dσ^*5pσ] excited state of 6 . Density functional theory (DFT) calculations revealed that the absorption and emission energies are inversely proportional to the number of metal ions (n) in polynuclear Au^I and Ag^I linear chain complexes without close metal???anion contacts. The emission energies are extrapolated to be 715 and 446 nm for the infinite linear Au^I and Ag^I chains, respectively, at metal???metal distances of about 2.93–3.02 Å. A QM/MM calculation on the model [Au₃(dcmp)₂Cl₂]⁺ system, with Au???Cl contacts of 2.90–3.10 Å, gave optimized Au???Au distances of 2.99–3.11 Å in its lowest triplet excited state and the emission energies were calculated to be at approximately 600–690 nm, which are assigned to a three‐coordinate Au^I site with its spectroscopic properties affected by Au^I???Au^I interactions.     

A picosecond transient absorption study of the intramolecular excited state proton transfer in 3-hydroxyflavone: assignment of the observed bands

The time-resolved transient absorption and gain of 3-hydroxyflavone in ethanol ( T = 300 and 173 K) and MCH ( T = 300 and 198 K) has been investigated. Two absorption bands (A and B) and two gain bands (E₁ and E₂) have been observed. Band A (lying in the 5200 Å spectral region) appears during and just after the excitation step and is assigned to the normal excited form N_x Band B (lying in the 6050 Å spectral region) appears after band A and is assigned to the tautomer form T. A gain band E₂ (lying at λ=5400 Å) appears at the same time as the B band and is attributed to the T→T emission. A gain band E₁ (lying at λ = 5730 Å) appears earlier than the E₂ band. Its origin is discussed and two possible assignments are presented: vibronic progression of the emission spectrum of T or formation of an intermediate species on the proton transfer pathway from N_x to T.     

Formation and decay of trifluoromethyl radicals during photodecomposition of the long-lived perfluoro-2,4-dimethyl-3-ethylpent-3-yl radical

UV irradiation of the long-lived radical [(CF₃)₂CF]₂C^·C₂F₅ (1) in a hexafluoropropylene trimer (HFPT) glassy matrix at 77 K and in a HFPT solution at 300 K leads to its decomposition to the ^·CF₃ radical and perfluoroolefin molecule. About 90% of the ^·CF₃ radicals formed recombine at 300 K. The remaining radicals add to the HFPT molecules generating the long-lived radicals [(CF₃)₂CF]₃C^·. Unlike the ^·CF₃ radicals produced by the photodecomposition of radicals 1, the ^·CF₃ radicals formed during radiolysis of HFPT are not stabilized in the glassy HFPT matrix at 77 K.     

Neutronenpulverdiffraktionsmessungen an [Zn(ND3)4]I2 bei 1,5 K, 10 K und 293 K: Wasserstoff-Brückenbindungen und Bewegungen von ND3-Molekülen

Neutron Powder Diffraction Measurements on [Zn(ND₃)₄]I₂ at 1.5 K, 10 K, and 293 K: Hydrogen Bonds and Dynamic of ND₃ Molecules Microcrystalline powder of [Zn(ND₃)₄]I₂ can be prepared by the reaction of gaseous NH₃ with dry ZnI₂ at room temperature within 8 h. Neutron powder diffraction measurements at 1.5 K, 10 K and 293 K were used to localize all hydrogen atoms. Isolated [Zn(ND₃)₄]²⁺ tetrahedra are three dimensionally linked with 2- and 3-centre (bent and bifurcated) N? D …? I^?-hydrogen bonds. Ammonia molecules are ordered at 1.5 K. Room temperature high thermal displacement parameters for D hint to the fact that NH₃-dynamics take place. Lattice parameters 300 K [10 K; 1,5 K]: a = 10.3783(8) Å [10.3407(4) Å; 10.3381(5)], b = 7.5239(6) Å [7.3960(2) Å; 7.3935(4) Å], c = 13.088(1) Å [12.9731(4) Å; 12.9695(6) Å], space group: Pnma.     

Cu3BiS3: Two-Dimensional Coordination Induces Out-of-Plane Phonon Scattering Enabling Ultralow Thermal Conductivity

The discovery of compounds with low thermal conductivity and the understanding of their microscopic mechanisms are of great challenges and scientific significance. Herein, we report a unique ternary sulfide compound, Cu₃BiS₃, in which all Cu atoms are coordinated within a two-dimensional [CuS₃] triangle plane. This local coordination leads to efficient out-of-plane phonon scattering and an ultralow thermal conductivity. Through DFT phonon spectrum calculations and analyses, we reveal that the lowest vibration frequency decreases from 2 THz for high-dimensional [CuS₄] tetrahedral coordinated Cu atoms in CuBiS₂ (CN=4, with an average Cu−S bond length of 2.328 Å) to 1.5 THz for low-dimensional [CuS₃] triangular coordinated Cu atoms in Cu₃BiS₃ (CN=3, with a shorter Cu−S bond length of 2.285 Å). This is due to the out-of-plane thermal vibration of the Cu atoms in the latter. Consequently,Cu₃BiS₃ exhibits one of the lowest values of κ_lat (0.32 W/m K) among its peer, with a 36 % reduction compared to CuBiS₂ (0.50 W/m K). This groundbreaking discovery highlights the significant role of 2D local coordination in reducing thermal conductivity through characteristic out-of-plane phonon scattering, while also contributing to a large Grüneisen parameter (2.06) in Cu₃BiS₃.     

Chemically activated 3-methyl-l-butene and 2-methyl-l-butene from photolysis of diazomethane–isobutene–neopentane–oxygen mixtures

An experimental study of the decomposition kinetics of chemically activated 2-methyl-l-butene and 3-methyl-l-butene produced from photolysis of diazomethane-isobutene-neopentane-oxygen mixtures is reported. The experimental rate constants for 3-methyl-l-butene decomposition were 1.74 ± 0.44 × 10⁸ sec^?1 and 1.01 ± 0.25 × 10⁸ sec^?1 at 3660 and 4358 Å, respectively. 2-Methyl-l-butene experimental decomposition rate constants were found to be 5.94 ± 0.59 × 10⁷ sec^?1 at 3660 Å and 3.42 ± 0.34 × 10⁷ sec^?1 at 4358 Å. Activated complex structures giving Arrhenius A-factors calculated from absolute rate theory of 10^{16.6 ± 0.5} sec^?1 for 3-methyl-l-butene and 10^{16.2 ± 0.4} sec^?1 for 2-methyl-l-butene, both calculated at 1000°K, were required to fit RRKM theory calculated rate constants to the experimental rate constants at reasonable E₀ and E* values. Corrected calculations (adjusted E₀ values) on previous results for 2-pentene decomposition gave an Arrhenius A-factor of 10^{16.45 ± 0.35} sec^?1 at 1000°K. The predicted A-factors for these three alkene decompositions giving resonance-stabilized methylully radicals are in good internal agreement. The fact that these A-factors are only slightly less than those for related alkane decompositions indicates that methylallylic resonance in the decomposition products leads to only a small amount of tightening in the corresponding activated complexes. This tightening is a significantly smaller factor than the large reduction in the critical energy due to resonance stabilization.     

Neutron powder profile studies of the gamma uranium trioxide phases

Neutron powder profile studies show the existence of three phases in gamma uranium trioxide between 373°K and 77°K. The three phases are closely related and the transitions smooth and displacive. At 373°K, γ-UO₃ is tetragonal, with a = 6.9013 (5) and c = 19.9754 (18) Å, and space group $\text{I4}{}_1\text{amd(}\text{D}{}^{19}{}_{\mathrm{4h}}\text{)}$. At 323°K, γ-UO₃ becomes orthorhombic, space group F_ddd(D²⁴_2h), with the cell dimensions (293°K) a = 9.787 (3), b = 19.932 (4) and c = 9.705 (3) Å. There is a further transition between 293°K and 77°K, and, at 77°K, the orthorhombic dimensions of the pseudocell are a = 9.8225 (7), b = 19.8487 (15), and c = 9.6318 (7) Å. The neutron diffraction studies show that, in all three phases, the coordination polyhedra of the two crystallographically distinct uranium atoms are octahedral and (dodecahedral-2) respectively. At 293°K, the shortest UO distance is 1.796 (6) Å, and thus there are no pure uranyl bonds, in agreement with the infrared spectrum. The UO distances are precise to about ± 0.006 Å, about ten times the precision of an earlier X-ray single-crystal study, in which the conclusions were in conflict with the infrared spectrum. The structure is made up of parallel chains of edge-fused U(2) octahedra, cross-linked by U(1) dodecahedra. The atomic shifts are not great in going from 373°K to 77°K; at 293°K the data will refine in the pseudotetragonal cell as well as the true orthorhombic cell, and the 77°K data will refine in the F_ddd cell.