The crystal structure of K6[CdO4] and Rb2CdO2 has been determined from single crystal X-ray diffraction data and refined toR=0.058 (K6[CdO4]) andR=0.088 (Rb2CdO2). K6[CdO4] crystallizes hexagonal, space group P63mc with lattice constantsa=867.42 (6),c=665.5 (1) pm,c/a=0.767 andZ=2. It is isotypic with Na6[ZnO4]. Rb2CdO2 is orthorhombic, space group Pbcn witha=1045.0 (2),b=629.1 (1),c=618.3 (1) pm,Z=4, and crystallizes with the K2CdO2 structure type. The crystal structures can be deduced from the motif of a closest packed arrangement of O2– with hexagonal (K6[CdO4]) or cubic (Rb2CdO2) stacking. The tetrahedra occupied by Cd2+ are isolated (K6[CdO4]) or edge-shared (formation of infinite SiS2-like chains [CdO4/2]) (Rb2CdO2). The powder diffraction pattern of Rb6[CdO4] [a=906.6 (1),c=694.3 (1) pm] and Rb2Cd2O3 [a=642.6 (2),b=679.0 (1),c=667.9 (2) pm, =115.2 (1)] confirm isotypie with K6[CdO4] and K2Cd2O3 respectively.
Herrn Prof. Dr.Gutman zum 65. Geburtstag gewidmet. 相似文献
The polymerization of acrylonitrile (AN) in aqueous nitric acid initiated by " cage " vanadyl polycarboxylate (P=VO)-thiourea (TU) complex was investigated. The overall rate ofpolymerization isThe relationship between the induction period (τ) and the temperature of polymerization as well as the concentrations of reactants can be expressed as follows :The molecular weight of polyacrylonitrile increases with increasing monomer concentration and decreases with increasing temperature of polymerization and concentrations of vanadyl polycarboxylate and thioureaThe polymerization mechanism was proposed and discussed. 相似文献
Pulsed‐laser induced polymerization is modeled via an approach presented in a previous paper.[1] An equation for the time dependence of free‐radical concentration is derived. It is shown that the termination rate coefficient may vary significantly as a function of time after applying the laser pulse despite of the fact that the change in monomer concentration during one experiment is negligible. For the limiting case of t ≫ c–1 (kpM)–1, where c is a dimensionless chain‐transfer constant, kp the propagation rate coefficient and M the monomer concentration, an analytical expression for kt is derived. It is also shown that time‐resolved single pulse‐laser polymerization (SP–PLP) experiments can yield the parameters that allow the modeling of kt in quasi‐stationary polymerization. The influence of inhibitors is also considered. The conditions are analyzed under which M (t) curves recorded at different extents of laser‐induced photo‐initiator decomposition intersect. It is shown that such type of behavior is associated with a chain‐length dependence of kt. 相似文献
The kintetic studies of polymerization of methyl methacrylate initiated with nickel ethyl acetoacetate gave the following equations. Rp=K1[Monomer]1.4[Chelate]0.5, in the absence ox aniline Rp=K2[Monomer]1.2[Chelate]0.5 [Aniline]0,5, in the presence of aniline. Some aromatic amines such as aniline markedly accelerated the polymerization, while pyridine had no such effect. The activation energy of initiation was 24.8 kcal/mol in the absence of aniline, and 8.8 kcal/mol in the presence of aniline. Both the kinetic data and the infrared spectra of the polymer indicated that the polymerization was radical in nature. The reaction order against monomer varied, as the temperature differed. The spectrophotometric investigation indicated formation of a complex between the chelate and the monomer, or amines. It also showed that the formation of a complex was not a factor which controlled the rate of initiation. A scheme of initiation mechanism was presented on the basis of the above experimental evidence. 相似文献
The ex‐situ qualitative study of the kinetic formation of the poly‐oxo cluster U38, has been investigated after the solvothermal reaction. The resulting products have been characterized by means of powder XRD and scanning electron microscopy (SEM) for the solid phase and UV/Vis, X‐ray absorption near edge structure (XANES), extended X‐ray absorption fine structure (EXAFS), and NMR spectroscopies for the supernatant liquid phase. The analysis of the different synthesis batches, stopped at different reaction times, revealed the formation of spherical crystallites of UO2 from t=3 h, after the formation of unknown solid phases at an early stage. The crystallization of U38 occurred from t=4 h at the expense of UO2, and is completed after t=8 h. Starting from pure uranium(IV) species in solution (t=0–1 h), oxidation reactions are observed with a UIV/UVI ratio of 70:30 for t=1–3 h. Then, the ratio is inversed with a UIV/UVI ratio of 25/75, when the precipitation of UO2 occurs. Thorough SEM observations of the U38 crystallites showed that the UO2 aggregates are embedded within. This may indicate that UO2 acts as reservoir of uranium(IV), for the formation of U38, stabilized by benzoate and THF ligands. During the early stages of the U38 crystallization, a transient crystallized phase appeared at t=4 h. Its crystal structure revealed a new dodecanuclear moiety (U12), based on the inner hexanuclear core of {U6O8} type, decorated by three additional pairs of dinuclear U2 units. The U12 motif is stabilized by benzoate, oxalates, and glycolate ligands. 相似文献
Two Keggin-type heteropolyanions were synthesized and characterized by X-ray crystal structure and elemental analysis as well as infrared spectroscopy. Both K3[VMo12O40]19H2O (1) and [Ni(H2O)6][H3PMoVMoVI11O40]230 H2O (2) were prepared in aqueous solution. Compound 1 crystallized in the space group Pm-3m, a=10.6513(1) Å, V=1208.4(3) Å3, Z=1. Compound 2 crystallized in the space group R-3 with a=b=13.9669(2) Å, c=42.0075(5) Å, V=7096.71(2) Å3, Z=3. The compound 1 contains a {K6VMo12O40} group in which six potassium ions form a regular {K6} octahedron. The heteropolyanion [VMo12O40]3– was capped by six potassium ions and enclosed by {K6} octahedron. A three-dimensional structure was formed by the buildup of {K3[VMo12O40]}n. Compound 2 contains a one-electron reduced heteropolyanion [H3PMoVMoVI11O40]1–. Ni2+ coordinated by six water molecules as the counter cation balances the negative charge of the molecule. 相似文献
The kinetics of oxidation-reduction reaction between N,N-diethylhydroxylamine (DEHAN) and nitrous acid in nitric acid solution
have been studied by spectrophotometry at 9.5°C. The rate equation is −d[HNO2]/dt=K[HNO2]·[DEHAN][HNO3] and the rate constantK=12.81 (mol/l)−2·min−1. A possible mechanism has been suggested on the basis of chemical analysis and Raman spectra. The activation energyE and the thermodynamic functions ΔH#, ΔG# and ΔS# are also calculated. 相似文献
Abstract The chain transfer constant of the polymethyl methacrylate radical for N,N-dimethylaniline was determined in two solvents, benzene and dimethyl phthalate. Plots were made using1/Pn=kt°Rp/kp2[M]2η + CS1 [S1]/[M] + CS2 [S2]/[M] +CM where η=viscosity of monomer-solvents mixture, kt°=rate coefficient of termination when η=1 cP, S1=benzene or dimethyl phthalate, S2=N,N-dimethylaniline, and other symbols have their usual meanings. The plots agreed well for the two solvents. If the plots were made without considering the viscosity term, two separate lines resulted for the two solvents. Thus it is essential to consider the viscosity of the polymerizing system in the analysis of chain transfer reactions when the termination reaction is diffusion-controlled and the viscosities of the monomer and solvent differ markedly. 相似文献
The polymerization kinetics in water of acrylylglycinamide (AG) initiated by K2S2O8 was studied over the temperature range 40.0 to 60.0°C. Monomer concentration was varied from 7.8 × 10?3 to 31.2 × 10?3M and catalyst from 1.85 × to 11.10 × 10?5M. The rate expression is ?d[M]/dt = Rp, = k1.22[K2S2O8]0.5[M]1.22, and the overall empirical rate constant, k1.22 = 1.14 × 1011e?15,800/RT 1.0.72 mole?0.72 min?1. To explain the dependence on monomer, a kinetic scheme which includes a bimolecular reaction (k2) between monomer and initiator is suggested. The simplified expression which describes the initial rate of polymerization is: ?d[M]/dt = Rp, = k4(2[I]/k5)1/2[M](k1 + k2[M])1/2, where k1, k2, k4 and k5 are rate constants for S2O8 = decomposition, a bimolecular reaction between monomer and initiator, propagation, and termination, respectively. Individual bimolecular rate constants are expressed in liter/mole-min. The equation predicts a dependence on monomer concentration between 1.0 and 1.5 with 1.5 being approached a t high monomer concentrations. Plots of RP2/[M]2 versus [M] are linear, as predicted by the postulated reaction route and values for k2 and k4/k51/2 were obtained from the slopes and intercepts of these plots. The temperature dependence of the bimolecular monomer-initiator reaction is k2 = 5.19 × 1021e?36,000/RT. Instead of the usual behavior, the k4/k51/2 ratio was found to decrease with temperature and the difference of activation energies, (E4 ? E5/2), is ?1.50 kcal. The temperature dependence of the propagation to square root of the termination rate constant ratio is k4/k51/2 = 6.16e1500/RT. These rather unusual results may be related to the ability of AG polymers in water to form thermally reversible gels; even above the gel melting points, the polymers are considerably aggregated in solution. This would tend to make the bimolecular termination reaction more temperature dependent and also account for the high values (59–69) for the k4/k51/2 ratios. For similar temperatures, the overall rate constants for AG are approximately four times those for acrylamide. 相似文献
The reaction of tert.-butyl carbodiimide with one equivalent of LiNHtBu in tetrahydrofuran at-78 °C produces {Li[C(NtBu)2(HNtBu)]}2-(THF) (1), which is an eight-membered Li2C2N4 ring; the deprotonation of (1) with two equivalents of n-BuLi in tetrahydrofuran at -78 °C and recrystallisation of the product from n-pentane yielded the unsolvated dimer {Li2[C(NtBu)3]}2 (2), which adopts the structure of a distorted hexagonal prism. 相似文献
The oxidation of [CoII(nta)(ox)(H2O)2]3− and [CoII(nta)(ph)(H2O)2]3− (nta = nitrilotriacetate, ox = oxalic acid and ph = phthalic acid) by periodate have been studied kinetically in aqueous
solution over 20–40 °C and a variety of pH ranges. The rate of oxidation of [CoII(nta)(ox)(H2O)2]3− by periodate, obeys the following equation: d[CoIII]/dt = [CoII(nta)(ox)(H2O)23−][H5IO6] {k4K5 + (k5K6K2/[H+]} while the reaction of [CoII(nta)(ph)(H2O)2]3− with periodate in aqueous acidic medium obeys the following rate law: d[CoIII]/dt = k6K8[CoII]T [IVII]T/{1 + [H+]/K7 + K8[IVII]T}. Initial cobalt(III) products were formed and slowly converted to final products, fitting an inner-sphere mechanism. Thermodynamic
activation parameters have been calculated. A common mechanism for the oxidation of ternary nitrilotriacetatocobalt(II) complexes
by periodate is proposed and supported by an excellent isokinetic relationship between ΔH* and ΔS* values for these reactions. 相似文献
This work was aimed at studying variations in the termination mechanism occurring during the after‐effects of a light‐induced polymerization of a dimethacrylate monomer after the irradiation had been discontinued. The experimental method was based on differential scanning calorimetry. The initiation was stopped at various moments of the reaction corresponding to different degrees of double‐bond conversion (starting conversions). Three termination models: monomolecular, bimolecular, and mixed were used to calculate the ratio of the bimolecular termination and propagation rate coefficients ktb/kp and/or the monomolecular termination rate coefficient ktm. The models were determined over short time intervals (conversion increments) of the dark reaction giving different values of rate coefficients for each time interval (interval approximation method). Two‐stage statistical analysis was used to find the model that best reproduced the experimental data obtained for each conversion increment. This enabled variations in the termination mechanism during the after‐effects to be followed. It was found that the termination mechanism changed with the time of the dark reaction from the bimolecular reaction to the mixed reaction when the light was cut off at low and medium double‐bond conversions. At higher starting conversions a monomolecular termination mechanism dominated from the beginning of the dark reaction. The mixed termination model was the only model to describe correctly the variations of rate coefficients in the dark, i. e., the increase in ktm and the decreasein the ktb/kp ratio. 相似文献
Summary: The range of validity of two popular versions of the nitroxide quasi‐equilibrium (NQE) approximation used in the theory of kinetics of alkoxyamine mediated styrene polymerization, are systematically tested by simulation comparing the approximate and exact solutions of the equations describing the system. The validity of the different versions of the NQE approximation is analyzed in terms of the relative magnitude of (dN/dt)/(dP/dt). The approximation with a rigorous NQE, kc[P][N] = kd[P N], where P, N and P N are living, nitroxide radicals and dormant species respectively, with kinetic constants kc and kd, is found valid only for small values of the equilibrium constant K (10−11–10−12 mol · L−1) and its validity is found to depend strongly of the value of K. On the other hand, the relaxed NQE approximation of Fischer and Fukuda, kc[P][N] = kd[P N]0 was found to be remarkably good up to values of K around 10−8 mol · L−1. This upper bound is numerically found to be 2–3 orders of magnitude smaller than the theoretical one given by Fischer. The relaxed NQE is a better one due to the fact that it never completely neglects dN/dt. It is found that the difference between these approximations lies essentially in the number of significant figures taken for the approximation; still this subtle difference results in dramatic changes in the predicted course of the reaction. Some results confirm previous findings, but a deeper understanding of the physico‐chemical phenomena and their mathematical representation and another viewpoint of the theory is offered. Additionally, experiments and simulations indicate that polymerization rate data alone are not reliable to estimate the value of K, as recently suggested.
Validity of the rigorous nitroxide quasi‐equilibrium assumption as a function of the nitroxide equilibrium constant. 相似文献
Kinetics and mechanism of the reduction of dihydroxydiperiodatoargentate(III) by 1,2-butanediol in alkaline medium were studied by spectrophotometry in the 298.2–318.2 K range. The reaction rate showed pseudo-first order dependence in oxidant and 1 < nap< 2 in reductant. The associated rate law is: -d[Ag(III)]t/dt = { kK1K2K3[CH3CH2CHOHCH2OH]2 [Ag(III)]t/[H2IO3-4][OH-]+K1K2+K1K2K3[CH3CH2CHOHCH2OH] = k.[Ag(III)]t. A plausible mechanism involving a pre-equilibrium of adduct formation between the complex and reductant was proposed. The activation parameters along with rate constants of the rate-determining step were calculated. 相似文献
Abstract Two 36-membered (2+2) Schiff-base macrocycles have been prepared and characterised without the use of template ions or high dilution techniques. Rather, intramolecular hydrogen-bonding and π-π interactions promote the isolation of these “figure-of-eight” products in good yields. Two cobalt(III) complexes of the 18-membered (1+1) Schiff-base macrocycles are formed when cobalt(II) is used as a template. The structures of one “figure-of-eight' metal-free (2+2) macrocycle and of one dicobalt(III) complex of a (1+1) macrocycle, in which the cobalt centres are octahedral, are presented. [H4L1tBu]2: C70H70N8O9, space group P-1, a=7.747(1), b=17.106(3), c=23.662(4) å, α=104.83(3), β=95.86(3), γ=97.80(3)°, U=2972.4(8) å3, Z=2, Dc=1.30 g cm?3, T=100K, 554 parameters, R1=0.072 [for 4137 reflections having F>4[(F)], wR2=0.191 and goodness of fit 0.97 (for all 8343 independent F2 data). [Co2(L1tBu)(OAc)2(pyridine)] 1.5DMF 0.5MeCN, C47.5H49N7O9.5Co2, space group P2(1)/n, a=13.067(2), b=26.071(4), c=14.023(3) å, β=93.02(1)°, U=4770.6(15) å3, Z=4, Dc=1.38 g cm?3, T=168 K, 622 parameters, R1=0.075 [for 5233 reflections having F > 4[?F?], wR2=0.133 and good ness of fit 1.08 (for all 8512 independent F2 data). 相似文献
The configurations of calix[4]arenes may be modified by the formation of donor-acceptor complexes which make use of the basicity of the oxygen atoms of the macrocycle. The complex [t-butylcalix[4]arene methyl ether][AlMe3]2,2, exhibits the previously unseen 1,2-alternate geometry, while [t-butylcalix[4]arene methyl ether][MeAlCl2]2,3, and [t-butylcalix[4]arene methyl ether][EtAlCl2]2,4, show the 1,3-alternate configuration.2 crystallizes in the triclinic space groupPl witha=11.14(1),b=11.60(1),c=12.02(1) Å, =77.32(8), =67.91(8), and =69.34(8)o withDc=1.06 g cm–3 forZ=1. Refinement based on 1270 observed reflections led toR=0.106.3 as the benzene solvate belongs to the monoclinic space groupC2/c witha=12.116(2),b=21.557(7),c=23.470(6) Å, and =104.05(2)o withDc=1.13 g cm–3 forZ=4. Refinement based on 2335 observed reflections led toR=0.075.4 crystallizes in the monoclinic space groupC2/c witha=12.062(4),b=21.175(6),c=21.596(5) Å, and =100.78(4)o withDc=1.18 g cm–3 forZ=4. Refinement based on 2529 observed reflections gaveR=0.082. The Al-O lengths in all three complexes are normal for donor-acceptor interactions.
Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82053 (45 pages). 相似文献
The self‐complementary, ethylene‐linked U*[ca]A(*) dinucleotide analogues 8, 10, 12, 14, 16 , and 18 , and the sequence‐isomeric A*[ca]U(*) analogues 20, 22, 24, 26, 28 , and 30 were obtained by Pd/C‐catalyzed hydrogenation of the corresponding, known ethynylene‐linked dimers. The association of the ethylene‐linked dimers was investigated by NMR and CD spectroscopy. The U*[ca]A(*) dimers form linear duplexes and higher associates (K between 29 and 114M ?1). The A*[ca]U(*) dimers, while associating more strongly (K between 88 and 345M ?1), lead mostly to linear duplexes and higher associates; they form only minor amounts of cyclic duplexes. The enthalpy–entropy compensation characterizing the association of the U*[cx]A(*) and A*[cx]U(*) dimers (x=y, e, and a) is discussed. 相似文献
The polymerization of di-2[2-(2-methoxyethoxy)ethoxy]ethyl itaconate (1) with dimethyl 2,2-azobisisobutyrate (2) was studied, in benzene, kinetically and spectroscopically with the electron paramagnetic resonance (EPR) method. The polymerization rate (Rp) at 50°C is given by the equation:Rp=k[2]0.48 [1]2.4. The overall activation energy of polymerization was calculated to be 34 kJ·mol–1. From an EPR study, the polymerization system was found to involve EPR-observable propagating polymer radicals of 1 under the actual polymerization conditions. Using the polymer radical concentration, the rate constants of propagation (kp) and termination (kt) were determined. With increasing monomer concentration,kp(1.54.3 L·mol–1·s–1 at 50°C) increases andkt(1.0·1044.2·104 L·mol–1·s–1 at 50°C) decreases, which seems responsible for the high dependence ofRp on the monomer concentration. The activation energies of propagation and termination were calculated to be 11 kJ·mol–1 and 84 kJ·mol–1, respectively. For the copolymerization of 1(M1) and styrene (M2) at 50°C in benzene the following copolymerization parameters were found:r1=0.2,r2=0.53, Q1=0.57, ande1=+0.7. 相似文献