Degradation on freezing dilute polystyrene solutions in p-xylene

Chain scission was observed during the crystallization of p-xylene in dilute polystyrene solutions. Degradation yields were determined by gel permeation chromatography, as a function of the number of freeze-and-thaw cycles, polymer concentration, and initial polymer molecular weight (M). The rate constant for chain scission K_c increases with the polymer chain length, from 0.021%/cycle at M = 110·10³ to 4.7%/cycle at M = 8.5·10⁶. Over the two decades range of investigated molecular weights, K_c follows an empirical scaling law of the form K_c ～ (M ? M_lim)^1.17578, where M_lim is a limiting molecular weight ? 29,000 g. mol^?1 below which no degradation could be induced. Some propensity for midchain scission was detected, although this tendency was much weaker in comparison to flow-induced degradation. A chain scission model based on crack propagation failed to reproduce the experimental results. To explain the observed dependence of K_c with the square of the radius of gyration, an interfacial stress transmission mechanism between the crystallization fronts and the polymer coil has been proposed. © 1994 John Wiley & Sons, Inc.     

Very low-pressure pyrolysis. VII. The decomposition of methylhydrazine, 1,1-dimethylhydrazine, 1,2-dimethylhydrazine,and tetramethylhydrazine. Concerted deamination and dehydrogenation of methylhydrazine

The rate constants (k_uni) for the first-order disappearance of the title molecules have been determined under VLPP conditions. The k_uni are not the rate constants of ultimate interest since they reflect the fact that energy transfer competes with the chemical decomposition. Use of the Rice-Ramsperger-Kassel-(Marcus) [RRK(M)] theory allows the determination of the high-pressure rate constants (k_α), if the mode of decomposition is known. The heats of formation of the radicals NH₂, CH₃NH, and (CH₃)₂N are known. These values should be usable for prediction of the activation energy for N? N bond homolysis in the hydrazines. Measured rate constants for UDMH and TMH bear this out, but the rate constant for MMH does not. This and other evidence lead to the conclusion that MMH decomposes via molecular concerted elimination of NH₃ and H₂ not and by N? N bond scission. The following values are preferred from this work (θ = 2.303RT in kcal/mole). Mode of decomposition is N—N bond scission unless noted otherwise in parenthesis: .     

Very low-pressure pyrolysis. IV. The decomposition of i-propyl iodide and n-propyl iodide

The decomposition rate constant of i-PrI under conditions of very low-pressure pyrolysis (VLPP) is completely consistent with the well-known high-pressure Arrhenius parameters and the RRK(M) theory. The decomposition of n-PrI under the same conditions proceeds via two pathways, the anti-Markownikoff dehydroiodination and C? I bond scission. The data, analyzed by taking into account the mutual interaction of the two pathways, is completely consistent with the known Arrhenius parameters for the bond scission step and, when combined with a reasonable A-factor, yields an activation energy for HI elimination which is as predicted for these semi-ion pair transition states.     

The Critical Role of Reductive Steps in the Nickel‐Catalyzed Hydrogenolysis and Hydrolysis of Aryl Ether C−O Bonds

The hydrogenolysis of the aromatic C?O bond in aryl ethers catalyzed by Ni was studied in decalin and water. Observations of a significant kinetic isotope effect (k_H/k_D=5.7) for the reactions of diphenyl ether under H₂ and D₂ atmosphere and a positive dependence of the rate on H₂ chemical potential in decalin indicate that addition of H to the aromatic ring is involved in the rate‐limiting step. All kinetic evidence points to the fact that H addition occurs concerted with C?O bond scission. DFT calculations also suggest a route consistent with these observations involving hydrogen atom addition to the ipso position of the phenyl ring concerted with C?O scission. Hydrogenolysis initiated by H addition in water is more selective (ca. 75 %) than reactions in decalin (ca. 30 %).     

Rates and equilibria in the reaction system Br + i-C4H10 ⇌ HBr + t-C4H9. The heat of formation of the t-butyl radical

The very low pressure reactor (VLPR) technique has been used to measure the bimolecular rate constant of the title reaction at 300 K. The rate constant is given by log k₁ (1/mol s) = (11.6 ± 0.4) ? (5.9 ± 0.6)/θ the equilibrium constant has also been measured at the same temperature and is given by K₁ = (5.6 ± 1) × 10^?3 and hence log k_?1 (1/mol s) = 9.5 ± 0.1. The results show that the reaction Br + t? C₄H₉ → HBr + i? C₄H₈ is unimportant under the present experimental conditions. Assigning the entropy of t-butyl radical to be 74 ± 2 eu which is in the possible range, the value of K₁ gives ΔH (t-butyl) = 9.1 ± 0.6 kcal/mol^?1. This yields for the bond dissociation, DH° (t-butyl-H) = 93.4 ± 0.6 kcal/mol. Both of these values are found to be in good agreement with recent VLPP studies.     

Theory of titration curves. VIII. A new interpretation of acid-base titration curves for polymeric acids. The significance of the Kern-equation parameters pKa and n

The Kern equation, pH = pK_a + n logf/(1 ? f), has been widely used in interpreting the potentiometric titration curves obtained on titrating polymeric acids with bases, and various interpretations of its empirical parameters pK_a and n have been advanced. Values of n similar to those generally obtained for polymeric acids are also obtained for dibasic and other monomeric polybasic acids for which the ratios of successive dissociation constants, K_i/K_(i+1), exceed the corresponding statistical values by a constant factor. The dependences of n on this factor and on the basicity of the acid are described. It is argued that the traditional picture of a dissociation constant varying continuously with the fraction titrated cannot be correct, and that the data can be more properly interpreted in terms of a constant non-statistical free-energy contribution to the successive values of K_i/K_(i+1).     

Kinetic Study of the Substitution Reactions of Benzyl Halides and Halide Ions in Acetone

The kinetics of the Finkelstein reactions of benzyl halides and halide ions in dry acetone was studied by using the GC and HPLC methods. The method of conductivity is used to measure the degree of dissociation of alkali halide in acetone. The dissociation of the salt and the common-ion effect were used to correct the halide ion concentration. Let k_xx′- and K_xx′- represent the corrected second-order rate constant and equilibrium constant for the PhCH₂X-X′- reaction, respectively, then at 25° k_xx-?= 1.83x×10^?3 M^?1 s^?1, K_II-=4.48×10⁴, k_Brcl-=0.377 M^?1 s^?1, K_Brcl-=874; and k_Br-=7.88 M^?1 s^?1, K_IBr-=19.2. The entropy of activation is always negative which implies that the activated complex is more solvated than the reactants. There is a good correlation between the enthalpy of activation and the C-X bond energy. The enthalpy of activation is much smaller than the corresponding C-X bond energy which implies that both the bond formation and the bond dissociation are taking place simultaneously in the transition state. A modified Taft equation is used to correlate the Finkelstein reactions of alkyl halides (RCH₂X). However, the benzyl halides can not be fit by the correlation equation due to the strong electrical polar effects. Discussion of the Finkelstein reactions of organic halides in terms of rate constants, equilibrium constants, solvent effects, and the thermodynamic parameters is also presented.     

Kinetics of the iodine atom catalyzed gas phase geometrical isomerization of diiodoethylene. Rotation about a single bond

The spectrophotometric determination of the rate of iodine atom catalyzed geometrical isomerization of diiodoethylene in the gas phase from 502.8 to 609.1°K leads to a rate constant for the bimolecular reaction between I and trans-diiodoethylene of log k_t?c(M^?1 sec^?1) = 8.85 ± 0.12 ? (11.01 ± 0.30)/θ. Estimates of the entropy and enthalpy change for the addition of I atoms to trans-diiodoethylene (process a.b) lead to log K_a.b(M^?1) = ?2.99 ? 4.0/θ, and thus to log k_c (sec^?1) = log k_t?c – log K_ab = 11.8 ?7.0/θ for the rate constant for rotation about the single bond in the adduct radical. The theory for calculation of the rotation rate constant is presented and it is shown that while the exact value depends on the barrier height, a value of 6.8 kcal/mole for this quantity leads to log k (sec^?1) = 11.8 ?6.7/θ. The activation energy points to a better value of the group contribution to heat of formation of the group C -(I)₂(H)(C) than one based on bond additivity.     

DNA与金属锇配合物相互作用的表面电化学研究

采用自己建立的DNA表面电化学研究微量方法, 研究了单双链DNA与两种锇配合物(联吡啶锇和二氯菲咯啉锇)的相互作用. 研究发现, 两种锇配合物都是通过静电作用与DNA结合, 其作用方式不受溶液离子强度的影响. 并计算得到了联吡啶锇和二氯菲咯啉锇与dsDNA和ssDNA相互作用的多个热力学和动力学参数, 如结合常数K_3＋或K_2＋, 结合常数比K_3＋/K_2＋, 离子强度为零时的极限比 , 结合自由能ΔG_b, 解离速度常数k, 结合位点数s.     

The very-low-pressure study of the kinetics and equilibrium: Cl + CH4 ⇄ CH3 + HCl at 298 K. The heat of formation of the CH3 radical

The bimolecular rate constant for the direct reaction of chlorine atoms with methane was measured at 25°C by using the very-low-pressure-pyrolysis technique. The rate constant was found to be In addition, the ratio k₁/k_?1 was observed with about 25% accuracy: K₁₍₂₉₈₎ = 1.3 ± 0.3. This gives a heat of formation of the methyl radical ΔH° _{f 298}(CH₃) = 35.1 ± 0.15 kcal/mol. A bond dissociation energy BDE (CH₃ ? H) = 105.1 ± 0.15 kcal/mol in good agreement with literature values was obtained.     

Ionic conduction in LiSCN/dimethylformamide/poly(propylene oxide) system I. Dissociation equilibria of LiSCN

Dissociation equilibria of lithiumthiocyanate (LiSCN) in N,N-dimethylformamide (DMF) solutions of poly (propylene oxide) (PPO) were investigated by using infrared spectroscopy. The stretching bands due to the thiocyanate ions SCN^?1 and the LiSCN ion pair were found at 2058 and 2072 cm^?1, respectively. At high LiSCN concentration C of ca. 20 wt %, another weak band due to the dimer (LiSCN)₂ was observed. From the ratio of the areas of the absorption bands, the dissociation constant K₁ for the equilibrium LiSCN ? Li⁺ + SCN^?1 and that K₂ for (LiSCN)₂ ? 2LiSCN have been determined. With increasing DMF content, K₁ increases from 1 × 10^?4 for bulk PPO to 4.8 × 10^?1 for pure DMF at 299 K. Log K¹ is not linear against inverse of the dielectric constant ? of the medium and decreases with increasing temperature. The enthalpy (ΔH) and entropy (ΔS) changes for the dissociation of LiSCN are both negative. ©1995 John Wiley & Sons, Inc.     

Poly(arylsulfone imide) as E-beam resist: Synthesis and radiolysis

Aromatic diamines containing ? SO₂? and ? S? moieties have been used to prepare soluble polyimides with ditrifluoromethyl methane bis(phthalic anhydride) (F-series polyimides) and polyamic acid with pyromellitic dianhydride (P-series). Gamma radiolysis gave G(S) values for scission between 1 to 2 with no crosslinking. Significant weight loss occurred with radiolysis is attributable to efficient ? SO₂? bond scission for the F-series polyimides, as well as imidization in the cases of P-series polyamic acids.     

Calculation of normal vibrations as applied to the study of properties of inorganic compounds

Using the Morse potential and spectroscopic constants it has been shown for diatomic molecules that the force constant of the bond (Kq) is proportional to its dissociation energy (D) so that Kq can be regarded as a measure of the bond strength. An analysis of K_MA of a series of complexes clearly indicates the correlation between the force constants K_MA and the MA bond length, thermodynamic constants of the MA bond dissociation, and reaction rates of the MA bond rupture.     

Electron Paramagnetic Resonance Study of Radical Pairs and Isolated Radicals Trapped in Irradiated Long-Chain Hydrocarbons at Low Temperatures

The intrinsic characteristics of radical pairs produced in squalane and in cetane receiving high gamma-dose are extensively studied with the EPR technique at temperatures from 77°K up to 150°K. The spectra of the paired radicals occur at g=4 with a very low transition probability in contrast to that of isolated radicals which appear at g=2 A well-resolved hyperfine spectrum corresponding to the species (CH₃CH₂.CH₂CH₃) is observed in cetane. The isothermal decay rates of radical pairs in cetane below 100°K are significantly slow; however, the decay kinetics at 150°K is first order with rate constant=1.86 min^?1. A relatively slower decay rate is obtained for isolated radicals suggesting that the decay mechanism of paired radicals is through geminate recombination. The relative inter-radical distance in radical pairs is known from a decay curve as a function of temperature. The yields of radical pairs are low in both matrices, only few percents of those of isolated radicals. The formation mechanisms of paired radicals with direct radiolytic bond scission process are discussed in connection with the experimental observations.     

Thermodynamic trans-Effects of the Nucleotide Base in the B12 Coenzymes

The thermodynamic effects of the nucleotide coordination on the Co-C bond strengths in the B ₁₂ coenzymes were analyzed. Methyl group transfer reactions from methylcob( III )inamides to cob( II )inamides and cob( I )inamides in neutral aqueous solution were used in equilibration experiments to determine the effect fo the intramolecular coordination of the nucleotide function on the Co-C bond dissociation energies of methylcob( III )alamin ( 4 ). In the equilibrium between 4 , cob( I )inamide ( 11 ), cob( I )alamin ( 10 ) and methylcob( III )inamide 6 (Scheme 2), 4 and 11 were found to predominate ( 4 + 11 ? 10 + 6 , equilibrium constant K_I/III≈0.004), while the equilibrium between 4 , cob( II )inamide 9 , cob( II )alamin ( 5 ), and 6 (Scheme 1) proved to be well balanced ( 4 + 9 ? 5 + 6 , equilibrium constant K_II/III=0.60). These equilibrium values indicate the nucleotide coordination to stabilize the Co–C bond in 4 both against homolysis (slight effect) and against nucleophilic heterolysis (considerable effect). They reflect a stabilization of the complete corrins 4 and 5 by the nucleotide coordination, which is also indicated for 4 and 5 by their (nucleotide) basicity. The latter information, where available for other organocobalamins, allows the analysis of the thermodynamicnucleotide trans effect there as well: e.g. in coenzyme B ₁₂ ( 1 ), the nucleotide coordination is found this way to weaken the Co–C bond towards homolysis by ca. 0.7 kcal/mol.     

Revised mechanism of carboxylation of ribulose‐1,5‐biphosphate by rubisco from large scale quantum chemical calculations

Here, we describe a computational approach for studying enzymes that catalyze complex multi‐step reactions and apply it to Ribulose 1,5‐bisphosphate carboxylase–oxygenase (Rubisco), the enzyme that fixes atmospheric carbon dioxide within photosynthesis. In the 5‐step carboxylase reaction, the substrate Ribulose‐1,5‐bisphosphate (RuBP) first binds Rubisco and undergoes enolization before binding the second substrate, CO₂. Hydration of the RuBP.CO₂ complex is followed by C C bond scission and stereospecific protonation. However, details of the roles and protonation states of active‐site residues, and sources of protons and water, remain highly speculative. Large‐scale computations on active‐site models provide a means to better understand this complex chemical mechanism. The computational protocol comprises a combination of hybrid semi‐empirical quantum mechanics and molecular mechanics within constrained molecular dynamics simulations, together with constrained gradient minimization calculations using density functional theory. Alternative pathways for hydration of the RuBP.CO₂ complex and associated active‐site protonation networks and proton and water sources were investigated. The main findings from analysis of the resulting energetics advocate major revision to existing mechanisms such that: hydration takes place anti to the CO₂; both hydration and C C bond scission require early protonation of CO₂ in the RuBP.CO₂ complex; C C bond scission and stereospecific protonation reactions are concerted and, effectively, there is only one stable intermediate, the C3‐gemdiolate complex. Our main conclusions for interpreting enzyme kinetic results are that the gemdiolate may represent the elusive Michaelis–Menten‐like complex corresponding to the empirical K_m (=K_c) with turnover to product via bond scission concerted with stereospecific protonation consistent with the observed catalytic rate. © 2018 Wiley Periodicals, Inc.     

Acid-base properties of solid polyelectrolytes: Dissociation of cellulosic polyanions

The potentiometric titration behavior of nitrogen dioxide and periodate-chlorite oxycellulose has been studied both in the presence and absence of sodium chloride. The apparent pK values in general increase with increasing degree of dissociation, but the reverse is true for periodate-chlorite oxycellulose in the absence of added salt. This behavior is interpreted in terms of electrostatic and hydrogen bonding interaction of carboxyl groups. The Gibbs–Donnan theory of polyaid dissociation was applied to calculate the intrinsic dissociation constant pK′₀. Assuming a model having a uniform potential distributin throughout the fiber, good agreement with the expected value of 3.24 was found (compared to pK′₀ = 3.25 calculated for periodate-chlorite oxycellulose).     

Proton‐controlled nitroxide mediated radical polymerization of styrene

The pyridyl alkoxyamine, which is composed of the 1‐phenylethyl radical and a pyridyl nitroxide fragments, displays protonation‐controlled C? ON bond homolysis. Its dissociation rate constant k_d value is approximately halved at 100 °C in tert‐butyl benzene when it is protonated by one equivalent of trifluoroacetic acid. Moreover, the bulk polymerization of styrene at 125 °C is performed with a good control over the molecular weight and the dispersity when initiated with this alkoxyamine under its basic and acidic forms but the protonation has induced a strong decreased polymerization rate. In contrast, in the case of n‐butyl acrylate, the control over the polymerization is lost for the protonated pyridyl alkoxyamine because the pyridyl nitroxide is less thermally stable under its acidic form. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Dissociation behaviors of carboxyl and amine groups on carboxymethyl‐chitosan in aqueous system

Several carboxymethyl‐chitosan (CMCS) samples with different deacetylation degree and/or substituted degree were prepared from the carboxymethylation reaction of chitosan under soft conditions. The products were dissolved in standard HCl aqueous solution to carry out potentiometric titration by using NaOH as titrating solution at different ionic strengths. Then the dissociation behaviors of protonated carboxyl and amine groups were investigated under their degree of dissociation (α) and protonation constant (pK_α) had been calculated. Moreover, influences of the intrinsic and extrinsic parameters on the dissociation behavior of CMCS were also considered in this article. As a result, dissociations of carboxyl and amine on CMCS exhibited unusual behaviors in comparison with carboxyl of carboxymethyl‐cellulose and amine groups of chitosan, respectively. The pK_α values of carboxyl declined slightly at early dissociation stage but subsequently maintained constant. In contrast, the pK_α of ammonium increased with its dissociation degree despite that there was an inflexed change on its dissociation curve. The potentiometric behavior of carboxyl was hardly affected by variation of deacetylation degree or substituted degree. However, these intrinsic parameters played more important role on dissociations of ammonium on CMCS. The ionic strength of media could bring screening effect on dissociaciation of both sorts of ionizable groups of CMCS. By increasing the ionic strength of media, screening effect on dissociations increased significantly. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1419–1429, 2008     

O + NNH: A possible new route for NOX formation in flames

We propose a new high temperature pathway for NO formation that involves the reaction of NNH with oxygen atoms. This reaction forms the HNNO* energized adduct via a rapid combination reaction; HNNO* then rapidly dissociates to NH + NO. The rate constant for O + NNH ? NH + NO is calculated via a QRRK chemical activation analysis to be 3.3 × 10¹⁴ T^?0.23exp(+510/T) cm³ mol^?1 s^?1. This reaction sequence can be an important or even major route to NO formation under certain combustion conditions. The presence of significant quantities of NNH results from the reaction of H with N₂. The H + N₂ ? NNH reaction is only ca. 6 kcal/mol endothermic with a relatively low barrier. The reverse reaction, NNH dissociation, has been reported in the literature to be enhanced by tunneling. Our analysis of NNH dissociation indicates that tunneling dominates. We report a two-term rate constant for NNH dissociation: 3.0 × 10⁸ + [M] {1.0 × 10¹³T^0.5exp(?1540/T)} s^?1. The first term accounts for pressure-independent tunneling from the ground vibrational state, while the second term accounts for collisional activation to higher vibration states from which tunneling can also occur. ([M] is the total concentration in units of mol cm^?3.) Use of this dissociation rate constant and microscopic reversibility results in a large rate constant for the H + N₂ reaction. As a result, we find that NNH ? H + N₂ can be partially equilibrated under typical combustion conditions, resulting in NNH concentrations large enough for it to be important in bimolecular reactions. Our analysis of such reactions suggests that the reaction with oxygen atoms is especially important. © 1995 John Wiley & Sons, Inc.