Relation between lyoluminescence and radical reaction rate constants

Light emission during the dissolving of irradiated sugars (lyoluminescence, LL) allows the estimation of absorbed dose. The use of 1-mannose as LL substance and the correlation between the concentration of paramagnetic centres and LL yield in the presence of [Fe(CN)₆]^4? and CNS^? anions demonstrated the possibility to measure relative rate constants of the reaction of mannose peroxy-radicals with different substances soluble in water.     

The hydroxyl radical reaction rate constants and atmospheric reaction products of three siloxanes

The relative rate technique has been used to measure the hydroxyl radical (OH) reaction rate constant of hexamethyldisiloxane (MM, (CH₃)₃Si-O-Si(CH₃)₃), octamethyltrisiloxane (MDM, (CH₃)₃Si-O-Si(CH₃)₂-O-Si(CH₃)₃), and decamethyltetrasiloxane (MD₂M, (CH₃)₃Si-O-Si(CH₃)₂-O-Si(CH₃)₂-O-Si(Ch₃)₃). Hexamethyldisiloxane, octamethyltrisiloxane, and decamethyltetrasiloxane react with OH with bimolecular rate constants of 1.32 ± 0.05 × 10⁻¹² cm³molecule⁻¹s⁻¹, 1.83 ± 0.09 × 10⁻¹² cm³ molecule⁻¹s⁻¹, and 2.66 ± 0.13 × 10⁻¹² cm³molecule⁻¹s⁻¹, respectively. Investigation of the OH + siloxane reaction products yielded trimethylsilanol, pentamethyldisiloxanol, heptamethyltetrasiloxanol, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, and other compounds. Several of these products have not been reported before because these siloxanes and the proposed reaction mechanisms yielding these products are complicated. Some unusual cyclic siloxane products were observed and their formation pathways are discussed in light of current understanding of siloxane atmospheric chemistry. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 445–451, 1997.     

Quantum chemical prediction of pathways and rate constants for reaction of cyanomethylene radical with NO

High-level ab initio calculations have been performed to study the mechanism and kinetics of the reaction of the cyanomethylene radical (HCCN) with the NO. The species involved have been optimized at the B3LYP/6-311++G(3df,2p) level, and their corresponding single-point energies are improved by the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311++G(3df,2p) approach. From the calculated potential energy surface, we have predicted the favorable pathways for the formation of several isomers of a HCCN-NO complex. Barrierless formation of HCN + NCO (P1) is also possible. Formation of HCNO + CN (P3) is endoergic but may become significant at high temperatures. To rationalize the scenario of our calculated results, we also employ the Fukui functions and hard-and-soft acid-and-base (HSAB) theory to seek possible clues. The predicted total rate coefficient, k(total), at He pressure 760 Torr can be represented with the equation k(total) = 1.40 × 10(-7) T(-2.01) exp(3.15 kcal mol(-1)/RT) at T = 298-3000 K in units of cm(3) molecule(-1) s(-1). The predicted total rate coefficients at some available conditions (He pressures of 6, 18, and 30 Torr in the temperature of 298 K) are in reasonable agreement with experimental observation. In addition, the rate constants for key individual product channels are provided in different temperature and pressure conditions.     

Calculation of proton coupling constants for dibenzothiophene radical anion

The calculation of proton hyperfine coupling constants for dibenzothiophene radical anion is reported.     

Reactivity of superoxide anion radical with a perchlorotriphenylmethyl (trityl) radical

The reaction of superoxide radical with a tricarboxylate derivative of perchlorotriphenylmethyl radical (PTM-TC) is studied. PTM-TC is a stable ("inert") free radical, which gives a single sharp electron paramagnetic resonance (EPR) peak in aqueous solutions. PTM-TC also gives a characteristic optical absorption at 380 nm. Superoxide, on reaction with PTM-TC, induced a decrease in the intensity of the EPR signal and optical absorption of PTM-TC at 380 nm. The signal loss was specific to superoxide and linearly dependent on the superoxide flux in the system. Competitive kinetics experiments revealed that PTM-TC reacts with superoxide with an apparent second-order rate constant of 8.3x10(8) M(-1) s(-1). Electrochemical and mass spectrometric analyses of the reaction suggested the formation of perchlorotriphenylmethane and molecular oxygen as products. The high sensitivity of detection of PTM-TC combined with the high rate constant of the reaction of superoxide with PTM-TC may offer a potential opportunity for measurement of superoxide in biological systems. In conclusion, the PTM-TC molecule has high sensitivity and specificity for superoxide radicals and thus may enable quantitative detection of superoxide generation in biological systems using EPR and/or spectrophotometric methods.     

Outer-sphere oxidation of the superoxide radical anion

Efforts to use the Marcus model to describe oxidations of the superoxide radical anion (O(2-)) by transition-metal complexes have failed dramatically, with discrepancies between theory and experiment spanning 13 orders of magnitude. As a result, the prevailing view is that these reactions involve some complex interactions that are not yet understood. We now show that once the familiar form of the Marcus cross relation (MCR) is modified to allow for the relatively small size of O(2-), excellent agreement is obtained between theory and experiment. This simple finding dispels the decades of uncertainty surrounding these reactions and provides a reliable method for determining whether oxidations of (O2)- occur via inner- or outer-sphere pathways. More generally, the modified MCR is applicable to any homogeneous electron-transfer process characterized by significant differences in size between electron donors and acceptors.     

The rate constants for the reaction of the hydroxyl radical with methyl,n-propyl,and n-butyl nitrites

The kinetics of the reaction of OH radicals with methyl, n-propyl, and n-butyl nitrite have been studied in a discharge flow system under pseudo first-order conditions. The OH radicals were generated by the reaction of H atoms with NO₂ and the concentration of OH; monitored by resonance fluorescence, was followed as a function of time in an excess of each nitrite. Values of k(CH₃ONO) = (0.6 ± 0.09) × 10⁹ dm³ mol^?1 s^?1 k(n – C₃H₇ONO) = (1.39 ± 0.20) × 10⁹ dm³ mol^?1 s^?1, and k(n – C₄H₉ONO) = (2.89 ± 0.43) × 10⁹ dm³ mol^?1 s^?1 at 295 K were obtained. These results agree with previous relative rate measurements from this laboratory but the value for k (CH₃ONO) is a factor of 7 greater than the value obtained by relative rate measurements elsewhere using a different OH source.     

Self- and cross-termination rate constants for the isopropylol radical and its anion in aqueous solution

Rate constants for the self- and cross-termination of the isopropylol radical [(CH₃)₂?OH] and its anion [(CH₃)₂?O^?] in aqueous solution are determined by kinetic electron spin resonance. Whereas the self-termination of the neutral radical occurs close to the diffusion-controlled limit, the cross- and self-terminations involving the anion are slower and reflect effects of charge repulsion and steric constraints by solvation.     

Reactivity of superoxide radical anion and hydroperoxyl radical with alpha-phenyl-N-tert-butylnitrone (PBN) derivatives

Nitrones have exhibited pharmacological activity against radical-mediated pathophysiological conditions and as analytical reagents for the identification of transient radical species by electron paramagnetic resonance (EPR) spectroscopy. In this work, competitive spin trapping, stopped-flow kinetics, and density functional theory (DFT) were employed to assess and predict the reactivity of O(2)(*-) and HO(2)(*) with various para-substituted alpha-phenyl-N-tert-butylnitrone (PBN) spin traps. Rate constants of O(2)(*-) trapping by nitrones were determined using competitive UV-vis stopped-flow method with phenol red (PR) as probe, while HO(2)(*) trapping rate constants were calculated using competition kinetics with 5,5-dimethylpyrroline N-oxide (DMPO) by employing EPR spectroscopy. The effects of the para substitution on the charge density of the nitronyl-carbon and on the free energies of nitrone reactivity with O(2)(*-) and HO(2)(*) were computationally rationalized at the PCM/B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) level of theory. Theoretical and experimental data show that the rate of O(2)(*-) addition to PBN derivatives is not affected by the polar effect of the substituents. However, the reactivity of HO(2)(*) follows the Hammett equation and is increased as the substituent becomes more electron withdrawing. This supports the conclusion that the nature of HO(2)(*) addition to PBN derivatives is electrophilic, while the addition of O(2)(*-) to PBN-type compounds is only weakly electrophilic.     

亚磺化脱卤研究 VII. 全氟碘代烷与SO~3^-的反应

全氟碘代烷R~FI[R~F=CF~3(CF~2)~5(1a);H(CF~2)~4(1b);H(CF~2)~8(1c);Cl(CF~2)~6(1d);Cl(CF~2)~8(1e)Cl(CF~2)~8(1f);;NaO~3S(CF~2)~20(CF~2)~2(1g);NaO~3S(CF~2)~2O(CF~2)~4(1h)]及α,ω-二碘化物I(CF~2)~nI[n=4(1i);n=6(1j)]与Ce^4^+-亚硫酸氢钠于70℃左右、pH3～4的溶液中反应,形成相应的全氟烷基亚磺酸盐,产率70～85%。全氟氯代烷和全氟溴代烷在同样条件下未能发生反应。     

Rate constants for the reaction of the chloromethyne radical with alkynes

Absolute rate constants were measured for the gas phase reactions of the CCl(X?²Π) radical produced in the flash photolysis of CHBr₂Cl with a representative series of alkynes. The rate of addition to the triple bond follows the trend established for the addition CCl(X?²Π) to olefinic double bonds and the acetylenic triple bond by the latter reagent. A linear correlation between log k and ionization potentials of the alkynes reflects the electrophilic nature of the reaction.     

Rate constants for the reaction of the bromomethyne radical with alkynes

Absolute rate constants were measured for the gas phase reactions of the CBr($\text{～}\text{X}$²Π) radical produced in the flash photolysis of CHBr₃ with a series of alkynes. The rate of addition to the triple bond followed the trend established for addition to double bonds and a linear correlation between log k and ionization potentials reflects the electrophilic nature of the reaction. In comparison with published data on CCl, S(³P₂) the reactivity follows the trend CBr ≈ CCl ≈ S(³P₂) > O(³P₂).     

Rate constants for radical recombination. III. The trifluoromethyl radical

Products of the radical reactions arising from t-Bu₂O₂, CF₃I, and CH₃I at 146°C in the vapor phase have been measured over a 33-fold range of CH₃I/CH₃I ratios and shown to be governed by the rapidly established equilibrium Together with K estimated by thermochemical methods, the results yield, for the rate of recombination for CF₃· radicals, k_r = 10^{9.7 ± 0.5} M^?1 sec^?1.