Temperature dependence of the spin-lattice relaxation rates in the triplet state of pyrazine at low temperatures

The electron spin-Lattice relaxation, (SLR) rate constants have been measured for the triplet state at pyrazine-d₄ in a cyclohexane matrix from 1 to 8 K by resolving the phosphorescent decay curve into three exponential components. A matrix kinetics is presented that gives the relationship between the SLR rate constants and the observed decay component rate constants and the decay component intensities of the three zero-field levels. An exact numerical procedure based on the Newton-Raphson technique is described in detail for systems of two and three coupled levels. This procedure was used to find the SLR rate constants starting from either the decay component rate constants or the decay component intensities. Both the matrix kinetics and the numerical procedure should be useful in many other studies involving multilevel systems whose sublevels are connected by some combination of SLR and microwaves. The observed SLR rate constants are best explained as a function of temperature as the sum of a direct process (linear in T) and a Raman process (T² dependence). Their relative magnitudes indicate that the Raman process is probably antharmonic. Vancus mechanisms for SLR in this system are examined, and a short review of SLR theory and experiment is given. The nature of the zero-field sublevels in pyrazine-J₄ is also discussed as is the effect of the choice of matrix solvent upon SLR rate constants and the phosphorescent spectrum.     

Author index to volume 49

The de-excitation rate constants of Ne(³P₂, ³P₀ and ³P₁) by N₂ and SF₆ were measured using a pulse radiolysis method combined with optical absorption spectroscopy. A new absorption law which relates the relative concentration S of absorbing atoms to the measured transmittance T, i.e. in S = Σ¹¹_{i = 0}a_iTⁱ, was used for analyzing the data. The presence of a small amount of SF₆ in the sample gas mixtures permitted removal of some artifacts due to thermal electrons for determining the rate constants.     

Rate constants for the reactions of Ar+ with CO2 and SO2 as a function of temperature (300–1500 K)

We have measured the rate constants for the reactions of Ar⁺ with CO₂ and SO₂ from 300 to 1500 K in a high temperature flowing afterglow. For the reaction with CO₂, we have found that all modes of energy, i.e., translation, rotation, and vibration, affect the rate constant to the same degree up to a total energy of 0.4 eV. Above 0.4 eV total energy, internal energy decreases the rate constants more effectively than does translational energy. For the reaction of Ar⁺ with SO₂, the rate constants go through a minimum at about 900 K. By comparing our results to drift tube data, we derive rate constants for reaction from the υ=0 and υ>0 vibrational levels. At low energy, the vibrationally excited SO₂ molecules react with Ar⁺ approximately twice as fast as the ground state molecules. Both vibrational modes have similar temperature dependences.     

Estimation of O-H bond dissociation energies in alcohols and acids from kinetic data

The O-H bond dissociation energies (D _O-H) in five alcohols and six acids have been determined from experimental data (rate constants of radical reactions). The ratio of the rate constants of the reactions R¹O˙+RH→R¹OH+R˙ and R ⁱ O˙+RH→R ⁱ OH+R˙ and the intersecting parabolas method are used in the estimation procedure. The D _O-H values are used to calculate the activation energies and rate constants for hydrogen abstraction from 2-methylbutane, butene-1, and cumene by alkoxyl and carboxyl radicals. The geometric parameters of the transition state are calculated for these reactions.     

Kinetic and mechanistic study of the reduction of 2,6-dichlorophenol indophenol by dithionites

The reaction of 2,6-dichlorophenolindophenol (DCPI) and dithionites is studied kinetically by applying the stopped-flow technique. Reaction rate constants are given for the pH range 1.30–6.80. The reaction was found to follow first-order kinetics with respect to each of the reactants. For pH 3.97, 5.10 and 6.80, the second-order reaction rate constant was determined by applying four different technique. Mean values of k = 172±5, 200±2 and 276±4 l mol^?1s^?1 are given for pH 3.97, 5.10 and 6.80, respectively. A mechanism is proposed for the reaction, which suggests partial reactions of all possible species of DCPI and dithionites at any pH. An equation for the calculation of k at any pH is derived, which gives k as a function of [H⁺], the partial reaction rate constants and the dissociation constants of DCPI and H₂S₂O₄. Values of reaction rate constants of all possible partial reactions are also presented.     

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 reactions of benzyl and methyl-substituted benzyl radicals with O2 and NO

Rate constants for reactions of benzyl, o-niethylbenzyl and p-meihylbenzyl radicals with O₂ and NO have been measured at room temperature. The radicals were generated by UV flash photolysis and the time decay measured by absorption at ≈ 300 nm. The rate constants are: benzyl (0.99 ± 0.07 and 9.5 ± 1.2), o-methylbenzyl (1.2 ± 0.07 and 8.6 ± 0.8) and p-mithyl-benzyl (1.1= 0.10 and 8.9 = 0.9) for O₂ and NO respectively in units of 10^?12 cm³ molecule^?1 s^?1.     

Quenching of singlet oxygen by hindered amine light stabilisers. A flash photolytic study

Rate constants for the quenching of singlet oxygen (¹O₂, ¹δ_g) for a series of piperidines, piperidine-N-oxyl free radicals and some commercially used hindered amine light stabilisers (HALS) have been measured by a laser flash photolysis method. Quenching rate constants are in the order: piperidine-N-oxyl free radicals ≤ secondary piperidines < tertiary piperidines. For some commercial HALS, ¹O₂ quenching rate constants and the light protective effect towards polypropylene photo-oxidation have been compared. No correlation has been found between the stabilizing action and the quenching efficiency towards ¹O₂. The data obtained point to little contribution of singlet oxygen to the key steps of polyolefin photo-oxidation.     

1H NMR study of the reversible cis / trans isomerization of {(μ-CH2)(μ-CO)[η5-C5H5Fe(CO)]2}

The cis / trans isomerization of the bridging methylene complex {(μ-CH₂)(μ-CO)[η⁵-C₅H₅-Fe(CO)]₂} was studied in solution by ¹H NMR spectrometry, using solvents with different polarities (acetone-d₆, chloroform-d₁ and benzene-d₆). Equilibrium constants and rate constants for the forward and reverse steps were measured between 278 and 323 k. Both reactions show first-order kinetics. A possible mechanism for the isomerization is proposed, involving the breaking of a FeFe bond in the rate-determining step.     

Investigation of gas phase reactions of OH radicals with fluoromethane and difluoromethane using Ar-sensitized pulse radiolysis

Hydroxyl radical reactions were studied in gas phase CH₃F and CH₂F₂ systems using an Ar-sensitized pulse radiolysis-absorption spectroscopy technique. Bimolecular H-atom abstraction rate constants were determined empirically by fitting the observed OH decay data to a first-order law, then plotting the apparent first-order rate constants against substrate concentration. The resulting values of the bimolecular rate constants in cm³ molec^-1 s^-1 are (1.71 ± 0.24) × 10^-14 for OH + CH₃F and (0.88 ± 0.14) × 10^-14 for OH + CH₂F₂. Both values are in good agreement with previous results. Kinetic modeling showed a substantial contribution of radical-radical reactions to OH loss in both cases, but the data reduction procedure was successful in distinguishing the contribution due to the bimolecular OH-substrate abstraction process. Observed and computer simulated OH decay curves match satisfactorily for typical experiments, using initial OH concentrations and OH abstraction rate constants which were within ± 10% of the measured values.     

Self-diffusion and electron spin exchange of hydrophobic probes in dilute solution

Electron spin exchange rate constants have been measured by ESR spectroscopy for a nitroxide spin probe in a number of solvents, including water. The apparent collision rate constants (k _c ^′ ) calculated from the spin exchange rate constants showed marked deviations from the Smoluchowsky equation (k _c ^′ η=const), which were greatest in solvents of lowest viscosity. These effects are attributed to inefficiency of the spin exchange process. Self-diffusion coefficients (D) were measured for diamagnetic analogs of the nitroxide spin probe in similar solvent systems by pulsed field gradient NMR spectroscopy. TheD values gave reasonable agreement when corrected for viscosity (Dη=const). Collision rate constants calculated fromD were in good agreement with those measured by ESR in solvents of high viscosity. Thek _c ^′ value for the spin probe in water was significantly lower than that in isoviscous organic solvents. This effect is discussed in terms of a hydrophobic hydration shell for the spin probe which acts as an additional barrier to collision.     

The microwave spectrum,structure and centrifugal distortion constants of 2-chloroacrylonitrile

The microwave spectrum of 2-chloroacrylonitrile has been studied in the 26.5–40 GHz region. A total of 99 a- and b-type rotational transitions have been measured and assigned for CH₂ =C³⁵Cl(CN),yielding values for the rotational constants (in MHz): A = 6973.27, B = 3148.16, C = 2165.95. For CH₂=C³⁷Cl(CN) a total of 53 transitions have been measured and assigned and the rotational constants obtained are (in MHz): A = 6909.35, B = 3081.17, C = 2127.98. The distortion effects have also been studied and the quartic distortion constants have been evaluated. From the observed hyperfine structure, the chlorine nuclear quadrupole coupling constants have been obtained. The structure of vinyl cyanide and vinyl chloride can be transferred to account remarkably well for the observed rotational constants.     

The potentiometric determination of stability constants of sulfite and cobalt(II) in ionic strength 2.0 M (NaClO4). Use of the matrix method

Determination of the stability constants for the system Co(II)/SO₃²⁻, was made potentiometrically using SO₃²⁻/HSO₃⁻ as buffers. Computation and matrix methods lead to the following overall stepwise constants: β₁ = 4.3 x 10² M⁻¹, β₂ = 2.2 x 10⁴ M⁻² and β₃ = 3.0 x 10⁶ M⁻³. The partial constants (M⁻¹) obtained were: K₁ = 4.3 x 10², K₂ = 51 and K₃ = 1.4 x 10². The HSO₃⁻ competition was eliminated by using the extrapolation procedure.     

Positional chemoselectivity in the Zn(II)-mediated removal of phenol protecting groups

A protocol was developed for the chemoselective ortho-deprotection of polyphenolic substrates using readily available Zn^IIX₂ salts. This procedure provides exceptional positional selectivity for the deprotection of phenols that reside adjacent to directing carbonyl functionality in the presence of similar protecting groups at the meta and para positions. Good to excellent yields of the desired free phenols were obtained (?96%), and a wide assortment of protecting groups was readily removed under the reaction conditions.     

Collisional quenching of NO(A,v&#x27; =0) by various gases

The rate coefficients for the quenching of NO(A ²Σ, v' =0) by NO, N₂O, H₂O, O₂, SF₆ and CO₂ were measured to be (in units of 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹): 2.67±0.41, 3.79±0.49, 7.8±1.0, 1.46±0.25, 3.82±0.49, and 4.30±0.26, respectively. Upper limits for the quenching rate constants by N₃, Ar, Ne, H₂, and CF₄ were also measured. All experiments were carried out by monitoring the temporal profiles of A ²Σ, v' =O→X²Π, v″=3 fluorescence after 226 nm pulsed laser excitation of NO(X²Π) to NO(A²Σ,v'=0).     

Complex Formation Equilibria of Unusual Seven-Coordinate Fe(EDTA) Complexes with DNA Constituents and Related Bio-relevant Ligands

Seven-coordinate Fe(EDTA)?CL complexes, where L represents a DNA constituent (uracil, uridine, thymine, thymidine and inosine), methylamine, ammonium chloride or imidazole, were investigated to resolve the solution chemistry of this system. The results show formation of 1:1 complexes with DNA constituents and the other ligands, supporting the hepta-coordination mode of Fe(III) ion. Stability constants of the complexes were measured by potentiometric titration at 25?°C and ionic strength 0.1 mol?L^?1 NaNO₃. The hydrolysis constant of [Fe(EDTA)(H₂O)]^? and the formation constants of the complexes formed in solution were calculated using the non-linear least-squares program MINIQUAD-75. The concentration distributions of the various complex species were evaluated as a function of?pH. The thermodynamic parameters ??H ⁰ and ??S ⁰, calculated from the temperature dependence of the equilibrium constants, were determined for the Fe(EDTA)?Curacil complex. The effect of dioxane as a solvent on the protonation constant of uracil, hydrolysis constants of [Fe(EDTA)(H₂O)]^?, and the formation constants of the Fe(EDTA)?Curacil complex are discussed.     

Electron transfer reactions between the superoxide ion and quinones

The electron transfer reactions of the superoxide ion with benzoquinone, trimethylbenzoquinone, and menadione in dimethylformamide were studied. A procedure of the determination of the relative rate constants of these reactions was developed; the reaction of O? ₂ with butyl bromide was chosen as a standard one. The relative rate constants measured at 20,°, 35°, and 50°C were slightly dependent on the quinone structure. The relationship between the free energy ΔF*of the electron transfer reactions and the standard free energy ΔF^o was discussed. This relationship is proposed as ΔF* = αΔF^o + β, where the proportionality coefficient α is equal to 0.04–0.11 for exothermal reactions and to 0.90–0.96 for endothermal reactions.     

Non-linear perturbation functions in the study of electrode kinetics: Part III. The use of the current-time function I(t)=I0t−1/2 in the study of irreversible charge transfer reactions

Theoretical calculations for the study of irreversible charge transfer reactions with the current-time function I(t)=I₀t^?1/2 are given. Methods of determining the kinetic constants from the resulting potential-time functions are proposed. Experimental results obtained by this method for the system Zn²⁺/Zn(Hg) with various concentrations of NaNO₃ as supporting electrolyte are presented.     

Adiabatic channel study of the capture of nitrogen and oxygen molecules by an ion: effect of nuclear symmetry and spin-spin interaction

Within the adiabatic channel treatment of ionmolecule capture we have calculated the low temperature capture rate constants of N₂(¹Σ _g ⁺ ) and O₂(³Σ _g ^? )in collisions with positive and negative ions. In both cases, the charge-quadrupole and the anisotropic charge-induced dipole interactions produce noticeable deviations from the Langevin rate constant. The rate constants calculated with account for the anisotropic interaction, in addition, are substantially affected by nuclear symmetry; in the case of O₂(³Σ _g ^? ), the fine-structure spin-spin interaction strongly manifests itself in the rate constants.