The discrimination between triplet state and radical cation in the pulse radiolysis of bithiophene in CCl4

The triplet state (³2T) and the radical cation (2T⁺√) of 2,2′-bithiophene (2T) are characterized by pulse radiolysis in CCl₄. Two main absorption bands at 360 and 420 nm are respectively attributed to ³2T* and to 2T⁺√. The triplet, induced in an excited state through a Förster mechanism, undergoes a conformational rearrangement (k₆=(6.8±0.9)×10⁶ s⁻¹). The radical cation is produced both through a resonance charge transfer and a second diffusional process; the two oxidizing species are respectively CCl₄⁺√ and (CCl⁺₃Cl⁻)_solv through the mediation of a singlet excited state, ¹2T*.     

Structures and energetics of C3H6S isomers: a Gaussian-3 ab initio study

Twenty-two isomers/conformers of C₃H₆S^+√ radical cations have been identified and their heats of formation (ΔH_f) at 0 and 298 K have been calculated using the Gaussian-3 (G3) method. Seven of these isomers are known and their ΔH_f data are available in the literature for comparison. The least energy isomer is found to be the thioacetone radical cation (4⁺) with C_2v symmetry. In contrast, the least energy C₃H₆O^+√ isomer is the 1-propen-2-ol radical cation. The G3 ΔH_f298 of 4⁺ is calculated to be 859.4 kJ mol⁻¹, ca. 38 kJ mol⁻¹ higher than the literature value, ≤821 kJ mol⁻¹. For allyl mercaptan radical cation (7⁺), the G3 ΔH_f298 is calculated to be 927.8 kJ mol⁻¹, also not in good agreement with the experimental estimate, 956 kJ mol⁻¹. Upon examining the experimental data and carrying out further calculations, it is shown that the G3 ΔH_f298 values for 4⁺ and 7⁺ should be more reliable than the compiled values. For the five remaining cations with available experimental thermal data, the agreement between the experimental and G3 results ranges from fair to excellent.

Cation CH₃CHSCH₂^+√ (10⁺) has the least energy among the eleven distonic radical cations identified. Their ΔH_f298 values range from 918 to 1151 kJ mol⁻¹. Nevertheless, only one of them, CH₂=SCH₂CH₂^+√ (12⁺), has been observed. Its G3 ΔH_f298 value is 980.9 kJ mol⁻¹, in fair agreement with the experimental result, 990 kJ mol⁻¹.

A couple of reactions involving C₃H₆S^+√ isomers CH₂=SCH₂CH₂^+√ (12⁺) and trimethylene sulfide radical cation (13⁺) have also been studied with the G3 method and the results are consistent with experimental findings.     

Heats of formation of isomeric [C, H4, O], [C, H3, N] and [C, H5, N] radical cations

The heats of formation of six radical cations have been calculated using ab initio MO methods at the MP4/6-31 + G(2df, p) level with MP2/6-31G(d, p)-optimized geometries. The theoretical values for ΔH⁰_f,298 (kJ/mol) of the radical ions considered are (experimental values in parentheses): methanol CH₃OH^+√: 854 (845); methyleneoxonium CH₂OH^+√₂: 815 (816); methyleneimine CH₂NH^+√: 1076 (1054); aminomethylene HCNH^+√₂: 1040 (1079); methylamine CH₃NH^+√₂: 863 (843) and methyleneammonium CH₂NH^+√₃: 855 (958). The calculated results thus confirm the discrepancy between experiment and theory on the heats of formation of nitrogen-containing radical cations. In the latter, the distonic species are calculated to be more stable than their classical isomers. The higher stability of the distonic ions has also been discussed. The recommended heat of formation of the methyleneiminium cation CH₂NH⁺₂ is 754 kJ/mol.     

Redox reactions of 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) in aqueous solution

Pulse radiolysis technique has been employed to study the reactions of oxidizing (^√OH, N₃^√) and reducing radicals (e⁻_aq, CO₂^√−, acetone ketyl radical) with 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) at different pH. Hydroxyl radicals react mostly by addition reaction forming radical adducts (λ_max=420 nm) and the oxidation is only a minor process even in the alkaline region. The reaction with azide radicals produced phenoxyl radicals (λ_max=340 nm), which are formed on fast deprotonation of solute radical cation. Using PMZ^√+/PMZ and ABTS^√−/ABTS²⁻ as the reference couple, different methods are employed to determine the one-electron reduction potential of o-vanillin and the average value is estimated to be 1.076±0.004 V vs. NHE at pH 6. The phenoxyl radicals of o-vanillin were able to oxidize ABTS²⁻ quantitatively. The e_aq⁻ is observed to react with o-vanillin with rate constant value of 2×10¹⁰ dm³ mol⁻¹ s⁻¹. CO₂^√− and acetone ketyl radical are also observed to react with o-vanillin by electron transfer mechanism and showed the formation of transient absorption bands with λ_max at 350 and 390 nm at pH 4.5 and 9.7, respectively. The pK_a of the one-electron reduced species was determined to be 8.1. The results indicate that the aldehydic group is the most preferred site for electron addition.     

Excess volumes for binary liquid mixtures of methylethylketone with methylene chloride, 1,2-dichloroethane, trichloroethylene, tetrachloroethylene and cyclohexane at various temperatures

Dilatometric measurements of excess volumes V^E have been made for binary liquid mixtures of methylethylketone with methylene chloride (CH₂Cl₂), 1,2-dichloroethane (CH₂ClCH₂Cl) and tetrachloroethylene (CCl₂CCl₂) at 293.15 and 303.15 K, for mixtures of methylethylketone with trichloroethylene (CHClCCl₂) at 298.15 and 308.15 K, and for mixtures of methylethylketone with cyclohexane (c-C₆H₁₂) at 303.15 K. The values of V^E have been found to be highly positive for methylethylketone + c-C₆H₁₂, slightly positive for methylethylketone + CH₂Cl₂ and methylethylketone + CCl₂CCl₂, and slightly negative for methylethylketone + CHClCCl₂ and methylethylketone + CH₂ClCH₂Cl. The results indicate the existence of specific interactions of methylethylketone with CH₂Cl₂, CH₂ClCH₂Cl, CHClCCl₂ and CCl₂CCl₂.     

Fraction of excited-state oxygen formed as b Σg in solution-phase-photosensitized reactions II. Effects of sensitizer substituent

The fraction F_Σ of excited-state oxygen formed as b ¹Σ_g⁺ was determined for a series of triplet-state photosensitizers in CCl₄ solutions. F_Σ was determined by monitoring the intensities of (a) O₂(b ¹Σ_g⁺) fluorescence at 1926 nm (O₂(b ¹Σ_g⁺)→O₂(a ¹Δ_g) and (b) O₂(a ¹ Δ_g) phosphorescence at 1270 nm (O₂(a ¹Δ_g) → O₂(X³Σ_g⁻)). Oxygen excited states were formed by energy transfer from substituted benzophenones and acetophenones. The data indicate that F_Σ depends on several variables including the orbital configuration of the lowest triplet state and the triplet-state energy. The available data indicate that the sensitizer-oxygen charge transfer (CT) state is not likely to influence F_Σ strongly by CT-mediated mixing of various sensitizer-oxygen states.     

Flash photolysis/temperature jump study of the vibrational relaxation of N2O(010) by O(P) and NO

This survey begins with the photochemistry at 254 nm and 298 K in the system H₂O₂COO₂RH, the primary objective of which is to determine the rate constants for the reaction OH + RH → H₂O + R relative to the well-known rate constant for the reaction OH + CO → CO₂ + H. Inherent in the scheme is that the reaction HO₂+CO→OH+CO₂ is negligible compared with the OH reaction, and a literature consensus gives k_HO2 < 10⁻¹⁹ cm³ molecule⁻¹ s⁻¹, or some 10⁶ less than k_OH at 298 K. Theoretical calculations establish that the first stage in the HO₂ reaction is the formation of a free radical intermediate HO₂ + CO → HOOCO (perhydroxooxomethyl) which decomposes to yield the products, and that the rate of formation of the intermediate is equal to the rate of formation of the products. The structure of the intermediate and a reaction profile are shown.

High temperature rate data reported subsequent to the data in the consensus and theoretical calculations lead here to a recommendation that, in the range 250–800 K, k_HO2 = 3.45 × 10⁻¹²T^1/2 exp(1.15 × 10⁴/T) cm³ molecule⁻¹ s⁻¹, the hard-sphere-collision Arrhenius modification. This yields k_HO2(298) = 1.0 × 10⁻²⁷ cm³ molecule⁻¹ s⁻¹ or some 10¹⁴ slower than k_OH(298).     

Persulphate quenching of the excited state of ruthenium(II) tris-bipyridyl dication: thermal reactions

The rate constant for the reaction between the sulphate radical (SO₄^√−) and the ruthenium (II) tris-bipyridyl dication (Ru(bipy)₃²⁺) is (3.3±0.2)×10⁹ mol⁻¹ dm³ s⁻¹ in 1 mol dm⁻³ H₂SO₄ and (4.9±0.5)×10⁹ mol⁻¹ dm³ s⁻¹ in 0.1 mol dm⁻³, pH 4.7 acetate buffer. The SO₄^√−radical produced by the electron transfer quenching of Ru(bipy)₃^2+* by S₂O₈²⁻ reacts rapidly with both acetate buffer and chloride ions. These side reactions result in a reduction in the overall quantum yield of Ru(bipy)₃³⁺ production and reduced reaction selectivity when Ru(bipy)₃^2+* is quenched by persulphate.     

First steps in radiation-induced hydrogel synthesis: radical formation and oligomerization in dilute aqueous N-isopropylacrylamide solutions

The reactions of hydroxyl radical, hydrogen atom and hydrated electron intermediates of water radiolysis with N-isopropylacrylamide (NIPAAm) were studied by pulse radiolysis in dilute aqueous solutions. OH, H and e_aq⁻ react with NIPAAm with rate coefficient of (6.9±1.2)×10⁹, (6.6±1)×10⁹, and (1.0±0.2)×10¹⁰ mol⁻¹ dm³ s⁻¹. In OH and H radical addition to the double bond mainly -carboxyalkyl type radicals form, (OHCH₂CH^√C(N-i-C₃H₇)O and CH₃CH^√C(N-i-C₃H₇)O). In reaction of e_aq⁻ oxygen atom centered radical anion is produced (CH₂CHC^√(N-i-C₃H₇)O⁻), the anion undergoes reversible protonation with pK_a=8.7. There is also an irreversible protonation on the β-carbon atom that produces the same radical as forms in H atom reaction (CH₃CH^√C(N-i-C₃H₇)O). The -carboxyalkyl type radicals at low NIPAAm concentration (0.1–1 mmol dm⁻³) mainly disappear in self-termination reactions, 2k_t,m=8.4×10⁸ mol⁻¹ dm³ s⁻¹. At higher concentrations the decay curves reflect the competition of the self-termination and radical addition to monomer (propagation). The termination rate coefficient of oligomer radicals containing a few monomer units is 2k_t≈2×10⁸ mol⁻¹ dm³ s¹.     

基于N,N'-双(3-吡啶基)-对苯二甲酰胺和1,3,5-苯三甲酸的二维Co(Ⅱ)金属有机骨架的合成、结构和磁性

在水热条件下,基于配体N,N'-双(3-吡啶基)-对苯二甲酰胺(3-bptpa)和1,3,5-苯三甲酸(1,3,5-H₃btc),合成了一例具有二维格子结构的钴(Ⅱ)MOF[Co(3-bptpa)(1,3,5-Hbtc)]·2H₂O(1),并进行了红外光谱(FT-IR)、元素分析(EDS)、差热-热重分析(DTA-TG)、X射线单晶衍射(XRD)和磁学表征。 结果表明,每个1,3,5-Hbtc^2-提供1个螯合配位羧基和1个桥连配位羧基与Co(Ⅱ)离子配位。 中心对称的二聚体[Co(3-bptpa)(1,3,5-Hbtc)]₂通过桥连配位的羧基连接成1D梯形链,相邻的梯形链通过3-bptpa与Co(Ⅱ)的配位作用连接为2D格子,从而形成CoN₂O₄变形八面体的配位构型。对配合物1在16~300 K的磁化率数据,使用八面体场下旋轨耦合的各向同性的单离子近似和分子场理论进行分析,Co(Ⅱ)离子表现强的旋轨耦合作用(λ=-100.4 cm^-1),相邻的Co(Ⅱ)离子之间通过桥连配位的羧基传递弱的反铁磁相互作用(zj'=-0.618 cm^-1)。     

Reactions of oxidizing radicals with 2- and 3-aminopyridines: a pulse radiolysis study

Reactions of OH radicals and some one-electron oxidants with 2-aminopyridine (2-AmPy) and 3-aminopyridine (3-AmPy) were studied in aqueous solutions using pulse radiolysis technique. The OH adduct of 2-AmPy at pH 9 has an absorption maximum at 360 nm along with a weak absorption band in the visible region and was found to be reactive with oxygen. The rate constant for its reaction with O₂ was determined to be 1.0×10⁸ dm³ mol⁻¹ s⁻¹. At pH 4 also, the OH adduct of 2-AmPy has an absorption band at 360 nm. However, there are differences in the absorption at other wavelengths. From the plot of ΔOD vs. pH at 340 nm, the pK_a of the OH adduct was determined to be 6.5. Among the specific oxidants, only SO₄^−√ radicals were able to oxidize 2-AmPy. In the case of 3-aminopyridine (3-AmPy), the transient species formed by OH radical reaction at pH 9 has an absorption maximum at 410 nm with shoulder bands on both the sides. Its absorption spectrum at pH 4 was different indicating the existence of a pK value for the OH adduct. pK_a of 3-AmPy-OH radical adduct species was evaluated to be 5.7. This adduct species was also found to be reactive with oxygen (k=7.6×10⁶ dm³ mol⁻¹ s⁻¹). Specific one-electron oxidants like N₃^√, Br₂^−√ C₂^−√ and SO₄^−√ were able to oxidize 3-AmPy indicating that it is easier to oxidize 3-AmPy as compared to 2-AmPy.     

Properties of phenoxyl radical of dehydrozingerone, a probable antioxidant

Free radical reactions of dehydrozingerone (DZ), a methoxy phenol, were studied at dfferent pHs with a variety of oxidants using nanosecond pulse radiolysis technique. Hydroxyl radical (OH) reaction with the phenolic form at pH 6 led mainly to the formation of an OH-adduct absorbing at 460 nm in addition to a minor oxidation product. On the other hand, at pH 10 with the deprotonated phenoxide ion, the only reaction observable was oxidation generating a phenoxyl radical absorbing at 360 nm. HPLC analysis indicated formation of two different products at pH 6 from addition and oxidation reactions, whereas at pH 10, only the oxidation product was detectable. Reactions of more specific secondary oxidizing radicals, N3√, Br√, Br⁻₂√ and Tl(II) with DZ gave rise to the phenoxyl radical over the entire pH range. DZ in the phenoxide ion form reacted with nitrogen dioxide and trichloromethyl peroxyl radicals with rate constants 6×10⁸ and 8.8×10⁸ dm³ mol⁻¹ s⁻¹ respectively leading to the phenoxyl radicals. The DZ phenoxyl radical reacted with trolox C (an analogue of -tocopherol) with a rate constant of 8.3×10⁷ dm³ mol⁻¹ s⁻¹. One electron reduction potential of the DZ phenoxyl radical at pH 6 was determined to be +1.1 V vs NHE using N3√/N⁻₃ as the standard couple.     

Thermochemical study of the complex formation of ag(I) with some sulphur-containing aminopyridines

The enthalpies of formation of the complexes between the silver(I) ion and some sulphur-containing aminopyridines of general formula N(CH₂)_n−1-S-(CH₂)_m-NH₂ where n = 1, and m = 1,2; 1,3; 2,2; 2,3 have been determined by direct calorimetric titration at 25°C in 0.5 M (K)NO₃ solution. The corresponding entropy terms, ΔS, have been calculated using the obtained enthalpy values and the previously reported ΔG values.

In acid medium (pH < 3) coordination occurs through the thioether group and the protonated species AgLH³⁺₂ and AgL₂H⁵⁺₄ are enthalpy-stabilized and entropy-destabilized. At higher pH values (pH &>; 3) additional chelation through the pyridine nitrogen is obvious by a marked increase in the enthalpy of formation of the complexes AgL₂H⁴⁺₃, AgL₂H³⁺₂ and AgLH²⁺. The last complex dimerizes into a cyclic dimer Ag₂L, H⁴⁺₂. At still higher pH values (pH &>; 6) the participation of the amino group in the dimeric chelates Ag₂L₂H³⁺ and Ag₂L²⁺₂ is revealed again by a marked increase in the heat of complexation.     

An EPR/ENDOR study of the asymmetric hydrogen bond between the quinone electron acceptor and the protein backbone in Photosystem I

Hydrogen bonding to the photoaccumulated secondary acceptor radical anion A₁^√− in photosystem (PS) I has been studied using pulsed Q-band ENDOR spectroscopy. With deuterated quinone in protonated PS I particles it is demonstrated that the observed radical anion has only one hydrogen-bond hyperfine coupling (hfc) tensor with tensor components above the 2 MHz range. Below 2 MHz the protein matrix protons dominate and a second weak H-bond could not be detected. The spectral resolution of pulsed Q-band ENDOR is critically required to separate the signals of the H-bond proton from those of the primary chlorophyll acceptor, A₀^√−, which cannot be avoided to be formed to some extent in the photoaccumulation procedure. The determined H-bond hfc tensor of A₁^√− is found to be close to axial symmetry with a small isotropic component, as expected from a predominantly dipolar electron–proton spin interaction in a hydrogen-bond. The principal tensor components are A=(+)7.7, MHz A=(−)4.9 MHz, A_iso=(−)0.7 MHz. The magnitude of the dipolar tensor corresponds to an unusually short H-bond which can be estimated from the point-dipole approximation (1.5±0.1 Å). Based on previous studies with A- and B-branch specific site-directed mutants of the A₁ site of PS I and the chosen photoaccumulation protocol, the observed A₁^√− radical anion can be assigned to the Q_K–A site of the A-branch. The observed H-bond hfc tensor is compared to those determined for related quinone radical anions observed in frozen protic solution as well as in the Q_A site of type II bacterial reaction centers.     

Self-termination rate of para-substituted benzyl radicals in aqueous solutions: a time-resolved study

The self-termination rates of the benzyl radical (C₆H₅---CH₂^√) and para-substituted benzyl radicals (X---C₆H₄---CH₂^√) were studied in aqueous solutions. The Arrhenius parameters and activation energies were determined in the temperature range 275.5–328 K. The kinetic activation energies of these radicals were close to the dynamic activation energy of the solvent, indicating that the termination rate is controlled by diffusion. The values for the rate constants (2k_t (10⁹ dm³ mol⁻¹ s⁻¹)) and the activation energies (E (kJ mol⁻¹)) were 5.94±0.52 and 14.69±0.61 for CH₃O---C₆H₄---CH₂^√, 4.52±0.2 and 17.65±1.16 for CH₃7z.sbnd;C₆H₄---CH₂^√, 3.07±0.45 and 17.58±0.97 for H---C₆H₄---CH₂^√, 4.13±0.81 and 19.10±1.20 for Cl---C₆H₄---CH₂^√ and 4.17±0.44 and 14.62±0.52 for NO₂---C₆H₄---CH₂^√.     

Rotational excitation of ions produced by the He(2 S) Penning ionization of CO and N2

Rotational-state distributions of the CO⁺ (A–X, B–X) and N₂⁺(B–X) emissions produced by the collisions of He(2 ³S) with CO and N₂ were studied in the collision energy (E_R range 100–200 meV. The rotational populations of the emitting states can be fitte by single Boltzmann temperatures (T_R. The T_R (320 ± 30 K) for the ν′ = 3 and 4 levels of the CO⁺ (A²Π) state are nearly independent of, or slightly increase with, E_R, while T_R for the CO⁺(B²Σ⁺, ν′ = 0) state increases rapidly with E_R.The T_R (430 ± 20 K) for the N₂⁺(B²Σ⁺, ν′ = 0) state is nearly independent or slightly decreases with increasing E_R. Interactions providing these trends are discussed.     

New temperature effect on tunneling reaction in hydrogen, alkane, and ethanol in the solid phase

Rate constants for the tunneling reaction (HD + D → h + D₂) in solid HD increase steeply with increasing temperature above 5 K, while they are almost constant below 4.2 K. The apparent activation energy for the tunneling reaction above 5 K is 95 K, which is consistent with the energy (91–112 K) for vacancy formation in solid hydrogen. The results above 5 K were explained by the model that the tunneling reaction was accelerated by a local motion of hydrogen molecules and hydrogen atoms. The model of the tunneling reaction assisted by the local motion of the reactans and products was applied to the temperature dependence of the proton-transfer tunneling reaction (C₆H₆⁻ + C₂H₅OH → C₆H₇ + C₂H₅O⁻) in solid ethanol, the tunneling elimination of H₂ molecule of H₂ molecule ((CH₃)₂ CHCH(CH₃)₂⁺ → (CH₃)₂ C = C(CH₃)₂⁺ + H₂) in solid 2,3-dimethylbutane, and the selective tunneling reaction of H atoms in solid neo-C₅H₁₂-alkane mixtures.     

Non-collinear magnetic structure of the Sr2ErRuO6 double perovskite

The crystal and magnetic structure of Sr₂ErRuO₆ has been studied by means of neutron powder diffraction as well as magnetization and susceptibility measurements. Neutron diffraction profile measured at 50 K shows that the Ru⁵⁺ and Er³⁺ are ordered in the B-sites of the perovskite-type structure, while the Sr atoms occupy the A-site. This compound crystallizes with a monoclinic unit cell, space group P2₁/n and lattice parameters are approximately √2a_p × √2a_p × 2a_p. Magnetic susceptibility measurements reveal the existence of antiferromagnetic interactions in which Ru⁵⁺ and Er³⁺ sublattices are involved. The field dependence of the magnetization indicates the presence of a weak ferromagnetic component at the transition temperature, arising from the spin canting of the antiferromagnetically ordered Ru⁵⁺ and Er³⁺ moments. Thermal evolution of the neutron diffraction patterns indicate that the Nèel temperature is 36 K and the magnetic reflections can be indexed on the basis of a propagation vector k = [0, 0, 0]. The spin arrangement is described by the AxAz magnetic modes where the Ru⁵⁺ and Er³⁺ moments are mainly aligned along the c-axis of the structure, forming an angle of 6° with the c-axis in the case of the Er³⁺ sublattice and 15° for the Ru⁵⁺ moment.     

Nonlinear dielectric effect in carbon tetrachloride solutions of iodobenzene

The results of measurements of Δ/E² as a function of concentration for CCl₄ solutions of iodobenzene are given and compared with earlier results for CCl₄ solutions of fluoro-, chloro- and bromo-benzene. Terms contributing to the molar saturation constant for pure halogeno-derivatives of benzene have been calculated, using Kielich's theory. The change of sign of Δ/E² for bromo- and iodo-benzene is explained.