The mechanism of the photochemical reactions of SO2 with isobutane excited at 3130 Å

The mechanism of the reactions of electronically excited SO₂ with isobutane has been studied through the measurement of the initial quantum yields of product formation in 3130 Å irradiated gaseous binary mixtures of SO₂ and isobutane and ternary mixtures of SO₂, isobutane, C₆H₆ or CO₂. Under low-pressure conditions (P < 10 torr) the kinetic treatment of the present data shows that only one singlet and one triplet state, presumably the ¹B₁ and ³B₁ states, are involved in the photoreaction mechanism. The data give k_2a = 8.4 × 10⁹; SO₂(¹B₁) + isobutane → products (2a); k_5a ? k₅ = 8.7 × 10⁸ l./mol·sec; SO₂(³B₁) + isobutane → products (5a) SO₂(³B₁) + isobutane → (SO₂) + isobutane (5b) k_1a/k₁ = 0.145 ± 0.037; SO₂(¹B₁) + SO₂ → SO₂(³B₁) + SO₂ (1a) SO₂(¹B₁) + SO₂ → (2SO₂) (1b) k_2b/k₂ = 0.273 ± 0.018; SO₂(¹B₁) + isobutane → SO₂(³B₁) + isobutane (2b); SO₂(¹B₁) + isobutane → (SO₂) + isobutane (2c) error limits are ± 2 σ. The contribution from the excited SO₂(¹B₁) molecules to the quantum yields of the photolyses of SO₂–isobutane mixtures is not negligible. Under high-pressure conditions (P > 10 torr) the low-pressure mechanism coupled with the saturation effect on the phosphorescence lifetimes of SO₂(³B₁) molecules cannot alone rationalize the quantum yields. The evaluation suggests that some nonradiative intermediate state (X) is involved in the formation of “extra” triplet molecules. This ill-defined state decays largely nonradiatively to SO₂ in experiments at low pressures, X → SO₂ (12). In the presence of C₆H₆ the low-pressure data give k₇ = (8.5 ± 1.8) × 10¹⁰, and the high-pressure data give k₇ = (8.3 ± 0.6) × 10¹⁰ and (9.9 ± 0.9) × 10¹⁰l./mol·sec; SO₂(³B₁) + C₆H₆ → nonradiative products (7). These estimates are in good agreement with values directly measured from low-pressure lifetime studies, (8.1 ± 0.7) × 10¹⁰ and (8.8 ± 0.8) × 10¹⁰l./mol·sec.     

Photochemistry of the SO2, 2-pentene system at 3660 Å

The photolysis of SO₂ in the presence of cis- and trans-2-pentene has been investigated at 3660 Å and 22°C. Quantum yield measurements of the SO₂ photosensitized conversion of one isomer into the other are consistent with a mechanism in which the only participating excited electronic state of SO₂ is the SO₂(³B₁) state. Quantum yield measurements were made for a variation in P_SO2/P_isomer reactant ratios of 4.01–283 and 57.5–351 for the cis and trans isomers, respectively. The data are consistent with a mechanism in which a (SO₂-olefin)³ collision intermediate is the precursor to the photosensitized isomeric products. The intermediate undergoes unimolecular decay to yield the cis and trans isomers with probabilities of 0.26 ± 0.05 and 0.69 ± 0.04, respectively. Estimates of the quenching rate constants at 22°C for removal of SO₂(³B₁) molecules by cis- and trans-2-pentene are (0.633 ± 0.125) × 10¹¹ l./mole/sec and (1.00 ± 0.27) × 10¹¹ l./mole/sec, respectively. An experimentally determined photostationary composition, [trans-2-pentene]/[cis-2-pentene] = 2.3 ± 0.1 was in fair agreement with that of 1.7 ± 0.7 as predicted from kinetic data derived in this study.     

The photochemistry of sulfur dioxide excited within its first allowed band (3130 Å) and the “forbidden” band (3700–;4000 Å)

The quantum yields of SO₃ formation have been determined in pure SO₂ and in SO₂ mixtures with NO, CO₂, and O₂ using both flow and static systems. In separate series of experiments excitation of SO₂ was effected within the forbidden band, SO₂(³B₁) ← \documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm SO}_2 (\tilde X,^1 A_1 ) $$\end{document}, and within the first allowed singlet band at 3130 Å. The values of Φ were found to be sensitive to the flow rate of the reactants. These results and the apparently divergent quantum yield results of Cox [10], Allen and coworkers [24, 26, 29], and Okuda and coworkers [11] were rationalized quantitatively in terms of the significant occurrence of the reactions SO + SO₃ → 2SO₂ (2), and 2SO → SO₂ + S [or (SO)₂] (3), in experiments of long residence time. From the present rate data, values of the rate constants were estimated, k₂=(1.2±0.7) × 10⁶; k₃=(5±4) × 10⁵ l˙/mole · sec. Φ values from triplet excitation experiments at high flow rates of NO? SO₂ and CO₂? SO₂ mixtures showed the sole reactant with SO₂ leading to SO₃ formation in this system to be SO₂(³B₁); SO₂(³B₁) + SO₂ → SO₃ + SO(³Σ^?) (la); k=(4.2±0.4) × 10⁷ l./mole · sec. With excitation of SO₂ at 3130 Å both singlet and triplet excited states play a role in SO₃ formation. If the reactive singlet state is ¹B₁, the long-lived fluorescent state, SO₂(¹B₁) + SO₂ → SO₃ + SO (¹ Δ or ³Σ^?) (lb), then k=(2.2±0.5) × 10⁹ l./mole · sec. From the observed inhibition of SO formation by added nitric oxide, it was found that the SO₃-forming triplet state, generated in this singlet excited SO₂ system, had a relative reactivity toward SO₂ and NO which was equal within the experimental error to that observed here for the SO₂(³B₁) species. Either SO₂(³B₁) molecules were created with an unexpectedly high efficiency in 3130 Å excited SO₂(¹B₁) quenching collisions, or another reactive triplet (presumably ³A₂ or ³B₂) of almost identical reactivity to SO₂(³B₁) was important here.     

The photolysis of SO2 at 3130 Å in the presence of cis- and trans-2-pentene

The photolysis of SO₂ at 3130 Å, FWHM = 165 Å, and 22°C has been investigated in the presence of cis- and trans-2-pentene. Quantum yields for the SO₂ photosensitized isomerization of one isomer to the other have been made for a variation in the [SO₂]/[C₅H₁₀] ratio of 3.41–366 for cis-2-C₅H₁₀ and of 1.28–367 for trans-2-C₅H₁₀. A kinetic analysis of each of these systems permitted new estimates to be made for the SO₂ collisionally induced intersystem crossing ratio at 3130 Å from SO₂(¹B₁) to SO₂(³B₁). The estimates of k_1a/(k_1a + k_1b) obtained are 0.12 ± 0.01 and 0.12 ± 0.02 (two different kinetic analyses in the cis-2-C₅H₁₀ study) and 0.20 ± 0.05 and 0.20 ± 0.04 (two different kinetic analyses in the trans-2-C₅H₁₀ study). Collisionally induced intersystem crossing ratios of k_2a/(k_2a + k_2b) = 0.51 ± 0.10 and k_3a/(k_3a + k_3b) = 0.62 ± 0.12 were obtained for cis- and trans-2-pentene, respectively. Quenching rate constants at 22°C for removal of SO₂(³B₁) molecules by cis- and trans-2-C₅H₁₀ were estimated as (1.00 ± 0.29) × 10¹¹ l./mole·sec and (0.857 ± 0.160) × 10¹¹ l./mole/sec, respectively. Prolonged irradiations, extrapolated to infinite irradiation times, for mixtures initially containing SO₂ and pure isomer, either the cis or trans, yielded a photostationary composition of [trans-2-pentene]/[cis-2-pentene] = 2.1 ± 0.1.     

Two exponential decay of 3371 å laser excited CS2 fluorescence

We have observed time resolved CS₂ fluorescence excited by an N₂⁺ pulsed laser at 3371 Å. The optical absorption in this region is unassigned; we find two fluorescing states with collision free lifetimes of 2.9 ± 0.3 and 17 ± 2 μsec. Deactivation rates for both states are reported for the collision partners CS₂, Ar, O₂, and N₂. All rates are near gas kinetic; the 2.9 ± 0.3 μsec state exhibits exceptionally fast deactivation, with the rate constant for CS₂ being (7.9 ± 1.2) × 10⁻¹⁰ cm³/molecule sec.     

The photolysis of SO2 at 3080 Å in the presence of cis- and trans-1,2-difluoroethylene

The photolysis of SO₂ at 3080 Å, FWHM = 150 Å, and 22°C has been investigated in the presence of cis- and trans-C₂F₂H₂. Quantum yield measurements for the photosensitized isomerization of cis-C₂F₂H₂ to trans-C₂F₂H₂ have been made for a variation in the [SO₂]/[cis-C₂F₂H₂] ratio from 0.992 to 253. The results fit a mechanism which is consistent with the SO₂(³B₁) state being the reactive excited state of sulfur dioxide. A mechanism employing only the SO₂(¹B₁) and SO₂(³B₁) excited states is quite satisfactory to rationalize the data. A value for the SO₂ collisionally induced intersystem crossing efficiency from SO₂(¹B₁) to SO₂(³B₁) of 0.35 ± 0.14 was estimated while the cis-C₂F₂H₂ efficiency was found to be 0.030 ± 0.012. The rate constant at 22°C for the removal of SO₂(³B₁) molecules by cis-C₂F₂H₂ was found to be (1.43 ± 0.13) × 10¹⁰1./mole · sec. A photostationary composition, [cis]/[trans] = 1.0 ± 0.1, was found from prolonged irradiations of SO₂ in the presence of the cis and trans isomers.     

The photolysis of N2O at 2139 Å and 1849 Å

The method of chemical difference was utilized to accurately determine the relative importance of all the reaction steps in the direct photolysis of N₂O at 2139 Å (25° and 250°C) and 1849 Å (25° C), as well as in the Hg6(¹P₁)-sensitized photolysis of N₂O at 1849 Å (25°C). In all cases, the primary process is predominantly, if not exclusively, Experiments with trace amounts of C₃H₆ added showed a slight, but not significant, difference in product ratios (N₂ and O₂). From these experiments the quantum yield of O(³P) from all possible sources was estimated as 0.02 ± 0.02. Experiments with excess N₂ at 1849 Å indicated that O(¹S) was not produced in the direct photolysis. The O(¹S) yield is probably zero, and certainly <0.05. The O(¹D) atom can react with N₂O via The ratio k₂/k₃ was found to be 0.69 ± 0.05 in all cases. When combined with other data from our laboratory, the average value is 0.65 ± 0.07. This represents the value for translationally energetic O(¹D) atoms. When excess He was added to remove the excess translational energy, k₂/k₃ rose to 0.83 ± 0.06, which is in reasonable agreement with the value of 1.01 ± 0.06 found in another laboratory. We conclude that for O(¹D) atoms with no excess thermal energy, k₂/k₃ = 0.90 ± 0.10.     

Metal–Metal Distances at the Limit: Cr–Cr 1.73 Å – the Importance of the Ligand and its Fine Tuning

The synthesis and structure of a homobimetallic chromium complex is reported. The ligand used to stabilise the quintuply bonded metals is a sterically fine‐tuned guanidinate. A chromium–chromium bond length of 1.7293(12) Å was observed. It is the shortest metal–metal distance reported for a stable compound yet.     

Analysis of the diffuse bands near 6100 Å in the fluorescence spectrum of Cs2

The diffuse bands near 6100 Å in the laser-induced fluorescence spectrum of Cs₂ are analyzed through quantum-mechanical spectral simulations. These bands are interpreted as bound-free emission to the vibrational continuum of the ground state from an excited state of ion-pair character. The lower region of this state, which we have labeled E′, is described approximately by the spectroscopic constants, T_e = 19400 cm⁻¹, R_e = 9 Å, and ω_e = 13 cm⁻¹. Experiments with a single-mode Ar⁺ laser as excitation source clearly reveal fine structure in the E′ → X spectrum, which was not evident for multimode laser excitation. This fine structure confirms our analysis and supports our suggestion that extensive averaging over initial (υ′, J′) levels is responsible for the absence of fine structure in the spectra excited by a multimode laser. Various averaging mechanisms are investigated in the spectral calculations. The paper includes a brief review of other work on “structured continua” in diatomic spectra, and a semiclassical treatment of such structure, with emphasis on the distinction between “reflection” structure and “interference” structure.     

Kinetics of the reaction O(3P) + SO2 + M → SO3 + M over the temperature range of 299°–440°K

Rate constants for the reaction O(³P) + SO₂ + M have been determined over the temperature range of 299°–440°K, using a flash photolysis–NO₂ chemiluminescence technique. For M?Ar, the Arrhenius expression was obtained. At room temperature k₂^Ar = (1.05 ± 0.21) × 10^?33 cm⁶/molec²·sec. In addition, the rate constants k₂ = (1.37 + 0.27) × 10^?33 cm⁶/molec²·sec, k₂ = (9.5 ± 3.0) ± 10^?33 cm⁶/molec²·sec, k₃ = (1.1 ± 0.2) ± 10^?31 cm⁶/molec²·sec, and k₃ = (2.6 ? 0.9) ± 10^?31 cm⁶/molec²·sec were obtained at room temperature where k₃^M is the rate constant for the reaction O + NO + M → NO₂ + M. The rate data are compared and discussed with literature values.     

Kinetics of malachite green fading in water–ethanol–2‐propanol ternary mixtures

The rate constant of malachite green (MG⁺) alkaline fading was measured in water–ethanol–2‐propanol ternary mixtures. This reaction was studied under pseudo‐first‐order conditions at 283–303 K. It was observed that the observed reaction rate constants, k_obs, were increased in the presence of different weight percentages of ethanol and 2‐propanol. The fundamental rate constants of MG⁺ fading in these solutions were obtained by using the SESMORTAC model. In each series of experiments, the concentration of one alcohol was kept constant and the concentration of the second one was changed. It was observed that at the constant concentration of one alcohol and variable concentrations of the second one, with an increase in temperature, k₂ values decrease according to the trend of hydroxide ion nucleophilic parameter values and k₁ values increase. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 441–453, 2011     

On the Phase Shifts in the Dynamics of the H2O2–Na2S2O3–H2SO4–CuSO4 Oscillator Revealed by Comparison of Potentiometric Responses of Different Indicator Electrodes

The dynamics of the H₂O₂–Na₂S₂O₃–H₂SO₄–CuSO₄ homogeneous pH oscillator was studied in the flow reactor potentiometrically using different sensors: platinum electrode, Cu(II) ion‐selective electrode (Cu‐ISE), and pH‐electrode. It was found that for the flow rates close to two bifurcation values, between which the oscillations exist, there is a detectable phase shift between the response of the Cu‐ISE and other electrodes, while it practically vanishes for the intermediate flow rates. To explain both the oscillations of the Cu‐ISE potential and the relevant phase shift, the system's dynamics was studied both experimentally and numerically. The literature kinetic mechanism of the pH oscillator was extended for the dynamics of the copper(II) and copper(I) species in the form of thiosulfate complexes, and kinetic parameters of the redox equilibria, ensuring the oscillations, were estimated. It was found that the phase shift at the relatively low flow rates occurs due to limited efficiency of the supply of CuSO₄ catalyst, as the species of lowest concentration, to the reactor, and therefore it can be minimized either by increasing the flow rate of all reactants or, alternatively, by enhancing the model concentration of CuSO₄ in the feeding stream, for its fixed flow rate. This work is one more proof that it is useful to monitor the dynamics of the homogeneous oscillatory systems with more than one electrode, if the experimental potential–time courses are to be explained in terms of an appropriate kinetic mechanism.     

Die Schwingungsspektren und Kraftkonstanten der Anionen MoOF–5 MoOF2–5, MoF–6 und WOF–5

Inhaltsübersicht. Die Schwingungsspektren der Verbindungen CsMoOF₅, MWOF₅ (M = K, Rb, Cs), K₂MoOF₅, K₂MoOF₅ · H₂O, K₂MoOF₅ · KHF₂ und MMoF₅ (M = K, Cs) wurden aufgenommen und zugeordnet. Aus den so ermittelten Normalschwingungsfrequenzen wurden für die Ionen MoOF₅^–, WOF₅^–, MoOF₅^2– und MoF^–₅ Kraftkonstanten nach einem modifizierten Valenzkraftfeld berechnet. Anhand der erhaltenen Werte der Valenzkraftkonstanten werden die Bindungs-verhältnisse diskutiert. Vibrational Spectra and Force Constants of the Anions M0OF₅^–, MoOF₅^2–, MoF^–₆, and WOF₅^– Abstract. The vibrational spectra of CsMoOF₅, MWOF₅ (M = K, Rb, Cs), K₂MoOF₅, K₂MoOF₅ · H₂O, K₂MoOF₅ · KHF₂, and MMoF₅ (M = K, Cs) have been recorded and assigned. The vibrational frequencies were used to calculate MVFF constants for the ions MoOF₅^–, WOF₅^–, MoOF₅^2–, and MoF^–₆. The stretching force constants are discussed relative to the bond properties.     

β‐Tl2SO4

The ambient‐temperature form of dithallium sulfate, β‐Tl₂SO₄, is similar to β‐K₂SO₄ and is characterized by isolated sulfate tetrahedra and two different thallium sites with coordination numbers 9 and 11. All the atoms, except one O atom, lie on mirror planes. In spite of there being a high concentration of Tl⁺ cations, the stereochemical activity of the 6s² pairs is low, similar to that of isotypic Tl₂XO₄ compounds (X = Cr and Se). This behaviour is the consequence of both weak Tl—O bonds and strong X—O bonds, because in a Tl—O—X linkage the electronic cloud of the O²⁻ anion is strongly distorted and displaced towards X, resulting in a low negative charge in the face of the Tl atom. Consequently, the Coulombic repulsions between the lone pair and the O²⁻ anions are weak. All of the Tl₂XO₄ compounds exhibit the same open packing of A⁺ cations and [XO₄]²⁻ anions as their isotypic alkali counterparts.     

Kinetics of the anionic homopolymerizations of ß–myrcene and 4–methylstyrene in cyclohexane initiated by n–butyllithium

The kinetics of isothermal anionic homopolymerization of ß–myrcene (MYR) and 4–methylstyrene (4MS) in cyclohexane, initiated by n–butyllithium was studied at different temperatures (55, 63, and 71 ° C). The kinetic information obtained from the homopolymerizations was used to estimate the parameters of the Eyring equation, ΔH^?= 84080 J/mol and ΔS^?= ?21.9 J/mol·K for the MYR, and ΔH^?= 51250 J/mol and ΔS^?= ?116.8 J/mol·K for the 4MS, to calculate the apparent propagation coefficients $k_p^{\mathrm{app}}$ as a function of temperature. Finally, the parameters obtained for the Eyring equation were validated by applying them in a mathematical model representing the kinetics of nonisothermal (quasi‐adiabatic) polymerization experiments of MYR and 4MS. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2157–2165     

The Prominent Enhancing Effect of the Cation–π Interaction on the Halogen–Hydride Halogen Bond in M1⋅⋅⋅C6H5X⋅⋅⋅HM2

We designed M¹???C₆H₅X???HM² (M¹=Li⁺, Na⁺; X=Cl, Br; M²=Li, Na, BeH, MgH) complexes to enhance halogen–hydride halogen bonding with a cation–π interaction. The interaction strength has been estimated mainly in terms of the binding distance and the interaction energy. The results show that halogen–hydride halogen bonding is strengthened greatly by a cation–π interaction. The interaction energy in the triads is two to six times as much as that in the dyads. The largest interaction energy is ?8.31 kcal mol^?1 for the halogen bond in the Li⁺???C₆H₅Br???HNa complex. The nature of the cation, the halogen donor, and the metal hydride influence the nature of the halogen bond. The enhancement effect of Li⁺ on the halogen bond is larger than that of Na⁺. The halogen bond in the Cl donor has a greater enhancement than that in the Br one. The metal hydride imposes its effect in the order HBeH<HMgH<HNa<HLi for the Cl complex and HBeH<HMgH<HLi<HNa for the Br complex. The large cooperative energy indicates that there is a strong interplay between the halogen–hydride halogen bonding and the cation–π interaction. Natural bond orbital and energy decomposition analyses indicate that the electrostatic interaction plays a dominate role in enhancing halogen bonding by a cation–π interaction.