A study of the bimolecular intersystem crossing reaction induced in the first excited singlet of SO2 by collisions with O2 and other atmospheric gases

The relative intensities of phosphorescence of SO₂(³B₁) molecules have been determined following the optical excitation of SO₂(¹B₁) molecules by a 2662 Å laser pulse. From a kinetic treatment of these measurements, the intersystem crossing ratio, k_2b/(k_1b + k_2b), was determined; SO₂(¹B₁) + M → SO₂(³B₁) + M (2b); SO₂(¹B₁) + M → SO₂ + M (1b). With M = O₂, N₂, Ar, CO₂, and CO, k_2b/(k_1b + k_2b) = 0.030 ± 0.013, 0.034 ± 0.029, 0.025 ± 0.005, 0.052 ± 0.014, and 0.045 ± 0.028, respectively. These data allow a new, more quantitative evaluation of the extent of involvement of the “excess” triplet SO₂ in the 3130 Å-irradiated mixtures of SO₂ and CO at high pressures [5, 6]. The new data are also of direct interest in the determination of the theoretical maximum rates of photooxidation of SO₂ in the sunlight-irratiated atmosphere of the earth.     

A study of the intersystem crossing reaction induced in gaseous sulfur dioxide molecules by collisions with nitrogen and cyclohexane at 27°C

The quantum yields of the sulfur dioxide triplet (³SO₂)-sensitized phosphorescence of biacetyl (Φ_sens) were determined in experiments with N₂–SO₂–Ac₂ and c-C₆H₁₂–SO₂–Ac₂ mixtures excited at 2875 Å at 27°C. The fraction of the biacetyl triplets which reacts homogeneously by radiative or nonradiative decay reactions was determined in a series of runs at constant [SO₂]/[M] and [SO₂]/[Ac₂] ratios but at varied total pressure. A kinetic treatment of the Φ_sens results and singlet sulfur dioxide (¹SO₂) quenching rate constant data gave the following new kinetic estimates: ¹SO₂ + M → (SO₂–M) (1b) ¹SO₂ + M → ³SO₂ + M (2b); for ¹SO₂–N₂ collisions, k_2b/(k_1b + k_2b) = 0.033 ± 0.008; for ¹SO₂–c-C₆H₁₂ collisions, k_2b/(k_1b ± k_2b) = 0.073 ± 0.024; previous studies have shown this ratio to be 0.095 ± 0.005 for ¹SO₂–SO₂ collisions. It was concluded that the inter-system crossing ratio in ¹SO₂ induced by collision is relatively insensitive to the nature of the collision partner M. However, the individual rate constants for the collision-induced spin inversion of ¹SO₂ (k_2b) and the total ¹SO₂-quenching constants (k_1b + k_2b) are quite sensitive to the nature of M: k_2b/k_2a varies from 0.10 ± 0.03 for M = N₂ to 1.11 ± 0.37 for M = c-C₆H₁₂, and (k_1b + k_2b)/(k_1a + k_2a) varies from 0.29 for M = N₂ to 1.44 for M = c-C₆H₁₂; k_1a and k_1b are the rate constants for the reactions ¹SO₂ - SO₂ → (2SO₂) (1a) and ¹SO₂ + SO₂ → ³SO₂ + SO₂ (2a), respectively.     

Relative rate constants for reactions of HFC 152a, 143, 143a, 134a,and HCFC 124 with F or Cl atoms and for CF2CH3, CF2HCH2, and CF3CFH radicals with F2, Cl2, and O2

Relative rate experiments using UV photolysis of F₂ or Cl₂ have been used to determine rate constant ratios for several hydrofluorocarbon (HFC) reactions with Cl or F atoms and for HFC alkyl radicals with molecular halogens. For mixtures with F₂ present, dark reactions are, also, observed which are attributed to thermal dissociation of the F₂ to form F atoms. At 296 K, the rate of reaction (1a) [CF₂HCH₃ + F → CF₂CH₃ + HF] relative to (1b) [CF₂HCH₃ + F → CF₂HCH₂ + HF] is k_1a/k_1b = 0.73 (±0.13) and is independent of T (= 262–348 K). At 296 K, the ratio of reaction (2a) [CF₂HCH₂F + F → products] to that of (k_1a + k_1b) is (k_1a + k_1b)/k_2a = 2.7 (±0.4), and for reaction (2b) [CF₃CH₃ + F → products] (k_1a + k_1b)/k_2b = 22 ± 12. The temperature dependence (263–365 K) of the rate constant of reaction (3) [CF₃CFH₂ + Cl → products] relative to reaction (4) [CF₃CFClH + Cl → products] is k₃/k₄(±10%) = 1.55 exp(?300 K/T). For the alkyl radicals formed from HFC 152a (CF₂HCH₂ and CF₂CH₃) and from HFC 134a (CF₃CFH), rate constants for the reactions with F₂ and Cl₂ were measured relative to their reactions with O₂. The rate constant of reaction (5cl) [CF₂CH₃ + Cl₂ → CF₂ClCH₃ + Cl] relative to (5o) [CF₂CH₃ + O₂ → CF₂(O₂)CH₃] is k_5cl/k_5o(±15%) = 0.3 exp(200 K/T). For reaction (5f) [CF₂CH₃ + F₂ → CF₃CH₃ + F], k_5f/k_5o(±35%) = 0.23. The ratio for reaction (6f) [CF₂HCH₂ + F₂ → CF₂HCH₂F + F] relative to (6o) [CF₂HCH₂ + O₂ → CF₂HCH₂O₂] is k_6f/k_6o(±40%) = 1.23 exp(?730 K/T). The rate constant ratio for reaction (8cl) [CF₃CFH + Cl₂ → CF₃CFClH + Cl] relative to reaction (8o) [CF₃CFH + O₂ → CF₃CFHO₂] is k_8cl/k_8o(±18%) = 0.16 exp(?940 K/T). For reaction (8f) [CF₃CFH + F₂ → CF₃CF₂H + F], k_8f/k_8o(±35%) = 0.6 exp(?860 K/T). © 1993 John Wiley & Sons, Inc.     

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.     

The SO2-sensitized phosphorescence of biacetyl vapor in photolyses at 2650 and 2875 Å. The intersystem crossing ratio in sulfur dioxide

Quantum yields of the triplet sulfur dioxide (³SO₂)-sensitized phosphoresence (Φ_sens) in biacetyl (Ac₂) have been determined in experiments over a wide range of pressures of SO₂ and Ac₂. Excited singlet sulfur dioxide (¹SO₂) was generated using 2650-Å and 28757hyphen;Å light. The values of Φ_sens were dependent on the [SO₂]/[Ac₂] ratio, as anticpated theoretically. However, in runs at a fixed [SO₂]/[Ac₂] ratio, the measured Φ_sens values were dependent on the total pressure. This theoretically unexpected effect is probably largely the result of biacetyl triplet diffusion with deactivation at the cell wall. Treatment of the quantum yield data in terms of the complete mechanism gave new estimates of the following rate functions: ¹SO₂ + SO₂ → (2SO₂) (1), ¹SO₂ + SO₂ → ³SO₂ + SO₂ (2), k₂/(k₁ + k₂) = 0.082 ± 0.003 (2650 Å), 0.095 ± 0.005 (2875 Å) ³SO₂ + Ac₂ → SO₂ + ³Ac₂ (9a), ³SO₂ + Ac₂ → SO₂ + Ac₂ (9b), k_9a + k_9b = (8.4 ± 2.1) × 10¹⁰ (2650 Å), (8.1 ± 3.0) × 10¹⁰ l./mole-sec (2875 Å) ³SO₂ → SO₂ + hv_p (6), k₆ = (7.3 ± 1.3) × 10¹ sec^?1.     

The α‐effect exhibited in gas‐phase SN2@N and SN2@C reactions

In order to explore the existence of α‐effect in gas‐phase S_N2@N reactions, and to compare its similarity and difference with its counterpart in S_N2@C reactions, we have carried out a theoretical study on the reactivity of six α‐oxy‐Nus (FO^?, ClO^?, BrO^?, HOO^?, HSO^?, H₂NO^?) in the S_N2 reactions toward NR₂Cl (R = H, Me) and RCl (R = Me, i‐Pr) using the G2(+)_M theory. An enhanced reactivity induced by the α‐atom is found in all examined systems. The magnitude of the α‐effect in the reactions of NR₂Cl (R = H, Me) is generally smaller than that in the corresponding S_N2 reaction, but their variation trend with the identity of α‐atom is very similar. The origin of the α‐effect of the S_N2@N reactions is discussed in terms of activation strain analysis and thermodynamic analysis, indicating that the α‐effect in the S_N2@N reactions largely arises from transition state stabilization, and the “hyper‐reactivity” of these α‐Nus is also accompanied by an enhanced thermodynamic stability of products from the n_(N) → σ*_(O?Y) negative hyperconjugation. Meanwhile, it is found that the reactivity of oxy‐Nus in the S_N2 reactions toward NMe₂Cl is lower than toward i‐PrCl, which is different from previous experiments, that is, the S_N2 reactions of NH₂Cl is more facile than MeCl. © 2013 Wiley Periodicals, Inc.     

FTIR study of the Cl- and Br-atom initiated oxidation of trichloroethylene

The Cl- and Br- initiated oxidations of CHCl(DOUBLEBOND)CCl₂ in 700 torr of air at 296 K have been studied using a Fourier transform infrared spectrometer. Rate constants k(Cl+CHCl(DOUBLEBOND)CCl₂)=(7.2±0.8)×10⁻¹¹ and k(Br+CHCl(DOUBLEBOND)CCl₂)=(1.1±0.4)×10⁻¹³ cm³ molecule⁻¹ s⁻¹ were determined using a relative rate technique with ethane and ethylene as references, respectively. The major products observed were CHXClC(O)Cl, (X=Cl or Br), CHClO, and CCl₂O. Combining results obtained for the Cl-initiated oxidation of CHCl₂(SINGLEBOND)CHCl₂, we deduced that Cl-addition on trichloroethylene occurs via channel 1a, Cl+CHCl(DOUBLEBOND)CCl₂→ CHCl₂(SINGLEBOND)CCl₂, (100±12)%. Self-reaction of the subsequently generated peroxy radicals CHCl₂(SINGLEBOND)CCl₂O₂ leads to CHCl₂CCl₂O radicals which were found to decompose via channel 8a, CHCl₂C(O)Cl+Cl, (91±11)% of the time, and channel 8b, CHCl₂+CCl₂O, (9±2)%. The reaction Br+CHCl(DOUBLEBOND)CCl₂→CHBrCl(SINGLEBOND)CCl₂ (17a) accounted for ≥(96±11)% of the total reaction. Decomposition of the CHBrCl(SINGLEBOND)CCl₂O radicals proceeds (≥93±11)% via CHBrClC(O)Cl+Cl. As part of this work, k(Cl+CHCl₂C(O)Cl)=(3.6±0.6)×10⁻¹⁴ and k(Cl+CHCl₂(SINGLEBOND)CHCl₂)=(1.9±0.2)×10⁻¹³ cm³ molecule⁻¹ s⁻¹ were measured. Errors reported above include statistical uncertainties (2σ) and estimated systematic uncertainties. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet: 29: 695–704, 1997.     

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.     

Kinetics and mechanism of the reaction of Cl atoms with HO2 radicals

The kinetic and mechanism of the reaction Cl + HO₂ → products (1) have been studied in the temperature range 230–360 K and at total pressure of 1 Torr of helium using the discharge‐flow mass spectrometric method. The following Arrhenius expression for the total rate constant was obtained either from the kinetics of HO₂ consumption in excess of Cl atoms or from the kinetics of Cl in excess of HO₂: k₁ = (3.8 ± 1.2) × 10^?11 exp[(40 ± 90)/T] cm³ molecule^?1 s^?1, where uncertainties are 95% confidence limits. The temperature‐independent value of k₁ = (4.4 ± 0.6) × 10^?11 cm³ molecule^?1 s^?1 at T = 230–360 K, which can be recommended from this study, agrees well with most recent studies and current recommendations. Both OH and ClO were detected as the products of reaction (1) and the rate constant for the channel forming these species, Cl + HO₂ → OH + ClO (1b), has been determined: k_1b = (8.6 ± 3.2) × 10^?11 exp[?(660 ± 100)/T] cm³ molecule^?1 s^?1 (with k_1b = (9.4 ± 1.9) × 10^?12 cm³ molecule^?1 s^?1 at T = 298 K), where uncertainties represent 95% confidence limits. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 317–327, 2001     

Rate constants for the reactions of Cl atoms with HCOOH and with HOCO radicals

Rate constants have been measured at room temperature for the reactions of Cl atoms with formic acid and with the HOCO radical: Cl + HCOOH → HCl + HOCO (R1) Cl + HOCO → HCl + CO₂ (R2) Cl atoms were generated by flash photolysis of Cl₂ and the progress of reaction was followed by time‐resolved infrared absorption measurements using tunable diode lasers on the CO₂ that was formed either in the pair of reactions ( R1 ) plus ( R2 ), or in reaction ( R1 ) followed by O₂ + HOCO → HO₂ + CO₂ (R3) In a separate series of experiments, conditions were chosen so that the kinetics of CO₂ formation were dominated either by the rate of reaction ( R1 ) or by that of reactions ( R1 ) and ( R2 ) combined. The results of our analysis of these experiments yielded: k₁ = (1.8₃ ± 0.12) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ k₂ = (4.8 ± 1.0) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 85–91, 2000     

Emission studies of the mechanism of gaseous biacetyl photolysis at 3450, 3650, 3880, and 4358 a and 28°C

The quantum yields of phosphorescence (Φ_p) of biacetyl have been determined in pure biacetyl, biacetyl-SO₂, and biacetyl-c-C₆H₁₂ mixtures in experiments using bands of radiation centered at 3450, 3650, 3880, and 4348 Å. It has been shown that the unexpected effect of gas concentration on the quantum yields of the sulfur dioxide triplet-sensitized phosphorescence of biacetyl resulted largely from the significant destruction of biacetyl triplets at the wall of the cell. The kinetics of the variation of Φ_p with [Ac₂], wavelength of the absorbed light, and added gases provide new estimates of the energy relations and the rate constants for the decomposition reaction of vibrationally rich biacetyl molecules in the first excited singlet state (¹Ac₂?): ¹Ac₂? → products (1), ¹Ac₂? + Ac₂ → ¹Ac₂ + Ac₂ (2); the minimum energy necessary in ¹Ac₂? for reaction (1) to occur is estimated to be about 72.8 kcal/mole above the ground state of biacetyl: k₁/k₂ = (4.3 ± 0.1) × 10^?3M at 3450 Å, (4.07 ± 0.04) × 10^?4M at 3650 Å, and (5.6 ± 0.4) × 10^?5M at about 3800 Å. The variation of the rate constant ratio is shown to be consistent with the expectations of the simple theory of excited molecule decomposition. Biacetyl triplet (³Ac₂) rate constants were determined by measurements of Φ_p in O₂ and NO-containing mixtures: ³Ac₂ + S → (Ac₂–S, products) (8); for O₂ = S, k₈ = (5.76 ± 0.40) × 10⁸ (3650 Å experiments), (5.76 ± 0.27) × 10⁸ (4358 Å); for NO = S, k₈ = (3.34 ± 0.20) × 10⁹ (3650 Å), (3.33 ± 0.18) × 10⁹ 1./mole-sec (4358 Å). A comparison between these and previous findings of the SO₂ triplet (³SO₂)-sensitized excitation of biacetyl [5,6] show that the decomposition of the initial ³Ac₂ product of the exothermic energy transfer reaction ³SO₂ + Ac₂ → SO₂ + ³Ac₂ is unimportant.     

Polysulfonylamine. LXIX. Neuartige Pniktogendisulfonylamide: Synthese von Bismutdimesylamiden und Kristallstrukturen des zwölfgliedrigen Cyclodimers [Ph2BiN(SO2Me)2]2 und des ionischen Komplexes [H(OAsPh3)2]⊕(MeSO2)2N⊖

Polysulfonyl Amines. LXIX. Novel Pnictogen Disulfonylamides: Synthesis of Bismuth Dimesylamides and Crystal Structures of the Twelve-Membered Cyclodimer [Ph₂BiN(SO₂Me)₂]₂ and of the Ionic Complex [H(OAsPh₃)₂]^⊕(MeSO₂)₂N^? The novel bismuth(III or V) disulfonylamides Ph₂BiN(SO₂Me)₂ ( 1 ), PhBi[N(SO₂Me)₂]₂ ( 2 ), PhBi[N(SO₂Me)₂]Br ( 3 ), Bi[N(SO₂Me)₂]₂Cl ( 4 ), Bi[N(SO₂Me)₂]Cl₂ · 12-crown-4 ( 5 ) and Ph₃Bi[N(SO₂Me)₂]Cl ( 6 ) were obtained by acidolysis of Ph₃Bi with HN(SO₂Me)₂ (→ 1 ), by metathesis of AgN(SO₂Me)₂ with Ph₂BiCl (→ 1 ) or PhBiBr₂ (→ 2, 3 ), by condensation of BiCl₃ with Me₃SiN(SO₂Me)₂ (→ 4 ; in presence of 12-crown-4: → 5 ), or by oxidative addition of ClN(SO₂Me)₂ to Ph₃Bi (→ 6 ). Independently of the molar ratio employed, triphenylarsane oxide and dimesylamine form the crystalline 2/1 complex [H(OAsPh₃)₂]^⊕(MeSO₂)₂N^? ( 7 ). The crystal packing of 7 (monoclinic, space group C2/c) consists of discrete cations displaying crystallographic C_i symmetry and a strong O …? H …? O hydrogen bond (H atom located on a centre of symmetry, O …? O′ 241.2 pm, As? O …? O′ 120°, As? O 168.3 pm), and chiral anions with crystallographic C₂ symmetry (N? S 157.3 pm, S? N? S 122,9°). In the solid state, the bismuth(III) compound 1 (triclinic, space group P1 ) is a cyclodimer with crystallographic C_i symmetry, in which two Ph₂Bi^⊕ cations are connected through two (α-O, ω-O)-donating dimesylamide ligands to form a roughly twelve-membered [BiOSNSO]₂ ring (Bi? O 239.7 and 246.6, O? S 148.0 and 145.4, S? N 157.7 and 159.2 pm, Bi? O? S 126.6 and 127.5°). The bismuth atom adopts a pseudo-trigonal-bipyramidal geometry (O? Bi? O 165.4, C? Bi? C 93.0, O? Bi? C 83.8 to 86.5°). The essentially similar conformations of the discrete anion in 7 and of the bidentate bridging ligand in 1 are discussed in detail.     

Rate constants and Arrhenius parameters for the reactions of Cl atoms with the halogenated ethers: CF3CH2OCHF2, CF3CHClOCHF2, and CF3CH2OCClF2

Rate constants have been determined for the reactions of Cl atoms with the halogenated ethers CF₃CH₂OCHF₂, CF₃CHClOCHF₂, and CF₃CH₂OCClF₂ using a relative‐rate technique. Chlorine atoms were generated by continuous photolysis of Cl₂ in a mixture containing the ether and CD₄. Changes in the concentrations of these two species were measured via changes in their infrared absorption spectra observed with a Fourier transform infrared (FTIR) spectrometer. Relative‐rate constants were converted to absolute values using the previously measured rate constants for the reaction, Cl + CD₄ → DCl + CD₃. Experiments were carried out at 295, 323, and 363 K, yielding the following Arrhenius expressions for the rate constants within this range of temperature:Cl + CF₃CH₂OCHF₂: k = (5.1₅ ± 0.7) × 10⁻¹² exp(−1830 ± 410 K/T) cm³ molecule⁻¹ s⁻¹ Cl + CF₃CHClOCHF₂: k = (1.6 ± 0.2) × 10⁻¹¹ exp(−2450 ± 250 K/T) cm³ molecule⁻¹ s⁻¹ Cl + CF₃CH₂OCClF₂: k = (9.6 ± 0.4) × 10⁻¹² exp(−2390 ± 190 K/T) cm³ molecule⁻¹ s⁻¹ The results are compared with those obtained previously for the reactions of Cl atoms with other halogenated methyl ethyl ethers. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 165–172, 2001     

Kinetic study of some elementary reactions of sulfur compounds including reactions of S and SO with OH radicals

The reactions of S + OH → SO + H (1) and SO + OH → SO₂ + H (2) were studied in a discharge flow reactor coupled to an EPR spectrometer. The rate constants obtained under the pseudo-first-order conditions with an excess of S or SO were found to be k₁ = (6.6 ± 1.4) × 10^?11 and k₂ = (8.4 ± 1.5) × 10^?11 at room temperature. Units are cm³/molec·sec. Besides no reactivity was observed between S and CO₂ at 298 K and between CIO and SO₂ up to 711 K.     

A kinetics study of the reaction of Cl with NO2

The third order rate coefficients for the addition reaction of Cl with NO₂, Cl + NO₂ + M → ClNO₂ (ClONO) + M; k₁, were measured to be k₁(He) = (7.5 ± 1.1) × 10^?31 cm⁶ molecule^?2 s^?1 and k₁(N₂) = (16.6 ± 3.0) × 10^?31 cm⁶ molecule^?2 s^?1 at 298 K using the flash photolysis-resonance fluorescence method. The pressure range of the study was 15 to 500 torr He and 19 to 200 torr N₂. The temperature dependence of the third order rate coefficients were also measured between 240 and 350 K. The 298 K results are compared with those from previous low pressure studies.     

Gas‐phase reactions of Cl atoms with propane,n‐butane,and isobutane

Using the relative kinetic method, rate coefficients have been determined for the gas‐phase reactions of chlorine atoms with propane, n‐butane, and isobutane at total pressure of 100 Torr and the temperature range of 295–469 K. The Cl₂ photolysis (λ = 420 nm) was used to generate Cl atoms in the presence of ethane as the reference compound. The experiments have been carried out using GC product analysis and the following rate constant expressions (in cm³ molecule^?1 s^?1) have been derived: (7.4 ± 0.2) × 10^?11 exp [‐(70 ± 11)/ T], Cl + C₃H₈ → HCl + CH₃CH₂CH₂; (5.1 ± 0.5) × 10^?11 exp [(104 ± 32)/ T], Cl + C₃H₈ → HCl + CH₃CHCH₃; (7.3 ± 0.2) × 10^?11 exp[?(68 ± 10)/ T], Cl + n‐C₄H₁₀ → HCl + CH₃ CH₂CH₂CH₂; (9.9 ± 2.2) × 10^?11 exp[(106 ± 75)/ T], Cl + n‐C₄H₁₀ → HCl + CH₃CH₂CHCH₃; (13.0 ± 1.8) × 10^?11 exp[?(104 ± 50)/ T], Cl + i‐C₄H₁₀ → HCl + CH₃CHCH₃CH₂; (2.9 ± 0.5) × 10^?11 exp[(155 ± 58)/ T], Cl + i‐C₄H₁₀ → HCl + CH₃CCH₃CH₃ (all error bars are ± 2σ precision). These studies provide a set of reaction rate constants allowing to determine the contribution of competing hydrogen abstractions from primary, secondary, or tertiary carbon atom in alkane molecule. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 651–658, 2002     

FTIR studies of the kinetics and mechanism for the reaction of Cl atom with formylchloride

The rate constant for the reaction Cl + CHClO → HCl + CClO was determined from relative decay rates of CHClO and CH₃Cl inthe photolysis of mixtures containing Cl₂ (～1 torr), CH₃Cl (～1 torr), and O₂ (～0.1 torr) in 700 torr N₂. In such mixtures CHClO was generated in situ as a principal product prior to complete consumption of O₂. The value of k(Cl + CHClO)/k(Cl + CH₃Cl) = 1.6 ± 0.2(3σ) combined with the literature value of k(Cl + CH₃Cl) = 4.9 × 10^?13 cm³/molecule sec gives k(Cl + CHClO) = 7.8 × 10^?13 cm³/molecule sec at 298 ± 2 K, in excellent agreement with a previous value of (7.9 ± 1.5) × 10^?13 cm³/molecule sec determined by Sanhueza and Heicklen [J. Phys. Chem., 79 , 7 (1975)]. Thus this reaction is approximately 100 times slower than the corresponding reactions of aldehydes and alkanes with comparable C? H bond energies (≤95 kcal/mol).     

Kinetic flash spectroscopic study of the CH3O2?CH3O2 and CH3O2?SO2 reactions

Methylperoxy radicals were generated by the flash photolysis of azomethane–oxygen mixtures. The observed broadband spectrum of the CH₃O₂ radical is similar, but not identical to those reported previously. The CH₃O₂ decay followed second-order kinetics at high CH₃O₂ concentrations with k₄' = (2.5 ± 0.3) × 10⁸ liter/mol·sec (23 ± 2°C); 2CH₃O₂ → products (4). Because of the potential loss of CH₃O₂ through the reactions with HO₂ and CH₃O radicals subsequently formed in this system, simulations suggest that the true k₄ is in the range: 2.5 × 10⁸ ≥ k₄ ≥ 2.3 × 10⁸ liter/mol·sec. Deviations from linearity of the plot of the reciprocal of the CH₃O₂ absorbance versus time were seen at long times and were attributed to the reaction (5) with an apparent rate constant k₅' ? (1.6 ± 0.4) × 10⁵ liter/mol·sec; CH₃O₂ + Me₂N₂ → product (5). The CH₃O₂–SO₂ reaction, CH₃O₂ + SO₂ → products (16), was studied by observing CH₃O₂ decay in flashed mixtures of Me₂N₂, O₂, and SO₂. The results gave the apparent second-order rate constant k₁₆' ? (6.4 ± 1.4) × 10⁶ liter/mol·sec. It appears likely that each occurrence of reaction (5) and (16) is followed by the loss of an additional CH₃O₂ radical and that k₅ ? k₅'/2 and k₁₆ ? k₁₆'/2. Our findings suggest that a significant fraction of the SO₂ oxidation in a sunlight-irradiated NO_x?RH-polluted atmosphere, may occur by reaction with CH₃O₂ as well as from the HO and HO₂ reactions.     

Kinetics and mechanism of the reaction of sulphur(IV) with N,N′‐ethylene‐bis(sali‐cylidiniminato)manganese‐(III) in aqueous solution

The reaction of (diaqua)(N,N′‐ethylene‐bis(salicylidiniminato)manganese(III) with aqueous sulphite buffer results in the formation of the corresponding mono sulphito complex, [Mn(Salen)(SO₃)]⁻ (S‐bonded isomer) via three distinct paths: (i) Mn(Salen)(OH₂)₂⁺ + HSO₃⁻ → (k₁); (ii) Mn(Salen)(OH₂)₂⁺ + SO₃²⁻ → (k₂); (III) Mn(Salen)(OH₂)(OH) + SO₃²⁻ → (k₃) in the stopped flow time scale. The fact that the mono sulphito complex does not undergo further anation with SO₃²⁻/HSO₃⁻ may be attributed to the strong trans‐activating influence of the S‐bonded sulphite. The values of the rate constants (10⁻²k_i/dm² mol⁻¹ s⁻¹ at 25°C, I = 0.3 mol dm⁻³), ΔH_i^#/kJ mol⁻¹ and ΔS_i^#/J K⁻¹ mol⁻¹ respectively are: 2.97 ± 0.27, 42.4 ± 0.2, −55.3 ± 0.6 (i = 1); 11.0 ± 0.8, 33 ± 3, −75 ± 10 (i = 2); 20.6 ± 1.9, 32.4 ± 0.2, −72.9 ± 0.6 (i = 3). The trend in reactivity (k₂ > k₁), a small labilizing effect of the coordinated hydroxo group (k₃/k₂ < 2), and substantially low values of ΔS^# suggest that the mechanism of aqua ligand substitution of the diaqua, and aqua‐hydroxo complexes is most likely associative interchange (I_a). No evidence for the formation of the O‐bonded sulphito complex and the ligand isomerization in the sulphito complex, (Mn^III‐OSO₂ → Mn^III‐SO₃), ensures the selectivity of the Mn^III centre toward the S‐end of the S^IV species. The monosulphito complex further undergoes slow redox reaction in the presence of excess sulphite to produce Mn^II, S₂O₆²⁻ and SO₄²⁻. The formation of dithionate is a consequence of the fast dimerization of the SO₃₋. generated in the rate determining step and also SO₄²⁻ formation is attributed to the fast scavenging of the SO₃₋. by the Mn^III species via a redox path. The internal reduction of the Mn^III centre in the monosulphito complex is insignificant. The redox reaction of the monosulphitomanganese(III) complex operates via two major paths, one involving HSO₃₋ and the other SO₃²⁻. The electron transfer is believed to be outersphere type. The substantially negative values of activation entropies (ΔS^# = −(1.3 ± 0.2) × 10² and −(1.6 ± 0.2) × 10² J K⁻¹ mol⁻¹ for the paths involving HSO₃₋ and SO₃²⁻ respectively) reflect a considerable degree of ordering of the reactants in the act of electron transfer. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 627–635, 1999     

Radiolytic decomposition of methanesulfonyl chloride in liquid cyclohexane. A kinetic determination of the bond dissociation energies D(ME-SO2) and D(c-C6H11-SO2)

The gamma radiation induced decomposition of MeSO₂Cl in cyclohexane (RH) was studied between 60 and 150°C. Throughout this temperature range the reaction proceeds by a free radical chain mechanism. Its propagation is described by the following reactions: The kinetic analysis of the results of the experiments with added SO₂, which were carried out in the temperature range of 80 to 150°C, gives 14.94 ± 0.92 and 11.91 ± 0.82 kcal/mole for D(Me-SO₂) and D(c-C₆H₁₁-SO₂), respectively. These bond dissociation energies are considerably lower than the gas-phase values, and the possible cause of this difference is discussed. Present results also seem to indicate that D(MeSO₂-H) does not exceed 95 kcal/mole. Competitive experiments with added tetrachloroethylene result in where θ = 2.303RT in kcal/mole.