Measurement of state-to-state rotational transfer rates in collision of I2*(B 0u+, υ = 15, j) with 3He, 4He,Ne, Ar,H2, D2 and I2

The selective laser excitation and induced fluorescence observation technique has been used to study rotationally inelastic collisions of I₂^*(B 0_u⁺, υ = 15,j) with I₂, ³He, ⁴He, Ne, Ar, H₂ and D₂. For each collision partner, several initial rotational levels ranging from j_i = 12 up to j_i = 146 have been excited. For purely rotational transfer within the υ = 15 level, our data are perfectly consistent with energy sudden (eventually corrected) scaling laws. Thus, any thermally averaged rate constant, k(j_i → j_f), can be expressed as a function of the basis rate constants k(l → 0). Furthermore, these k(l → 0) are found to follow simple empirical fitting laws. Consequently any k(j_i → j_f) can be predicted given a set of two or three fitting parameters. Collisions with relatively heavy particles (I₂, Ar and Ne) are well described by using the inverse power fitting law k(l → 0) = b[l(l+1)]^?γ, where b = 1.7, 1.2 and 1.2×10^?10 cm³ s^?1 and γ = 1.08, 1.02 and 1.17 for I₂^*-Ne, I₂^*-Ar and I₂^*-I₂ collisions respectively. For collisions with light particles (³He, ⁴He, H₂ and D₂), k(l → 0) shows a sharp decrease with l which can be accounted for by a hybrid power-exponential fitting law k(l → 0) = b[l(l+1)]^?γ exp[-l(l+1)/l^* (l^*+1)], where b = 0.84, 0.71, 2.77 and 2.78×10^?10 cm³ s^?1l⁺ = 20.6, 23.1, 18.8 and 31.4, and γ = 0.66, 0.66, 0.78 and 0.91 for I₂^*-³He, I₂^*-He, I₂^*-H₂ and I₂^*-D₂ collisions, respectively. We confirm that the rotational transfer dynamics in heavy molecules is mainly governed by angular momentum exchange.     

Electronic nonadiabatic transitions in the reactive (Ar+ + H2, Ar + H+2, ArH+ + H) system. Numerical results for the collinear configuration

In this communication are presented exact quantum mechanical nonadiabatic electronic transition probabilities for the collinear reaction Ar⁺ + H₂(v_i = 0) → ArH⁺(v_f) + H. The calculations were performed using a potential surface calculated by the DIM method. It is established that large probabilities (≈ 1.0) can be obtained only if there is enough translational energy to overcome a potential barrier formed due to the crossing between v_i = 0 of the Ar⁺ + H₂ system and v_i = 2 of the Ar + H⁺₂ system. The threshold for the reaction is found to be 0.06 eV.     

Collision-induced intramolecular vibration—vibration energy transfer in CO2: CO2(0001) + H2/D2

Intramolecular vibration—vibration energy transfer cross sections have been calculated for CO₂(00⁰1) + H₂/D₂ → CO₂(11¹0) + H₂/D₂, → CO₂(10⁰0) + H₂/D₂, and → CO₂(20⁰0) + H₂/D₂ based on the mechanism that the energy mismatch is transferred to the translational motion. For CO₂ + H₂, the calculated cross section for CO₂(00⁰1) + H₂ → CO₂(10⁰0) + H₂ is in good agreement with experimental data. Cross sections for the processes (00⁰1 → 11¹O) and (00⁰1 → 20⁰0) are found to be too small compared with experimental data. For CO₂ + D₂, (00⁰1 → 10⁰0) is also the most important process and appears to represent experimental data at room temperature. The sum of three cross sections of CO₂ + H₂ is always greater than that of CO₂ + D₂ over the temperature range of 100–2500 K.     

Dynamical competition between reactive and reactive detachment channels in X− + H2 colliding systems (X = Cl,Br, I)

Two reactive processes are observed: X⁻ + H₂ → XH + H⁻ (R) and X⁻ + H₂ → XH + H + e⁻ (RD). The angular and energy distributions of the molecular XH products are measured at collision energies varying from 5 to 10 eV center of mass. These distributions obey identical rules in the three systems: (a) XH molecules formed by both R and RD processes are scattered at the same c.m. angle, respectively 55° ±10 for ClH, 80° ±20 for BrH and 90° ±20 for IH. (b) The rovibrational energy of the XH molecules, when formed by R processes, is limited to a small amount: ⩽ 2 eV for ClH, ⩽ 1.5 eV for BrH, ⩽ 1 eV for IH, whereas when formed by RD it extends to the highest amount available from the collision energy, up to the dissociation limit. The RD process is not observed experimentally in the I⁻/H₂ system. This dynamical behaviour is fully understood in terms of non-adiabatic interaction between the two lowest [XH₂]⁻ ionic surfaces, but the reason of the angular anisotropy is still not well understood.     

Temperature dependence of the probabilities of VV energy exchange between H2 and DCl

The temperature dependence of the removal of the vibrational energy of H₂ by DCl in H₂(1) + DCl(0) has been investigated over the range of 300–3000 K. The energy transfer probability of H₂(1) + DCl(0) → H₂(0) + DCl(1), where the vibrational energy of H₂(1) is removed by both the vibrational and rotational motions of DCl(0), is found to be strongly temperature dependent and increases with temperature closely following the relation log P α T^1/3. Over the temperature range it changes by two orders of magnitude. The probability of the near-resonant process H₂ (1) + DCl(O) → H₂(0) + DCl(2) is very close to that of the former at 300 K, but it increases only slightly as the temperature is raised to 3000 K. The sum of the probabilities of these two processes at 300 K is 3.4 × 10^?5, which agrees with the experimental value of 3.95 × 10^?5.     

The kinetics of ClO formation in the flash photolysis of chlorine–oxygen mixtures

The production of ClOO and ClO radicals following the flash photolysis of chlorine + oxygen mixtures has been studied. For the mechanism the following kinetic parameters were measured: k₃K = 1.3 × 10¹⁰ l²/mol²·sec; k₂/k₃ = 17; and k₃/?(ClOO; 250 nm) = 9.7 × 10⁵ cm/sec. Then k₃ = 5.9 × 10⁹ l/mol·sec, k₂ = 1.0 × 10¹¹ l/mol·sec, and ?(ClOO; 250 nm) = 6.1 × 10³ l/mol·cm. From limits established for the equilibrium constant K, ΔH°_f (ClOO) = 94 ± 2 kJ/mol.     

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.     

Quantum wave packet calculation of reaction probabilities,cross sections,and rate constants for H+ + LiH → Li + H reaction

The H⁺ + LiH → Li + H reactive scattering has been studied using a quantum real wave packet method. The state‐to‐state and state‐to‐all reaction probabilities for the entitled collision have been calculated at zero total angular momentum. The probabilities for J > 0 are estimated from the J = 0 results by using J‐shifting approximation based on the Capture model. The integral cross sections and thermal rate constants are then calculated. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Kinetic study of reactions of C2H5O2 with NO at 298 K and 0.55 – 2 torr

The kinetics of C₂H₅O₂ and C₂H₅O₂ radicals with NO have been studied at 298 K using the discharge flow technique coupled to laser induced fluorescence (LIF) and mass spectrometry analysis. The temporal profiles of C₂H₅O were monitored by LIF. The rate constant for C₂H₅O + NO → Products (2), measured in the presence of helium, has been found to be pressure dependent: k₂ = (1.25±0.04) × 10^?11, (1.66±0.06) × 10^?11, (1.81±0.06) × 10^?11 at P (He) = 0.55, 1 and 2 torr, respectively (units are cm³ molecule^?1 s^?1). The Lindemann-Hinshelwood analysis of these rate constant data and previous high pressure measurements indicates competition between association and disproportionation channels: C₂H₅O + NO + M → C₂H₅ONO + M (2a), C₂H₅O + NO → CH₃CHO + HNO (2b). The following calculated average values were obtained for the low and high pressure limits of k_2a and for k_2b : k = (2.6±1.0) × 10^?28 cm⁶ molecule^?2 s^?1, k = (3.1±0.8) × 10^?11 cm³ molecule^?1 s^?1 and k_2b ca. 8 × 10^?12 cm³ molecule^?1 s^?1. The present value of k, obtained with He as the third body, is significantly lower than the value (2.0±1.0) × 10^?27 cm⁶ molecule^?2 s^?1 recommended in air. The rate constant for the reaction C₂H₅O₂ + NO → C₂H₅O + NO₂ (3) has been measured at 1 torr of He from the simulation of experimental C₂H₅O profiles. The value obtained for k₃ = (8.2±1.6) × 10^?12 cm³ molecule^?1 s^?1 is in good agreement with previous studies using complementary methods. © 1995 John Wiley & Sons, Inc.     

The photolysis of water vapor at 1470 Å. H2 production in the primary process

The primary processes in the photolysis of water vapor at 1470 Å are due to H₂O + hν(λ = 1470 Å) → H₂ + O(¹D), H₂O + hν(λ = 1470 Å) → H + OH with the H₂ yield of the first process accounting for 23% of the overall H₂ production. The quantum yield of this process is estimated to be 0.08 by using O₂ as a scavenger for H-atoms. Secondary reactions involving the photolytic products and added O₂ are discussed.     

Dissociative excitation of water by metastable argon

The reaction Ar(²P_2,0) + H₂O → Ar + H + OH(A²Σ⁺)was studied in crossed molecular beams by observing the luminescence from OH(A²Σ⁺). No significant dependence of the spectrum on collision energy was found over the 22–52 meV region. Spectral simulation was used to obtain the OH(A) vibrational distribution and rotational temperature, assuming a Boltzmann rotational distribution. Since predissociation is known to strongly affect the rovibrational distribution, the individual rotational state lifetimes were included in the simulation program and were used to obtain the average vibrational state lifetimes. Excellent agreement with experiment was obtained for vibrational population ratios N₀/N₁/N₂ of 1.00/ 0.40/0.013 and a rotational temperature of 4000 K. Correction for the different average vibrational lifetimes gave formation rate ratios P₀/P₁/P₂ of 1.00/0.49/0.25. The differences between these results and those from flowing afterglow studies on the same system are discussed. Three reaction mechanisms are considered, and the vibrational prior distributions are calculated from a simple density-of-states model. Only fair agreement with experiment is obtained. The best agreement for the mechanisms giving OH(A) in two 2-body dissociation steps is obtained by assuming 1.0 eV of internal energy remains in the second step. The OH(A) vibrational population distribution of the present work is similar to that found in the photolysis of H₂O at 122 nm, where there is 1.10 eV of excess internal energy.     

High temperature pyrolysis of methane in shock waves. Rates for dissociative recombination reactions of methyl radicals and for propyne formation reaction

The pyrolysis of 2% CH₄ and 5% CH₄ diluted with Ar was studied using both a single–pulse and time–resolved spectroscopic methods over the temperature range 1400–2200 K and pressure range 2.3–3.7 atm. The rate constant expressions for dissociative recombination reactions of methyl radicals, CH₃ + CH₃ → C₂H₅ + H and CH₃ + CH₃ → C₂H₄ + H₂, and for C₃H₄ formation reaction were investigated. The simulation results required considerably lower value than that reported for CH₃ + CH₃ → C₂H₄ + H₂. Propyne formation was interpreted well by reaction C₂H₂ + CH₃ → P-C₃H₄ + H with ?? = 6.2 × ¹⁰12 exp(?17 kcal/RT) ^cm3 mol^?1 s^?1.     

Quantum Mechanics Rate Constant for the N+ND Reaction

We present nonadiabatic quantum dynamical calculations on the two coupled potential en-ergy surfaces (1²A′ and 2²A′) [J. Theor. Comput. Chem. 8, 849 (2009)] for the reaction. Initial state-resolved reaction probabilities and cross sections for the N+ND→N₂+D reaction and N′+ND→N′D+N reaction for collision energies of 5 meV to 1.0 eV are determined, re-spectively. It is found that the N+ND→N₂+D reaction is dominated in the N+ND reaction.In addition, we obtained the rate constants for the N+ND→N₂+D reaction which demand further experimental investigations.     

A flash photolysis-resonance fluorescence study of the reaction of atomic hydrogen with molecular oxygen H + O2 + M → HO2 + M

Using the technique of flash photolysis-resonance fluorescence, absolute rate constants have been measured for the reaction H + O₂ + M → HO₂+M over a temperature range of 220–360°K. Over this temperature range, the data could be fit to an Arrhenius expression of the following form: The units for k_Ar are cm⁶/mole-s. At 300°K the relative efficiencies for the third-body gases Ar:He:H₂:N₂:CH₄ were found to be 1.0:0.93:3.0:2.8:22. Wide variations in the photoflash intensity at several temperatures demonstrated that the reported rate constants were measured in the absence of other complex chemical processes.     

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.     

The Formation of OH*(<Superscript>2</Superscript>Σ<Superscript>+</Superscript>) Radical in the Reaction of Hydrogen with Oxygen behind a Shock Wave in Nonequilibrium Conditions

The mechanism of formation of the electronically excited radical OH*(A²Σ⁺) has been studied by analyzing calculations quantitatively describing the results of shock wave experiments carried out in order to determine the moment of maximum OH* radiation at temperatures T < 1500 K and pressures P ≤ 2 atm in the H₂ + O₂ mixtures diluted by argon when the vibrational nonequilibrium is a factor determining the mechanism and rate of the overall process. In kinetic calculations, the vibrational nonequilibrium of the initial H₂ and O₂ components, the HO₂, OH(X²Π), O₂*(¹Δ) intermediates, and the reaction product H₂O were taken into account. The analysis showed that under these conditions the main contribution to the overall process of OH* formation is caused by the reactions OH + Ar → OH* + Ar, H₂ + HO₂ → OH* + H₂O, H₂ + O*(¹D) → OH* + H, HO² + O → OH* + O² and H + H²O → OH* + H², which occur in the vibrational nonequilibrium mode (their activation barrier is overcome due to the vibrational excitation of reactants), and by H + O₃ → OH* + O₂ and H + H₂O₂ → OH* + H₂O, which are reverse to the reactions of chemical quenching.     

Kinetic study of the reaction of OH radicals with nitrosyl chloride as a possible source of ClOH

The reaction of OH with NOCl has been studied using the discharge flow reaction-EPR technique. The absolute rate constant is k₁ = (4.3±0.4)× 10^?13 cm³ molecule^?1 s^?1 at 298 K. A mass spectrometric investigation of the products shows that this reaction occurs via two primary steps, OH + NOCl → NO + ClOH(1a) and OH + NOCl → HONO + Cl (1b) with k_1a =k_1b.     

Orientation dependence in collision induced electronic relaxation studied through van der Waals complexes with isomeric structures

Weakly bound molecular complexes with more than one well-defined structures provide us with an unique opportunity to investigate dynamic processes induced by intermolecular interactions with specific orientations. The relative orientation of the two interacting molecules or atoms is defined by the complex structure. The effect of the orientation in the spin changing collisions glyoxal(S₁)+Ar → glyoxal(T₁)+Ar and acetylene (S₁)+Ar → acetylene(T)+Ar have been studied by measuring the intersystem crossing (ISC) rates of the glyoxal(S₁)·Ar and acetylene(S₁)·Ar complexes with different isomeric structures. Results show that there is a strong orientation dependence in the ISC of glyoxal(S₁) induced by interaction with the Ar atom: the Ar atom positioned in the molecular plane is much more effective than in the out-of-plane position in inducing the S₁ → T₁ transition of glyoxal. On the other hand, studies of acetylene(S₁)·Ar complexes indicate that the Ar-induced ISC rates are nearly identical for the in-plane and out-of-plane positions. Orientation dependence in the collision induced vibrational relaxation process C₂H₂(S₁,v _i )+Ar → C₂H₂(S₁,v _f <v _i )+Ar is also studied by measuring the vibrational predissociation rates of the acetylene(S₁)·Ar complex isomers. The results indicate that collisions of C₂H₂(S₁,v ₃=3, 4) with Ar at two orthogonal orientations are equally effective in causing vibrational relaxation of C₂H₂.     

Simultaneous vibration-vibration and vibration-translation transitions in H2 + D2 at high collision energies

The coupling between VV and VT transitions in H₂(2) + D₂(0) → H₂(1) + D₂(1) and D₂(2) +H₂(0) → D₂(1) + H₂(1) at high collision energies is investigated by use of the solution of the Schrodinger equation of motion. The coupling is sufficiently important that the energy exchange processes cannot be described by the VV mechanism alone.     

The NO- and NO2-catalyzed decomposition of I2 in shock waves

Measurements of the NO-catalyzed dissociation of I₂ in Ar in incident shock waves were carried out in the temperature range of 700°-1520°K and at total concentrations of 5 × 10^?6-6 × 10^?5 mol/cm³, using ultraviolet-visible absorption techniques to monitor the disappearance of I₂. It was shown that the main reaction responsible for the disappearance under these conditions is I₂ + NO → INO + I, for which a rate coefficient of (2.9 ± 0.5) × 10¹³ exp[-(18.0 ± 0.6 kcal/mol)/RT] cm²/mol·sec was determined. The INO formed dissociates rapidly in a subsequent reaction. The reaction, therefore, constitutes a “chemical model” for a “thermal collisional release mechanism.” Preliminary measurements of the rate coefficient for I₂ + NO₂ → INO₂ + I are also presented. Combined with information on the reverse reactions obtained in earlier room temperature experiments, these results lead to accurate values of ΔH°_f for INO and INO₂ equal to 29.7 ± 0.5 and 15.9 ± 1 kcal/mol, respectively.