An Ab initio determination of the rate constant for H2 + CN → H + HCN

The reactants, products, and saddle point for the reaction H₂ + CN → H + HCN have been studied by ab initio calculations. The computed structures, frequencies, and energetics are compared directly to available measurements and, indirectly, to experimental rateconstants. The theoretical rate constants used in the comparison are calculated with conventional transition state theory. By reduction of the computed reaction barrier to 4.1 kcal mol,^?1 good agreement with experimental rate constants is obtained over a 3250-K temperature range. This computed rate constant is well represented by the form 4.9 × 10_?18 T^2.45 e^{?1, 126/T} over the temperature range of 250 K–3500 K. Substantial reaction rate curvature is found due to low-frequency bending modes at the saddle point. The results for this reaction are compared to other abstraction reactions involving H atom transfer to identify correlations between reaction exothermicity and both abstraction barriers and reaction rate curvature.     

Thermal and state-selected rate constant calculations for O(3p) + H2 → OH + H and isotopic analogs

We present a new parametrization (based on ab initio calculations) of the bending potentials for the two lowest potential energy surfaces of the reaction O(³P) + H₂, and we use it for rate constant calculations by variational transition-state theory with multidimensional semiclassical tunneling corrections. We present results for the temperature range 250–2400 K for both the rate constants and the intermolecular kinetic isotope effects for the reactions of O(³P) with D₂ and HD. In general, the calculated rate constants for the thermal reactions are in excellent agreement with available experiments. We also calculate the enhancement effect for exciting H₂ to the first excited vibrational state. The calculations also provide information on which aspects of the potential energy surfaces are important for determining the predicted rate constants.     

Angular momentum conservation in the O + OH ↔ O2 + H reaction

We have studied the O + OH ↔ O₂ + H reaction on Varandas's DMBE IV potential using a variety of statistical methods, all involving the RRKM assumption for the HO₂* complex. Comparing our results using microcanonical variational transition‐state theory (μVT) with those using microcanonical/fixed‐J variational transition‐state theory (μVT‐J), we find that the effect of angular momentum conservation on the rate coefficient is imperceptible up to a temperature of about 700 K. Above 700 K angular momentum conservation increasingly reduces the rate coefficient, but only by approximately 21% even at 5000 K. Comparing our μVT‐J calculations with the quasi‐classical trajectory (QCT) results of Miller and Garrett [ 1 ], we confirm their conclusion that non‐RRKM dynamics of the HO₂* complex reduces the rate coefficient by about a factor of 2 independent of temperature. Our calculations of k^(c), the rate coefficient for HO₂* formation from O + OH, are in excellent agreement with the QCT results of Miller and Garrett. Although the differences are not large, we find k_CVT^(c) > k_μVT^(c) > k_μVT‐J^(c) > k_QCT^(c), where CVT stands for canonical variational transition‐state theory. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 753–756, 1999     

Effect of Solvation on Chemical Reactions,I. Addition of a Single Water Molecule to the H− + H2O → OH− + H2 Reaction

The gas phase chemical reaction, H^? + H₂O → H₂ + OH, and the effect of an additional water molecule on the reaction, H^?(H₂O) + H₂O → H₂ + OH(H₂O), have been investigated. The optimal structures and energies of the reactants, products, two stable intermediates, and the transition state connecting the two intermediates have been determined. The additional water molecule does not affect the potential surface congruently: it destabilizes the H(H₂O) minimum, but stabilizes the H₂ ?OH minimum and the transition state connecting the two intermediates. However, it stabilizes the products more than the H₂ ?OH^? minimum. Finally, in line with the reduction in the barrier height, the transition state for the H(H₂0) to H₂ ?OH^? isomerization moves further along the reaction path.     

Insights Gained by Modeling the Kinetics of Archetypal Hydrogen Atom Transfer Reactions: NH3 + ·H ⇆ H2N· + H2 and CH4 + ·H ⇆ H3C· + H2

Experimental measurements of the kinetics of the title reactions extend to temperature ranges of 1360 K for the ammonia‐hydrogen reaction and of 1602 K for the methane‐hydrogen reaction. Curved plots of ln(k) versus 1/T are obtained. Many theoretical calculations modeling these reactions routinely use tunneling corrections to match experiment. The steepness and curvatures of the plots are modeled successfully in this work and are shown to be caused solely by changes in the bond dissociation energies of the bonds involved in the reactions without invoking tunneling or any other adjustable parameters. The conclusion that tunneling does not contribute significantly to the rates in the temperature range of the measurements is in stark contrast with those theoretical calculations invoking large tunneling factors in the experimental temperature range. Support for the conclusion is provided by theoretical calculations of harmonic quantum transition state theory implementing instanton theory. There is direct experimental evidence that significant tunneling occurs in some H atom transfers, as with isotopomers of H₂ + ·H and other H transfers at very low temperatures. However, there is no direct experimental evidence of significant tunneling contributions to the rates of the title reactions in the temperature range of the measurements. Insights are gained into what specific forces must be overcome by the enthalpy of activation for reaction to occur.     

Deuterium enrichment of interstellar methanol explained by atom tunneling

We calculate, down to low temperature and for different isotopes, the reaction rate constants for the hydrogen abstraction reaction H + H(3)COH → H(2) + CH(2)OH/CH(3)O. These explain the known abundances of deuterated forms of methanol in interstellar clouds, where CH(2)DOH can be almost as abundant as CH(3)OH. For abstraction from both the C- and the O-end of methanol, the barrier-crossing motion involves the movement of light hydrogen atoms. Consequently, tunneling plays a dominant role already at relatively high temperature. Our implementation of harmonic quantum transition state theory with on the fly calculation of forces and energies accounts for these tunneling effects. The results are in good agreement with previous semiclassical and quantum dynamics calculations (down to 200 K) and experimental studies (down to 295 K). Here we extend the rate calculations down to lower temperature: 30 K for abstraction from the C-end of methanol and 80 K for abstraction from the OH-group. At all temperatures, abstraction from the C-end is preferred over abstraction from the O-end, more strongly so at lower temperature. Furthermore, the tunneling behavior strongly affects the kinetic isotope effects (KIEs). D + H(3)COH → HD + CH(2)OH has a lower vibrationally adiabatic barrier than H + H(3)COH → H(2) + CH(2)OH, giving rise to an inverse KIE (k(H)/k(D) < 1) at high temperature, in accordance with previous experiments and calculations. However, since tunneling is more facile for the light H atom, abstraction by H is favored over abstraction by D below ~135 K, with a KIE k(H)/k(D) of 11.2 at 30 K. The H + D(3)COD → HD + CD(2)OD reaction is calculated to be much slower than the D + H(3)COH → HD + CH(2)OH, in agreement with low-temperature solid-state experiments, which suggests the preference for H (as opposed to D) abstraction from the C-end of methanol to be the mechanism by which interstellar methanol is deuterium-enriched.     

Rate constant for the reaction Br + O3 → BrO + O2 from 248 to 418 K: Kinetics and mechanism

The rate constant for the Br + O₃ → BrO + O₂ reaction was measured by the discharge flow technique, employing resonance fluorescence detection of Br. Over the temperature range 248 to 418 K, in 1 to 3 torr of He, decays of Br in excess O₃ yield the value k₁ = (3.28 ± 0.40) × 10^?11 e^[?944±30]/T cm³ molecule^?1 s^?1. Cited uncertainties are at the 95% confidence level and include an estimate of the systematic errors. The rate constants for the reactions of O₃ with Br, Cl, F, OH, O, and N correlate with the electron affinities of the radicals suggesting that the reactions proceed through early transition states dominated by transfer of electron density from the highest occupied molecular orbital of ozone to the singly occupied radical MO. The implications of this new measurement of k₁ for stratospheric chemistry are discussed.     

H2O(HOD) + Cl→HCl(DCl) + OH(OD)反应动态学的理论研究

用从头算方法, 获得了H2O + Cl→HCl + OH(R1), HOD +Cl→DCl + OH(R2), HOD + Cl→HCl + OD(R3)反应的内禀反应坐标(IRC)。根据传统过渡态、变分过渡态理论及相应的隧道效应校正, 计算了反应的速率常数。对已有实验速率常数值的R1反应, 我们计算的结果和实验一致。根据Truhlar的振动选态公式, 分别讨论了激发HOD中OH, OD振动模式对反应速率的影响,得到激发HOD中的OH振动模式将有利于产物OD + HCl生成, 和实验的结论相一致。     

Full-dimensional quantum dynamics study of exchange processes for the D + H2O and D + HOD reactions

The exchange processes of D + H(2)O and D + HOD reactions are studied using initial state-selected time-dependent wave packet approach in full dimension. The total reaction probabilities for different partial waves, together with the integral cross sections, are obtained both by the centrifugal sudden (CS) approximation and exact coupled-channel (CC) calculations, for the H(2)O(HOD) reactant initially in the ground rovibrational state. In the CC calculations, small resonance peaks in the reaction probabilities and quick diminishing of the resonance peaks with the increase of total angular momenta J do not lead to clear step-like features just above the threshold in the cross sections for the title reactions, which are different in other isotopically substituted reactions where the hydrogen atom was included as the reactant instead of the deuterium atom [B. Fu, Y. Zhou, and D. H. Zhang, Chem. Sci. 3, 270 (2012); B. Fu and D. H. Zhang, J. Phys. Chem. A 116, 820 (2012)]. It is interesting that the shape resonance-induced features resulting from the reaction tunneling are significantly diminished accordingly in the reactions of the deuterium atom and H(2)O or HOD, owing to the weaker tunneling capability of the reagent deuterium atom in the title reactions than the reagent hydrogen atom in other reactions. In the CS calculations, the resonance peaks persist in many partial waves but cannot survive the partial-wave summations. The cross sections for the D(') + H(2)O → D(')OH + H and D(') + HOD → D(')OD + H reactions are substantially larger than those for the D(') + HOD → HOD(') + D reaction, indicating that the D(')/H exchange reactions are much more favored than the D(')/D exchange.     

Measurements of the bimolecular rate constants for S + O2 → SO + O and CS2 + O2 → CS + SO2 at high temperatures

The bimolecular reactions in the title were measured behind shock waves by monitoring the O-atom production in COS? O₂? Ar and CS₂? O₂? Ar mixtures over the temperature range between 1400 and 2200 K. A value of the rate constant for S + O₂ → SO + O was evaluated to be (3.8 ± 0.7) × 10¹² cm³ mol^?1 s^?1 between 1900 and 2200 K. This was connected with the data at lower temperatures to give an expression k₂ = 10^10.85 T^0.52 cm³ mol^?1 s^?1 between 250 and 2200 K. An expression of the rate constant for CS₂ + O₂ → CS + SO₂ was obtained to be k₂₁ = 10^12.0 exp(?32 kcal mol^?1/RT) cm³ mol^?1 s^?1 with an error factor of 2 between 1500 and 2100 K.     

Real wave packet and flux analysis studies of the H + F2 → HF + F reaction

The H + F₂ → HF + F reaction on ground state potential energy surface is investigated using the quantum mechanical real wave packet and Flux analysis method based on centrifugal sudden approximation. The initial state selected reaction probabilities for total angular momentum J = 0 have been calculated by both methods while the probabilities for J > 0 have been calculated by Flux analysis method. The initial state selected reaction probabilities, integral cross sections and rate coefficients have been calculated for a broad range of collision energy. The results show a large rotational enhancement of the reaction probability. Some resonances were seen in the state‐to‐state reaction probabilities while state‐to‐all reaction probabilities and the reaction cross section do not manifest any oscillations and the initial state selected reaction rate constants are sensitive to the temperature. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Variational transition state theory rate constants and H/D kinetic isotope effects for CH3 + CH3OCOH reactions

The rate constants and H/D kinetic isotope effect for hydrogen abstraction reactions involving isotopomers of methyl formate by methyl radical are computed employing methods of the variational transition state theory (VTST) with multidimensional tunneling corrections. The energy paths were built with a dual-level method using the moller plesset second-order perturbation theory (MP2) method as the low-level and complete basis set (CBS) extrapolation as the high-level energy method. Benchmark calculations with the CBS_D-T approach give an enthalpy of reaction at 0 K for R1 (−4.5 kcal/mol) and R2 (−4.2 kcal/mol) which are in good agreement with the experiment, that is, −4.0 and − 4.8 kcal/mol. For the reactional paths involving the isotopomers CH₃ + CH₃OCOH → CH₄ + CH₃OCO and CH₃ + CH₃OCOD → CH₃D + CH₃OCO, the value of k^H/k^D (T = 455 K) using the canonical VTST/small-curvature tunneling approximation method is 6.7 in close agreement with experimental value (6.2). © 2019 Wiley Periodicals, Inc.     

O + NNH: A possible new route for NOX formation in flames

We propose a new high temperature pathway for NO formation that involves the reaction of NNH with oxygen atoms. This reaction forms the HNNO* energized adduct via a rapid combination reaction; HNNO* then rapidly dissociates to NH + NO. The rate constant for O + NNH ? NH + NO is calculated via a QRRK chemical activation analysis to be 3.3 × 10¹⁴ T^?0.23exp(+510/T) cm³ mol^?1 s^?1. This reaction sequence can be an important or even major route to NO formation under certain combustion conditions. The presence of significant quantities of NNH results from the reaction of H with N₂. The H + N₂ ? NNH reaction is only ca. 6 kcal/mol endothermic with a relatively low barrier. The reverse reaction, NNH dissociation, has been reported in the literature to be enhanced by tunneling. Our analysis of NNH dissociation indicates that tunneling dominates. We report a two-term rate constant for NNH dissociation: 3.0 × 10⁸ + [M] {1.0 × 10¹³T^0.5exp(?1540/T)} s^?1. The first term accounts for pressure-independent tunneling from the ground vibrational state, while the second term accounts for collisional activation to higher vibration states from which tunneling can also occur. ([M] is the total concentration in units of mol cm^?3.) Use of this dissociation rate constant and microscopic reversibility results in a large rate constant for the H + N₂ reaction. As a result, we find that NNH ? H + N₂ can be partially equilibrated under typical combustion conditions, resulting in NNH concentrations large enough for it to be important in bimolecular reactions. Our analysis of such reactions suggests that the reaction with oxygen atoms is especially important. © 1995 John Wiley & Sons, Inc.     

H2 + CN(n=0,1)→H + HCN振动选态反应

用从头算方法获得了Ｈ２＋ＣＮ反应的内禀反应坐标（ＩＲＣ），沿着ＩＲＣ，计算了各垂直于ＩＲＣ的简正模所对应的频率（Ｗ）以及沿ＩＲＣ运动与垂直ＩＲＣ运动的简正模之间的耦合常数（ＢＫＦ），根据传统过渡态，变分过渡态理论和选态公式，计算了ｎＣＮ＝０及ｎＣＮ＝１时反应的速率常数，并得到了实验相一致的结果，还计算了ｎＣＨ＝１及ｎＣＮ＝１的Ｈ＋ＨＣＮ→Ｈ２＋ＣＮ反应速率常数，可供实验工作者参考。     

Collisional energy transfer in the reactions I + NO + M → INO + M and I + NO2 + M → INO2 + M

The low-pressure recombination rate constants of the reactions I + NO + M → INO + M (with 14 different M) and I + NO₂ + M → INO₂ + M (with 26 different M) have been measured at 330°K by laser flash photolysis. The collision efficiencies β_c are analyzed and compared with other thermal activation systems. Whereas β_c increases in one reaction with an increasing number of atoms in M, practically no such effect is found when, for the same M, different reactions with varying complexities of the reacting molecules are considered.     

Kinetics and mechanism for the CH2O + NO2 reaction: A computational study

The reactants, products, and transition states of the CH₂O + NO₂ reaction on the ground electronic potential energy surface have been searched at both B3LYP/6?311+G(d,p) and MPW1PW91/6?311+G(3df,2p) levels of theory. The forward and reverse barriers are further improved by a modified Gaussian‐2 method. The theoretical rate constants for the two most favorable reaction channels 1 and 2 producing CHO + cis‐HONO and CHO + HNO₂, respectively, have been calculated over the temperature range from 200 to 3000 K using the conventional and variational transition‐state theory with quantum‐mechanical tunneling corrections. The former product channel was found to be dominant below 1500 K, above which the latter becomes competitive. The predicted total rate constants for these two product channels can be presented by k_t (T) = 8.35 × 10^?11 T^6.68 exp(?4182/T) cm³/(mol s). The predicted values, which include the significant effect of small curvature tunneling corrections, are in quantitative agreement with the available experimental data throughout the temperature range studied (390–1650 K). © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 184–190, 2003     

Bayesian uncertainty quantification of recent shock tube determinations of the rate coefficient of reaction H + O2→ OH + O

We analyze the ignition delay in hydrogen–oxygen combustion and the important chain ‐branching reaction H + O₂→ OH + O that occurs behind the shock waves in shock tube experiments. We apply a stochastic Bayesian approach to quantify uncertainties in the theoretical model and experimental data. The approach involves a statistical inverse problem, which has four “components” as input information: (a) model, (b) prior joint probability density function (PDF) of the uncertain parameters, (c) experimental data, and (d) uncertainties in the scenario parameters. The solution of this statistical inverse problem is a posterior joint PDF of the uncertain parameters from which we can easily extract statistical information. We first perform a parametric study to investigate how the level of the total uncertainty (which we define as the sum of model uncertainty and experimental uncertainty) affects the uncertainty in the rate coefficient k₁ of the reaction H + O₂→ OH + O, which is “most likely” expressed by k₁=1.73×10²³T^?2.5exp(?11550/T) cm³ mol^?1 s^?1 over the experimental temperature range 1100–1472 K. We also introduce the idea of “irreducible” uncertainty when considering other parameters in the system. After statistically calibrating the parameters modeling the rate coefficient k₁, we predict its 95% confidence interval (CI) for different temperature regimes and compare the CI against the values of k₁ obtained deterministically. Our results show that a small uncertainty in gas temperature (±5 K) introduces appreciable uncertainty in k₁. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 586–597, 2012     

Canonical statistical adiabatic channel model calculations of the H + CH3 → CH4 recombination reaction on an ab initio potential energy surface. The role of the transitional modes

The canonical formalism of the statistical adiabatic channel model is used to calculate limiting high pressure rate constants for the H + CH₃ → CH₄ recombination reaction on a recently reported analytic potential energy surface based on ab initio calculations. An effective adiabatic channel potential which incorporates the G_?? matrix element of the twofold degenerate H₃C? H transitional bending mode, quartic anharmonicity, and state selected mode coupling effects is implemented. The rate constants calculated over the temperature range 200–1000 K are in very good agreement with recent canonical variational transition state theory calculations performed on the same surface. The comparison with experimental results is also discussed.     

The reaction H + D2 → HD + D: Distorted wave calculations at Etrans (υ = 0,j 0) = 0.55 and 1.3 eV

Static—static distorted wave and vibrationally adiabatic distorted wave calculations have been performed for the product rotational distributions of the H + D₂ → HD + D reaction using an accurate ab initio potential energy surface. Comparison is made with coupled states and quasiclassical trajectory calculations as well as with experimental measurements.     

Spezifisch π→π*-induzierte Reaktionen von γ-Dimethoxymethylcyclohexen-2-onen: 1,3-Umlagerung und Wasserstoffabstraktion durch das α-Kohlenstoffatom

When α,β-unsaturated γ-dimethoxymethyl cyclohexenones are excited to the S₂(π,π*) state, certain unimolecular reactions can be observed to compete with S₂ → S₁ internal conversion. These reactions do not occur from the S₁(n,π*) or the lowest T(π,π* and n,π*) states. They comprise the radical elimination of the formylacetal substituent (cf. 8 , 9 → 32 + 33 ), γ → α formylacetal migration (cf. 6 → 27 , 8 → 30 , 9 → 34 , 12 → 37 ), and a cyclization process involving the transfer of a methoxyl hydrogen to the α carbon and ring closure at the β position (cf. 6 → 28 , 8 → 31 , 12 → 38 , 20 → 40 + 41 ). The quantum yield of the ring closure 20a → 40a + 41a is 0.016 at ≤ 0.05M concentration. It is independent of the excitation wavelength within the π→π* absorption band (238–254 nm), but Φ ( 40a + 41a ) decreases at higher concentrations. According to the experimental data the reactive species of these specifically π→π*-induced transformations is placed energetically higher than the S₁(n,π*) state, and it is either identical with the thermally equilibrated S₂(n,π*) state, or reached via this latter state. The linear dienone 14 undergoes a similar π→π*-induced cyclization (→ 42 ) whereas the benzohomologue 26 proved unreactive, and the dienone 22 at both n → π and π→π* excitation only gives rise to rearrangements generally characteristic of cross-conjugated cyclohexadienones.