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1.
A detailed computational study has been performed on the mechanism and kinetics of the C2H + CH3CN reaction. The geometries were optimized at the BHandHLYP/6–311G(d, p) level. The single-point energies were calculated using the BMC-CCSD, MC-QCISD and QCISD(T)/6–311+G(2df, 2pd) methods. Five mechanisms were investigated, namely, direct hydrogen abstraction, C-addition/elimination, N-addition/elimination, C2H–to–CN substitution and H-migration. The kinetics of the title reaction were studied using TST and multichannel RRKM methodologies over a wide range of temperatures (150–3,000 K) and pressures (10?4–104 torr). The total rate constants show positive temperature dependence and pressure independence. At lower temperatures, the C-addition step is the most feasible channel to produce CH3 and HCCCN. At higher temperatures, the direct hydrogen abstraction path is the dominant channel leading to C2H2 and CH2CN. The calculated overall rate constants are in good agreement with the experimental data.  相似文献   

2.
Li Wang  Jing-yao Liu  Ze-sheng Li   《Chemical physics》2008,351(1-3):154-158
The dynamic properties of the hydrogen abstraction reactions of CF2H2 and CF3H with F atom are investigated in the temperature range of 182–2000 K. The minimum-energy path (MEP) is optimized at MP2/6-311 G(d, p) level, then the energy profiles are refined at the CCSD(T)/6-311++G(3df, 2pd) level (single-point). The theoretical rate constants, which are calculated by the variational transition state theory (VTST) including the small curvature tunneling (SCT) correction, are in good agreement with the experimental ones. It is found that the rate constant of the CF2H2 + F reaction are larger than that of the CF3H + F reaction and the activation energies exhibit in the just opposite order. This phenomenon can be rationalized by the hardness η of the halomethane molecules. The comparison of the two reactions with the CFH3 + F reaction is made. It is found that the rate constants decrease in the order of CFH3 + F > CF2H2 + F > CF3H + F. The effect of fluorine substitution leads to a dramatic increase in the activation energy and a decrease in the preexponential factor. We hope that present theoretical studies for these compounds can give further information concerning how fluorine substitution affects the rate constants of hydrogen abstraction reactions.  相似文献   

3.
The dynamics of the F atom reaction with propyne (CH(3)CCH) has been investigated using a universal crossed molecular beam apparatus. Two reaction channels have been clearly observed: H+C(3)H(3)F and HF+C(3)H(3). The substitution of F for H occurs mainly via a complex formation mechanism, producing reaction products with some contribution from a direct reaction mechanism. The HF product, however, appears to be dominantly forward scattered relative to the F atom beam direction, suggesting that the HF formation occurs via a direct abstraction mechanism. Branching ratios for the two observed reaction channels are also determined. The H formation channel is found to be the major reaction pathway, while the HF formation channel is also significant. From the measurements of DF versus HF products from the F atom reaction with deuterated propyne, the H atom picked up by the F atom in the reaction with normal propyne seems to come mostly from the CH(3) group. In addition, the H atom produced in the H atom formation channel appears to be mostly from the CH(3) group with some contribution from the CCH group.  相似文献   

4.
Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by 193 nm laser flash photolysis of silacyclopent-3-ene, have been carried out in the presence of ammonia, NH3. Second order kinetics were observed. The reaction was studied in the gas phase at 10 Torr total pressure in SF6 bath gas at each of the three temperatures, 299, 340 and 400 K. The second order rate constants (laser pulse energy of 60 mJ +) fitted the Arrhenius equation:noindent Experiments at other pressures showed that these rate constants were unaffected by pressure in the range 10–100 Torr, but showed small decreases in value at 3 and 1 Torr. There was also a weak intensity dependence, with rate constants decreasing at laser pulse energies of 30 mJ +. Ab initio calculations at the G3 level of theory, show that SiH2+NH3 should form an initial adduct (donor-acceptor complex), but that energy barriers are too great for further reaction of the adduct. This implies that SiH2+NH3 should be a pressure dependent association reaction. The experimental data are inconsistent with this and we conclude that SiH2 decays are better explained by reaction of SiH2 with the amino radical, NH2, formed by photodissociation of NH3 at 193 nm. The mechanism of this previously unstudied reaction is discussed.  相似文献   

5.
The multiple‐channel reactions SiH3 + SiH3CH3 → products and SiH3 + SiH2(CH3)2 → products are investigated by direct dynamics method. The minimum energy path (MEP) is calculated at the MP2/6‐31+G(d,p) level, and energetic information is further refined by the MC‐QCISD method. The rate constants for individual reaction channels are calculated by the improved canonical variational transition state theory (ICVT) with small‐curvature tunneling (SCT) correction over the temperature range of 200–2400 K. The theoretical three‐parameter expression k1(T) = 2.39 × 10−23T4.01exp(−2768.72/T) and k2(T) = 9.67 × 10−27T4.92exp(−2165.15/T) (in unit of cm3 molecule−1 s−1) are given. Our calculations indicate that hydrogen abstraction channel from SiH group is the major channel because of the smaller barrier height among eight channels considered. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010  相似文献   

6.
The dynamics of the Cl+SiH4 reaction has been studied using the universal crossed molecular beam method. Angular resolved time-of-flight spectra have been measured for the channelSiH3Cl+H. Product angular distributions as well as energy distributions in the center-of-mass frame were determined for the channel. Experimental results show that the SiH3Clproduct is mainly backward scattered relative to the Cl atom beam direction, suggestingthat the channel takes place via a typical SN2 type reaction mechanism.  相似文献   

7.
A mass spectrometric analysis of the positive ions and neutral products in a silane glow discharge has been performed. The active species, created by dissociation, disproportionation, and ion-molecule reactions are mainly SiH2, SiH3, and H. A calculation of the distribution of the SiH n + ions shows that the silane concentration monitors the abundance of SiH 3 + . The diffusional transport of radicals toward the discharge-tube walls can explain the observed deposition rates. The study of SiH4-SiD4 and SiH4-D2 plasmas emphasizes several reactions which modify the free-radical populations depending on the discharge conditions: disproportionation, termination, recombination, and abstraction. Heterogeneous reactions have also been observed: etching of the film by H atoms and direct incorporation of hydrogen in the growing film. A general scheme for the plasma deposition mechanism is proposed.  相似文献   

8.
Experimental data (the rate constants and activation energies) for seven reactions of direct substitution of one atom for another D + CH3R CH2DR + H, D + NH3 DNH2 + H, D + H2O HOD + H, F + CH3X CH3F + X (X = F, Cl, Br, and I) involving atoms D and F and molecules C2H6, H2O, NH3, CH3F, CH3Cl, CH3Br, and CH3I are analyzed using the parabolic model of a bimolecular radical reaction. The activation energies for the thermally neutral analogs of these substitution reactions are calculated. Atomic substitution involving deuterium atoms has a lower activation energy of a thermally neutral reaction than radical abstraction or substitution.  相似文献   

9.
By means of the dual‐level direct dynamics method, the mechanisms of the reactions, CH3CF2Cl + OH → products (R1) and CH3CFCl2 + OH → products (R2), are studied over a wide temperature range 200–2000 K. The optimized geometries and frequencies of the stationary points are calculated at the MP2/6‐311G(d,p) level, and then the energy profiles of the reactions are refined with the interpolated single‐point energy method at the G3(MP2) level. The canonical variational transition‐state theory with the small‐curvature tunneling (SCT) correction method is used to calculate the rate constants. For the title reactions, three reaction channels are identified and the H‐abstraction channel is the major pathway. The results indicate that F substitution has a significant (reductive) effect on hydrochlorofluorocarbon reactivity. Also, for all H‐abstraction reaction channels the variational effect is small and the SCT effect is only important in the lower temperature range on the rate constants calculation. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010  相似文献   

10.
1,1,2,2,3,3,4‐Heptafluorocyclopentane (F7A) has considerable potential to be a new halon replacement due to its environmental friendliness and low‐toxicity. However, the reaction processes of F7A with hydroxyl and hydrogen free radicals, which are of great importance for investigating its fire suppression mechanisms, are still unclear. In this paper, ab inito and density functional theory are used to deduce the possible reaction pathways for the reactions of F7A with hydroxyl and hydrogen free radicals at the CCSD/cc‐pVDZ//B3LYP/6‐311++G (d,p) level of theory. Two distinct reaction pathways including ten elementary reaction channels for F7A with hydroxyl free radical, and five distinct reaction pathways including twenty elementary reaction channels for F7A with hydrogen free radical are investigated. The geometries, vibrational frequencies and reaction energy barriers are also determined. Based on the calculated results, the possible reaction mechanisms are proposed and discussed. The most feasible reaction channel for F7A with hydroxyl free radical is that leads to CH(OH)CH2(CF2)3+·F, and the most feasible reaction channel for F7A with hydrogen free radical is that leads to (CF2)3CH2CH·+HF. The study is helpful to further study its fire suppression mechanisms and promote it to be a new generation of halon replacement.  相似文献   

11.
The HF infrared chemiluminescence from the reactions of F atoms with B2H6, CH4, CH3F, CH2F2, CH2Cl2, CH3ONO. CH3NO2, NH3 (and ND3). PH3 and HNCO has been observed from a 300 K flowing-afterglow reactor. Experiments were done for a range of CH4 and F atom concentrations to identify conditions which were free of vibrational relaxation and secondary reactions, and these conditions were used to assign initial HF(v) vibrational distributions for each reaction. The emission intensity from each reaction also was compared to that from CH4 in order to obtain the relative HF formation rate constants at 300 K. Since the absolute rate constant for F + CH4 is well established, the combination of all of these data provides absolute rate constants for HF(v) formation at 300 K. The ND3 reaction was studied to obtain information on more vibrational levels in order to better estimate the HF(v = 0) and DF(v = 0) components of the ammonia distributions. With NH3 and ND3 there is no significant isotope effect on the energy disposal. Except for NHCO, for which an addition-elimination channel is possible, the HF(v) distributions are inverted and <fv > = 0.60. Differences between the HF(v) distributions reported here and some other reports in the literature are noted: the present data are discussed as representative of direct H atom abstraction for 300 K Boltzmann conditions. The HCl infrared chemiluminescence from the F + CHCl2 secondary reaction also was observed; the HCl(v) distribution was v1: v2: v3: v4: v5 - 0.47: 0.23: 0.18: 0.08: 0.04.  相似文献   

12.
The hydrogen abstraction reaction F+CH3OH has two possible reaction pathways: HF+CH3O and HF+CH2OH. Despite the absence of intrinsic barriers for both channels, the former has a branching ratio comparable to the latter, which is far from the statistical limit of 0.25 (one out of four available H atoms). Furthermore, the measured branching ratio of the two abstraction channels spans a large range and is not quantitatively reproduced by previous theoretical predictions based on the transition-state theory with the stationary point information calculated at the levels of M?ller-Plesset perturbation theory and G2. This work reports a theoretical investigation on the kinetics and the associated branching ratio of the two competing channels of the title reaction using a quasi-classical trajectory approach on an accurate full-dimensional potential energy surface (PES) fitted by the permutation invariant polynomial-neural network approach to ca. 1.21x105 points calculated at the explicitly correlated (F12a) version of coupled cluster singles doubles and perturbative triples (CCSD(T)) level with the aug-cc-pVDZ basis set. The calculated room temperature rate coeffcient and branching ratio of the HF+CH3O channel are in good agreement with the available experimental data. Furthermore, our theory predicts that rate coeffcients have a slightly negative temperature dependence, consistent with barrierless nature of the reaction.  相似文献   

13.
Infrared chemiluminescence under conditions of arrested relaxation has been applied to the study of the hydrogen and deuterium abstraction reactions of HCOOH, DCOOH and H2CO with F atoms. Two distinctly different modes of product excitation are observed, depending upon whether the reaction proceeds via the formyl or carboxyl hydrogen. Reaction at the formyl hydrogen (or deuterium) causes substantial inversion in the diatomic product internal energy distributions. The F + H2CO and F + DCOOH reactions respectively channel 56% and 54% of the available energy into vibration in the product diatomic when they occur at the formyl site. In both cases the product energy distributions are qualitatively similar to those observed in direct reactions of triatomic systems on repulsive energy surfaces. In contrast to these, reaction at the carboxyl hydrogen of DCOOH gives an HF2 product vibrational distribution having a Boltzmann equilibrium shape at a temperature of 4300 K. The ratio of HF to DF product from the F + DCOOH study shows that reaction occurs at the carboxyl hydrogen approximately twice as often as at the formyl site. Comparison with triatomic reactions involving the same mass-combinations implies that abstraction of the formyl hydrogen occurs via single-collision, direct encounters, whereas reaction at the carboxyl site involves a long-lived complex in which extensive randomisation of the reaction exoergicity among all the product vibrational modes can occur.  相似文献   

14.
The multichannel reactions (1) HOCl + F --> products and (2) HOBr + F --> products have been investigated using the dual-level direct dynamics method. The minimum energy paths (MEPs) are calculated at both the MPW1K/6-311G(d,p) and QCISD/6-311G(d,p) levels, then the single-point energies are further corrected at the QCISD(T)/6-311++G(3df,3pd) level of theory. There are hydrogen-bonded complexes with the energies less than those of the reactants or products located at the entrance or exit channel of both hydrogen abstraction reactions; while for the halogen abstraction channels only one complex exists at the reactant side in the bromine abstraction channel. The rate constants are evaluated by the improved canonical variational transition-state theory (ICVT). The agreement of the rate constants with available experimental values for two reactions at room temperature is good. Theoretical results indicate that for the reaction HOCl + F, hydrogen abstraction channel leading to the formation of HF + ClO will predominate the reaction over the whole temperature range, and the reaction of HOBr + F may proceed mainly through the bromine abstraction channel at the lower temperature while the contribution of hydrogen abstraction channel will become significant as the temperature increases. Because of lack of the kinetic data of these reactions, the present theoretical results are expected to be useful and reasonable to estimate the dynamical properties of these reactions over a wide temperature range where no experimental value is available.  相似文献   

15.
The mechanism of the gas‐phase reactions of SiHn+ (n = 1,2) with NF3 were investigated by ab initio calculations at the MP2 and CAS‐MCSCF level of theory. In the reaction of SiH+, the kinetically relevant intermediates are the two isomeric forms of fluorine‐coordinated intermediate HSi‐F‐NF2+. These species arise from the exoergic attack of SiH+ to one of the F atoms of NF3 and undergo two competitive processes, namely an isomerization and subsequent dissociation into SiF+ + HNF2, and a singlet‐triplet crossing so to form the spin‐forbidden products HSiF+ + NF2. The reaction of SiH2+ with NF3 involves instead the concomitant formation of the nitrogen‐coordinated complex H2Si‐NF3+ and of the fluorine‐coordinated complex H2Si‐F‐NF2+. The latter isomer directly dissociates into NF2+ + H2SiF, whereas the former species preferably undergoes the passage through a conical intersection point so to form a H2SiF‐NF2+ isomer, which eventually dissociates into H2SiF+ and NF2. © 2012 Wiley Periodicals, Inc.  相似文献   

16.
The title reaction was studied in a standard flow system with F atoms produced by RF discharge in F2-He mixture. Analysis was by gas chromatography using electron capture detection. There were two major products, identified as CF2BrCF2H and CF2BrCF2Br, plus presumably HF which was not detectable. The overall rate of disappearance of reactant was found to be of mixed one and one-half order, indicating a complex reaction. A mechanism is proposed comprising six steps and involving two radical species CF2Br?FBr (R1) and CF2Br?F2. The 300 K rate constant for the initial step F + reactant → HF + R1 is evaluated to be 2.2 × 10?13 cm3/molec·s, which fits in with rates of other saturated hydrocarbon reactants containing one hydrogen atom, thus supporting the view that in this class of reactants the rates of reactions of the type F + saturated hydrocarbon depend mainly on the number of hydrogen atoms in the reactant.  相似文献   

17.
One route to break down halomethanes is through reactions with radical species. The capability of the artificial force‐induced reaction algorithm to efficiently explore a large number of radical reaction pathways has been illustrated for reactions between haloalkanes (CX3Y; X=H, F; Y=Cl, Br) and ground‐state (2Σ+) cyano radicals (CN). For CH3Cl+CN, 71 stationary points in eight different pathways have been located and, in agreement with experiment, the highest rate constant (108 s?1 M ?1 at 298 K) is obtained for hydrogen abstraction. For CH3Br, the rate constants for hydrogen and halogen abstraction are similar (109 s?1 M ?1), whereas replacing hydrogen with fluorine eliminates the hydrogen‐abstraction route and decreases the rate constants for halogen abstraction by 2–3 orders of magnitude. The detailed mapping of stationary points allows accurate calculations of product distributions, and the encouraging rate constants should motivate future studies with other radicals.  相似文献   

18.
Challenges associated with the theoretical and experimental kinetics of the F+NH3→HF+NH2 reaction suggest the need for a more‐precise potential surface. We have investigated the reactants and the products of the reaction, as well as the transition state and two complexes, with rather rigorous ab initio methods. The F????NH3 complex existing in the entrance valley is predicted to lie 13.7 kcal mol?1 below the reactants. A small classical barrier of 2.0 kcal mol?1 separates this entrance well from products HF+NH2. These results explain the observation by Persky of unprecedented inverse temperature dependence for the F+NH3 rate constants. The strong hydrogen‐bonded complex FH????NH2 exists in the exit valley, and with a binding energy of 9.9 kcal mol?1 relative to separated products. The vibrational frequencies of all stationary points are predicted with the CCSD(T)/aug‐cc‐pVQZ method.  相似文献   

19.
20.
The association reaction between silyl radical (SiH3) and H2O2 has been studied in detail using high-level composite ab initio CBS-QB3 and G4MP2 methods. The global hybrid meta-GGA M06 and M06-2X density functionals in conjunction with 6-311++G(d,p) basis set have also been applied. To understand the kinetics, variational transition-state theory calculation is performed on the first association step, and successive unimolecular reactions are subjected to Rice–Ramsperger–Kassel–Marcus calculations to predict the reaction rate constants and product branching ratios. The bimolecular rate constant for SiH3–H2O2 association in the temperature range 250–600 K, k(T) = 6.89 × 10?13 T ?0.163exp(?0.22/RT) cm3 molecule?1 s?1 agrees well with the current literature. The OH production channel, which was experimentally found to be a minor one, is confirmed by the rate constants and branching ratios. Also, the correlation between our theoretical work and experimental literature is established. The production of SiO via secondary reactions is calculated to be one of the major reaction channels from highly stabilized adducts. The H-loss pathway, i.e., SiH2(OH)2 + H, is the major decomposition channel followed by secondary dissociation leading to SiO.  相似文献   

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