Mechanistic aspects of the abstraction of an allylic hydrogen in the chlorine atom reaction with 2-methyl-1,3-butadiene (isoprene)

The different channels for the abstraction of an allylic hydrogen in the chlorine atom reaction with isoprene were explored using ab initio methodology. It is shown that the metathesis reaction proceeds through an association-elimination mechanism in which a weakly bound intermediate (HCl.C(5)H(7)(*)) is formed first (formal addition). Further evolution by HCl elimination leads to the final C(5)H(7)(*) radical. QCISD(T)/aug-cc-pVDZ//MP2/6-31G(d,p) calculations show that for two of the possible pathways the barrier heights involved are moderate and the formation of the intermediates are exergonic (DeltaG < 0). Therefore, the mechanism proposed is both kinetically and thermodynamically feasible. The pressure dependence experimentally observed for the Cl + isoprene reaction can be rationalized in terms of the association-elimination mechanism proposed.     

Understanding the complex dissociation dynamics of energy selected dichloroethylene ions: neutral isomerization energies and heats of formation by imaging photoelectron-photoion coincidence

The dissociative photoionization of 1,1-C(2)H(2)Cl(2), (E)-1,2-C(2)H(2)Cl(2), and (Z)-1,2-C(2)H(2)Cl(2) has been investigated at high energy and mass resolution using the imaging photoelectron photoion coincidence instrument at the Swiss Light Source. The asymmetric Cl-atom loss ion time-of-flight distributions were fitted to obtain the dissociation rates in the 10(3) s(-1) < k < 10(7) s(-1) range as a function of the ion internal energy. The results, supported by ab initio calculations, show that all three ions dissociate to the same C(2v) symmetry ClC═CH(2)(+) product ion. The 0 K onset energies thus establish the relative heats of formation of the neutral isomers, that is, the isomerization energies. The experimental rate constants, k(E), as well as ab initio calculations indicate an early isomerization transition state and no overall reverse barrier to dissociation. The major high energy channels are the parallel HCl loss and the sequential ClC═CH(2)(+) → HCCH(+) + Cl process, the latter in competition with a ClC═CH(2)(+) → ClCCH(+) + H reaction. A parallel C(2)H(2)Cl(2)(+) → C(2)HCl(2)(+) + H channel also weakly asserts itself. The 0 K onset energy for the sequential Cl loss reaction suggests no barrier to the production of the most stable acetylene ion product; thus the sequential Cl-atom loss is preceded by a ClC═CH(2)(+) → HC(Cl)CH(+) reorganization step with a barrier lower than that of the second Cl-atom loss. The breakdown diagram corresponding to this sequential dissociation reveals the internal energy distribution of the first C(2)H(2)Cl(+) daughter ion, which is determined by the kinetic energy release in the first, Cl loss reaction at high excess energies. At low kinetic energy release, this distribution corresponds to the predicted two translational degrees of freedom, whereas at higher energies, the excess energy partitioning is characteristic of only one translational degree of freedom. New Δ(f)H(o)(298K) of 3.7, 2.5, and 0.2 ± 1.75 kJ mol(-1) are proposed for 1,1-C(2)H(2)Cl(2), (E)-1,2-C(2)H(2)Cl(2), and (Z)-1,2-C(2)H(2)Cl(2), respectively, and the proton affinity of ClCCH is found to be 708.6 ± 2.5 kJ mol(-1).     

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。     

Accurate ab initio potential energy surface, thermochemistry, and dynamics of the Cl(2P, 2P(3/2) + CH4 → HCl + CH3 and H + CH3Cl reactions

We report a high-quality, ab initio, full-dimensional global potential energy surface (PES) for the Cl((2)P, (2)P(3/2)) + CH(4) reaction, which describes both the abstraction (HCl + CH(3)) and substitution (H + CH(3)Cl) channels. The analytical PES is a least-squares fit, using a basis of permutationally invariant polynomials, to roughly 16,000 ab initio energy points, obtained by an efficient composite method, including counterpoise and spin-orbit corrections for the entrance channel. This composite method is shown to provide accuracy almost equal to all-electron CCSD(T)/aug-cc-pCVQZ results, but at much lower computational cost. Details of the PES, as well as additional high-level benchmark characterization of structures and energetics are reported. The PES has classical barrier heights of 2650 and 15,060 cm(-1) (relative to Cl((2)P(3/2)) + CH(4)(eq)), respectively, for the abstraction and substitution reactions, in good agreement with the corresponding new computed benchmark values, 2670 and 14,720 cm(-1). The PES also accurately describes the potential wells in the entrance and exit channels for the abstraction reaction. Quasiclassical trajectory calculations using the PES show that (a) the inclusion of the spin-orbit corrections in the PES decreases the cross sections by a factor of 1.5-2.5 at low collision energies (E(coll)); (b) at E(coll) ≈ 13,000 cm(-1) the substitution channel opens and the H/HCl ratio increases rapidly with E(coll); (c) the maximum impact parameter (b(max)) for the abstraction reaction is ~6 bohr; whereas b(max) is only ~2 bohr for the substitution; (d) the HCl and CH(3) products are mainly in the vibrational ground state even at very high E(coll); and (e) the HCl rotational distributions are cold, in excellent agreement with experiment at E(coll) = 1280 cm(-1).     

Quantum dynamical study of heavy-light-heavy reactions: application to the (Cl + CH<Subscript>4</Subscript> → HCl + CH<Subscript>3</Subscript>) reaction

A time dependent quantum-mechanical QM study is performed on the Cl + CH₄ → HCl + CH₃ reaction, using a pseudotriatomic ab initio based surface. Probabilities present some clear peaks versus t, which we assign to transition state resonances where the light H atom oscillates between heavy Cl and CH₃ groups. For ground-state reactants, the reactivity is of quantum origin. The reaction occurs through an abstraction mechanism, following both direct and an indirect mechanisms. The calculations show the participation of a short-lived collision complex in the microscopic reaction mechanism. This theoretical result and other oscillating properties found here could, however, be related to the existence of resonance for the production of HCl, as suggested by experimentalists.     

A fully ab initio potential energy surface for ClH2 reactive system

An ab initio analytical potential energy surface called BW3 for the ClH2 reactive system is presented. The fit of this surface is based on about 1 200 ab initio energy points, computed with multi-reference configuration interaction(MRCI) and scaling external correlation (SEC) method and a very large basis set. The precision in the fit is very high. The BW3 surface could reproduce correctly the dissociation energy of H2 and HCl, and the endothermicity of the Cl + H2 abstraction reaction. For the Cl + H2 abstraction reaction, the saddle point of BW3 lies in collinear geometries, and the barrier height is 32.84 kJ/mol; for the H + ClH exchange reaction, the barrier of BW3 is also linear, with a height of 77.40 kJ/mol.     

A gas-phase crossed-beam study of OH produced in the radical-radical reaction of O((3)P) with iso-propyl radical (CH3)2CH

The gas-phase radical-radical reaction dynamics of ground-state atomic oxygen [O((3)P)] with iso-propyl radicals, (CH(3))(2)CH, were investigated by applying a combination of high-resolution laser-induced fluorescence spectroscopy in a crossed-beam configuration and ab initio calculations. The nascent distributions of OH (X(2)Π: υ' = 0) from the major reaction channel O((3)P) + (CH(3))(2)CH → C(3)H(6) (propene) + OH showed substantial internal excitations with a bimodal feature of low- and high-N' components with neither spin-orbit nor Λ-doublet propensities. Unlike previous kinetic results, proposed to proceed only through the direct H-atom abstraction process, on the basis of the population analysis and comparison with the statistical theory, the title reaction can be described in terms of two competing mechanisms at the molecular level: direct abstraction process and indirect short-lived addition-complex-forming process with a ratio of 1.25?:?1.     

Searching for resonances in the reaction Cl+CH4-->HCl+CH3: quantum versus quasiclassical dynamics and comparison with experiments

A quantum-mechanical (QM) and quasiclassical trajectory (QCT) study was performed on the title reaction, using a pseudotriatomic ab initio based surface. Probabilities and integral cross sections present some clear peaks versus the collision energy E(col), which we assign to Feshbach resonances of the transition state, where the light H atom oscillates between the heavy Cl and CH(3) groups. For ground-state reactants, reactivity is essentially of quantum origin (QCT observables and oscillations are smaller, or much smaller, than QM ones), and the calculated integral cross section and product distributions are in reasonable agreement with the experiment. The reaction occurs through an abstraction mechanism, following both a direct and an indirect mechanism. The quasiclassical trajectory calculations show the participation of a short-lived collision complex in the microscopic reaction mechanism. Finally, QCT differential cross sections of Cl+CH(4)-->HCl (nu(')=0 and 1)+CH(3) oscillate versus E(col), whereas experimentally this only occurs for HCl (nu(')=1). This theoretical result and other oscillating properties found here could, however, be related to the existence of a Feshbach resonance for the production of HCl (nu(')=1), as suggested by experimentalists.     

CH2Cl与OH自由基反应机理的理论研究

The reaction mechanism of CH2Cl radical with OH radical to produce HCCl+H2O,HCOCl+H2 and H2CO+HCl has been studied by using quantum chemistry ab initio calculations. The optimized geometrical parameters,and vibrational frequencies of all species were obtained at the UMP2（FC）level of theory in conjunction with 6-311++G* basis set. Besides,the zero-point energies（ZPE）,relative energies and total energies of all species were calculated using Gaussian-3（G3）model. The results of theoretical study indicate that the activated intermediate CH2ClOH is first formed through a barrierless process,followed by atoms migration,radical groups rotation and bonds fission to produce HCCl+H2O,HCOCl+H2 and H2CO+HCl,respectively. And all channels are exothermic by 72.81,338.54 and 354.08 kJ/mol. The reaction heat of reactants to H2CO+HCl is 281.27 kJ/mol more than that of reactants to HCCl+H2O. This result accords with that of experiments.     

Reaction of chlorine atom with trichlorosilane from 296 to 473 K

The reaction of trichlorosilane (HSiCl(3)) with atomic chlorine (Cl) has been investigated by using infrared kinetic spectroscopy of the HCl product. The overall second order rate constant for the reaction has been determined as a function of temperature by using pseudo-first-order kinetic methods. Formation of HCl (nu=0) was monitored on the (nu=1<--0) R(2) line at 2944.914 cm(-1) and that of HCl (nu=1) on the (nu=2<--1) R(2) line at 2839.148 cm(-1). The overall second order rate constant was determined to be (2.8+/-0.1)x10(-11) cm(3) molecule(-1) s(-1) at 296 K. The rate constant shows no pressure dependence and decreases slightly with increased temperature [k=(2.3+/-0.2)x10(-11)e((66+/-3)/T) cm(3) molecule(-1) s(-1)]. Substantial vibrational excitation is measured in the HCl product, with the fraction of HCl (nu=1)/HCl (total)=0.41+/-0.08. These observations are consistent with the reaction being a barrierless hydrogen abstraction reaction. The experimental results are supported by ab initio quantum chemical calculations that show the transition state for abstraction to lie below the energy of the reactants, in disagreement with previously published calculations.     

Direct Ab Initio Dynamics Study on the Reaction of CH3CHF2 (HFC‐152a) with the Cl Atom

A direct ab initio dynamics method is used to investigate the hydrogen‐abstraction reaction CH₃CHF₂+Cl. One transition state is located for α‐H abstraction, and two are identified for β‐H abstraction. The potential‐energy surface (PES) is obtained at the G3(MP2)//MP2/6‐311G(d, p) level. Furthermore, the rate constants of the three channels are evaluated by using canonical variational transition‐state theory (CVT) with small‐curvature tunneling (SCT) contributions over a wide temperature range of 200–2500 K. The dynamic calculations show that the reaction proceeds mainly by α‐H abstraction over the whole temperature range. The calculated rate constants and branching ratios are both in good agreement with the available experimental values.     

Photodissociation of propargyl chloride at 193 nm

The photodissociation of propargyl chloride (C3H3Cl) has been studied at 193 nm. Ion imaging experiments with state-selective detection of the Cl atoms and single-photon ionization of the C3H3 radicals were performed, along with measurements of the Cl + C3H3 and HCl + C3H2 recoil kinetic energy distributions, using a scattering apparatus with electron bombardment ionization detection to resolve the competing Cl and HCl elimination channels. The experiments allow the determination of the Cl (2P3/2) and Cl (2P1/2) (hereafter Cl) branching fractions associated with the C-Cl bond fission, which are determined to be 0.5 +/- 0.1 for both channels. Although prior translational spectroscopy studies by others had concluded that the low velocity signal at the Cl+ mass was due to daughter fragments of the HCl elimination products, the present work shows that Cl atoms are produced with a bimodal recoil kinetic energy distribution. The major C-Cl bond fission channel, with a narrow recoil kinetic energy distribution peaking near 40 kcal/mol, produces both Cl and Cl, whereas the minor (5%) channel, partitioning much less energy to relative kinetic energy, produces only ground spin-orbit state Cl atoms. The maximum internal energy of the radicals produced in the low-recoil-kinetic-energy channel is consistent with this channel producing electronically excited propargyl radicals. Finally, in contrast to previous studies, the present work determines the HCl recoil kinetic energy distribution and identifies the possible contribution to this spectrum from propargyl radicals cracking to C3+ ions in the mass spectrometer.     

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.     

Theoretical study of H-abstraction reactions from CH3Cl and CH3Br molecules by ClO and BrO radicals

The rate constants of the H-abstraction reactions from CH(3)Cl and CH(3)Br molecules by ClO and BrO radicals have been estimated over the temperature range of 300-2500 K using four different levels of theory. Calculations of optimized geometrical parameters and vibrational frequencies are performed using B3LYP and MP2 methods combined with the cc-pVTZ basis set. Single-point energy calculations have been carried out with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (perturbatively) electron excitations CCSD(T) using the cc-pVTZ and cc-pVQZ basis sets. Canonical transition-state theory combined with an Eckart tunneling correction has been used to predict the rate constants as a function of temperature. In order to choose the appropriate levels of theory with chlorine- and bromine-containing species, the reference reaction Cl ((2)P(3/2)) + CH(3)Cl → HCl + CH(2)Cl (R(ref)) was first theoretically studied because its kinetic parameters are well-established from numerous experiments, evaluation data, and theoretical studies. The kinetic parameters of the reaction R(ref) have been determined accurately using the CCSD(T)/cc-pVQZ//MP2/cc-pVTZ level of theory. This level of theory has been used for the rate constant estimation of the reactions ClO + CH(3)Cl (R(1)), ClO + CH(3)Br (R(2)), BrO + CH(3)Cl (R(3)), and BrO + CH(3)Br (R(4)). Six-parameter Arrhenius expressions have been obtained by fitting to the computed rate constants of these four reactions (including cis and trans pathways) over the temperature range of 300-2500 K.     

F~2+2HCl→2HF+Cl~2反应机理的密度泛函理论研究

用密度泛函理论(DFT)B3LYP方法,在6-311G^*^*基组下,计算研究了反应F~2+2HCl→2HF+Cl~2的机理。求得各可能反应途径的系列过渡态,并通过振动分析和内禀反应坐标(IRC)分析加以证实。比较反应能垒(理论计算活化能)发现,标题反应若以分子与分子作用机理进行,则需克服的最大能垒为150.63kJ.mol^-^1;若以F~2分子先裂解为F原子再反应的机理进行,则需越过能垒154.82kJ.mol^-^1,求得反应F+HCl→HF+Cl的线形和三角形两种过渡态,以三角形较稳定;求得反应HCl+Cl→H+Cl~2的两种过渡态,以线形较稳定。     

Structure and bonding of H3CCuH

The reaction CH₄ + Cu(²P) → H₃CCuH → H₃C-CU + H and H₃C + CuH has been subjected to ab initio SCF calculations within the RHF formalism using a split valence shell gaussian basis set. The three electronic states (²A₁m ²E and ²E) of H₃CCuH studied had C_3V symmetry. The molecular structure and computed properties of the H₃CCuH ground state (²A₁) suggested that this molecule can be considered as a methyl radical attached to a closed-shell CuH. The results also indicate that the formation of H₃CCuH may occur through either an insertion or via an abstraction mechanism, nevertheless, the insertion mechanism appears to be the most favoured.     

Bimolecular reactions of vibrationally excited molecules. Roaming atom mechanism at low kinetic energies

Quasiclassical trajectory calculations have been performed for the H + H'X(v) → X + HH' abstraction and H + H'X(v) → XH + H' (X = Cl, F) exchange reactions of the vibrationally excited diatomic reactant at a wide collision energy range extending to ultracold temperatures. Vibrational excitation of the reactant increases the abstraction cross sections significantly. If the vibrational excitation is larger than the height of the potential barrier for reaction, the reactive cross sections diverge at very low collision energies, similarly to capture reactions. The divergence is quenched by rotational excitation but returns if the reactant rotates fast. The thermal rate coefficients for vibrationally excited reactants are very large, approach or exceed the gas kinetic limit because of the capture-type divergence at low collision energies. The Arrhenius activation energies assume small negative values at and below room temperature, if the vibrational quantum number is larger than 1 for HCl and larger than 3 for HF. The exchange reaction also exhibits capture-type divergence, but the rate coefficients are larger. Comparisons are presented between classical and quantum mechanical results at low collision energies. At low collision energies the importance of the exchange reaction is enhanced by a roaming atom mechanism, namely, collisions leading to H atom exchange but bypassing the exchange barrier. Such collisions probably have a large role under ultracold conditions. The calculations indicate that for roaming to occur, long-range attractive interaction and small relative kinetic energy in the chemical reaction at the first encounter are necessary, which ensures that the partners can not leave the attractive well. Large orbital angular momentum of the primary products (equivalent to large rotational excitation in a unimolecular reaction) is favorable for roaming.     

Role of O2 + QOOH in low-temperature ignition of propane. 1. Temperature and pressure dependent rate coefficients

The kinetics of the reaction of molecular oxygen with hydroperoxyalkyl radicals have been studied theoretically. These reactions, often referred to as second O(2) addition, or O(2) + QOOH reactions, are believed to be responsible for low-temperature chain branching in hydrocarbon oxidation. The O(2) + propyl system was chosen as a model system. High-level ab initio calculations of the C(3)H(7)O(2) and C(3)H(7)O(4) potential energy surfaces are coupled with RRKM master equation methods to compute the temperature and pressure dependence of the rate coefficients. Variable reaction coordinate transition-state theory is used to characterize the barrierless transition states for the O(2) + QOOH addition reactions as well as subsequent C(3)H(6)O(3) dissociation reactions. A simple kinetic mechanism is developed to illustrate the conditions under which the second O(2) addition increases the number of radicals. The sequential reactions O(2) + QOOH → OOQOOH → OH + keto-hydroperoxide → OH + OH + oxy-radical and the corresponding formally direct (or well skipping) reaction O(2) + QOOH → OH + OH + oxy-radical increase the total number of radicals. Chain branching through this reaction is maximized in the temperature range 600-900 K for pressures between 0.1 and 10 atm. The results confirm that n-propyl is the smallest alkyl radical to exhibit the low-temperature combustion properties of larger alkyl radicals, but n-butyl is perhaps a truer combustion archetype.     

Kinetics and mechanism of the gas phase reaction of chlorine atoms with i-propanol

FTIR smog chamber techniques and ab initio calculations have been used to investigate the kinetics and mechanism of the reaction of Cl atoms with i-propanol in 700 Torr of N(2) at 296 K. The reaction is observed to proceed with a rate constant of k(1) = (8.28 +/- 0.97) x 10(-11) cm(3) molecule(-1) s(-1) and gives CH(3)C(OH)CH(3) and CH(3)CH(OH)CH(2) radicals in yields of 85 +/- 7 and 15 +/- 7%, respectively. Calculations indicate that abstraction of the secondary H can proceed through a lower energy pathway than the primary. Rapid decomposition of the chlorination product CH(3)CCl(OH)CH(3) complicates its direct detection, likely due to heterogeneous chemistry. IR spectra for the chlorides CH(3)CCl(OH)CH(3) and CH(3)CH(OH)CH(2)Cl were inferred experimentally and assignments confirmed via comparison with ab initio computed spectra.