Product distributions, rate constants, and mechanisms of LiH+H reactions

We present a quantum-mechanical investigation of the LiH depletion reaction LiH+H-->Li+H2 and of the H exchange reaction LiH+H'-->LiH'+H. We report product distributions, rate constant, and mechanism of the former, and rate constant and mechanism of the latter reaction. We use the potential-energy surface by Dunne et al. [Chem. Phys. Lett. 336, 1 (2001)], the real-wave-packet method by Gray and Balint-Kurti [J. Chem. Phys. 108, 950 (1998)], and the J-shifting approximation. The 1H2 nuclear-spin statistics and progressions of vib-rotational states (v',j') rule both initial-state-resolved and thermal product distributions, which have saw-toothed shapes with odd j' preferred with respect to even j'. At high collision energies and temperatures, we obtain a regular 3-to-1 intensity alternation of rotational states. At low collision energies and temperatures, the degeneracy and density of many H2 levels can, however, give more irregular distributions. During the collision, the energy flows from the reactant translational mode to the product vibration and recoil ones. The rate constants of both reactions are not Arrhenius type because the reactions are barrier-less. The low-temperature, LiH depletion rate constant is larger than the H exchange one, whereas the contrary holds at high temperature. The real-time mechanisms show the nuclear rearrangements of the nonreactive channel and of the reactive ones, and point out that the LiH depletion is preferred over the H exchange at short times. This confirms the rate-constant results.     

Chemically activated reactions on the C7H5 energy surface: propargyl + diacetylene, i-C5H3 + acetylene, and n-C5H3 + acetylene

This study uses computational chemistry and statistical reaction rate theory to investigate the chemically activated reaction of diacetylene (butadiyne, C(4)H(2)) with the propargyl radical (C˙H(2)CCH) and the reaction of acetylene (C(2)H(2)) with the i-C(5)H(3) (CH(2)CCCC˙H) and n-C(5)H(3) (CHCC˙HCCH) radicals. A detailed G3SX-level C(7)H(5) energy surface demonstrates that the C(3)H(3) + C(4)H(2) and C(5)H(3) + C(2)H(2) addition reactions proceed with moderate barriers, on the order of 10 to 15 kcal mol(-1), and form activated open-chain C(7)H(5) species that can isomerize to the fulvenallenyl radical with the highest barrier still significantly below the entrance channel energy. Higher-energy pathways are available leading to other C(7)H(5) isomers and to a number of C(7)H(4) species + H. Rate constants in the large multiple-well (15) multiple-channel (30) chemically activated system are obtained from a stochastic solution of the one-dimensional master equation, with RRKM theory for microcanonical rate constants. The dominant products of the C(4)H(2) + C(3)H(3) reaction at combustion-relevant temperatures and pressures are i-C(5)H(3) + C(2)H(2) and CH(2)CCHCCCCH + H, along with several quenched C(7)H(5) intermediate species below 1500 K. The major products in the n-C(5)H(3) + C(2)H(2) reaction are i-C(5)H(3) + C(2)H(2) and a number of C(7)H(4) species + H, with C(7)H(5) radical stabilization at lower temperatures. The i-C(5)H(3) + C(2)H(2) reaction predominantly leads to C(7)H(4) + H and to stabilized C(7)H(5) products. The title reactions may play an important role in polycyclic aromatic hydrocarbon (PAH) formation in combustion systems. The C(7)H(5) potential energy surface developed here also provides insight into several other important reacting gas-phase systems relevant to combustion and astrochemistry, including C(2)H + the C(3)H(4) isomers propyne and allene, benzyne + CH, benzene + C((3)P), and C(7)H(5) radical decomposition, for which some preliminary analysis is presented.     

Absolute rate constants for the reactions between amino and alkyl radicals at 298 K

The absolute rate constants for the reactions of NH₂ radicals with ethyl, isopropyl, and t-butyl radicals have been measured at 298 K, using a flash photolysis–laser resonance absorption method. Radicals were generated by flashing ammonia in the presence of an olefin. A new measurement of the NH₂ extinction coefficient and oscillator strength at 597.73 nm was performed. The decay curves were simulated by adjusting the rate constants of both the reaction of NH₂ with the alkyl radical and the mutual interactions of alkyl radicals. The results are k(NH₂ + alkyl) = 2.5 (±0.5), 2.0 (±0.4), and 2.5 (±0.5) × 10¹⁰ M^?1·s^?1 for ethyl, isopropyl, and t-butyl radicals, respectively. The best simulations were obtained when taking k(alkyl + alkyl) = 1.2, 0.6, and 0.65 × 10¹⁰M^?1·s^?1 for ethyl, isopropyl, and t-butyl radicals, respectively, in good agreement with literature values.     

Collision between polymeric radicals for termination rate constants

The motion of each polymeric radical during a collision between the polymeric radicals with the same radius is treated as completely random motion. The result obtained is: k_t = 0.250k_s (where k_t is the chain-termination rate constant and k_s is the reaction rate constant between radical chain ends). On taking the motion of the primary radical during a collision between a primary radical and a large polymeric radical to be completely random, the result obtained is: k_ti = 0.250k_si (where k_ti is the primary radical termination rate constant and k_si is the reaction rate constant between primary radical and radical chain end). On substituting k_s for k_si in the second equation, the rate constant obtained becomes the chain termination rate constant between the very small polymeric radical and the very large polymeric radical, and identical to the former equation. This identity indicates that the effect of the difference of the size of the polymeric radicals on the collision process relating to the chain termination rate constant should not be large.     

Disproportionation reactions between CF2H and C2H5 radicals in the gas phase

The disproportionation and combination reactions between CF₂H and C₂H₅ radicals have been studied in the gas phase from room temperature to 70°C. For the pathway which yields CF₂H₂ + C₂H₄, relative to CF₂HC₂H₅, Δ = 0.072 ± 0.019. The competing reaction channel that produces CF₂ + C₂H₆ is approximately four times as efficient, with Δ = 0.265 ± 0.038. With CF₂D radicals an isotope effect for the CF₂ + C₂H₅D reaction pathway was observed with Δ ? 0.2.     

First rate constants for reactions of OH radicals with amides

Rate constants have been measured for the reaction of OH radicals with four amides, R₁N(CH₃)—C(O)R₂ (R₁ = H or Methyl, R₂ = Methyl or Ethyl), at 300 and 384 K using flash photolysis/resonance fluorescence. Reactants are introduced under slow flow conditions and are controlled by two independent methods, gas saturation and continuous injection. It turns out that the reactivities of the amides are considerably lower than those of the corresponding amines. The pattern of rate constants obtained at 300 K: 14, 21, 5.2, and 7.6 · 10⁻¹² cm³/s for N,N-Dimethylacetamide (dmaa), N,N-Dimethylpropionamide (dmpa), N-Methylacetamide (maa), and N-Methylpropionamide (mpa), respectively, indicates a single, dominating reaction center and strong electronic effects of the substituents at both sides of the amide function. Correspondingly, the observed negative temperature dependence (E/R = − 400 to − 600 K) excludes a direct abstraction mechanism. © 1997 John Wiley & Sons, Inc.     

Theoretical study on the rate constants for the C2H5 + HBr --> C2H6 + Br reaction

The reaction C(2)H(5) + HBr --> C(2)H(6) + Br has been theoretically studied over the temperature range from 200 to 1400 K. The electronic structure information is calculated at the BHLYP/6-311+G(d,p) and QCISD/6-31+G(d) levels. With the aid of intrinsic reaction coordinate theory, the minimum energy paths (MEPs) are obtained at the both levels, and the energies along the MEP are further refined by performing the single-point calculations at the PMP4(SDTQ)/6-311+G(3df,2p)//BHLYP and QCISD(T)/6-311++G(2df,2pd)//QCISD levels. The calculated ICVT/SCT rate constants are in good agreement with available experimental values, and the calculate results further indicate that the C(2)H(5) + HBr reaction has negative temperature dependence at T < 850 K, but clearly shows positive temperature dependence at T > 850 K. The current work predicts that the kinetic isotope effect for the title reaction is inverse in the temperature range from 200 to 482 K, i.e., k(HBr)/k(DBr) < 1.     

Accurate ab initio predictions of ionization energies of hydrocarbon radicals: CH2, CH3, C2H, C2H3, C2H5, C3H3, and C3H5

The ionization energies for methylene (CH2), methyl (CH3), ethynyl (C2H), vinyl (C2H3), ethyl (C2H5), propargyl (C3H3), and allyl (C3H5) radicals have been calculated by the wave-function-based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled-cluster level with single and double excitations plus a quasiperturbative triple excitation [CCSD(T)]. When it is appropriate, the zero-point vibrational energy correction, the core-valence electronic correction, the scalar relativistic effect correction, the diagonal Born-Oppenheimer correction, and the high-order correlation correction have also been made in these calculations. The comparison between the computed ionization energy (IE) values and the highly precise experimental IE values determined in previous pulsed field ionization-photoelectron (PFI-PE) studies indicates that the CCSD(T)/CBS method is capable of providing accurate IE predictions for these hydrocarbon radicals achieving error limits well within +/-10 meV. The benchmarking of the CCSD(T)/CBS IE predictions by the PFI-PE experimental results also lends strong support for the conclusion that the CCSD(T)/CBS approach with high-level energy corrections can serve as a valuable alternative for reliable IE determination of radicals, particularly for those radicals with very unfavorable Franck-Condon factors for photoionization transitions near their ionization thresholds.     

Ab initio rate constants from hyperspherical quantum scattering: application to H+C2H6 and H+CH3OH

The dynamics and kinetics of the abstraction reactions of H atoms with ethane and methanol have been studied using a quantum mechanical procedure. Bonds being broken and formed are treated with explicit hyperspherical quantum dynamics. The ab initio potential energy surfaces for these reactions have been developed from a minimal number of grid points (average of 48 points) and are given by analytical functionals. All the degrees of freedom except the breaking and forming bonds are optimized using the second order perturbation theory method with a correlation consistent polarized valence triple zeta basis set. Single point energies are calculated on the optimized geometries with the coupled cluster theory and the same basis set. The reaction of H with C2H6 is endothermic by 1.5 kcal/mol and has a vibrationally adiabatic barrier of 12 kcal/mol. The reaction of H with CH3OH presents two reactive channels: the methoxy and the hydroxymethyl channels. The former is endothermic by 0.24 kcal/mol and has a vibrationally adiabatic barrier of 13.29 kcal/mol, the latter reaction is exothermic by 7.87 kcal/mol and has a vibrationally adiabatic barrier of 8.56 kcal/mol. We report state-to-state and state-selected cross sections together with state-to-state rate constants for the title reactions. Thermal rate constants for these reactions exhibit large quantum tunneling effects when compared to conventional transition state theory results. For H+CH3OH, it is found that the CH2OH product is the dominant channel, and that the CH3O channel contributes just 2% at 500 K. For both reactions, rate constants are in good agreement with some measurements.     

Collision theory thermal rate constants for H + H2 reactions on the SLTH potential surface

Rate constants for the reactions H + D₂ → HD + D and D + H₂ → HD + D were computed from collision theory using an excitation function derived assuming that reaction proceeds vibrationally adiabatically on the SLTH potential surface at fixed approach angles with reaction probability described by the line-of-centers model and an angle-dependent barrier. Most of the experimentally observed steric factor is accounted for by the excitation function of this model. The computed increase of activation energy with temperature can be understood as the consequence of opening up the cone of acceptance for the incoming atom with increasing relative translational energy.     

Photoionization of hot radicals: C2H5, n-C3H7, and i-C3H7

The combination of ion-imaging and vacuum-ultraviolet (vuv) single-photon ionization is used to study the internal energy dependence of the relative photoionization yields of the C(2)H(5),n-C(3)H(7), and i-C(3)H(7) radicals following the 266 nm photodissociation of the corresponding alkyl iodides. The comparison of the ion images obtained by vuv photoionization of the radical with those obtained by two-photon-resonant, three-photon ionization of the complementary I (2)P(32) and I*(2)P(12) atoms allows the extraction of the internal energy dependence of the cross sections. Factors influencing the appearance of the ion images in the different detection channels are discussed, including the secondary fragmentation of the neutral radicals, Franck-Condon factors for the photoionization process, and the unimolecular fragmentation of the parent photoions.     

Determination of the rate constants for the gas-phase reactions of methyl butenol with OH radicals,ozone, NO3 radicals,and Cl atoms

Using a relative rate method, rate constants for the gas-phase reactions of 2-methyl-3-buten-2-ol (MBO) with OH radicals, ozone, NO₃ radicals, and Cl atoms have been investigated using FTIR. The measured values for MBO at 298±2 K and 740±5 torr total pressure are: k_OH=(3.9±1.2)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹, k_O3=(8.6±2.9)×10⁻¹⁸ cm³ molecule⁻¹ s⁻¹, k=(8.6±2.9)×10⁻¹⁵ cm³ molecule⁻¹ s⁻¹, and k_Cl=(4.7±1.0)×10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. Atmospheric lifetimes have been estimated with respect to the reactions with OH, O₃, NO₃, and Cl. The atmospheric relevance of this compound as a precursor for acetone is, also, briefly discussed. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 589–594, 1998     

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.     

Rate constants for tsomerization of C6H5C(SC6H5)2 ĆH2 radicals to C6H5Ć(SC6H5)CH2SC6H5 radicals based on EPR data

Conclusions The parameters of the Arrhenius equation for the isomerization of PhC(SPh)₂H₂ radicals to Ph(SPh)CH₂. SPh radicals are in agreement with an intramolecular character of the rearrangement, with a 1,2-migration of the thiyl group. These parameters were calculated on the basis of the data that were obtained by the EPR method in the range 10–70°.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2596–2597, November, 1982.     

Reflected shock tube studies of high-temperature rate constants for OH + C2H2 and OH + C2H4

The reflected shock tube technique with multi-pass absorption spectrometric detection of OH-radicals at 308 nm (corresponding to a total path length of approximately 4.9 m) has been used to study the reactions, OH + C(2)H(2)--> products (1) and OH + C(2)H(4)--> C(2)H(3) + H(2)O (2). The present optical configuration gives a S/N ratio of approximately 1 at approximately 0.5-1.0 x 10(12) radicals cm(-3). Hence, kinetics experiments could be performed at [OH](0) = approximately 4-20 ppm thereby minimizing secondary reactions. OH was produced rapidly from the dissociations of either CH(3)OH or NH(2)OH (hydroxylamine). A mechanism was then used to obtain profile fits that agreed with the experiment to within <+/-5%. The derived Arrhenius expressions, in units of cm(3) molecule(-1) s(-1) are: k(1) = (1.03 +/- 0.24) x 10(-10) exp(-7212 +/- 417 K/T) for 1509-2362 K and k(2) = (10.2 +/- 5.8) x 10(-10) exp(-7411 +/- 871 K/T) for 1463-1931 K. The present study is the first ever direct measurement for reaction (1) at temperatures >1275 K while the present results extend the temperature range for (2) by approximately 700 K. These values are compared with earlier determinations and with recent theoretical calculations. The calculations agree with the present data for both reactions to within +/-10% over the entire T-range.     

Novel products from C6H5 + C6H6/C6H5 reactions

To date only one product, biphenyl, has been reported to be produced from C(6)H(5) + C(6)H(6)/C(6)H(5) reactions. In this study, we have investigated some unique products of C(6)H(5) + C(6)H(6)/C(6)H(5) reactions via both experimental observation and theoretical modeling. In the experimental study, gas-phase reaction products produced from the pyrolysis of selected aromatics and aromatic/acetylene mixtures were detected by an in situ technique, vacuum ultraviolet (VUV) single photon ionization (SPI) time-of-flight mass spectrometry (TOFMS). The mass spectra revealed a remarkable correlation in mass peaks at m/z = 154 {C(12)H(10) (biphenyl)} and m/z = 152 {C(12)H(8) (?)}. It also demonstrated an unexpected correlation among the HACA (hydrogen abstraction and acetylene addition) products at m/z = 78, 102, 128, 152, and 176. The analysis of formation routes of products suggested the contribution of some other isomers in addition to a well-known candidate, acenaphthylene, in the mass peak at m/z = 152 (C(12)H(8)). Considering the difficulties of identifying the contributing isomers from an observed mass number peak, quantum chemical calculations for the above-mentioned reactions were performed. As a result, cyclopenta[a]indene, as-indacene, s-indacene, biphenylene, acenaphthylene, and naphthalene appeared as novel products, produced from the possible channels of C(6)H(5) + C(6)H(6)/C(6)H(5) reactions rather than from their previously reported formation pathways. The most notable point is the production of acenaphthylene and naphthalene from C(6)H(5) + C(6)H(6)/C(6)H(5) reactions via the PAC (phenyl addition-cyclization) mechanism because, until now, both of them have been thought to be formed via the HACA routes. In this way, this study has paved the way for exploring alternative paths for other inefficient HACA routes using the PAC mechanism.     

Rotational spectrum and carbon-13 hyperfine structure of the C3H, C5H, C6H, and C7H radicals

By means of Fourier transform microwave spectroscopy of a supersonic molecular beam, we have detected the singly substituted carbon-13 isotopic species of C(5)H, C(6)H, and C(7)H. Hyperfine structure in the rotational transitions of the lowest-energy fine structure component ((2)Pi(12) for C(5)H and C(7)H, and (2)Pi(32) for C(6)H) of each species was measured between 6 and 22 GHz, and precise rotational, centrifugal distortion, Lambda-doubling, and (13)C hyperfine coupling constants were determined. In addition, resolved hyperfine structure in the lowest rotational transition (J = 32-->12) of the three (13)C isotopic species of C(3)H was measured by the same technique. By combining the centimeter-wave measurements here with previous millimeter-wave data, a complete set of (13)C hyperfine coupling constants were derived to high precision for each isotopic species. Experimental structures (r(0)) have been determined for C(5)H and the two longer carbon-chain radicals, and these are found to be in good agreement with the predictions of high-level coupled-cluster calculations. C(3)H, C(5)H, and C(7)H exhibit a clear alternation in the magnitude and sign of the (13)C hyperfine coupling constants along the carbon-chain backbone. Because the electron spin density is nominally zero at the central carbon atom of C(3)H, C(5)H, and C(7)H, and at alternating sets of carbon atoms of C(5)H and C(7)H, owing to spin polarization, almost all of the (13)C coupling constants at these atoms are small in magnitude and negative in sign. Spin-polarization effects are known to be important for the Fermi-contact (b(F)) term, but prior to the work here they have generally been neglected for the hyperfine terms a, c, and d.     

Ab initio study of C + H3+ reactions

The reaction C + H3+ --> CH(+) + H2 is frequently used in models of dense interstellar cloud chemistry with the assumption that it is fast, i.e. there are no potential energy barriers inhibiting it. Ab initio molecular orbital study of the triplet CH3+ potential energy surface (triplet because the reactant carbon atom is a ground state triplet) supports this hypothesis. The reaction product is 3 pi CH+; the reaction is to exothermic even though the product is not in its electronic ground state. No path has been found on the potential energy surface for C + H3+ --> CH2(+) + H reaction.