Hue Minh Thi Nguyen

Experimental and theoretical study of the reaction of the ethynyl radical with acetylene (HCC+HCCH)

The absolute rate coeAcient of the reaction of the ethynyl radical with acetylene was measured using a pulsed laser photolysis/chemiluminescence (PLP/CL) technique in which HCC radicals were generated by excimer laser photodissociation of acetylene at 193 nm, and pseudo-first-order exponential decays of thermalized HCC were monitored in real time by the CH(A 2 D! X 2 P) CL resulting from their reaction with O2. The rate coeAcient as determined using this single, absolute technique over the extended temperature range of 2956 T OKU < 800 was found to exhibit no temperature dependence, the result being kacetyleneaO1:3 0:1U10 ˇ10 cm 3 molecule ˇ1 s ˇ1 . B3LYP and CCSD(T)/ 6-311++G(d,p) calculations revealed that while the direct H-abstraction has a barrier of40 kJ mol ˇ1 , the terminal Caddition is barrier-free giving the 2-ethynyl-vinyl radical (HCBC‐CH@CH) which either dissociates directly into diacetylene (HCBC‐CBCHa H) or first rearranges to 1-ethynyl-vinyl (HCBC‐C@CH2) before undergoing a H-loss. In both these pathways, all intermediates and transition structures lie energetically well below the reactants and therefore both fragmentation pathways are open to thermal reactants. An additional C4H2a H forming pathway through the vinylidene HC@CH‐CH@C was identified; it is expected to contribute at higher flame temperatures. ” 2000 Published by Elsevier Science B.V.

Ab initio study on the oxidation of NCN by OH: prediction of the individual and total rate constants.

The mechanism for the reaction of NCN with OH has been investigated by ab initio molecular orbital and transition-state theory calculations. The potential energy surface (PES) was calculated by the highest level of the modified GAUSSIAN-2 (G2M) method, G2M(CC1). The barrierless association process of OH + NCN --> OH...NCN (van der Waals, vdw) was also examined at the UCCSD(T)/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) and CASPT2(13,13)/ANO-L//B3LYP/6-311+G(d,p) levels. The predicted heats of reaction for the production of H + NCNO, HNC + NO, HCN + NO, and N(2) + HOC, 7.8, -53.2, -66.9, and -67.7, respectively, are in excellent agreement with the experimental values, 8.2 +/- 1.3, -52.3 +/- 1.7 (or 55.7 +/- 1.7), -66.3 +/- 0.7, and -68.1 +/- 0.7 kcal/mol. The kinetic results indicate that, in the temperature range of 300-1000 K, the formation of trans,trans-HONCN (LM2) is dominant. Over 1000 K, formation of H + NCNO is dominant, while the formation of HCN + NO becomes competitive. The rate constants for the low-energy channels given in units of cm(3) molecule(-1) s(-1) can be represented by the following: k(1)(LM2) = 1.51 x 10 (15)T(-8.72) exp(-2531/T) at 300-1500 K in 760 Torr N(2); k(2)(H+NCNO) = 5.54 x 10 (-14)T(-0.97) exp(-3669/T) and k(3)(HCN+NO) = 7.82 x 10 (-14)T(0.44) exp(-2013/T) at 300-2500 K, with the total rate constant of k(t) = 3.18 x 10 (2)T(-4.63) exp(-740/T), 300-1000 K, and k(t) = 2.53 x 10 (-14)T(1.13) exp(-489/T) in the temperature range of 1200-2500 K. These results are recommended for combustion modeling applications.

The reaction of C2H with H2: Absolute rate coefficient measurements and ab initio study

In this work, a pulsed laser photolysis/chemiluminescence (PLP/CL) technique was used to measure absolute rate coefficients for the reaction of C2H+H2→products over the temperature range 295–666 K. Ethynyl radicals were produced pulsewise by excimer laser photolysis of acetylene at 193 nm and real-time pseudo-first-order decays of C2H were monitored by the CH(A 2Δ→X 2Π) chemiluminescence resulting from their reaction with O2. Over the experimental temperature range, the results indicate that the rate coefficient exhibits a non-Arrhenius behavior in line with theoretical predictions, khydrogen(T)=3.92×10−19 T2.57±0.30 exp[−(130±140) K/T] cm3 molecule−1 s−1. Experiments were supplemented by ab initio molecular orbital calculations up to the coupled-cluster theory including all single and double excitations plus perturbative corrections for the triples, UCCSD(T), with the 6-311++G(d,p) basis set for geometry optimizations and the aug-cc-pVTZ for electronic energy single points, revealing that the direct hydrogen abstraction yielding HC≡CH+H is the only product channel of any importance. There is also no important crossing between the doublet and quartet energy surfaces. Finally, geometry optimizations at the UCCSD(T)/6-311++G(2df,2p) level have shown that the transition structure for H-abstraction is linear; harmonic vibration frequencies at this level, and single-point UCCSD(T)/aug-cc-pVTZ energies for these geometries result in an adiabatic barrier height for H-abstraction, including harmonic vibration zero point energies, of 12.8 kJ/mol, while the classical potential energy barrier is 9.2 kJ/mol.In this work, a pulsed laser photolysis/chemiluminescence (PLP/CL) technique was used to measure absolute rate coefficients for the reaction of C2H+H2→products over the temperature range 295–666 K. Ethynyl radicals were produced pulsewise by excimer laser photolysis of acetylene at 193 nm and real-time pseudo-first-order decays of C2H were monitored by the CH(A 2Δ→X 2Π) chemiluminescence resulting from their reaction with O2. Over the experimental temperature range, the results indicate that the rate coefficient exhibits a non-Arrhenius behavior in line with theoretical predictions, khydrogen(T)=3.92×10−19 T2.57±0.30 exp[−(130±140) K/T] cm3 molecule−1 s−1. Experiments were supplemented by ab initio molecular orbital calculations up to the coupled-cluster theory including all single and double excitations plus perturbative corrections for the triples, UCCSD(T), with the 6-311++G(d,p) basis set for geometry optimizations and the aug-cc-pVTZ for electronic energy single points, revealing that the direct hydr...

Pulsed laser photolysis and quantum chemical-statistical rate study of the reaction of the ethynyl radical with water vapor

The rate coefficient of the gas-phase reaction C(2)H + H(2)O-->products has been experimentally determined over the temperature range 500-825 K using a pulsed laser photolysis-chemiluminescence (PLP-CL) technique. Ethynyl radicals (C(2)H) were generated by pulsed 193 nm photolysis of C(2)H(2) in the presence of H(2)O vapor and buffer gas N(2) at 15 Torr. The relative concentration of C(2)H radicals was monitored as a function of time using a CH* chemiluminescence method. The rate constant determinations for C(2)H + H(2)O were k(1)(550 K) = (2.3 +/- 1.3) x 10(-13) cm(3) s(-1), k(1)(770 K) =(7.2 +/- 1.4) x 10(-13) cm(3) s(-1), and k(1)(825 K) = (7.7 +/- 1.5) x 10(-13) cm(3) s(-1). The error in the only other measurement of this rate constant is also discussed. We have also characterized the reaction theoretically using quantum chemical computations. The relevant portion of the potential energy surface of C(2)H(3)O in its doublet electronic ground state has been investigated using density functional theory B3LYP6-311 + + G(3df,2p) and molecular orbital computations at the unrestricted coupled-cluster level of theory that incorporates all single and double excitations plus perturbative corrections for the triple excitations, along with the 6-311 + + G(3df,2p) basis set [(U)CCSD(T)6-311 + + G(3df,2p)] and using UCCSD(T)6-31G(d,p) optimized geometries. Five isomers, six dissociation products, and sixteen transition structures were characterized. The results confirm that the hydrogen abstraction producing C(2)H(2)+OH is the most facile reaction channel. For this channel, refined computations using (U)CCSD(T)6-311 + + G(3df,2p)(U)CCSD(T)6-311 + + G(d,p) and complete-active-space second-order perturbation theory/complete-active-space self-consistent-field theory (CASPT2/CASSCF) [B. O. Roos, Adv. Chem. Phys. 69, 399 (1987)] using the contracted atomic natural orbitals basis set (ANO-L) [J. Almlof and P. R. Taylor, J. Chem. Phys.86, 4070 (1987)] were performed, yielding zero-point energy-corrected potential energy barriers of 17 kJ mol(-1) and 15 kJ mol(-1), respectively. Transition-state theory rate constant calculations, based on the UCCSD(T) and CASPT2/CASSCF computations that also include H-atom tunneling and a hindered internal rotation, are in perfect agreement with the experimental values. Considering both our experimental and theoretical determinations, the rate constant can best be expressed, in modified Arrhenius form as k(1)(T) = (2.2 +/- 0.1) x 10(-21)T(3.05) exp[-(376 +/- 100)T] cm(3) s(-1) for the range 300-2000 K. Thus, at temperatures above 1500 K, reaction of C(2)H with H(2)O is predicted to be one of the dominant C(2)H reactions in hydrocarbon combustion.

Experimental and theoretical study of the gas phase reaction of ethynyl radical with methane (HCC+CH4)

Abstract Absolute rate coefficients of the reaction of ethynyl radical with methane were measured for the first time at higher temperatures by a pulsed laser photolysis/chemiluminescence (PLP/CL) technique. Ethynyl radicals (HCC) radicals were generated pulsewise upon excimer laser photodissociation of acetylene at 193 nm and pseudo-first-order exponential decays of thermalized HCC were monitored in real-time by the CH ( A 2 Δ → X 2 Π ) chemiluminescence produced by their reaction with O 2 . The rate coefficients k (HCC+CH 4 ), over 295⩽T ( K ) , exhibit strong non-Arrhenius behaviour, being k(T)=1.39×10 −18 T 2.34±0.40 exp [(380±180) K /T] cm 3 molecule −1 s −1 . Calculations at the CCSD(T)/aug-cc-pvTZ level reveal that the direct H-abstraction yielding HCCH+CH 3 has the lowest energy barrier of about 10 kJ mol −1 .

Triplet–singlet energy gaps in iodo-carbenes (I–C–X): Remarkable discrepancy between theory and experiment

The triplet–singlet energy gaps in iodo-carbenes, I–C–X with X = H, F, Cl, Br and I, were computed using the MP2, CCSD(T), CASSCF/CASPT2, MR-SDCI, MR-ACPF and B3LYP methods, with basis sets up to 6-311++G(3df,2p) and aug-cc-pvTZ and effective core potentials. Corrections for relativistic effects were also incorporated. Our results indicate that diiodo-carbene (CI2) is likely to possess a singlet ground state lying around 30 kJ mol−1 below the lowest triplet state, thus at variance with that of a recent negative ion photoelectron spectroscopic study (R. L. Schwartz, G. E. Davico, T. M. Ramont and W. C. Lineberger, J. Phys. Chem. A, 1999, 103, 8213). In addition, the present study confirms the singlet character of other iodo-carbenes with substantial triplet–singlet gaps and also points out the remarkably large fluctuations and discrepancies in the absolute values of the energy splittings, not only between experiment and theory, but also between ab initio quantum chemical methods.

An experimental and theoretical study of the reaction of ethynyl radicals with nitrogen dioxide (HC≡C+NO2)

A pulsed laser photolysis/chemiluminescence (PLP/CL) technique was used to determine absolute rate constants of the reaction C2H+NO2→products over the temperature range 288–800 K at a pressure of 5 Torr (N2). The reaction has a large rate constant that decreases with increasing temperature. It may be expressed in simple Arrhenius form as k1(T)=(7.6±1.0)×10−11 exp[(130±50) K/T], although there is an indication of a downward curvature for T>700 K. A three-parameter Arrhenius fit to the data, which takes this into account gives k1(T)=(9.7±1.5)×10−9T−0.68 exp[(158±65) K/T]. Our experiments also show that the 293 K rate constant is invariant to pressure between 2 and 11 Torr (N2). We have also characterized the C2H+NO2 reaction theoretically. A large portion of the potential energy surface (PES) of the [C2,H,N,O2] system has been investigated in its electronic (singlet) ground-state using DFT with the B3LYP/6-311++G(3df,2p) method and MO computations at the CCSD(T)/6-311++G(d,p) level of theory. Seventeen isom...

Dynamics of the reactions of O(1D) with CD3OH and CH3OD studied with time-resolved Fourier-transform IR spectroscopy.

We investigated the reactivity of O((1)D) towards two types of hydrogen atoms in CH(3)OH. The reaction was initiated on irradiation of a flowing mixture of O(3) and CD(3)OH or CH(3)OD at 248 nm. Relative vibration-rotational populations of OH and OD (1 ≤ v ≤ 4) states were determined from their infrared emission recorded with a step-scan time-resolved Fourier-transform spectrometer. In O((1)D) + CD(3)OH, the rotational distribution of OD is nearly Boltzmann, whereas that of OH is bimodal; the product ratio [OH]/[OD] is 1.56 ± 0.36. In O((1)D) + CH(3)OD, the rotational distribution of OH is nearly Boltzmann, whereas that of OD is bimodal; the product ratio [OH]/[OD] is 0.59 ± 0.14. Quantum-chemical calculations of the potential energy and microcanonical rate coefficients of various channels indicate that the abstraction channels are unimportant and O((1)D) inserts into the C-H and O-H bonds of CH(3)OH to form HOCH(2)OH and CH(3)OOH, respectively. The observed three channels of OH are consistent with those produced via decomposition of the newly formed OH or the original OH moiety in HOCH(2)OH or decomposition of CH(3)OOH. The former decomposition channel of HOCH(2)OH produces vibrationally more excited OH because of incomplete intramolecular vibrational relaxation, and decomposition of CH(3)COOH produces OH with greater rotational excitation, likely due to a large torque angle during dissociation. The predicted [OH]/[OD] ratios are 1.31 and 0.61 for O((1)D) + CD(3)OH and CH(3)OD, respectively, at collision energy of 26 kJ mol(-1), in satisfactory agreement with the experimental results. These predicted product ratios vary weakly with collision energy.

Theoretical study of the reaction of the ethynyl radical with ammonia (C2H + NH3): hydrogen abstraction versus condensation

Portions of the potential energy surface (PES) related to the reaction between the ethynyl radical and ammonia (C2H + NH3) have been investigated in detail using both MO and DFT methods up to geometry optimizations using the coupled-cluster theory with large basis sets. Several (C2H4N) intermediates and transition structures for unimolecular rearrangements between them have been characterized. Calculations at the CCSD(T)/6-311++G(3df,2p) + ZPE level show that the C2H + NH3 reaction has two main entrance channels: H-abstraction and condensation. The relative energies (kcal mol−1) along the H-abstraction pathway are as follows: 1 C2H + NH3 (0) → pre-reaction complex CO2 (−2.9) → TS (−1.8) → post-reaction complex CO3 (−28.4) → HCCH + NH2 (−26.6). This channel thus starts by formation of a weak complex HCC…H3N, which after H-atom transfer gives rise to another weak complex between the products, HCCH⋯NH2. The energies (kcal mol−1) along the condensation pathway are: 1 C2H + NH3 (0) → pre-association complex CO1 (−6.1) → TS (−3.0) → adduct HCC–NH3 (−7.6) → TS (4.3) → H2N–CCH + H (−14.2). Although both complex CO1 and primary adduct HCC–NH3 are slightly more stable than CO2 and CO3, the transition structure for conversion of the adduct has a substantially higher energy than the reactants and is fairly rigid, whereas the transition state for H-abstraction lies below the reactant limit and is rather loose. Therefore, H-abstraction is calculated to be clearly favored over condensation at all temperatures. The predicted barrier-free main channel is consistent with recent experimental results showing the title reaction to be a fast process exhibiting a negative temperature dependence. In view of the small energy barrier related to the novel condensation pathway, it might contribute at high temperatures in a significant way to the products formation.

Kinetic study of the 2-naphthyl (C10H7) radical reaction with C2H2.

The kinetics for the gas-phase reaction of 2-naphthyl radical with acetylene has been measured by monitoring the C10H7O2 radical in the visible region employing cavity ringdown spectrometry (CRDS) using 2-C10H7Br as a radical source photolyzing at 193 nm in the presence of a small fixed amount of O2 at 40 Torr pressure with Ar as a diluent. Absolute rate constants measured at temperatures between 303 and 448 K can be expressed by the following Arrhenius equation: k(T) = (3.36 +/- 0.63) x 10(11) exp[-(817 +/- 34)/T] cm3 mol(-1) s(-1). Theoretically, the potential energy surfaces (PESs) for the reactions of acetylene with 1- and 2-C10H7 radicals have been calculated with the G2MS//B3LYP/6-311+G(d,p) method. The PESs show that the reactions of 1- and 2-C10H7 with C2H2 occur first by forming adducts with 2.6 and 2.9 kcal/mol barriers, respectively. The rate constants for the stabilization and decomposition of the adducts have been predicted by RRKM/ME calculations. The mechanisms for the decomposition of the two adducts were predicted to be distinctively different under experimental conditions; the excited 1-C10H7C2H2 radical produces primarily acenaphthylene because of its low formation barrier, while the excited 2-C10H7C2H2 radical can be effectively stabilized by collisional quenching due to its high exit barrier. The predicted rate constant for the 2-C10H7 reaction with C2H2 is in reasonable agreement with the experimental values under the conditions employed.