Reaction Product Branching Research Articles

Ab Initio Chemical Kinetics for H + NCN: Prediction of NCN Heat of Formation and Reaction Product Branching via Doublet and Quartet Surfaces

The reaction of NCN with H atoms has been investigated by ab initio MO and RRKM theory calculations. The mechanisms for formation of major products on the doublet and quartet potential energy surfaces have been predicted at the CCSD(T) level of theory with the complete basis set limit. In addition, the heat of formation for NCN predicted at this rigorous level and those from five isogyric reactions are in close agreement with the best value based on the isodesmic process, (3)CCO + N2 = (3)NCN + CO, 109.4 kcal/mol, which lies within the two existing experimental values. The rate constants for the three possible reaction channels, H + NCN → CH + N2 (k(P1)), HCN + (4)N (k(QP1)), and HNC + (4)N (k(QP2)), have been calculated in the temperature range 298-3000 K. The results show that k(P1) is significantly higher than k(QP1) and k(QP2) and that the total rate constant agrees well with available experimental values in the whole temperature range studied. The kinetics of the reverse CH + N2 reaction has also been revisited at the CCSD(T)/CBS level; the predicted total rate constants at 760 Torr Ar pressure can be represented by kr = 4.01 × 10(-15) T(0.90) exp(-17.42 kcal mol(-1)/RT) cm(3) molecule(-1) s(-1) at T = 800-4000 K. The result agrees closely with the most recent experimental data and the best theoretical result of Harding et al. (J. Phys. Chem. A 2008, 112, 522) as well as that of Moskaleva and Lin (Proc. Combust. Inst. 2000, 28, 2393) evaluated with a steady-state approximation after a coding error correction made in this study.

Cyclic Versus Linear Isomers Produced by Reaction of the Methylidyne Radical (CH) with Small Unsaturated Hydrocarbons

The reactions of the methylidyne radical (CH) with ethylene, acetylene, allene, and methylacetylene are studied at room temperature using tunable vacuum ultraviolet (VUV) photoionization and time-resolved mass spectrometry. The CH radicals are prepared by 248 nm multiphoton photolysis of CHBr(3) at 298 K and react with the selected hydrocarbon in a helium gas flow. Analysis of photoionization efficiency versus VUV photon wavelength permits isomer-specific detection of the reaction products and allows estimation of the reaction product branching ratios. The reactions proceed by either CH insertion or addition followed by H atom elimination from the intermediate adduct. In the CH + C(2)H(4) reaction the C(3)H(5) intermediate decays by H atom loss to yield 70(+/-8)% allene, 30(+/-8)% methylacetylene, and less than 10% cyclopropene, in agreement with previous RRKM results. In the CH + acetylene reaction, detection of mainly the cyclic C(3)H(2) isomer is contrary to a previous RRKM calculations that predicted linear triplet propargylene to be 90% of the total H-atom coproducts. High-level CBS-APNO quantum calculations and RRKM calculations for the CH + C(2)H(2) reaction presented in this manuscript predict a higher contribution of the cyclic C(3)H(2) (27.0%) versus triplet propargylene (63.5%) than earlier predictions. Extensive calculations on the C(3)H(3) and C(3)H(2)D system combined with experimental isotope ratios for the CD + C(2)H(2) reaction indicate that H-atom-assisted isomerization in the present experiments is responsible for the remaining discrepancy between the new RRKM calculations and the experimental results. Cyclic isomers are also found to represent 30(+/-6)% of the detected products in the case of CH + methylacetylene, together with 33(+/-6)% 1,2,3-butatriene and 37(+/-6)% vinylacetylene. The CH + allene reaction gives 23(+/-5)% 1,2,3-butatriene and 77(+/-5)% vinylacetylene, whereas cyclic isomers are produced below the detection limit in this reaction. The reaction exit channels deduced by comparing the product distributions for the aforementioned reactions are discussed in detail.

Response to Comment on “High-Level ab Initio and Density Functional Study of the CH + NO Reaction Product Branching”

Response to Comment on “High-Level ab Initio and Density Functional Study of the CH + NO Reaction Product Branching”

Gas-phase reaction dynamics studied by ion imaging

Ion imaging is a multiplex detection technique that employs multi-photon ionization and two-dimensional time-of-flight mass spectrometry to obtain detailed information on the dynamics that govern the making and breaking of chemical bonds. It has primarily been used to study gas-phase unimolecular photofragmentations, but can also be used to study bimolecular reactions and photoelectron processes. In many cases ion imaging permits the simultaneous measurement of reaction product branching ratios, quantum-state populations and velocity distributions. © 1998 John Wiley & Sons, Ltd.

Detailed study of the dynamics of the O+(4S)+ HCN reaction. A case study of ion–molecule reactions in the spacecraft environment

Guided-ion beam (GIB) cross-section and time-of-flight (TOF) measurements are presented for the O+(4S)+ HCN reaction from 0.2–25 eV centre-of-mass (cm) collision energies. The reaction exhibits a charge-transfer and three chemical reaction channels, NO+, CHO+ and CO+. Charge-transfer predominates above ca. 1 eV. The GIB measurements are augmented with selected ion flow tube (SIFT) rate constant measurements at 310 and 485 K. The total SIFT thermal reaction rate coefficients at 310 and 485 K are, 2.5 ± 0.6 × 10–9 cm3 s–1 and 2.3 ± 0.6 × 10–9 cm3 s–1. These values are compared to the 2.0 ± 0.5 × 10–9 cm3 s–1 reaction rate coefficient derived from the 0.2 eV GIB total cross-section.The analysis of the TOF spectra with the osculating complex model reveals that product ion formation is mediated by long-lived intermediate complexes at low collision energies, while a direct mechanism dominates for the majority of the product ions at high energy. Contributions from both direct and complex-mediated mechanisms are observed at intermediate energies. The complex-mediated products show a near-statistical partitioning of reaction energy which is substantiated by a phase-space-theory based calculation. The chemical reaction product branching ratios are discussed in the context of recent ab initio[O—H—C—N]+ hypersurface calculations (A. Luna, A. Mebel and K. Morokuma, personal communication). The application of the measurements to an understanding of atmosphere–spacecraft interactions is also discussed.