Ab initio study of C4H3 potential energy surface and reaction of ground-state carbon atom with propargyl radical

Theoretical Study on the Reaction of Nitric Oxide with Propargyl Radical

The reaction of nitric oxide (NO) with propargyl radical (C₃H₃) was investigated at the CCSD(T)/cc-pVTZ//B3LYP/6-311++G(df, pd) level of theory. The rate coefficients of the system were determined by using the RRKM-CVT method with Eckart tunneling correction over a temperature range of 200-800 K and a pressure range of 1.0 × 10^-4 to 10.0 bar. Eight channels proceeding via the barrierless formation of excited intermediate ONCH₂CCH or CH₂CCHNO at the first step were explored. Three favorable channels (i.e., channels producing adduct of ONCH₂CCH and CH₂CCHNO and products of HCN and H₂CCO) were confirmed. The rate coefficients of channels producing adduct of ONCH₂CCH and CH₂CCHNO are comparable and have weak negative temperature dependence and positive pressure dependence. Channel producing products of HCN and H₂CCO is more important at low pressure and high temperature and less important after pressure greater than 1.0 × 10^-2 bar (with a branching ratio less than 6% at 0.1 bar).

Sub-Doppler infrared spectroscopy and formation dynamics of triacetylene in a slit supersonic expansion

Infrared spectroscopy and formation dynamics of triacetylene are investigated in a slit jet supersonic discharge and probed with sub-Doppler resolution (≈60 MHz) on the fundamental antisymmetric CH stretch mode (ν5). The triacetylene is generated in the throat of the discharge by sequential attack of ethynyl radical with acetyelene and diacetylene: (i) HCCH → HCC + H, (ii) HCC + HCCH → HCCCCH + H, (iii) HCC + HCCCCH → HCCCCCCH + H, cooled rapidly in the slit expansion to 15 K, and probed by near shot-noise-limited absorption sensitivity with a tunable difference-frequency infrared laser. The combination of jet cooled temperatures (Trot = 15 K) and low spectral congestion permits (i) analysis of rotationally avoided crossings in the ν5 band ascribed to Coriolis interactions, as well as (ii) first detection of ν5 Π-Π hot band progressions built on the ν12 sym CC bend and definitively assigned via state-of-the-art ab initio vibration-rotation interaction parameters (αi), which make for interesting comparison with recent spectroscopic studies of Doney et al. [J. Mol. Spectrosc. 316, 54 (2015)]. The combined data provide direct evidence for significantly non-equilibrium populations in the CC bending manifold, dynamically consistent with a strongly bent radical intermediate and transition states for forming triacetylene product. The presence of intense triacetylene signals under cold, low density slit jet conditions provides support for (i) barrierless addition of HCC with HCCCCH and (ii) a high quantum yield for HCCCCCCH formation. Complete basis set calculations for energetics [CCSD(T)-f12/VnZ-f12, n = 2,3] and frequencies [CCSD(T)-f12/VdZ-f12] are presented for both radical intermediate and transition state species, predicting collision stabilization in the slit jet expansion to be competitive with unimolecular decomposition with increasing polyyne chain length.

Ab initio/Rice-Ramsperger-Kassel-Marcus study of the singlet C4H4 potential energy surface and of the reactions of C2(XΣg+1) with C4H4(XA1g+1) and C(D1) with C3H4 (allene and methylacetylene)

Ab initio modified Gaussian-2 G2M(RCC,MP2) calculations have been performed for various isomers and transition states on the singlet C4H4 potential energy surface. The computed relative energies and molecular parameters have then been used to calculate energy-dependent rate constants for different isomerization and dissociation processes in the C4H4 system employing Rice-Ramsperger-Kassel-Marcus theory and to predict branching ratios of possible products of the C2(1Sigmag+)+C2H4, C(1D)+H2CCCH2, and C(1D)+H3CCCH reactions under single-collision conditions. The results show that C2 adds to the double C=C bond of ethylene without a barrier to form carbenecyclopropane, which then isomerizes to butatriene by a formal C2 "insertion" into the C-C bond of the C2H4 fragment. Butatriene can rearrange to the other isomers of C4H4, including allenylcarbene, methylenecyclopropene, vinylacetylene, methylpropargylene, cyclobutadiene, tetrahedrane, methylcyclopropenylidene, and bicyclobutene. The major decomposition products of the chemically activated C4H4 molecule formed in the C2(1Sigmag+)+C2H4 reaction are calculated to be acetylene+vinylidene (48.6% at Ecol = 0) and 1-buten-3-yne-2-yl radical [i-C4H3(X2A'), H2C=C=C=CH*]+H (41.3%). As the collision energy increases from 0 to 10 kcal/mol, the relative yield of i-C4H3+H grows to 52.6% and that of C2H2+CCH2 decreases to 35.5%. For the C(1D)+allene reaction, the most important products are also i-C4H3+H (55.2%) and C2H2+CCH2 (30.1%), but for C(1D)+methylacetylene, which accesses a different region of the C4H4 singlet potential energy surface, the calculated product branching ratios differ significantly: 65%-69% for i-C4H3+H, 18%-14% for C2H2+CCH2, and approximately 8% for diacetylene+H2.

Identification and Chemistry of C4H3 and C4H5 Isomers in Fuel-Rich Flames

Quantitative identification of isomers of hydrocarbon radicals in flames is critical to understanding soot formation. Isomers of C4H3 and C4H5 in flames fueled by allene, propyne, cyclopentene, or benzene are identified by comparison of the observed photoionization efficiencies with theoretical simulations based on calculated ionization energies and Franck-Condon factors. The experiments combine molecular-beam mass spectrometry (MBMS) with photoionization by tunable vacuum-ultraviolet synchrotron radiation. The theoretical simulations employ the rovibrational properties obtained with B3LYP/6-311++G(d,p) density functional theory and electronic energies obtained from QCISD(T) ab initio calculations extrapolated to the complete basis set limit. For C4H3, the comparisons reveal the presence of the resonantly stabilized CH2CCCH isomer (i-C4H3). For C4H5, contributions from the CH2CHCCH2 (i-C4H5) and some combination of the CH3CCCH2 and CH3CHCCH isomers are evident. Quantitative concentration estimates for these species are made for allene, cyclopentene, and benzene flames. Because of low Franck-Condon factors, sensitivity to n-isomers of both C4H3 and C4H5 is limited. Adiabatic ionization energies, as obtained from fits of the theoretical predictions to the experimental photoionization efficiency curves, are within the error bars of the QCISD(T) calculations. For i-C4H3 and i-C4H5, these fitted adiabatic ionization energies are (8.06 +/- 0.05) eV and (7.60 +/- 0.05) eV, respectively. The good agreement between the fitted and theoretical ionization thresholds suggests that the corresponding theoretically predicted radical heats of formation (119.1, 76.3, 78.7, and 79.1 kcal/mol at 0 K for i-C4H3, i-C4H5, CH3CCCH2, and CH3CHCCH, respectively) are also quite accurate.

The Addition of Hydrogen Atoms to Diacetylene and the Heats of Formation of i-C4H3 and n-C4H3

In this article, we discuss in detail the addition of hydrogen atoms to diacetylene and the reverse dissociation reactions, H + C(4)H(2)<==>i-C(4)H(3) (R1) and H + C(4)H(2)<==>n-C(4)H(3) (R2). The theory utilizes high-level electronic structure methodology to characterize the potential energy surface, Rice-Ramsperger-Kassel-Marcus (RRKM) theory to calculate microcanonical/J-resolved rate coefficients, and a two-dimensional master-equation approach to extract phenomenological (thermal) rate coefficients. Comparison is made with experimental results where they are available. The rate coefficients k1(T, p) and k2(T, p) are cast in forms that can be used in chemical kinetic modeling. In addition, we predict values of the heats of formation of i-C(4)H(3) and n-C(4)H(3) and discuss their importance in flame chemistry. Our basis-set extrapolated, quadratic-configuration-interaction with single and double excitations (and triple excitations added perturbatively), QCISD(T), predictions of these heats of formation at 298 K are 130.8 kcal/mol for n-C(4)H(3) and 119.3 kcal/mol for the i-isomer; multireference CI calculations with a nine-electron, nine-orbital, complete-active-space (CAS) reference wavefunction give just slightly larger values for these parameters. Our results are in good agreement with the recent focal-point analysis of Wheeler et al. (J. Chem. Phys. 2004, 121, 8800-8813), but they differ substantially for DeltaH0(f 298)(n-C(4)H(3)) with the earlier diffusion Monte Carlo predictions of Krokidis et al. (Int. J. Chem. Kinet.2001, 33, 808-820).

Thermochemistry of disputed soot formation intermediates C4H3 and C4H5

Accurate isomeric energy differences and standard enthalpies of formation for disputed intermediates in soot formation, C(4)H(3) and C(4)H(5), have been determined through systematic extrapolations of ab initio energies. Electron correlation has been included through second-order Z-averaged perturbation theory (ZAPT2), and spin-restricted, open-shell coupled-cluster methods through triple excitations [ROCCSD, ROCCSD(T), and ROCCSDT] utilizing the correlation-consistent hierarchy of basis sets, cc-pVXZ (X = D, T, Q, 5, and 6), followed by extrapolations to the complete basis set limit via the focal point method of Allen and co-workers. Reference geometries were fully optimized at the ROCCSD(T) level with a TZ(2d1f,2p1d) basis set. Our analysis finds that the resonance-stabilized i-C(4)H(3) and i-C(4)H(5) isomers lie 11.8 and 10.7 kcal mol(-1) below E-n-C(4)H(3) and E-n-C(4)H(5), respectively, several kcal mol(-1) (more, less) than reported in recent (diffusion Monte Carlo, B3LYP density-functional) studies. Moreover, in these systems Gaussian-3 (G3) theory suffers from large spin contamination in electronic wave functions, poor reference geometries, and anomalous vibrational frequencies, but fortuitous cancellation of these sizable errors leads to isomerization energies apparently accurate to 1 kcal mol(-1). Using focal-point extrapolations for isodesmic reactions, we determine the enthalpies of formation (delta(f)H(0) (composite function)) for i-C(4)H(3), Z-n-C(4)H(3), E-n-C(4)H(3), i-C(4)H(5), Z-n-C(4)H(5), and E-n-C(4)H(5) to be 119.0, 130.8, 130.8, 78.4, 89.7, and 89.1 kcal mol(-1), respectively. These definitive values remove any remaining uncertainty surrounding the thermochemistry of these isomers in combustion models, allowing for better assessment of whether even-carbon pathways contribute to soot formation.