UV to Near-IR CO Emissions from O + C2H2 and O + C3O2 Flames at Low Pressure and High Temperature

The reaction of O(3P) with alkynes: a dynamic and computational study focusing on formyl radical production

Production of formyl radical, HCO, from reactions of O(³P) with alkynes (acetylene, propyne, 1-butyne, and 1-pentyne) has been investigated using cavity ringdown laser absorption spectroscopy (CRDLAS) and computational methods. No HCO was detected from reaction with acetylene, while the amount of HCO increased for propyne and 1-butyne, dropping off somewhat for 1-pentyne. These results differ from trends previously observed for reactions of O(³P) with alkenes, which exhibit the largest HCO production for the smallest alkene and drop off as the alkene size increases. Computational studies employing density functional and coupled cluster methods have been employed to investigate the triplet and singlet state pathways for HCO production. Because intersystem crossing (ISC) has been shown to be important in these processes, the minimum energy crossing point (MECP) between the triplet and singlet surfaces has been studied. We find the MECP for propyne to possess C₁ symmetry and to lie lower in energy than previous studies have found. Natural Bond Orbital and Natural Resonance Theory analyses have been performed to investigate the changes in spin density and bond order along the reaction pathways for formation of HCO. Explanations are suggested for the trend in HCO formation observed for the alkynes. The trend in alkyne HCO yield also is compared and contrasted with the trend previously observed for the alkenes.

Dissection of the multichannel reaction of acetylene with atomic oxygen: from the global potential energy surface to rate coefficients and branching dynamics

A global potential energy surface for the O(³P) + C₂H₂reaction is developed and the quasi-classical trajectory study on the potential energy surface reproduce the rate coefficient and product branching ratio.

Trajectory surface hopping study of the O((3)P) + C2H2 reaction dynamics: effect of collision energy on the extent of intersystem crossing

Intersystem crossing (ISC) dynamics plays an important role in determining the product branching in the O((3)P) + C2H2 reaction despite the necessarily small spin-orbit coupling constant values. In this study we investigate the effect of collision energy on the extent of the contribution of a spin non-conserving route through ISC dynamics to the product distributions at the initial collision energies 8.2, 9.5, and 13.1 kcal/mol. A direct dynamics trajectory surface hopping method is employed with potential energy surfaces generated at the unrestricted B3LYP/6-31G(d,p) level of theory to perform nonadiabatic dynamics. To make our calculation simpler, nonadibatic transitions were only considered at the triplet-singlet intersections. At the crossing points, Landau-Zener transition probabilities were calculated using spin-orbit coupling constant values computed at the same geometry. The Landau-Zener model for the title reaction is validated against a more rigorous Tully's fewest switches method and found to be working reasonably well as expected because of weak spin-orbit coupling. We have compared our results with the recent crossed molecular beam experiments and observed a very good agreement with respect to the primary product branching ratios. Our calculation revealed that there is no noticeable effect of the initial collision energy on the overall product distributions that corroborates the recent experimental findings. Our calculation indicates, however, that the extent of intersystem crossing contributions varies significantly with collision energy, needed to be verified, experimentally.

Quantum Chemical and Theoretical Kinetics Study of the O(3P) + C2H2 Reaction: A Multistate Process

The potential energy surfaces of the two lowest-lying triplet electronic surfaces 3A'' and 3A' for the O(3P) + C2H2 reaction were theoretically reinvestigated, using various quantum chemical methods including CCSD(T), QCISD, CBS-QCI/APNO, CBS-QB3, G2M(CC,MP2), DFT-B3LYP and CASSCF. An efficient reaction pathway on the electronically excited 3A' surface resulting in H(2S) + HCCO(A2A') was newly identified and is predicted to play an important role at higher temperatures. The primary product distribution for the multistate multiwell reaction was then determined by RRKM statistical rate theory and weak-collision master equation analysis using the exact stochastic simulation method. Allowing for nonstatistical behavior of the internal rotation mode of the initial 3A'' adducts, our computed primary-product distributions agree well with the available experimental results, i.e., ca. 80% H(2S) + HCCO(X2A'' + A2A') and 20% CH2(X3B1) + CO(X1sigma+) independent of temperature and pressure over the wide 300-2000 K and 0-10 atm ranges. The thermal rate coefficient k(O + C2H2) at 200-2000 K was computed using multistate transition state theory: k(T) = 6.14 x 10(-15)T (1.28) exp(-1244 K/T) cm3 molecule(-1) s(-1); this expression, obtained after reducing the CBS-QCI/APNO ab initio entrance barriers by 0.5 kcal/mol, quasi-perfectly matches the experimental k(T) data over the entire 200-2000 K range, spanning 3 orders of magnitude.

Vibrational and Rotational Distributions of the CH(A2Δ) Product of the C2H + O(3P) Reaction Studied by Fourier Transform Visible (FTVIS) Emission Spectroscopy

The C(2)H + O((3)P) --> CH(A) + CO reaction is investigated using Fourier transform visible emission spectroscopy. The O((3)P) and C(2)H radicals are produced by simultaneous 193 nm photolysis of SO(2) and C(2)H(2) precursors, respectively. The nascent vibrational and rotational distributions of the CH(A) product are obtained under time-resolved, but quasi-steady-state, conditions facilitated by the short lifetime of the CH(A) emission. The vibrational temperature of the CH(A) product is found to be appreciably hotter (2800 +/- 100 K) than the rotational distributions in the v' = 0 (1400 +/- 100 K) and v' = 1 (1250 +/- 250 K) levels. The results suggest that the reaction may proceed through an electronically excited HCCO() intermediate; moreover, the vibrational excitation compared to rotational excitation is higher than expected based on a statistical distribution of energy and may be the result of geometrical changes in the transition state. The CH(A) emission is also observed in a C(2)H(2)/O/H reaction mixture using a microwave discharge apparatus to form O atoms, with subsequent H atom production. The nascent rotational and vibrational distributions of the CH(A) determined by the microwave discharge apparatus are very similar to the CH(A) distributions obtained in the photodissociation experiment. The results support the idea that the C(2)H + O((3)P) reaction may play a role in low-pressure C(2)H(2)/O/H flames, as previously concluded.

Vibrational Distributions of the CO(v) Products of the C2H2+ O(3P) and HCCO + O(3P) Reactions Studied by FTIR Emission

The C2H2 + O(3P) and HCCO + O(3P) reactions are investigated using Fourier transform infrared (FTIR) emission spectroscopy. The O(3P) radicals are produced by 193 nm photolysis of an SO2 precursor or microwave discharge in O2. The HCCO radical is either formed in the first step of the C2H2 + O(3P) reaction or by 193 nm photodissociation of ethyl ethynyl ether. Vibrationally excited CO and CO2 products are observed. The microwave discharge experiment [C2H2 + O(3P)] shows a bimodal distribution of the CO(v) product, which is due to the sequential C2H2 + O(3P) and HCCO + O(3P) reactions. The vibrational distribution of CO(v) from the HCCO + O(3P) reaction also shows its own bimodal shape. The vibrational distribution of CO(v) from C2H2 + O(3P) can be characterized by a Boltzmann plot with a vibrational temperature of approximately 2400 +/- 100 K, in agreement with previous results. The CO distribution from the HCCO + O(3P) reaction, when studied under conditions to minimize other processes, shows very little contamination from other reactions, and the distribution can be characterized by a linear combination of Boltzmann plots with two vibrational temperatures: 2320 +/- 40 and 10 300 +/- 600 K. From the experimental results and previous theoretical work, the bimodal CO(v) distribution for the HCCO + O(3P) reaction suggests a sequential dissociation process of the HC(O)CO++ --> CO + HCO; HCO --> H + CO.