Reaction of CH with H2O: Temperature Dependence and Isotope Effect

The CH(X2Π) + H2O reaction: two transition state kinetics

The reaction of ground state methylidyne (CH) with water vapor (H2O) is theoretically re-investigated using high-level coupled cluster computations in combination with semi-classical transition state theory (SCTST) and two-dimensional master equation simulations. Insertion of CH into a H-O bond of H2O over a submerged barrier via a well-skipping mechanism yielding solely H and CH2O is characterized. The reaction kinetics is effectively determined by the formation of a pre-reaction van der Waals complex (PRC, HC-OH2) and its subsequent isomerization to activated CH2OH in competition with PRC re-dissociation. The tunneling effects are found to be minor, while variational effects in the PRC → CH2OH step are negligible. The calculated rate coefficient k(T) is nearly pressure-independent, but strongly depends on temperature with pronounced down-up behavior: a high value of 2 × 10-10 cm3 s-1 at 50 K, followed by a fairly steep decrease down to 8 × 10-12 cm3 s-1 at 900 K, but increasing again to 5 × 10-11 cm3 s-1 at 3500 K. Over the T-range of this work, k(T) can be expressed as: k(T, P = 0) = 2.31 × 10-11 (T/300 K)-1.615 exp(-38.45/T) cm3 s-1 for T = 50-400 K k(T, P = 0) = 1.15 × 10-12 (T/300 K)0.8637 exp(892.6/T) cm3 s-1 for T = 400-1000 K k(T, P = 0) = 4.57 × 10-15 (T/300 K)3.375 exp(3477.4/T) cm3 s-1 for T = 1000-3500 K.

CRAHCN-O: A Consistent Reduced Atmospheric Hybrid Chemical Network Oxygen Extension for Hydrogen Cyanide and Formaldehyde Chemistry in CO2-, N2-, H2O-, CH4-, and H2-Dominated Atmospheres

Hydrogen cyanide (HCN) and formaldehyde (H₂CO) are key precursors to biomolecules such as nucleobases and amino acids in planetary atmospheres. However, many reactions which produce and destroy these species in atmospheres containing CO₂ and H₂O are still missing from the literature. We use a quantum chemistry approach to find these missing reactions and calculate their rate coefficients using canonical variational transition state theory and Rice-Ramsperger-Kassel-Marcus/master equation theory at the BHandHLYP/aug-cc-pVDZ level of theory. We calculate the rate coefficients for 126 total reactions and validate our calculations by comparing with experimental data in the 39% of available cases. Our calculated rate coefficients are most frequently within a factor of 2 of experimental values and generally always within an order of magnitude of these values. We discover 45 previously unknown reactions and identify 6 from this list that are most likely to dominate H₂CO and HCN production and destruction in planetary atmospheres. We highlight ¹O + CH₃ → H₂CO + H as a new key source and H₂CO + ¹O → HCO + OH as a new key sink, for H₂CO in upper planetary atmospheres. In this effort, we develop an oxygen extension to our consistent reduced atmospheric hybrid chemical network (CRAHCN-O), building off our previously developed network for HCN production in N₂-, CH₄-, and H₂-dominated atmospheres (CRAHCN). This extension can be used to simulate both HCN and H₂CO production in atmospheres dominated by any of CO₂, N₂, H₂O, CH₄, and H₂.

Theoretical Study on the Dynamics and Kinetics of the Reaction of CH2OH with OH

The stochastic one-dimensional chemical master equation (CME) simulation method was used to investigate the dynamics of the reaction of CH₂OH with OH. A multiwell multichannel potential energy surface (PES) was constructed at the CCSD(T)/Aug-cc-pVTZ//CBS-QB3 and QCISD(T)/Aug-cc-pVTZ//CBS-QB3 levels of theory. The constructed PES consisted of three chemically activated intermediates and two van der Waals complexes. The fractional population analysis unraveled the role of the energized intermediates and van der Waals complexes in the early stages of this complex reaction. The CME calculations provided the phenomenological rate constants through analysis of the eigenvalues and eigenvectors of collision matrices while Leonard-Jones potential was used to model the collisions. The CME results indicated that CH₂O and H₂O were the major products, in accordance with the literature. Also, the findings declared the temperature and pressure independence of the reaction over a wide range of temperature (250 to 2400 K) and pressure (0.1 to 7 atm). Furthermore, the efficiency of tunneling on the hydrogen transfer isomerization reaction of trans-HCOH to CH₂O was confirmed over the temperature range of 250 to 3000 K. The rate constants for different reaction channels are reported.

Product Study of the Reaction of CH3 with OH Radicals at Low Pressures and Temperatures of 300 and 612 K†

The product distribution for the title reaction was studied using our time-of-flight mass spectrometer (TOFMS) connected to a tubular flow reactor. The methyl and hydroxyl radicals were produced by an excimer laser pulse (lambda = 193 nm) photolyzing acetone and nitrous oxide in the presence of excess water or hydrogen. Helium was used as the bath gas; the total density was held constant at 1.2 x 10(17) cm(-3). At 300 K the observations were consistent with singlet methylene ((1)CH(2)) and water as the main product channel with a small contribution of methanol. In contrast, at about 610 K three channels-formaldehyde isomers and methanol in addition to (1)CH(2) + H(2)O-are formed with similar yields. When acetone-d(6) was used, the production of both CHDO and CD(2)O was observed, indicating that two different formaldehyde-producing channels are operating simultaneously. These experimental results are compared with RRKM and master equation calculations on the basis of the properties of the methanol potential energy surface from a recent ab initio study.