Reaction of CH with H₂O:  Temperature Dependence and Isotope Effect

Abstract

The reactions of CH(ν=0) and CD(ν=0) radicals with H2O and D2O were investigated in the temperature range between 291 and 723 K. The reactions of CH(ν=1) and CD(ν=1,2) with H2O and D2O were studied at 293 K. CH(D) radicals were generated by multiphoton dissociation of CH(D)Br3 using pulsed laser photolysis and the resulting CH(D) time profiles were monitored by laser induced fluorescence. The reactions of CH(D) in the ν = 0 state exhibit a negative temperature dependence and are independent of total pressure between 20 and 200 Torr at room temperature. The rate coefficients can be fitted over the experimental temperature range by the following expressions: k(T)CH+H 2 0 = (1.56 ± 0.25) × 10-11(T/298)-1.42 ± 0.31 cm3 molecule-1 s-1; k(T)CH+D 2 0 = (1.49 ± 0.25) × 10-11(T/298)-1.04 ± 0.29 cm3 molecule-1 s-1; k(T)CD+H 2 0 = (1.38 ± 0.21) × 10-11(T/298)-1.16 ± 0.25 cm3 molecule-1 s-1; k(T)CD+D 2 0 = (1.71 ± 0.28) × 10-11(T/298)-1.13 ± 0.32 cm3 molecule-1 s-1. The errors on the parameters correspond to ±1 σ standard deviation from nonlinear least-squares fitting, and the data were weighted by 1/σ2 using a σ of 20%. Deuteration of either the CH radical or H2O has no discernible effect on the kinetics of the reaction, suggesting formation of an intermediate complex prior to the insertion of CH(D) into the OH(D) bond. Rate coefficients for CH(ν=1) and CD(ν=1,2) were found to be faster than those for the radicals in the vibrational ground state. The results are compared with previous experimental and theoretical studies, and the mechanism is discussed.

Unusual Low-Temperature Reactivity of Water: The CH + H2O Reaction as a Source of Interstellar Formaldehyde?

“…15 The present work confirms this result by employing an efficient collision partner at 296 K, SF 6 , in the carrier gas mixture, which is characterized by an average energy loss per collision substantially larger than those of rare gases. Although the rate constants obtained by the earlier temperature dependent studies 7,8 differ by a factor of two, indicating possible systematic errors in the gasphase water concentration measurement, both sets of rate constants increase with decreasing temperature, a trend that is supported by the statistical calculations 15 also displayed in Figure 3.…”

“…Two different values of the TS energy for passage from the prereactive complex to the adduct were required in the calculations, -16.2 kJ/mol and -7.2 kJ/mol to furnish good agreement with the experiments of Zabarnick et al 8 and Blitz et al 7 respectively. The present study extends these measurements by demonstrating an order of magnitude increase of the rate constant between 296 K and 50 K; in good agreement with the calculated values employing a TS energy of -16.2 kJ/mol over the 296-100 K range.…”

The gas-phase chemistry of carbon chains in dark cloud chemical models

“…The 2 C n H radicals are characterized by doublet ground states and are very reactive species. The CH radical is reactive with molecules (Canosa et al 1997, Daugey et al 2005, Loison et al 2006, Loison & Bergeat 2009, Goulay et al 2009, Blitz et al 1999, Blitz et al 1997, Blitz et al 2012) and atoms ). The C 2 H radical is also reactive with molecules (Pedersen et al 1993, Goulay et al 2011, Woon 2006 and atoms .…”

The reaction of methylidene (CH) with methanol isotopomers

“…has no signiÐcant isotope e †ect although it too occurs without an activation barrier. 6 The rationale that we suggested for this observation was that the rate determining initial step in the reaction was the formation of an intermediate complex with lone pairs of the oxygen atom, before insertion into the OÈH bond. The idea of an initial complex is supported by theoretical calculations.12 The reaction of CH with methanol is an interesting extension of these studies as potentially both mechanisms could occur, with the CH radical either inserting directly into the CÈH bonds of the methyl fragment or forming an initial complex with the lone pairs of the oxygen before inserting into either the OÈH or CÈO bond.…”