Formation of HCOH + H₂ through the reaction CH₃ + OH. Experimental evidence for a hitherto undetected product channel

Abstract

In an extension of our earlier studies at lower temperatures [4,51 the title reaction was measured directly in a flow reactor at temperatures of 600 and 700 K. The pressure of 0.65 mb was chosen that low in order to reduce the contribution of the stabilization channel. OH was used in an excess over CH3. Both reactants along with the reaction products were monitored by mass spectrometry. CH3 profiles served as the major observable quantity for the extraction of rate data. This had to be done by using computer simulation since it was impossible to work under pseudo-first-order conditions.The obtained total rate coefficients were divided into channel rate coefficients by means of branching ratios as determined by the mass spectrometric measurement of the reaction products. For CH3 + OH, this led to a rate coefficient, k l , into the stabilization channel, and another one, kl,+f referring to the sum of two Hz-eliminating channels yielding the biradical HCOH and to a minor extent H2CO. These latter channels have not been measured before.In order to distinguish between them we switched over from OH to OD to get(1'0 -HzCO + HD so that the biradical and/or aldehyde channels could be determined by their by-products Hz and HD, respectively. The use of OD makes it also possible to measure the channelthrough its by-product, HDO.within our error limits no significant isotope effect takes place. units of cm, molec, and s:A comparison of the rate coeficients of both systems, i.e., CH3 + OH and CH3 + OD, indicates thatFor the rate coefficient into the HCOH channel, we arrive at a preliminary Arrhenius expression in k1, = 9.1 x 10-l~ exp(-1500/~).The H2CO channel could not be detected at our lower temperature rendering us with a rate coefficient at 700 K klf(700 K) = 1.7 X 10-l'Since simulation is needed for the deduction of the total rate coefficients as well as of the branching ratios, an uncertainty factor of 1.5 has to be attributed to these numbers. 0

VULCAN: An Open-source, Validated Chemical Kinetics Python Code for Exoplanetary Atmospheres

“…Physically, they are responsible for the step with highest energy barrier, breaking and forming the bond between carbon and oxygen. Unfortunately, these reactions rates are not well studied and the measured rates reported in the literature are inconsistent (Baulch et al 1994;Humpfer et al 1994;Hidaka et al 1989). (See the discussion in Visscher & Moses 2011; In our network, we adopt the rate coefficients for reaction (13) from Jasper et al (2007) by considering its more recent publication date and more thorough treatment.…”

Nonlinear intensity dependence in the infrared multiphoton excitation and dissociation of methanol pre-excited to different energies

“…55 Further reaction channels have been investigated through bimolecular reaction of electronically excited O( 1 D 2 ) with CH 4 56 -58 or through the recombination reaction of CH 3 with OH. 59 Hack and Thiesemann determined CH 2 O, 1 CH 2 , OH and ground state O( 3 P) from the unimolecular dissociation of chemically activated CH 3 OH by LIF. 56 At an excitation energy of 559 kJ mol Ϫ1 with respect to the vibrational ground state of CH 3 OH, they found that the unimolecular decay process is dominated by the OH-channel ͑90%͒ whereas the other decay channels are of minor importance: 6% for CH 2 O, 2% for 1 CH 2 and for O( 3 P).…”

“…56 At an excitation energy of 559 kJ mol Ϫ1 with respect to the vibrational ground state of CH 3 OH, they found that the unimolecular decay process is dominated by the OH-channel ͑90%͒ whereas the other decay channels are of minor importance: 6% for CH 2 O, 2% for 1 CH 2 and for O( 3 P). Humpfer et al investigated the bimolecular reaction of CH 3 with OH in a flow reactor at 700 K and a total pressure of 65 Pa. 59 For the reaction channels leading to HCOH, 1 60 although their analysis has been subject to questions. 61,62 The calculated transition state structures 53 and available thermochemical data 41,54 have been used to calculate k(E) for reaction channels ͑1͒-͑6͒ in Table I.…”

“…61,62 The calculated transition state structures 53 and available thermochemical data 41,54 have been used to calculate k(E) for reaction channels ͑1͒-͑6͒ in Table I. A comparison of the calculated k i (E) for the reactions ͑2͒-͑4͒ with the experimental results from Humpfer et al 59 clearly shows that the calculated k 3 (E) is significantly too large. To get a better agreement between experiment and theory the calculated vibrational frequencies at the transition state for reaction ͑3͒ have been modified to reduce the preexponential Arrhenius factor A 3ϱ determined for Tϭ700 K by a factor of 3.…”

Evolution of CO on Titan

“…We examined the work of the Oser group (Oser et al 1992, Humpfer et al 1995. They describe difficult and complicated experiments, and the analysis of the data is subject to many uncertainties.…”