The branching ratio in the thermal decomposition of H{sub 2}CO

The thermal decomposition of H{sub 2}CO has been investigated in reflected shock waves experiments at temperatures between 2,004--2,367 K. The quantitative temporal formation of H-atoms in the reactions, (1a) H{sub 2}CO + Kr {yields} HCO + H + Kr and HCO + Kr {yields} CO + H + Kr, were measured by the atomic resonance absorption spectrometric (ARAS) technique. The product HCO-radicals instantaneously decompose giving a second H-atom. The experiments were carried out under conditions where secondary reaction perturbations were negligible. The observed H-atom profiles could be reproduced using a two step mechanism, reactions (1a) and (1b), H{sub 2}CO + Kr {yields} H{sub 2} + CO + Kr. The resulting values for the branching ratio, k{sub 1a}/(k{sub 1a} + k{sub 1b}) range between 6.7--12.2%. The data yield second-order rate constants, k{sub 1a} = 1.019 {times} 10{sup {minus}9} exp({minus}38706 K/T) and k{sub 1b} = 4.658 {times} 10{sup {minus}9} exp({minus}32110 K/T) cm{sup 3}/molecule s, respectively. The rate data and branching ratio results are compared to earlier determinations. Lastly, the data are theoretically rationalized using three theoretical formalisms. Single channel theoretical calculations are carried out with the semiempirical Troe and with the RRKM-Gorin methods, and these are compared to multichannel RRKM calculations using the Unimol code