Thermal decomposition of ethanol. I. Ab Initio molecular orbital/Rice–Ramsperger–Kassel–Marcus prediction of rate constant and product branching ratios

The unimolecular decomposition of ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}$ has been investigated at the G2M (RCC2) level of theory. The decomposition reaction was found to be dependent strongly on pressure and temperature. Among the eight product channels identified, the ${\mathrm{H}}_2\mathrm{O}$ -elimination process (1) via a four-member-ring transition state is dominant below 10 atm in the temperature range of 700–2500 K. At the high—pressure limit and over 1500 K, cleavage of the C–C bond by reaction (2) producing ${\mathrm{CH}}_3+{\mathrm{CH}}_2\mathrm{OH}$ is predicted to be dominant while the ${\mathrm{CH}}_3{\mathrm{CH}}_2+\mathrm{OH}$ channel (8) also becomes competitive. The predicted high-pressure rate constants for the two major product channels can be given by $k_1{\text{=7.0×10}}^{13} \exp \text{(−34\hspace{0.05em}200/}\mathrm{T})$ and $k_2{\text{=3.7×10}}^{26}\hspace{0.17em}{\mathrm{T}}^{\text{−2.95}} \exp \text{(−45\hspace{0.05em}600/}\mathrm{T}\mathrm{) }{\mathrm{s}}^{\text{−1}},$ which compare reasonably with earlier data and with our preliminary experimental result obtained in a shock tube and static cell study. At the internal energy corresponding to the $\mathrm{O}{(}^1\mathrm{D)+}{\mathrm{C}}_2{\mathrm{H}}_6$ reaction (140.7 kcal/mol above ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\mathrm{),}$ the predicted branching ratios for the production of ${\mathrm{CH}}_3,$ ${\mathrm{C}}_2{\mathrm{H}}_5,$ and ${\mathrm{H}}_2$ are in qualitative agreement with the result of a recent cross-molecular beam experiment.