Thermal decomposition of iso-propanol: First-principles prediction of total and product-branching rate constants

The unimolecular decomposition of ${\mathrm{iso-C}}_3{\mathrm{H}}_7\mathrm{OH}$ has been studied with a modified GAUSSIAN-2 method. Among the six low-lying product channels identified, the ${\mathrm{H}}_2\mathrm{O}$ -elimination process (2) via a four-member-ring transition state is dominant below 760 Torr over the temperature range 500–2500 K. At higher pressures and over 1200 K, the cleavage of a C-C bond by reaction (1) producing ${\mathrm{CH}}_3+{\mathrm{CH}}_3\mathrm{C(H)OH}$ is predicted to be dominant. The predicted low- and high-pressure limit rate constants for these two major product channels can be given by $k_1^0{\text{=6.3×10}}^{42}{\mathrm{ T}}^{\text{−16.21}} \exp \text{(−47\hspace{0.05em}400/}\mathrm{T}\mathrm{),}$ $k_2^0{\text{=7.2×10}}^{44}{\mathrm{ T}}^{\text{−14.70}} \exp \text{(−35\hspace{0.05em}700/}{\mathrm{T) cm}}^3 {\mathrm{molecule}}^{\text{−1}} {\mathrm{s}}^{\text{−1}},$ $k_1^{\infty }{\text{=8.0×10}}^{29}{\mathrm{ T}}^{\text{−3.75}} \exp \text{(−45\hspace{0.05em}800/}\mathrm{T}\mathrm{),}$ and $k_2^{\infty }{\text{=2.0×10}}^6{\mathrm{ T}}^{\text{2.12}} \exp \text{(−30\hspace{0.05em}700/}{\mathrm{T) s}}^{\text{−1}},$ respectively. Predicted $k_1$ values compare reasonably with available experimental data; however, $k_2$ values are lower than the experimentally determined apparent rate constant for ${\mathrm{C}}_3{\mathrm{H}}_6$ formation, which may derive in large part from secondary radical reactions. Other minor decomposition products were predicted to have the barriers ${\mathrm{H}}_2+{\mathrm{CH}}_3{\mathrm{C(O)CH}}_3,$ $E_3^0\text{=82.8\hspace{0.05em}}\mathrm{kcal/mol};$ ${\mathrm{H}}_2\mathrm{O}{+}^1{\mathrm{CH}}_3{\mathrm{CCH}}_3,$ $E_4^0\text{=77.9\hspace{0.05em}}\mathrm{kcal/mol};$ ${\mathrm{CH}}_4+{\mathrm{CH}}_3\mathrm{C(H)O},$ $E_5^0\text{=84.3\hspace{0.05em}}\mathrm{kcal/mol};$ and ${\mathrm{CH}}_4{+}^1{\mathrm{CH}}_3\mathrm{COH},$ $E_6^0\text{=81.9\hspace{0.05em}}\mathrm{kcal/mol}.$ The triplet-singlet energy gap for ${\mathrm{CH}}_3{\mathrm{CCH}}_3$ was predicted to be 5.2 kcal/mol, favoring the singlet state.