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

The unimolecular decomposition of iso-C3H7OH has been studied with a modified GAUSSIAN-2 method. Among the six low-lying product channels identified, the H2O-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 CH3+CH3C(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)=6.3x10(42) T-16.21 exp(-47 400/T), k(2)(0)=7.2x10(44) T-14.70 exp(-35 700/T) cm(3) molecule(-1) s(-1), k(1)(infinity)=8.0x10(29) T-3.75 exp(-45 800/T), and k(2)(infinity)=2.0x10(6) T-2.12 exp(-30 700/T) s(-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 C3H6 formation, which may derive in large part from secondary radical reactions. Other minor decomposition products were predicted to have the barriers H-2+CH3C(O)CH3, E-3(0)=82.8 kcal/mol; H2O+(CH3CCH3)-C-1, E-4(0)=77.9 kcal/mol; CH4+CH3C(H)O, E-5(0)=84.3 kcal/mol; and CH4+(CH3COH)-C-1, E-6(0)=81.9 kcal/mol. The triplet-singlet energy gap for CH3CCH3 was predicted to be 5.2 kcal/mol, favoring the singlet state. (C) 2002 American Institute of Physics.