Isomerisation reactions of alkoxy radicals:: theoretical study and structure-activity relationships

Thermochemical and kinetic parameters for 1,5-H isomerisation reactions of alkoxy radicals up to C-8 have been determined theoretically using density functional theory. Pressure dependence (through RRKM statistical calculations) as well as tunneling corrections have been taken into account. The results of calculations are validated by available experimental relative rate constants. These results show that the set of alkoxy radicals studied can be divided into three categories according to the H-abstraction site involved in the isomerisation reaction (primary, secondary and tertiary). Values for kinetic parameters: pre-exponential factors, activation energies and rate constants are proposed for each category. In particular, the following rate constant values are predicted: k(isom)=6.2x10(5) s(-1), 9.3x10(6) s(-1) and 4.5x10(8) s(-1) for 1,5-H transfer from a primary group (-CH3), secondary group (-CH2-) and tertiary group (>CH-), respectively, at 298 K and 1 atm pressure. An uncertainty factor of about 5 is estimated for calculated rate constants. These results corroborate Atkinson's recommendations except for the third group for which our value is two orders of magnitude larger. Another result of this study is that the pressure dependence of the rate constant for the isomerisation reaction is weak except for abstraction of a tertiary H-atom where k(isom) (298 K, 1 atm) is 40% of the infinite pressure rate constant. It can be also stressed that, where the isomerisation is possible, it will always be the dominant pathway with respect to the reaction with O-2, but it may be in competition with the decomposition reaction. We show that this is also the case in upper tropospheric conditions (0.2 atm and 220 K).