Kinetic Studies of OH Reactions with a Series of Methyl Esters

Absolute rate constants have been measured for the gas-phase reactions of hydroxyl radicals with a series of methyl esters: methyl propionate (k ₁), methyl butyrate (k ₂), methyl valerate (k ₃), and methyl caproate (k ₄). Experiments were carried out using the pulsed laser photolysis-laser induced fluorescence technique over the temperature range 253-372 K. The obtained kinetic data were used to derive the following Arrhenius expressions: k ₁ = (1.45 ± 0.42) × 10^-12 exp[-(148 ± 86)/T]; k ₂ = (0.96 ± 0.29) × 10^-12 exp[(380 ± 91)/T]; k ₃ = (1.37 ± 0.64) × 10^-12 exp[(401 ± 142)/T]; k ₄ = (2.46 ± 1.04) × 10^-12 exp[(326 ± 130)/T] (in units of cm³ molecule^-1 s^-1). At room temperature, the rate constants obtained (in units of 10^-12 cm³ molecule^-1 s^-1) were as follows: methyl propionate (0.83 ± 0.09); methyl butyrate (3.30 ± 0.25); methyl valerate (4.83 ± 0.55); methyl caproate (7.15 ± 0.70). Our results are compared with the previous determinations and discussed in terms of structure-activity relationships.