Studies of site selective hydrogen atom abstractions by Cl atoms from isobutane and propane by laser flash photolysis/IR diode laser spectroscopy

Abstract

The kinetics of chlorine atom abstractions from normal and selectively deuterated propane and isobutane have been measured at room temperature and 195 K using a laser flash photolysis system, and following the course of the reaction via IR diode laser absorption measurements of HCl product. In conjunction with the kinetic measurements, a comparison of the HCl signal heights from pairs of measurements on normal and selectively deuterated systems has allowed the determination of the branching fractions of the reactions at the primary, secondary (propane) and tertiary (isobutane) positions. The kinetic data (all in units of cm³ molecule⁻¹ s⁻¹) for the reaction of Cl atoms with propane ((1.22 ± 0.02) ×10⁻¹⁰, 195 K; (1.22 ± 0.03) ×10⁻¹⁰ 298 K) and isobutane ((1.52 ± 0.02) ×10⁻¹⁰, 195 K; (1.25 ± 0.04) ×10⁻¹⁰, 298 K) are generally in good agreement with literature data. No data are available for comparison with our measurements for the reactions of Cl atoms with CH₃CD₂CH₃ ((1.02 ± 0.03) ×10⁻¹⁰, 195 K; (1.09 ± 0.02) ×10⁻¹⁰, 298 K) or (CH₃)₃CD ((1.32 ± 0.03) ×10⁻¹⁰, 195 K; (1.12 ± 0.04) ×10⁻¹⁰, 298 K). Rate coefficients at 195 K for the reactions of Cl atoms with ethane ((5.04 ± 0.08) ×10⁻¹¹ and n-butane ((2.19 ± 0.03) ×10⁻¹⁰) were also measured. The branching fractions for abstraction at the primary position increased with temperature for both propane ((40 ± 3)% at 195 K to (48 ± 3)% at 298 K) and isobutane ((49 ± 4)% at 195 K to (62 ± 5)% at 298 K). The direct measurements from this study are in good agreement with most calculations based on structure activity relationships.