Rate Constants and Kinetic Isotope Effects in the Reactions of Atomic Chlorine with<i>n</i>-Butane and Simple Alkenes at Room Temperature

Abstract

Rate constants for the reactions of chlorine atoms with n-butane and simple alkenes, as well as most of their deuterated analogs, were studied at room temperature using two independent techniques. Using a fast flow discharge system (FFDS), the decay of chlorine atoms in 1 Torr He was followed using resonance fluorescence at 135 nm. In relative rate (RR) studies the decay of the organic was followed, relative to a reference compound, using GC-FID when they both reacted with chlorine atoms. These RR measurements were performed at 1 Torr in N2 and at 1 atm in both N2 and air. The results of the FFDS and RR studies at 1 Torr were generally in excellent agreement with each other and, where available, with literature data. Discrepancies appear to exist at 1 atm for isoprene, where the addition portion of the rate constant measured in this laboratory is 40% higher than a value for k∞ reported recently by Bedjanian et al.40 Our rate constant for 1,3-butadiene is 25% smaller than that of Bierbach et al.39 Rate constants measured in these studies which have not been previously reported in the literature are as follows (in units of cm3 molecule-1 s-1, with the errors being the statistical 2σ errors): C3D6 (4.1 ± 0.8) × 10-11 in 1 Torr He, (4.3 ± 1.0) × 10-11 in 1 Torr N2, and (2.3 ± 0.3) × 10-10 in 1 atm N2 or air; 1-C4H8 (1.0 ± 0.1) × 10-10 in 1 Torr He, (1.2 ± 0.2) × 10-10 in 1 Torr N2, and (2.2 ± 0.3) × 10-10 at 1 atm N2 or air; 1-C4D8 (1.0 ± 0.2) × 10-10 in 1 Torr He, (1.2 ± 0.2) × 10-10 in 1 Torr N2, and (2.0 ± 0.4) × 10-10 in 1 atm N2 or air; n-C4D10 (1.6 ± 0.1) × 10-10 averaged over all pressures and carrier gases. Deuteration results in a normal kinetic isotope effect (KIE) for direct hydrogen abstraction, but an inverse kinetic isotope effect for addition to the double bond. The KIE (kH/kD) for n-butane was measured to be 1.4 ± 0.2. For ethene, an inverse KIE was measured, 0.74 ± 0.06 at 1 atm in N2 or air compared to an average of 0.35 at 1 Torr in N2 or He reported in earlier studies.29 The KIEs for the larger alkenes were unity within experimental error. For propene at 1 Torr, the inverse KIE for addition is largely counterbalanced by a normal KIE for abstraction of an allylic hydrogen. For the larger alkenes, this result is consistent with expectations because addition is close to the high-pressure limit even at 1 Torr and abstraction is expected to play a minor role in the overall reaction. The atmospheric implications of these measurements are discussed.