Laser Initiated Chain Reactions: The Kinetics of the Chlorine/Cyclohexane/Oxygen Chain System

The combination of laser photolysis with infrared chemiluminescence detection has proven an effective technique for determining the propagation rate coefficients of halogen + RH chain reactions. This thesis will extend the technique to cyclic alkanes by determining the propagation rate coefficients of the Cl2/ cyclohexane chain system as well as the rate coefficient for the primary chain termination mechanism, R + O2 → RO2. The following reaction scheme is supported by the kinetic analysis: Cl2 → 2Cl Laser Photolysis Cl + RH → HCl(v) + R First Propagation Step (k1) R + Cl2 → RCl + Cl Second Propagation Step (k2) where RH = cyclohexane, R = cyclohexyl radical, and RCl = cyclohexyl chloride. The addition of a radical scavenger such as oxygen will terminate the chain reaction by the following mechanisms: Cl + O2 + M → ClO2 + M (k3) R + O2 → RO2 (k4) Laser photolysis/ chemiluminescence experiments were performed under first-order conditions on slowly flowing mixtures of cyclohexane, chlorine, oxygen, diluted in argon at reduced pressures (5 to 50 Torr). Chlorine is photolyzed by a Nd:YAG laser emitting a pulse of third harmonic (355 nm) light to initiate the chain reaction. The first propagation step is sufficiently exothermic as to generate HCl in the v = 1 vibrational energy level which subsequently returns to the ground state by fluorescence emission at 3.5 μm which is monitored via a bandpass filter with a cryogenically cooled (77 K) mercury-cadmium-telluride (HgCdTe) detector. This technique requires observation of HCl(v) only under the appropriate experimental conditions to determine the rate coefficients k1, k2, kv, and k4. Signal-to-noise challenges limit the technique from determining k3, the scavenging of Cl atom by oxygen, because of the necessity of a third-body collision for this reaction. A detailed derivation of the equations describing the time-dependent concentrations in this kinetic scheme under pseudo first-order conditions is included. Computer simulations were used to model the fluorescence intensity profile in an effort to determine the experimental parameters that best serves this study. Nonlinear least-squares fits of the fluorescence intensity profiles yields values for the rate coefficients when using the expression for [HCl(v)]. The values for the rate coefficients determined by this study were: k1 = (1.8 ± 0.2) x 10-10 cc·molecule-1·sec-1, k2 = (3.1 ± 0.2) x 10-11 cc·molecule-1·sec-1, kv = (1.7 ± 0.5) x 10-11 cc·molecule-1·sec-1, and k4 = (1.2 ± 0.3) x 10-11 cc·molecule-1·sec-1. A comparison with literature values for these rate coefficients will be made followed by a discussion of the success and challenge of this technique. This technique is useful for studying reactions of the halogen + hydrocarbon type where intermediates other than HCl(v) are difficult to monitor. Improvements in detector design could allow this technique in theory to perform real-time kinetic analysis and thus provide a better understanding of these chain systems