David Dolson (Advisor)
Master of Science (MS)
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.
Department or Program
Department of Chemistry
Year Degree Awarded
Copyright 2007, all rights reserved. This open access ETD is published by Wright State University and OhioLINK.