Experimental and Theoretical Determinations of the Mechanisms, Kinetics, and Thermochemistry of Chlorinated Species

J.W. Hudgens, J.A. Manion, C. Gonzalez, K.K. Irikura, and T.C. Allison

Objective: To measure and predict the reaction mechanisms, kinetics, and thermochemistry of C₃ (and larger) chlorinated species.

Problem: Within incinerators and plasmas, the reactions of chlorine with unsaturated C₃ chemical species are believed to engage in sequences that synthesize highly chlorinated by-products and pollutants. Yet, when attempting to formulate a numeric model that describes the production of such chlorinated species, one finds that no reliable reaction mechanisms, rate coefficients, or thermochemical data are available. The absence of such data persists because many practical obstacles have hindered experimental measurements of these properties. Moreover, in the absence of benchmark experimental data, the ab initio computational community has generally ignored the entire chemical class of C₃ chlorinated species, and thus, no broad overview for these species exists.

Approach: The research acquires new experimental kinetic and thermochemical data and involves extensive, state-of-the-art ab initio calculations, enabling interpretation of the data. We attempt to formulate or adapt models that allow us to predict trends across the entire chemical class. Cavity ring-down (CRD) absorption spectroscopy is used to measure real-time kinetic data and gas-chromatography/mass spectrometry (GC-MS) is used to measure the reaction end-products. The ab initio methods use density functional, Möller-Plesset, and multi-reference codes and formulations involving isogyric and isodesmic reactions.

Results and Future Plans: Two successful studies have shown: (1) new insights into the reactions of chlorine atoms with unsaturated C₃ species and (2) the inability of older theories to predict the properties of perchlorinated compounds. In the first study, we used CRD and GC-MS experiments to measure the reaction rate coefficients and end-products of the reactions, Cl + allene and Cl + propargyl chloride. Both reactions form energized radicals that isomerize before forming products. By using ab initio calculations to predict each governing reaction surface (e.g., Figure 1), we could accurately predict the observed products and confirm the governing mechanisms. The work also lead to new thermochemical enthalpies for several chlorinated C₃ radicals and stable species.

Figure 1. Ab initio energy diagram that explains the products observed from Cl + allene.