Unimolecular HCl Elimination from 1,2-Dichloroethane:  A Single Pulse Shock Tube and ab Initio Study

Abstract

Thermal decomposition of 1,2-dichloroethane (1,2-DCE) has been studied in the temperature range of 1050-1175 K behind reflected shock waves in a single pulse shock tube. The unimolecular elimination of HCl is found to be the major channel through which 1,2-DCE decomposes under these conditions. The rate constant for the unimolecular elimination of HCl from 1,2-dichloroethane is found to be 10 13.98(0.80 exp(-57.8 ( 2.0/ RT) s -1 , where the activation energy is given in kcal mol -1 and is very close to that value for CH 3 CH 2 Cl (EC). Ab initio (HF and MP2) and DFT calculations have been carried out to find the activation barrier and the structure of the transition state for this reaction channel from both EC and 1,2-DCE. The preexponential factors calculated at various levels of theory (HF/6-311++G**, MP2/6-311++G**, and B3LYP/6-311++G**) are (≈10 15 s -1 ) significantly larger than the experimental results. If the torsional mode in the ground state is treated as free internal rotation the preexponential factors reduce significantly, giving excellent agreement with experimental values. The DFT results are in excellent (fortuitous?) agreement with the experimental value for activation energy for 1,2-DCE while the MP2 and HF results seem to overestimate the barrier. However, DFT results for EC is 4.5 kcal mol -1 less than the previously reported experimental values. At all levels, theory predicts an increase in HCl elimination barrier on β-Cl substitution on EC. † Part of the special issue "Donald Setser Festschrift".

Mixed Quantum-Classical Reaction Path Dynamics of C2H5F → C2H4 + HF

“…[19][20][21][22]24,25 Some of these later experimental studies have also included computational modeling results. 23,25 Nonstatistical vibrational energy distributions of the HF product for the unimolecular decomposition of fluoroethane and related molecules have been obtained by monitoring HF infrared emission following infrared multiphoton dissociation 17 and chemically activated decomposition. 19 The explanation, elaborated upon by Zamir and Levine, 36 was that the available energy may be divided into two portions: the barrier height of the reverse reaction and the energy in excess of this barrier height.…”

Kinetics of the thermal decomposition of tetramethylsilane behind the reflected shock waves between 1058 and 1194 K

“…In fact, this method was used extensively by various groups. [24][25][26] Three pressure transducers (PCB 113A22) were mounted towards the end of the driven section. The mounted pressure transducers were used to measure the shock velocity and thereby to calculate both the primary and reflected shock temperatures.…”

A second-generation constrained reaction volume shock tube

“…The earliest facilities to accomplish this were single pulse shock tubes, [11][12][13] which used a ball valve to confine the post-reflected-shock test gas mixture within the test section after it was cooled by the arrival of the expansion wave. In this way, post-shock gas samples could be obtained and analyzed without being diluted by other gases in the shock tube.…”

“…To summarize the above review, two separate techniques have generally been employed to confine a gas mixture to part of a shock tube: a ball valve [11][12][13] and a gate valve [14][15][16][17] (note that the use of thin diaphragms for gas separation in shock tube driven sections 18 is uncommon because the diaphragm fragments created as the incident shock wave shatters the diaphragm interfere with diagnostics located in the test section). Of the two techniques, we consider the gate valve to be superior for our application because it is thinner and occupies less volume than a ball valve (particularly for our relatively large 11.53-cm diameter tube).…”