Structure-reactivity correlations for reactions between H/D atoms with selected ethers: Reaction-rate coefficients from direct shock-tube measurements and transition-state theory

Abstract

Shock-tube experiments at elevated pressures between 2.0 and 2.7 bar were carried out to study H-atom abstractions between D atoms and selected ether compounds: dimethyl ether (DME), diethyl ether (DEE), dimethoxymethane (DMM), and methyl propyl ether (MPE). D-atom resonance absorption spectrometry (D-ARAS) behind reflected shock waves was used to monitor the consumption of D atoms. To study the bimolecular reactions between D atoms and the specific ether, gas mixtures of the selected ether compound and C₂D₅I diluted in argon (bath gas) were prepared; C₂D₅I was used as a precursor for D atoms. This innovative approach using Dual ARAS (D-ARAS and H-ARAS) allows the distinct detection of precursor decay followed by H-atom abstraction reactions and ether decay followed by H-atom release. For the study of the reaction D + DME → HD + products, the experiments covered a temperature range of 940–1050 K; for the reaction D + DEE → HD + products, the temperature range was 980–1260 K; for the reaction D + DMM → HD + products, the temperature range was 930–1300 K; and for the reaction D + MPE → HD + products, the temperature spans a range of 1000–1350 K. Experimentally determined rate coefficients have been expressed by the following Arrhenius equations:

k_total(D+DME)(T) = 1.9×10⁻¹⁰ exp (−31.4 kJ/mol / RT) cm³s⁻¹,

k_total(D+DEE)(T) = 1.7×10⁻¹⁰ exp (−22.4 kJ/mol / RT) cm³s⁻¹,

k_total(D+DMM)(T) = 2.7×10⁻¹⁰ exp (−27.3 kJ/mol / RT) cm³s⁻¹,

and k_total(D+MPE)(T) = 5.1×10⁻¹⁰ exp (−31.5 kJ/mol / RT) cm³s⁻¹.

The experimental results show an uncertainty of ±30 % and were supplemented by transition-state theory (TST) calculations based on molecular properties and energies from computations at the G4 level of theory. TST computations were conducted for H-atom abstraction from various types of primary and secondary carbon bonds. Bond-specific reaction rate-coefficient expressions were derived from theory and compared with experimental results to establish correlations between molecular structure and reactivity.