The Measurement of the Rate Parameters for the Reactions i‐C₃H and n‐C₃H + HI → C₃H₈ + I^• over the Temperature Range 293–623 K: Implications for the Standard Heat of Formation of the Propyl Radicals

Abstract

An investigation of the absolute rate constants of the metathesis reactions i‐C3H7• + HI → C3H8 + I• (1) and n‐C3H7• + HI → C3H8 + I• (2) was performed using a newly developed apparatus. The obtained Arrhenius expressions are k1 = 1.94 (±0.42) × 10−11 exp(−6.190 (±0.855)/RT) and k2 = 6.49 (±1.86) × 10−11 exp(−11.084 (±0.913)/RT) (R = 8.314 J K−1 mol−1) over the temperature range 293–623 K (A in cm3 molec−1 s−1, Ea in kJ mol−1). Two Knudsen reactors of different geometry coupled to single‐photon (VUV) photoionization mass spectrometry (SPIMS) were successively used. The two propyl isomers were generated externally to and upstream of the reactor and led to thermalized i‐ and n‐C3H7• free radicals. A minor correction to k2 for the wall loss of n‐C3H7• (kw) was applied throughout the temperature range, whereas no significant wall loss of i‐C3H7• could be measured. The obtained results are based on the disappearance of propyl radicals with increasing amounts of HI. Thermochemical parameters of propyl free radicals resulting from the present kinetic measurements are discussed and point toward a slightly lower value for the standard heat of formation of ΔfH2980(i‐C3H7•) compared to the currently accepted value, whereas no significant adjustment for n‐C3H7• seems to be necessary. The recommended values are ΔfH2980(i‐C3H7•) = 86.6 ± 2.1 kJ mol−1 and ΔfH2980(n‐C3H7•) = 101.1 ± 2.1 kJ mol−1 resulting from a third law evaluation using S2980(i‐C3H7•) = 289.9 and S2980(n‐C3H7•) = 290.3 J K−1 mol−1, both values being very close to the values measured by W. Tsang, namely 88 ± 2 and 100 ± 2 kJ mol−1, respectively.

Evaluated Site-Specific Rate Constants for Reaction of Isobutane with H and CH3: Shock Tube Experiments Combined with Bayesian Model Optimization

“…One of the biggest uncertainties for the H + isobutane reaction is from I atom chemistry in our shock tube experiments. The model includes recombination of I with isobutyl and tert -butyl, but we are unsure if I atoms can also abstract hydrogens from isobutane via the following reactions: This reaction has not been studied under any conditions, but Leplat and Rossi studied the reverse reaction of reaction (HI + tert -butyl) at temperatures from 293 to 623 K. They also studied analogous reactions to the reverse reaction of reaction , including HI + 1-propyl → propane + I. Reaction of I with isobutane will compete with reaction of I with our radical scavenger toluene: We used a rate constant from Walsh et al from 1966, which is to our knowledge the only experimental value available for the forward or reverse reaction.…”

The Kinetics of the Reaction C2H5 • + HI → C2H6 + I• over an Extended Temperature Range (213–623 K): Experiment and Modeling

“…The reader is referred to Leplat et al for a more detailed account of additional background for Reaction (R1) [10], which presents an experimental measurement of 1 that is approximately a factor of 20 lower at 300 K compared to the results of Seetula et al [11]. So far, sophisticated ab initio electronic structure calculations coupled to chemical-kinetic activation mechanisms have not been able to clearly distinguish between the two sets of results for hydrogen metathesis reactions that differ by factors of 20 and more for more complex alkyl free radicals [10,12,13]. We hope that future, hopefully imminent, developments will allow a quantitative evaluation of the two sets of results on the level of sophisticated ab initio electronic structure theory coupled to detailed chemical kinetic models.…”

Kinetics of isopropanol decomposition and reaction with H atoms from shock tube experiments and rate constant optimization using the method of uncertainty minimization using polynomial chaos expansions (MUM‐PCE)

“…For acetone, the model predicts only 37‐58% is due to H attack on the tertiary hydrogen (R1504), with an approximately equal amount produced from I attack at this same site (R1527). According to the rate constants from Leplat and Rossi 71,72 and thermochemistry, 73 I is more selective than H for the tertiary carbon, thus producing a greater proportion of acetone. To properly constrain the reactions of interest (R1503 and R1504), we must deconvolve the secondary chemistry.…”

“…Few rate constants exist for many reactions involving I atoms, including reaction of I with isopropanol. We estimated these rates by analogy based on the experimental studies of Leplat and Rossi 71,72 on R + HI reactions, taking tert ‐butyl + HI ⇌ isobutane + I as an analog of I + tertiary α and 1‐propyl + HI ⇌ propane + I as the analog of I + primary β. The extent of these reactions in our system depends on the concentration of I and thus depends also on both how much I is scavenged by 135TMB and the equilibrium between I and I 2 .…”

“…t-C 3 H 6 OH will decompose, as usual, to make acetone and H. While we have considered reaction of I with the primary hydrogens of isopropanol, literature rate constants 71,72 and thermodynamics 73 predict that I atoms will heavily favor H-abstraction from the tertiary site. The variations seen in Figure 5 are likely not from the reactions of OH and CH 3 with isopropanol, as these radicals are present in all mixtures and should-unlike I atoms-lead to approximately the same product ratio as H. Regardless, the secondary chemistry means that acetone/propene product ratios do not equate exactly with relative H + isopropanol rates, so we turn to chemical modeling methods to better analyze our data.…”