The Measurement of the Rate Parameters for the Reactions i-C3Hinline image and n-C3Hinline image + HI → C3H8 + I over the Temperature Range 293–623 K: Implications for the Standard Heat of Formation of the Propyl Radicals



An investigation of the absolute rate constants of the metathesis reactions i-C3Hinline image + HI → C3H8 + I (1) and n-C3Hinline image + 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-C3Hinline image free radicals. A minor correction to k2 for the wall loss of n-C3Hinline image (kw) was applied throughout the temperature range, whereas no significant wall loss of i-C3Hinline image 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 Δfinline image(i-C3Hinline image) compared to the currently accepted value, whereas no significant adjustment for n-C3Hinline image seems to be necessary. The recommended values are Δfinline image(i-C3Hinline image) = 86.6 ± 2.1 kJ mol−1 and Δfinline image(n-C3Hinline image) = 101.1 ± 2.1 kJ mol−1 resulting from a third law evaluation using inline image(i-C3Hinline image) = 289.9 and inline image(n-C3Hinline image) = 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.