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The kinetics of the reactions C2H3 + H2 → H + C2H4 (1) and CH3 + H2 → H + CH4 (2) have been studied in the temperature ranges 499−947 K (reaction 1) and 646−1104 K (reaction 2) and He densities (6−18) × 1016 atoms cm-3 by laser photolysis/photoionization mass spectrometry. Rate constants were determined in time-resolved experiments as a function of temperature. Ethylene was detected as a primary product of reaction 1. Within the above temperature ranges the experimental rate constants can be represented by Arrhenius expressions k1 = (3.42 ± 0.35) × 10-12 exp(−(4179 ± 67 K)/T) cm3 molecule-1 s-1 and k2 = (1.45 ± 0.18) × 10-11 exp(−(6810 ± 102 K)/T) cm3 molecule-1 s-1. Experimental values of k2 are in agreement with the available literature data. The potential energy surface and properties of the transition state for reactions (1, −1) were studied by ab initio methods. Experimental and ab initio results of the current study were analyzed and used to create a transition state model of the reaction. The resul...

The kinetics of the reaction CH[sub 3]CHCl + O[sub 2] [l reversible] CH[sub 3]ChClO[sub 2] [yields] products (1) has been studied at temperatures 296-839 K and He densities of (3-49) [times] 10[sup 16] molecule cm[sup [minus]3] by laser photolysis/photoionization mass spectrometry. Rate constants were determined in time-resolved experiments as a function of temperature and bath gas density. At low temperatures (298-400 K) the rate constants are in the falloff region under the conditions of the experiments. Relaxation to equilibrium in the addition step of the reaction was monitored within the temperature range 520-590 K. Equilibrium constants were determined as a function of temperature and used to obtain the enthalpy and entropy of the addition step of the reaction (1). At high temperatures (750-839 K) the reaction rate constant is independent of both pressure and temperature within the uncertainty of the experimental data and equal to (1.2 [+-] 0.4) [times] 10[sup [minus]14] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1]. Vinyl chloride (C[sub 2]H[sub 3]Cl) was detected as a major product of reaction 1 at T = 800 K. 29 refs., 3 figs., 6 tabs.

Rate constants for the unimolecular decomposition of iso-C 3 H 7 have been determined by laser flash photolysis coupled with photoionization mass spectrometry, over the temperature range 720-910 K. The reaction was studied in He, at densities of (3-30)×10 6 atoms cm -3 . More limited measurements were made for Ar and N 2 . The reaction is in the falloff region under all conditions studied. Three methods of data analysis were employed: (i) A transition state model was constructed by reference to literature values of dissociation and association limiting high-pressure rate constants over the temperature range 177-910 K

The unimolecular decomposition of C[sub 2]H[sub 5] in helium has been investigated near the low-pressure limit (T = 876-1094 K; P =0.8-14.3 Torr). Rate constants (k[sub 1]) have been determined as a function of temperature and pressure in the indicated ranges in time-resolved experiments. The reaction was isolated for quantitative study in a heated tubular reactor coupled to a photoionization mass spectrometer. Weak collision effects (fall-off behavior) were analyzed using a master equation analysis. Values of [l angle][Delta]E[r angle][sub down] for the exponential down energyloss probability were obtained for each experiment performed. The microcanonical rate constants, k[sub 1](E), needed to solve the master equation were obtained from a transition state model for the reaction which is described. The temperature dependence of these [l angle][Delta]E[r angle][sub down] determinations was apparent and fits the expression [l angle][Delta]E[r angle][sub down] = 0.255T[sup 1.0([+-]0.1)] cm[sup [minus]1]. It is shown that this expression(derived from experiments conducted between 876 and 1094 K) provides a reasonable representation of observed weak collision effects in helium down to 285K. Values for [l angle][Delta]E[r angle][sub down]for C[sub 2]H[sub 5] decomposition in other bath gases were obtained by reexamining published data on the fall-off of the C[sub 2]H[sub 5] decomposition inmore » helium (200-1100 K); k[sub 1][sup 0]=6.63 [times] 10[sup 9]T[sup [minus]4.99] exp([minus]20,130 K/T) cm[sup 3]molecule[sup [minus]1]s[sup [minus]1]. Prior published experiments on both the forward and reverse reactions (C[sub 2]H[sub 5] +(M) [leftrightarrow] C[sub 2]H[sub 4] + H + (M)) in the fall-off region were reevaluated expression for the high-pressure limit rate constant for the temperature range 200-1100 K,k[sub 1][sup [infinity]] = 1.11 [times] 10[sup 10]T[sup 1.037] exp(-18,504/T) s[sup [minus]1]s. 49 refs., 12 figs., 5 tabs.« less