KINETICS AND THERMOCHEMISTRY OF THE REACTION OF 1-CHLOROETHYL RADICALWITH MOLECULAR-OXYGEN

The kinetics of the reaction CH3CHCl + O-2 reversible arrow CH3CHClO2 --> products (1) has been studied at temperatures 296-839 K and He den sities of(3-49) x 10(16) molecule cm(-3) by laser photolysis/photoioni zation mass spectrometry. Rate constants were determined in time-resol ved experiments as a function of temperature and bath gas density. At low temperatures (298-400 K) the rate constants are in the falloff reg ion under the conditions of the experiments. Relaxation to equilibrium in the addition step of the reaction was monitored within the tempera ture range 520-590 K. Equilibrium constants were determined as a funct ion of temperature and used to obtain the enthalpy and entropy of the addition step of the reaction (1). At high temperatures (750-839 K) th e reaction rate constant is independent of both pressure and temperatu re within the uncertainty of the experimental data and equal to (1.2 /- 0.4) x 10-(14) cm(3) molecule(-1) s(-1). Vinyl chloride (C2H3Cl) wa s detected as a major product of reaction 1 at T = 800 K. The rate con stant of the reaction CH3CHCl + Cl-2 --> products (6) was determined a t room temperature and He densities of(9-36) x 10(16) molecule cm(-3) using the same technique. The value obtained is k(6) = (4.37 +/- 0.69) x 10(-12) cm(3) molecule(-1) s(-1). An estimate of the high-pressure limit for reaction 1 was determined using this measured k(6) and the k (1)/k(6) ratio obtained by Kaiser et al.: K-1(infinity) (T = 298K) (1. 04 +/- 0.22) x 10(-11) cm(3) molecule(-1) s(-1). In a theoretical part of the study, structure, vibrational frequencies, and energies of nin e conformations of CH3CHClO2 were calculated using ab initio UHF/6-31G and MP2/6-31G** methods. The theoretical results are used to calcula te the entropy change of the addition reaction Delta S degrees(298) = -152.3 +/- 3.3 J mol(-1) K-1. This entropy change combined with the ex perimentally determined equilibrium constants resulted in a CH3CHCl-O- 2 bond energy Delta H degrees(298) = -131.2 +/- 1.8 kJ mol(-1). The ro om-temperature entropy (S degrees(298) = 341.0 +/- 3.3 J mol(-1) K-1) and the heat of formation (Delta H-f degrees(298) = -54.7 +/- 3.7 kJ m ol(-1)) of the CH3CHClO2 adduct were obtained.