The kinetics and the mechanism of the thermal decomposition of H2S and subsequent reactions have been studied. The rate constant for the initiation reaction H2S + M → products (1) was determined by a shock tube−infrared emission spectroscopy at temperatures 2740−3570 K to be k 1 = 10-10.44±0.31 exp[−(268.6±18.4)kJ mol-1/RT] cm3 molecule-1 s-1, which is about one-fifth to one-tenth of the recent results reported by Woiki and Roth (J. Phys. Chem. 1994, 98, 12958) and Olschewski et al. (J. Phys. Chem. 1994, 98, 12964). An ab initio (MRCI+Q) calculation suggested that a spin-forbidden product channel (→S(3P) + H2) is energetically favorable compared to a H−S bond fission channel; that is, the singlet−triplet intersystem crossing occurs at an energy lower than the dissociation threshold for HS + H by about 17 kJ mol-1. The present rate constant for reaction 1 could be well reproduced by an unimolecular decomposition theory with the calculated energy for the crossing and with a reasonable collision parameter, βc. The rate constants for important subsequent reactions, S(3P) + H2 → products (3) and S(3P) + H2S → products (4), were also determined by a laser photolysis−shock tube−atomic resonance absorption spectrometry method:  k 3 = 10-9.58±0.16 exp[−(82.5±4.0) kJ mol-1/RT] (1050−1660 K) cm3 molecule-1 s-1, and k 4 = 10-9.86±0.17 exp[−(30.9±4.1) kJ mol-1/RT] (1050−1540 K) cm3 molecule-1 s-1. The ARAS measurement of H atoms revealed that the main products for reaction 3 are HS + H at pressures below 2 atm.

Thermal decomposition of COS was investigated by shock tubes between 1140—3230 K. The decay of COS and S was monitored by IR emissions and atomic resonance absorption spectrometry (ARAS) coupled with laser flash photolysis technique, respectively. The rate constants for the reactions COS + M → CO + S + M (1) and COS + S → CO + S2 (2) were determined as k1 = (4.07 ± 1.83) × 10−10exp (−257 ± 24kJ/RT), T: 1900—3230 K and k2 = (3.91 ± 1.18) × 10−11exp (−28.3 ± 0.9 kJ/RT) cm3 molecule−1 s−1, T: 1140—1680 K.

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