KINETIC-STUDIES ON THE PYROLYSIS OF H2S

The kinetics and the mechanism of the thermal decomposition of H2S and subsequent reactions have been studied. The rate constant for the ini tiation reaction H2S + M --> products (1) was determined by a shock tu be-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] cm(3) molecul e(-1) s(-1), which is about one-fifth to one-tenth of the recent resul ts reported by Woiki and Roth (J. Phys. Chem. 1994, 98, 12958) and Ols chewski et al, (J. Phys, Chem. 1994, 98, 12964). An nb initio (MRCI+Q) calculation suggested that a spin-forbidden product channel (-->S(P-3 ) + H-2) is energetically favorable compared to a H-S bond fission cha nnel; that is, the singlet-triplet intersystem crossing occurs at an e nergy lower than the dissociation threshold for HS + H by about 17 kJ mol(-1). The present rate constant for reaction 1 could be well reprod uced by an unimolecular decomposition theory with the calculated energ y for the crossing and with a reasonable collision parameter, beta(c). The rate constants for important subsequent reactions, S(P-3) + H-2 - -> products (3) and S(P-3) + H2S --> products (4), were also determine d by a laser photolysis-shock tube-atomic resonance absorption spectro metry method: k(3) = 10(-9.58+/-0.16) exp[-(82.5+/-4.0) kJ mol(-1)/RT] (1050-1660 K) cm(3) molecule(-1) s(-1), and k(4) = 10(-9.86+/-0.17) e xp[-(30.9+/-4.1) kJ mol(-1)/RT] (1050-1540 K) cm(3) 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.