Temperature and Pressure Dependence of the Reaction

Experimental data for the unimolecular decomposition of CS2 from the literature are analyzed by unimolecular rate theory with the goal of obtaining rate constants for the reverse reaction S + CS (+M) → CS2 (+M) over wide temperature and pressure ranges. The results constitute an important input for the kinetic modeling of CS2 oxidation. CS2 dissociation proceeds as a spin-forbidden process whose detailed properties are still not well understood. The role of the singlet−triplet transition involved is discussed. ■ INTRODUCTION The oxidation of CS2 leads to a variety of sulfur-containing species by a mechanism that has been the subject of a number of kinetic modeling studies (for a summary of earlier work, see ref 1). As the agreement between measurements and kinetic models leaves a lot to desire, it appears necessary to inspect the rate constants of individual reaction steps and their accuracies in more detail again. One of these reactions is the combination of S atoms with CS, + + → + S CS ( M) CS ( M) 2 (R−1) It is potentially important as a terminating step in the low-temperature oxidation of CS2. As there are apparently no direct measurements of the rate of this reaction, one has to rely on experimental data for the reverse thermal decomposition of CS2, + → + + CS ( M) S CS ( M) 2 (R1) and convert these with the equilibrium constant = − K k k / c 1 1 (1) This is the issue of the present article. The problem looks easier than it really is. Reaction R−1 first leads into bonding electronically excited triplet states of CS2 which then undergo triplet−singlet spin inversions into the singlet electronic ground state of CS2. The detailed dynamics of this mechanism may result in complicated temperature and pressure dependences of the overall rate constants k1 and k−1. In order to unravel these, help from quantum chemical calculations will be required. The thermal dissociation of CS2 (eq R1) has been studied in shock waves since the mid-1960s. The results for the lowpressure rate constants k1,0 in the bath gas M = Ar are in fair agreement (within about a factor of 3), although the role of the