A shock-tube study of the kinetics of decomposition of carbon disulphide

The rate of decomposition of carbon disulphide in shock waves through CS2 + Ar mixtures has been studied spectroscopically between 2250 and 3350 °K. Mainly, the rate of appearance of CS has been measured, but in some instances it has been possible also to follow the rate of appearance of S2 and the rate of disappearance of CS2. The rate obeys mixed second-order kinetics of the type –d[CS2]/dt = k∞ [Ar] [CS2] + k0[CS2]2. The term that is second-order in [CS2] appears to correspond to a unimolecular decomposition of the CS2 in which the collision partner is another CS2, rather than to a true bimolecular reaction. k0 is found to be about 20 times as large as k∞. The rate constant for decomposition at infinite dilution in argon is given in Arrhenius form by k∞ = 1015.9 exp ( – 81.8 kcal/RT) cm3 mole-1 s-1. The application of R. R. K. H. theory to this yields as the most probable result, k∞ = (PZ/3!) (96 kcal/RT)3 exp ( – 96 kcal/RT), in which P is about 0.1. Thus four effective oscillators may be contributing energy to the decomposition process, the rate-determining step of which is likely to be a transition from the ground state to an excited singlet state at a crossing point located at an energy of about 96 kcal. In this respect CS2 is an interesting contrast to CO2, where the transition is probably singlet-triplet. The firmness of these conclusions is limited by the scarcity of knowledge concerning excited electronic states of CS2.