The collisional removal of the carbene CCl2(X̃(0,0,0)) and CCl2(Ã 1B1(0,7,0)) by rare gases and simple molecules Original Research Article

Absolute rate constants have been measured at room temperature for the collisional removal of dichlorocarbene, CCl2, in its X̃ 1A1(0,0,0) ground electronic state by simple alkenes, CF2CCl2, He and Ar. Rate data for CCl2 in the first singlet electronically excited state, à 1B1(0,7,0), obtained by time-resolved emission, are reported for rare gases, simple alkenes (C2H4, C3H6, 1-C4H8, i-C4H8, 1,3-butadiene), NO, O2, N2, CH4, HCl and the precursor CF2CCl2 itself as collision partners. The rate constants for the removal of the Ã(0,7,0) state are found to be close to those of the collision number (≈10−10 cm3 molecule−1 s−1), indicating the role played by long-range attractive forces in the collisional quenching and analogy to the behaviour of other three-atom carbenes in analogous electronic states. The kinetic data for the à and X̃ states have been analysed within the framework of several kinetic models. The application of Parmenter’s potential-well model, based on the attractive component of the potential, indicates that the quenching partners fall into three main groups for the quenching behaviour of the A state. Rare gases exhibit the lowest rate constants. Quenching by CH4 and double-bonded molecular partners, permitting cyclic addition, is more rapid. Collisional removal by the third group comprising the inorganic diatoms N2, O2, NO and HCl, is found to be the most rapid. The potential-well depth for CCl2(Ã) has been evaluated to be (εAA/k)1/2=34.6 K1/2. A collision-complex model also groups the reactants into three sets, roughly similar to those in Parmenter’s plot, one physical and the other two chemical in nature, but the correlation is poor. For quenching partners where reactive channels are available, the collision-complex model indicates that, once the transition state is formed, removal then proceeds via different channels. Correlation of rate constants for the removal of the à state with ionisation potential, to examine the role of charge transfer, was found to be limited in scope. Finally, a Parmenter’s plot for the limited body of available new data for the X̃ ground state is presented.