Experimental and modelling study of the multichannel thermal dissociations of CHF and CHF.

The thermal unimolecular dissociation of CHF was studied in shock waves by monitoring the UV absorption of a dissociation product identified as CHF. It is concluded that, under conditions applied, the formation of this species corresponds to a minor, spin-allowed, dissociation channel of about 3% yield. Near to the low-pressure limit of the reaction, on the other hand, the energetically more favourable dissociation leads to CH + HF on a dominant, spin-forbidden, pathway. By considering the multichannel character of the reaction, it is shown that, in contrast to the low-pressure range, the high-pressure range of the reaction should be dominated by CHF formation. The channel-switching probably takes place at pressures higher than those applied in the present work. In addition to the two dissociation channels of CHF producing CH + HF and CHF + H, a third, spin-allowed, dissociation channel leading to CHF + H was also considered and estimated to proceed with a yield smaller than 0.5%. Besides the dissociation of CHF, the dissociation of CHF was studied by monitoring the UV spectrum of CHF. Details of this spectrum were investigated. Similar to CHF, the dissociation of CHF can proceed on several dissociation channels, under the present conditions either to CHF + H or to CF + H. After modelling single-channel falloff curves for all reaction pathways, coupling effects of multichannel dissociations were interpreted in the framework of multichannel unimolecular rate theory.