REACTION OF EXCITED CADMIUM (P-3J) ATOMS WITH ALKANE HYDROCARBONS - QUENCHING CROSS-SECTIONS AND PRIMARY CDH YIELDS

Absolute cross sections for the quenching of Cd(3PJ) by several alkane hydrocarbons have been determined using the resonance-radiation flash photolysis technique. Yields of CdH from the alkanes as compared to the CdH yield in the reaction of Ca(3PJ) with H2 were also determined. The Cd(3PJ) quenching cross sections for the normal alkanes are four orders of magnitude smaller than the cross section for quenching by H2, even though CdH formation is exothermic for the alkanes and only thermoneutral for H2. The CdH yields from Cd(3PJ) quenching for propane and isobutane are equal within experimental error to the CdH yield for H2, and it is suggested that this yield is probably unity. In marked contrast to the quenching of the valence isoelectronic species Hg(3P0) and Hg(3P1) by the alkanes, there is virtually no dependence of the Cd(3PJ) quenching rate on C-H bond strength, and none of the theories advanced to explain the Hg(3PJ) interaction with C-H bonds can account for the Cd(3PJ) results. A mechanism is tentatively suggested in which a loose complex is formed between Cd(3PJ) and alkane molecules, followed by a low-probability potential surface crossing which has little dependence on C-H bond strength, or at least can occur with primary C-H bonds. It is also suggested that differences between Cd(3PJ) and Hg(3PJ) quenching by the alkanes can be understood if the mechanistic pathways which account for the Hg(3PJ) results are merely energetically inaccessible to the lower-energy Cd(3PJ) state at moderate temperatures. © 1979 American Chemical Society.