✍ Scribed by Kenneth L. Demerjian; Jack G. Calvert
No coin nor oath required. For personal study only.
The mechanism of the electronically excited sulfur-dioxide sensitized isomerization of cis-2-butene has been studied through the measurement of the initial quantum yields of tram-2-butene formation in 3130-A irradiated gaseous binary mixtures of SO2 and cis-2butene and ternary mixtures of SO,, cis-2-butene, and Cop. The kinetic treatment of the present data from the SOz-C4H8 mixtures and those of recent similar studies of Penzhorn and Gusten (31 and Cox [4] are all consistent with the involvement of only the long-lived fluorescent 'B1 and phosphorescent 3B1 states of SOz in the isomerization mechanism. The data give kzs + k2b) = 0.21 f 0.04; SOz('B1) + SO, -+ SOz(3B1) 4-SO2(2a); SOz(lB1) + SO, --f (2S02) (2b). The analogous intersystem crossing ratio for the SO~('B1)-cis-2butene quenching collisions is the largest observed among the many quencher molecules studied; the value lies in the range of 0.85 to 0.37, with the extremes representing different choicles of alternative data and possible mechanisms. From the present data the ratio of the SOz(3B,) quenching rate constant with SO2 to that with cis-2-butene as quencher is estimated to be (2.7 f 1.2) X in good agreement with our directly measured ratio from lifetime studies (2.91 f 0.23) X (30-321, and the value found in isomerization studies by Cox (2.40 f 0.09) X (41. The simple two-excited state mechanism, which is seemingly applicable to the relatively low-pressure binary SOrbutene mixture results, is not adequate to explain the data obtained in experiments with large quantities of added
Several alternative mechanisms can be used to rationalize these results, but all alternatives must incorporate some other excited SO, species (X) as well as SOZ(~BI) and SOz('B1). The kinetics suggest that the ill-defined state is unreactive toward the olefin and decays nonradiatively to SO2 largely in experiments at the lower pressures, X -+ SO2 (1 1); it may generate SOz( 3B1) in a collisionally induced process at high added inert gas (COZ) pressures, X + COZ + SOz(.'B1) + COz (loa) and X + COZ -+ SO, + COz (lob). The data give k l l / ( k l ~R + k l ( ~b ) = 0.026 mole/l.
Here an "excess" quantum yield of isomerization is observed.
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