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The mechanism of the isomerization of cis ‐2‐butene photosensitized by sulfur dioxide excited within the first allowed band at 3130Å
Author(s) -
Demerjian Kenneth L.,
Calvert Jack G.
Publication year - 1975
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550070107
Subject(s) - chemistry , isomerization , photochemistry , excited state , intersystem crossing , quenching (fluorescence) , reaction rate constant , 2 butene , butene , ternary operation , fluorescence , kinetics , catalysis , organic chemistry , atomic physics , ethylene , physics , quantum mechanics , computer science , singlet state , programming language
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 trans ‐2‐butene formation in 3130‐A irradiated gaseous binary mixtures of SO 2 and cis ‐2‐butene and ternary mixtures of SO 2 , cis ‐2‐butene, and CO 2 . The kinetic treatment of the present data from the SO 2 C 4 H 8 mixtures and those of recent similar studies of Penzhorn and Güsten [3] and Cox [4] are all consistent with the involvement of only the long‐lived fluorescent 1 B 1 and phosphorescent 3 B 1 states of SO 2 in the isomerization mechanism. The data give ( k 2a + k 2b )=0.21±0.04; SO 2 ( 1 B 1 ) + SO 2 → SO 2 ( 3 B 1 ) + SO 2 (2a); SO 2 ( 1 B 1 ) + SO 2 → (2SO 2 ) (2b). The analogous intersystem crossing ratio for the SO 2 ( 1 B 1 )‐ cis ‐2‐butene 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 choices of alternative data and possible mechanisms. From the present data the ratio of the SO 2 ( 3 B 1 ) quenching rate constant with SO 2 to that with cis ‐2‐butene as quencher is estimated to be (2.7±1.2) × 10 −3 , in good agreement with our directly measured ratio from lifetime studies (2.91±0.23) × 10 −3 [30−32], and the value found in isomerization studies by Cox (2.40±0.09) × 10 −3 [4]. The simple two‐excited state mechanism, which is seemingly applicable to the relatively low‐pressure binary SO 2 ‐butene mixture results, is not adequate to explain the data obtained in experiments with large quantities of added CO 2 gas. Here an “excess” quantum yield of isomerization is observed. Several alternative mechanisms can be used to rationalize these results, but all alternatives must incorporate some other excited SO 2 species (X) as well as SO 2 ( 3 B 1 ) and SO 2 ( 1 B 1 ). The kinetics suggest that the ill‐defined state is unreactive toward the olefin and decays nonradiatively to SO 2 largely in experiments at the lower pressures, X → SO 2 (11); it may generate SO 2 ( 3 B 1 ) in a collisionally induced process at high added inert gas (CO 2 ) pressures, X + CO 2 → SO 2 ( 3 B 1 ) + CO 2 (10a) and X + CO 2 → SO 2 + CO 2 (10b). The data give k 11 /( k 10a + k 10b )=0.026 mole/ l .