The Stereoisomerization of 11-cis-Retinal

The thermodynamic parameters of the stereoisomerization of 11-cis-retinal to the all-trans configuration have been measured in n-heptane between 20 and 80° (C). ΔF = -1100 to -1400 cal mole-1, ΔH = +150 cal mole-1, and ΔS = +4.4 e.u. The kinetic parameters have been measured in several solvents, which differ in their polarity, and in the presence of I2 as catalyst. For the uncatalyzed isomerizations, the Arrhenius energy (Ea) varies between 22,400 and 26,200 cal mole-1, and the Arrhenius or frequency factor (A) between about 108 and 1011 sec-1. In the presence of I2, A increases with relatively little change in Ea. In terms of the theory of absolute reaction rates, this means that chiefly the configurational parameter, ΔS‡, changes; ΔH‡ varies only slightly. The implications of these findings for the mechanism of isomerization are discussed.