The ΔH for Thermal trans-Stilbene/cis-Stilbene Isomerization. Do S0 and T1 Potential Energy Curves Cross?

Article abstract of DOI:10.1021/j100295a022

Equilibrium compositions of trans- and cis-stilbene, attained with atomic iodine as a catalyst in benzene or tert-butylbenzene solutions, were measured in the 303.2-461.2 K range.Significant differences from literature values (Fischer, G.; Muszkat, K.A.; Fischer, E.J.Chem.Soc.B 1968 156) are traced to probable interference in earlier analyses by bibenzyl, present as a contaminant in stilbene samples.The temperature dependence of the equilibrium constant K for the trans <*> cis reaction gives ΔH = 4.59 +/- 0.09 kcal/mol and ΔS = 1.05 +/- 0.24 eu.The ΔH value is sufficiently large to require intersection of S₀ and T₁ potential energy curves close to the perpendicular geometry.