Question

Using a molecular mechanics program, calculate the energy difference or the difference in heats of formation between Z- and E-stilbene. How do your computed values compare to the difference in heats of hydrogenation of the two isomers.

Answer 1

A photochemical reaction occurs when internal conversion and relaxation of an excited state leads to a ground state isomer of the initial substrate molecule, or when an excited state undergoes an intermolecular addition to another reactant molecule in the ground state. The cis-trans photochemical isomerization of stilbene is a reaction of the first kind, as shown in the following diagram. Both cis and trans-stilbene undergo ? ? ?* electron excitation by absorption of uv light. Whereas isolated double bonds require 180 nm light for such excitation, conjugation with the phenyl substituents lowers the transition energy to about 300 nm, a more easily achieved source. The molar absorptivity of the cis-isomer is less than that of the trans-isomer because steric crowding of the ortho sites causes the phenyl groups to twist slightly out of coplanarity.
The stability of the stereoisomers of stilbene is due to a 62 kcal/mole barrier to rotation about the double bond produced by the ?-bond. This bonding is absent in the ? ? ?* excited state (magenta curve in the diagram). Both the initial S₁ states formed from the cis and trans ground states are slightly twisted (the cis by 25