Photochemical studies of unimolecular processes VI. The unimolecular reactions of C ₆ H ₈ isomers and the interpretation of their photolyses

The thermal unimolecular isomerization of hexa-1, trans-3,5-triene to the cis-compound is shown to obey the rate expression $k$$_{1}$ = (4.5 $\pm $1.5) $\times $10$^{12}$ exp (-181.3 $\pm $2.0 kJ mol$^{-1}$/$RT$)s$^{-1}$. At low pressures of cyclohexa-1,3-diene, it undergoes a thermal unimolecular conversion to benzene and hydrogen via cyclohexa-1,4-diene; the Arrhenius parameters found for this process are $k$$_{2}$ = (4.7 $\pm $2.2) $\times $10$^{13}$ exp (-258 $\pm $ 4 kJ mol$^{-1}$/$RT$)s$^{-2}$. From these, other kinetic and thermodynamic data, the theory of unimolecular reactions was used to calculate the quantum yields of the main products in the gas phase photolyses of cis- and trans-hexa-1,3,5-triene and cyclohexa-1,3-diene as a function of pressure. Good agreement with experiment was obtained assuming that all the observed photochemical reactions involved vibrationally excited molecules formed by internal conversion of the initially populated excited electronic state. A limited amount of cis-trans isomerization may occur in the excited electronic states of the hexatrienes. A step-ladder deactivation model with $\Delta $$E$ = 25 kJ mol$^{-1}$ for the parent molecule gave good agreement with the experiments, in which there is clear evidence of multi-step quenching. It is deduced from their photochemistry that the trans and cis hexa-1,3,5-trienes differ in enthalpy by 11 kJ mol$^{-1}$.