Conrotatory photochemical ring opening of alkylcyclobutenes in solution. Atest of the hot ground-state mechanism

Quantum yields for photochemical ring opening of six alkylcyclobutene have been measured in hexane solution using 228-nm excitation, which selectively populates the lowest pi ,R(3s) excited singlet states of these: molecules and has been shown previously to lead to ring opening with clean conrotator y stereochemistry. The compounds studied in this work-1,2-dimethylcyclobute ne (1), cis- and trans-1,2,3,4-tetramethylcyclobutene (cis- and trans-5), h examethylcyclobutene (8), and cis- and trans-tricyclo[6.4.0.0(2,7)]dodec-1( 2)-ene (cis- and trans-9)-were selected so as to span a broad range in mole cular weight and as broad a range as possible in Arrhenius parameters for-t hermal (ground-state) ring opening. RRKM calculations have been carried out to provide estimates; of the rate constants fbr ground-state ring opening of each of the compounds over a range of thermal energies from 20 000 to 49 000 cm(-1). These have been used to estimate upper limits for the quantum yields of ring opening via a hot ground-state mechanism, assuming a value o f k(deact) = 10(11) s(-1) for the rate constant for collisional deactivatio n by the solvent,hat internal conversion to the ground state from the lowes t Rydberg state occurs with dose to unit efficiency, and that ergodic behav ior is followed. The calculated quantum yields are significantly lower than the experimental values in all cases but one (1). This suggests that the R ydberg-derived ring opening of alkylcyclobutenes is a true excited-state pr ocess and rules out the hot groundstate mechanism for the reaction.