Substituent Effects in Buta-1,3-diene Photochemistry: A CAS-SCF Study of 2,3-Dimethylbutadiene and 2-Cyanobutadiene Excited-State Reaction Paths

The low-energy reaction paths on the first excited state of buta-1,3-diene (butadiene), 2,3-dimethylbutadiene (2,3-DMB), and 2-cyanobutadiene (2-CNB) have been studied at the CAS-SCF level with 4-31G and DZ+d basis sets. Intrinsic reaction coordinate calculations indicate that each excited state pathway connects both the s-cis and s-trans excited state equilibrium structures of 2,3-DMB and 2-CNB to a conical intersection of the ground and excited state. The geometry of the lowest energy conical intersection points have been fully optimized. The conical intersections provide ultrafast radiationless decay channels and thus are a central feature in the mechanism of butadiene photolysis. The excited state reaction paths of the model (butadiene) and substituted systems involve a similar reaction coordinate corresponding to the deformation of the butadiene moiety by out-of-plane motion of the carbon framework coupled with asynchronous rotation of the terminal methylenes. The comparison of the excited state pathways for parent and substituted butadienes provides a basis for the rationalization of the dramatic substituent effects observed in the photoproduct quantum yield ratio of 2,3-DMB and 2-CNB. The result supports the idea that substituents mainly affect the nature of the ground-state relaxation occurring just after the decay. However, this effect is intimately related to the particular structure and dynamics that the photoexcited reactant acquires during its excited-state lifetime.