SUBSTITUENT EFFECTS IN BUTA-1,3-DIENE PHOTOCHEMISTRY - A CAS-SCF STUDY OF 2,3-DIMETHYLBUTADIENE AND 2-CYANOBUTADIENE EXCITED-STATE REACTIONPATHS

The low-energy reaction paths on the first excited state of buta-1,3-d iene (butadiene), 2,3-dimethylbutadiene (2,3-DMB), and 2-cyanobutadien e (2-CNB) have been studied at the CAS-SCF level with 4-31G and DZ+d b asis sets. Intrinsic reaction coordinate calculations indicate that ea ch excited state pathway connects both the s-cis and s-trans excited s tate equilibrium structures of 2,3-DMB and 2-CNB to a conical intersec tion of the ground and excited state. The geometry of the lowest energ y conical intersection points have been fully optimized. The conical i ntersections provide ultrafast radiationless decay channels and thus a re a central feature in the mechanism of butadiene photolysis. The exc ited state reaction paths of the model (butadiene) and substituted sys tems involve a similar reaction coordinate corresponding to the deform ation of the butadiene moiety by out-of-plane motion of the carbon fra mework coupled with asynchronous rotation of the terminal methylenes. The comparison of the excited state pathways for parent and substitute d butadienes provides a basis for the rationalization of the dramatic substituent effects observed in the photoproduct quantum yield ratio o f 2,3-DMB and 2-CNB. The result supports the idea that substituents ma inly affect the nature of the ground-state relaxation occurring just a fter the decay. However, this effect is intimately related to the part icular structure and dynamics that the photoexcited reactant acquires during its excited-state lifetime.