Increasing the heavy atom effect of xenon by adsorption to zeolites: Photolysis of 2,3-diazabicyclo[2.2.2]oct-2-ene

The distribution of products in the photolysis of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) is determined by the electronic spin state of the cyclohexane-1,4-diyl intermediate. Tripler cyclohexane-1,4-diyl yields 1,5-hexadiene (HD) while singlet cyclohexane-1,4-diyl produces bicyclo[2.2.0]hexane (BCH) as the stable product. Intersystem crossing (ISC) between the two diyls is enhanced by an external heavy atom effect. Direct photolysis (366 nm) of DBO in NaY zeolite containing adsorbed xenon yields a product ratio of 75:25 in favor of the tripler product, HD. This is a 24% increase in triplet product from direct photolysis in n-octane. The dramatic enhancement of ISC may be caused by polarization of the Xe atom in the faujasite cage, thereby allowing it to reach its full potential as a heavy atom perturbant. The 3:1 product ratio derives from complete equilibration of the three tripler electron spin states and the singlet spin state either in the short-lived diazenyl diradical or on a high-energy surface of the 1,4-cyclohexanediyl in which the singlet and triplet states are degenerate. The same 75:25 product ratio is achieved (without Xe adsorbed to the zeolite) when the Na+ cation of the zeolite is exchanged with Cs+ cation. This is not surprising since Xe and Csf are isoelectronic and therefore should share similar spin-orbit coupling characteristics. Also reported are the product ratios when photolysis of DBO is carried out in zeolites containing other monovalent cations (Li+, Na+, K+, Rb+).