Bond selective photochemistry in CH2BrI through electronic excitation at 210 nm

To explore the possibility of bond selective photochemistry in an excited electronic state, we have studied the photolysis of CH2BrI in a molecular beam at 210 nm. Following the direct local excitation of a repulsive transition on the C–Br bond at 210 nm, the fragments were detected by time-of-flight mass spectrometry. The dominant channel was found to be C–Br fission (60%) releasing an average of 15 kcal/mol into translation with the remainder reacting to form CH2+IBr and CH2+I+Br. There was no evidence for the primary fission of the C–I bond, making this the first clear example of the selective cleavage of a stronger bond in a molecule over the weakest one.