Bond-selective photodissociation of aliphatic disulfides.

The photochemistry of aliphatic disulfides is presented. The photolysis products are photoionized with coherent vacuum ultraviolet radiation and analyzed by time-of-flight mass spectrometry. With 248-nm excitation, the predominant dissociation pathway is S-S bond cleavage. With 193-nm excitation, S-S bond cleavage, C-S bond cleavage, and molecular rearrangements are all observed as primary processes. The branching ratio for S-S bond cleavage relative to C-S bond cleavage is typically 1-2 orders of magnitude greater at 248 run than 193 run. This wavelength dependence cannot be explained readily by photodissociation from the ground electronic state. The ground state S-S bond energy, ∼ 280 kJ/mol, is much larger than the C-S bond energy, ∼ 235 kJ/mol. If dissociation occurred from the ground state, higher wavelength radiation would be expected to favor the lower energy process, but the opposite effect is observed. Thus, excited state photochemistry is indicated. These results are discussed with respect to the differences between low and high energy collision-induced dissociation of peptides that contain disulfide linkages and to the possibility of achieving bond-selective photodissociation of such ions.