Double Cation Formation from the Photo-Fragmentation of the closo-Carboranes

Abstract

Time-of-flight mass analysis with multi-stop coincidence detection has been used to study the multi-cation ionic fragmentation via fission of three isomeric carborane icosahedral cage compounds closo-1,2-orthocarborane, closo-1,7-metacarborane, closo-1,12-paracarborane (C₂B₁₀H₁₂) following inner-shell excitation at or above the B 1s regime. Photoelectron-photoion-photoion coincidence (PEPIPICO) spectroscopy was used to study the dominant fission routes in the core level excitation regime. Series of ion pairs are identified, where asymmetric fission dominates, leading to ion pairs of different mass. The fragmentation yields and charge separation mass spectra of all three isomers are generally quite similar in that the ion pairs (H⁺, Y⁺), (Y⁺, Y₁₁⁺), and (Y₃⁺, Y₉⁺) dominate, where Y⁺ denotes the mass region from B⁺ to CH⁺. Slight differences are observed at the B 1s-threshold, where the H⁺ and BH₂⁺/CH⁺ ion pairs dominate for ortho- and meta-carborane, while (Y⁺, Y₁₁⁺) ion pairs dominate the multi-photofragment ion yield of paracarborane. These similarities and distinct differences in charge separation are discussed by considering the energetics of these three major species of ion pairs, as well as charge distributions in closo-carboranes and charge distributions in the carborane cage. It is shown that product formation via charge separation is driven by electronic relaxation, so that the lowest energy products are not necessarily formed.