Ion Mobility Mass Spectrometry of Au₂₅(SCH₂CH₂Ph)₁₈ Nanoclusters

Ion mobility mass spectrometry (IM-MS) can separate ions based on their size, shape, and charge as well as mass-to-charge ratios. Here, we report experimental IM-MS and IM-MS/MS data of the Au₂₅(SCH₂CH₂Ph)₁₈⁻ nanocluster. The IM-MS of Au₂₅(SCH₂CH₂Ph)₁₈⁻ exhibits a narrow, symmetric drift time distribution that indicates the presence of only one structure. The IM-MS/MS readily distinguishes between the fragmentation of the outer protecting layer, made from six [−SR−Au−SR−Au−SR−] “staples’ where R = CH₂CH₂Ph, and the Au₁₃ core. The fragmentation of the staples is characterized by the predominant loss of Au₄(SR)₄ from the cluster and the formation of eight distinct bands. The consecutive eight bands contain an increasing variety of Au_lS_mR_n⁻ product ions due to the incremental fragmentation of the outer layer of Au₂₁X₁₄⁻, where X = S or SCH₂CH₂Ph. The mobility of species in each individual band shows that the lower mass species exhibit greater collision cross sections, facilitating the identification of the Au_lS_mR_n⁻ products. Below the bands, in the region 1200−2800 m/z, product ions relating to the fragmentation of the Au₁₃ core can be observed. In the low mass 50−1200 m/z region, fragment ions such as Au(SR)₂⁻, Au₂(SR)₃⁻, Au₃(SR)₄⁻, and Au₄(SR)₅⁻ are also observed, corresponding to the large fragments Au_25-x(SR)_18-(x+1). The study shows that most of the dominant large fragments are of the general type Au₂₁X₁₄^∓, and Au₁₇X₁₀^∓ with electron counts of 8 and 6 in negative and positive mode, respectively. This suggests that geometric factors may outweigh electronic factors in the selection of Au₂₅(SR)₁₈ structure.