Ion Mobility Mass Spectrometry of Au₂₅(SCH₂CH₂Ph)₁₈Nanoclusters

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 Au 25 (SCH 2 CH 2 Ph) 18 − nanocluster. The IM-MS of Au 25 (SCH 2 CH 2 Ph) 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 = CH 2 CH 2 Ph, and the Au 13 core. The fragmentation of the staples is characterized by the predominant loss of Au 4 (SR) 4 from the cluster and the formation of eight distinct bands. The consecutive eight bands contain an increasing variety of Au l S m R n − product ions due to the incremental fragmentation of the outer layer of Au 21 X 14 −, where X = S or SCH 2 CH 2 Ph. 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 l S m R n − products. Below the bands, in the region 1200−2800 m / z, product ions relating to the fragmentation of the Au 13 core can be observed. In the low mass 50−1200 m / z region, fragment ions such as Au(SR) 2 −, Au 2 (SR) 3 −, Au 3 (SR) 4 −, and Au 4 (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 Au 21 X 14 ∓, and Au 17 X 10 ∓ 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 Au 25 (SR) 18 structure.