Doubly charged cluster ions [(NaCl)(m)(Na)(2)](2+): magic numbers, dissociation, and structure

Doubly charged sodium chloride cluster ions [(NaCl)(m)(Na)(2)](2+) with m = 11-62, were produced by electrospray ionization and examined by tandem mas s spectrometry in a quadrupole ion trap. Variations in the ion intensity wi th cluster size indicate the existence of magic numbers at m = 11, 12, 17, 20, 71, 26, 30, 34, 36, 44, 54, and 61. The magic number clusters were stud ied by collision-induced dissociation (CID). The largest clusters (m > 35) fragment mainly by neutral losses. Cluster ions containing 19-35 NaCl molec ules are prone to fission with charge separation to form a singly charged m agic number cluster ion (a regular crystallite comprised of a x b x c ions, where a, b, and c are small odd integers) along with other lower abundance singly charged clusters. Small doubly charged clusters (m < 19) could not be isolated for study by tandem mass spectrometry unless an unusually large isolation window (10-30 mass/charge units) was used. Structures of three t ypes are proposed to account for the dissociation behavior of the magic num ber dusters: (1) combination blocks, where a block is a singly charged magi c number cluster a x b x c, including the special case of a perfect sheet ( a x b x 1), (2) merged blocks, and (3) regular crystallites with single-poi nt defects (a x b x c - 1, where a, b, and c are small odd integers). Elect rospray ionization (ESI) proves to be a good way to generate salt clusters in the gas phase and ion trap tandem mass spectrometry is well suited to th eir characterization (C) 2000 Elsevier Science B.V.