Abstract
Structures of small sodium halide nanocrystal（cluster）cations and anions, NanFn－1＋, NanIn－1＋, Nan－1Fn－, and Nan－1In－, have been studied by ion mobility mass spectrometry coupled with quantum chemical calculations. In this spectrometry, structural information of atomic and molecular cluster ions was obtained from the measurement of collision cross sections of the ions with a buffer gas in an ion drift cell. As a result of the comparison between the experimental cross sections thus measured and those obtained by theoretical calculations, it was found that the NanFn－1＋ cluster ions have substructures of bulk face-centered-cubic crystals in most of the cluster size. Among such structures, we predominantly observed cuboid ions with near regular hexahedron such as n＝14（3×3×3）and 23（3×3×5）as magic numbers in the mass spectra. In addition, non-rock-salt “cage” type structures in which one sodium atom is encapsulated into the sodium fluoride cuboid lattice were also found to be stable for n＝7 and 10. Also the stable geometric structures of above four cluster systems were examined systematically in order to obtain the dependence of composite ion sizes on the stable structures. As for the Nan－1Fn－ anions and the NanIn－1＋ cations, cage type structures similar to those of Na7F6＋ and Na10F9＋ were observed at n＝7 and 10. However, similar cage structures were not detected for n＝7 and 10 of Nan－1In－ anions. This result indicates that the cage structures are sensitive to the radius of caged ions（excess atomic ions）: Inclusion of I－（ionic radius of 2.06 Å）causes large distortion in the structure, whereas they are stable for small caged ions such as Na＋（1.16 Å）and F－（1.19 Å）. The experiment of water and methanol adsorption reactions on NanFn－1＋ ions also showed that the water molecule is highly reactive at specific cluster size, e.g., n＝13 and 22.
