https://doi.org/10.1140/epjd/e2010-10526-y
Regular Article
Microhydration of the methylene blue cation in an electrospray ion source
1
Department of Physics, Tokyo Metropolitan
University, Hachioji,
Tokyo
192-0397,
Japan
2
Department of Chemistry, Tokyo Metropolitan
University, Hachioji, Tokyo
192-0397,
Japan
3
Atomic, Molecular and Optical Physics Laboratory,
RIKEN, Wako,
Saitama
351-0198,
Japan
a e-mail: majima@nucleng.kyoto-u.ac.jp
Received:
18
September
2010
Received in final form:
7
December
2010
Published online:
18
March
2011
Microhydrated methylene blue cations, MB+(H2O)n, are produced in an electrospray ion source and their size-distributions are measured as a function of the source temperature. A series of MB+(H2O)n ions is observed up to n ≃ 60. A striking feature observed in the mass spectra is that the series of hydrated ions starts at n = 4; intensities of n = 1–3 are extremely suppressed. The absence of n = 1–3 ions is well explained by the energetics concerning evaporation processes of water molecules, based on stable structures and the binding energies of MB+(H2O)n ions calculated by DFT calculations up to n = 5. MB+(H2O)n ions for n > 4 evaporate a single water molecule sequentially, while MB+(H2O)4 tends to fragment into MB+ and (H2O)4 rather than MB+(H2O)3 and an H2O molecule. We have observed a clear magic peak at n = 24, which strongly suggests that the MB+(H2O)24 ion is formed by attaching a neutral (H2O)20 cage onto an MB+(H2O)4 ion.
© EDP Sciences, Società Italiana di Fisica and Springer-Verlag 2011