Microhydration of the methylene blue cation in an electrospray ion source

T. Sato; T. Majima; Kenro Hashimoto; Kouhei Hashimoto; Y. Zama; J. Matsumoto; H. Shiromaru; K. Okuno; H. Tanuma; T. Azuma

doi:10.1140/epjd/e2010-10526-y

2022 Impact factor 1.8

Atomic, Molecular, Optical and Plasma Physics

Eur. Phys. J. D 63, 189-194 (2011)
https://doi.org/10.1140/epjd/e2010-10526-y

Regular Article

Microhydration of the methylene blue cation in an electrospray ion source

T. Sato¹, T. Majima¹^a^,b, Kenro Hashimoto², Kouhei Hashimoto², Y. Zama², J. Matsumoto², H. Shiromaru², K. Okuno¹, H. Tanuma¹ and T. Azuma¹^,3

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

Abstract

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.