Beryllium(II): The Strongest Structure-Forming Ion in Water? A QMCF MD Simulation Study

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Introduction
Beryllium is a naturally occurring metal that is found in beryl
and bertrandite rock and is well-known for its toxicity in
mammals.1,2 Beryllium has very specific physicochemical
properties, including low density, high melting point, and high
tensile strength of its alloys, that make it useful in the
manufacture of products ranging from space shuttles to golf
clubs.3,4 It is a good neutron moderator, and because of its low
weight and high rigidity, it is utilized as a material for highfrequency
drivers in acoustics. Beryllium-containing materials
have gained high importance in many key technologies including
nuclear fission and nuclear fusion; radiation sources; hightemperature
ceramics for microelectronics; and high-performance
alloys for naval, aircraft, and space technologies.4-6 The
best known health hazard related to beryllium is chronic
beryllium disease (CBD),7,8 but Be can also cause contact
dermatitis, and beryllium and its compounds are carcinogenic
for both animals and humans.9 Beryllium’s most fascinating
form is emerald, a beryllium-aluminum silicate whose luminous
color makes it a precious gemstone. Despite the importance of
this element, the chemistry of beryllium is relatively unexplored
compared to that of its neighboring elements10 because of a
growing conce
about the toxicity of this element and its
compounds, which has discouraged basic research on beryllium
and even industrial activities.1,9,11,12 Most industrial chemistry
of beryllium, and its toxicity, is based on equilibria in aqueous
solutions.1,2 Therefore, detailed knowledge of the behavior of
the Be2+ ion in aqueous solution, including a description of its
structure-forming effects, is desirable. For a structure-making
ion, the ordering of the bound water molecules must outweigh
the order of the solvent structure. Such ions will be highly
polarizing, and therefore, in particular, small and/or highly
charged metal ions such as Li+, Mg2+, Al3+, and first-row
transition-metal ions are considered strong structure formers.
On the other hand, larger and/or less polarizing ions such as
Rb+ and Cs+ exhibit structure-breaking effects.13,14 Various
experimental techniques [X-ray diffraction, neutron diffraction
(ND), Raman spectroscopy, IR spectroscopy]15-22 have yielded
a tetrahedral first hydration shell structure for Be2+ in aqueous
solution with ion-oxygen distances varying from 1.61 to 1.67
Å. An early classical MD simulation of BeCl2 in aqueous
solution gave an average first-shell distance of 1.75 Å with
coordination number 4.17 A Car-Parrinello MD simulation23
produced an rmax value of 1.65 Å for the first shell with a
coordination number of 4, with the second shell appearing very
tight. These effects can be attributed to the small periodic box
of about 10 Å containing 31 water molecules, which does not
provide realistic surroundings for the second hydration shell.
The different first-shell distances obtained from various simulation
techniques are compared in Table 1. Current advancements
in computational capacities open the way to utilizing more
sophisticated simulation techniques using combined quantum
mechanical and molecular mechanical (QM/MM) simulations24-28
to resolve such differences and ambiguities conce
ing the
influence of an ion on the surrounding solvent, and in this
context, an extended ab initio QM/MM MD simulation has been
performed on Be2+ in aqueous solution.29 The recently developed
quantum mechanical charge field (QMCF) approach for the
treatment of solvated systems further enhances the capabilities
of this methodology.30,31 This approach does not require the
construction of any other potential functions except those for
solvent-solvent interactions while maintaining all of the
advantages of large simulation boxes and ensuring the accuracy