Hydration of beryllium(ii) in aqueous solutions of common inorganic salts. A combined vibrational spectroscopic and ab initio molecular orbital study

Abstract

Raman spectra of aqueous beryllium perchlorate, chloride, nitrate, and sulfate solutions have been measured over a broad concentration (0.098-4.950 mol L(-1)) range. The Raman spectroscopic data suggest that the tetra-aqua beryllium(II) ion is thermodynamically stable in perchlorate, chloride, and nitrate solutions over the concentration range measured. No inner-sphere complexes in these solutions could be detected spectroscopically except in very concentrated beryllium nitrate solutions. Beryllium sulfate solutions however, show a different picture, namely the existence of a thermodynamically stable beryllium sulfato complex most likely monodentate even at very low concentrations. At very high beryllium sulfate concentrations, a small quantity of a bidentate sulfato complex was found. With a temperature increase, the sulfato complex formation increases and this demonstrates the entropically driven sulfato complex formation. Furthermore, with increased temperature the hydrolysis increases, measured by the formation of hydrogen sulfate. Ab initio geometry optimizations and frequency calculations are reported for beryllium-water clusters with only inner sphere waters, clusters with an inner sphere and an incomplete second hydration, and clusters with a higher number of waters in the second hydration sphere. The cluster, [Be(OH2)(12)(2+)] (Be[4 + 8]) with 4 water molecules in the first sphere and 8 water molecules in the second sphere gave sufficiently realistic frequencies for BeO4 skeleton in comparison to the experimental ones. However, the cluster, [Be(OH2)(18)(2+)] (Be[6 + 12]) with 6 water molecules in the inner sphere and 12 water molecules in the outer sphere on an energy minimum gave unrealistically low BeO4 frequencies. This fact demonstrates that a six-fold coordination of Be2+ can be ruled out.

An ab initio study of beryllium(II) hydration

“…Careful re-examination of the figures of Mason reveal that there is a small concentration dependence of the peak maximum, with the most dilute solution studied actually giving a peak maximum of around 1.65 ± 0.01 Å, with this value decreasing as the concentration is increased. For the totally symmetric vibrational frequency, one would expect an inverse relationship with Be-O distance, and this is borne out in both our experimental results [1] and in the more concentrated solutions in Ref. [33].…”

“…The experimental results give m 1 at 531 cm À1 , m 2 at 250-300 cm À1 (crystal), m 3 at $760 cm À1 , and m 4 at 348 cm À1 [1]. The values are mostly converged when one reaches the TZ basis set.…”

“…[33]. In our previous work [1], we examined several different salt solutions, ranging from fairly concentrated to fairly dilute, and were careful to suppress hydrolysis by adding excess acid.…”

“…In a recent paper by the author and collaborators, a thorough Raman study of beryllium perchlorate, chloride, nitrate, and sulfate in aqueous solution was presented [1]. To support the experimental assignments, ab initio calculations were carried out at the HF, MP2, and B3LYP levels, using the 6-31G * and 6-31+G * basis sets.…”

A brighter beryllium

“…Research carried out 2 on tetracoordinated complexes, for example, has provided further knowledge of the structural diversity of beryllium complexes. The possible coordination numbers of beryllium have also recently been clarified 3,4 and mechanistic studies are underway to elucidate solvent-exchange processes 5,6 .…”