How Counterions Affect the Solution Structure of Polyoxoaurates: Insights from UV/Vis Spectral Simulations and Electrospray Mass Spectrometry. Issue 23 (15th July 2014) - Kondinski, Aleksandar
Abstract: The solution structure of the two presently known polyoxoaurates, [Au4 As4 O20 ] 8– (Au4 As4 ) and [Au4 Se4 O16 ] 4– (Au4 Se4 ), was elucidated by a combination of time‐dependent density‐functional‐theory (TD‐DFT) calculations, experimental UV/Vis spectroscopy, and electrospray ionization mass spectrometry (ESI‐MS). The TD‐DFT‐predicted electronic spectra for molecular models including different numbers of counterions, along with the main Au4 As4 and Au4 Se4 structural motifs, were analyzed and compared with the experimental UV/Vis spectra of the two polyanions. The performed analysis revealed important structural differences between the arsenate and the selenite derivatives in aqueous solution. For the selenite oxoaurate, the bare Au4 Se4 polyanion itself shows very good agreement between predicted and experimental excitation energies. In contrast, the solution state of the arsenate oxoaurate appears to be best described by a dimeric assembly of the type [Na5 Au8 As8 O40 ] 11– (Na5 Au8 As8 ) with a stoichiometry and structure very similar to those reported for the solid state. Indirect experimental evidence that Na5 Au8 As8 is the dominant species in aqueous solution is provided by ESI‐MS measurements. Our work demonstrates that the combination of experimental UV/Vis spectroscopy and TD‐DFT calculations complemented with electrospray mass spectrometry may serve as an alternative strategy for rationalizing the solution chemistry of polyoxometalates, which cannot be studied by conventionally used spectroscopic techniques, such as NMR or EPR spectroscopy. Abstract : The solution structure of the two known polyoxoaurates, [Au4 As4 O20 ] 8– and [Au4 Se4 O16 ] 4–, was elucidated by time‐dependent DFT calculations, UV/Vis spectroscopy, and electrospray mass spectrometry. The combination of computational and experimental approaches allowed us to clarify the underlying structure–property relation and to explain the differences in the UV/Vis spectra of the two oxoaurates.