We report a combined theoretical and experimental study of the water octamer-h16. The calculations used the ring-polymer instanton method to compute tunnelling paths and splittings in full dimensionality. The experiments measured extensive high resolution spectra near 1.4 THz, for which isotope dilution experiments and group theoretical analysis support assignment to the octamer. Transitions appear as singlets, consistent with the instanton paths, which involve the breakage of two hydrogen-bonds and thus give tunneling splittings below experimental resolution.

The formation of like-charge guanidinium-guanidinium contact ion pairs in water is evidenced and characterized by X-ray absorption spectroscopy and first-principles spectral simulations based on molecular dynamics sampling. Observed concentration-induced nitrogen K-edge resonance shifts result from π* state mixing and the release of water molecules from each first solvation sphere as two solvated guanidinium ions associate into a stacked pair configuration. Possible biological implications of this counterintuitive cation-cation pairing are discussed.

Since their introduction into the commercial marketplace in 1991, lithium ion batteries have become increasingly ubiquitous in portable technology. Nevertheless, improvements to existing battery technology are necessary to expand their utility for larger-scale applications, such as electric vehicles. Advances may be realized from improvements to the liquid electrolyte; however, current understanding of the liquid structure and properties remains incomplete. X-ray absorption spectroscopy of solutions of LiBF4 in propylene carbonate (PC), interpreted using first-principles electronic structure calculations within the eXcited electron and Core Hole (XCH) approximation, yields new insight into the solvation structure of the Li(+) ion in this model electrolyte. By generating linear combinations of the computed spectra of Li(+)-associating and free PC molecules and comparing to the experimental spectrum, we find a Li(+)-solvent interaction number of 4.5. This result suggests that computational models of lithium ion battery electrolytes should move beyond tetrahedral coordination structures.