title: Revisiting the hydration structure of aqueous Na.
abstract: A combination of theory, X-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) are used to probe the hydration structure of aqueous Na+. The high spatial resolution of the XRD measurements corresponds to Qmax= 24 Å-1while the first-reported Na K-edge EXAFS measurements have a spatial resolution corresponding to 2k = Qmax= 16 Å-1. Both provide an accurate measure of the shape and position of the first peak in the Na-O pair distribution function, gNaO(r). The measured Na-O distances of 2.384 ± 0.003 Å (XRD) and 2.37 ± 0.024 Å (EXAFS) are in excellent agreement. These measurements show a much shorter Na-O distance than generally reported in the experimental literature (Na-Oavg∼ 2.44 Å) although the current measurements are in agreement with recent neutron diffraction measurements. The measured Na-O coordination number from XRD is 5.5 ± 0.3. The measured structure is compared with both classical and first-principles density functional theory (DFT) simulations. Both of the DFT-based methods, revPBE and BLYP, predict a Na-O distance that is too long by about 0.05 Å with respect to the experimental data (EXAFS and XRD). The inclusion of dispersion interactions (-D3 and -D2) significantly worsens the agreement with experiment by further increasing the Na-O distance by 0.07 Å. In contrast, the use of a classical Na-O Lennard-Jones potential with SPC/E water accurately predicts the Na-O distance as 2.39 Å although the Na-O peak is over-structured with respect to experiment.
authors.affiliation: Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99354, USA.
authors.forename: M
authors.initials: M
authors.lastname: Galib
authors.affiliation: Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99354, USA.
authors.forename: M D
authors.initials: MD
authors.lastname: Baer
authors.affiliation: X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: L B
authors.identifier: 0000000173171642
authors.initials: LB
authors.lastname: Skinner
authors.affiliation: Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99354, USA.
authors.forename: C J
authors.initials: CJ
authors.lastname: Mundy
authors.affiliation: Swiss Light Source, Paul Scherrer Institute (PSI), 5232, Villigen, Switzerland.
authors.forename: T
authors.initials: T
authors.lastname: Huthwelker
authors.affiliation: Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99354, USA.
authors.forename: G K
authors.initials: GK
authors.lastname: Schenter
authors.affiliation: X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: C J
authors.initials: CJ
authors.lastname: Benmore
authors.affiliation: Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, USA.
authors.forename: N
authors.identifier: 0000000169147914
authors.initials: N
authors.lastname: Govind
authors.affiliation: Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99354, USA.
authors.forename: J L
authors.initials: JL
authors.lastname: Fulton