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pH-dependent x-ray absorption spectra of aqueous boron oxides
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View: Figures


Image of FIG. 1.
FIG. 1.

Boron oxide speciation as a function of pH. Calculated for a 0.5 M boric acid solution, the plot shows the fraction of the total boron in each of the species (structure indicated on graph) as a function of pH. Circles—boric acid; squares—triborate; triangles—tetraborate; diamonds—borate.

Image of FIG. 2.
FIG. 2.

Experimental and theoretical B K-edge NEXAFS spectra showing the transition from trigonal to tetrahedrally coordinated boron with pH. The spectra are of 0.5 M boric acid solutions at a series of pH values, increasing pH top to bottom as indicated on figure. For each pH value the solid line is the experimental spectrum and the dotted line is the associated spectrum from calculations.

Image of FIG. 3.
FIG. 3.

Boron K-edge NEXAFS spectra for aqueous (solid line) and solid (dashed line) boric acid. Solid boric acid, sassolite, data provided by Fleet and Liu (Ref. 21).

Image of FIG. 4.
FIG. 4.

Calculations of borate NEXAFS spectra in the presence and absence of sodium cations and/or water. The upper section shows calculations for aqueous borate, aqueous sodium borate, and bare borate. There are no significant spectral changes between the three. Similarly, there are no significant spectral changes between aqueous boric acid and bare boric acid shown in the lower section.

Image of FIG. 5.
FIG. 5.

Radial distribution functions calculated from MD simulations. From top to bottom: O to H–water, H to O–water, O to O–water, and B to O–water. In each plot the dotted blue line is boric acid, the solid red line is sodium borate, and the dashed black line is borate (without sodium). As evidenced by the change in peak height at 2 Å (top plot), the cation disrupts the donation of hydrogen bonds to the borate oxygens. For boric acid there are also few donor hydrogen bonds because the lone pairs on the oxygens contribute to the pi system in the trigonal planar molecule. H to O–water bonds (upper middle plot) are less affected by the cation, but the freely rotating H's on borate lead to greater variability in hydrogen bonds donated to water. Interactions between H's and O's within the molecule also diminish the oxides ability to donate a hydrogen bond to water. The disruption of hydrogen bonds by the cation is even more evident in the O to O–water RDF (lower middle). The B to O–water RDF (bottom) gives a relatively broad peak indicative of weak hydrogen bonding to water.

Image of FIG. 6.
FIG. 6.

Single snapshot spectra and associated states (10% isosurface) for bare boric acid (upper) and fully hydrated boric acid (lower). The LUMO state, the empty 2pz orbital, responsible for the sharp low energy transition at 0 eV (energy adjusted to LUMO energy and unstretched) is nearly identical for the bare and hydrated cases. The LUMO states are the boxed images on the left. Water does not interact significantly with this empty 2pz orbital. The σ* states to the right are some of the many responsible for the high energy feature (∼8 eV). The local σ* character is preserved upon hydration and there is minimal mixing with water. The fine structure in the spectrum is averaged out in an ensemble of snapshots.

Image of Scheme 1.
Scheme 1.

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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: pH-dependent x-ray absorption spectra of aqueous boron oxides