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Govind, N.

Publications and source records attributed to Govind, N..

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Unraveling the Spectral Signatures of Solvent Ordering in K-edge XANES of Aqueous Na +

The aqueous solvation structure of the Na + ion is studied both by molecular dynamics (MD) simulations and K-edge X-ray absorption near edge spectroscopy (XANES). In this paper, we present a systematic study contrasting the differences in the predicted XANES spectra, using molecular configurations generated from classical and quantum MD. Using a set of suitable order parameters, we elucidate how the spectroscopic features are influenced by the local solvation structure around the Na + ion. Lastly, our findings suggest that XANES is sensitive to fluctuations between the first and second solvation shells and can distinguish between classical and quantum interaction representations used to generate molecular ensembles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Revisiting the hydration structure of aqueous Na +

In this paper, 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 –1 while 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, g NaO (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–O avg ~ 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.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Electron Transfer for Large Molecules through Delocalization

Electron transfer for large molecules lies in between a Marcus-Theory two-state transfer and a Landauer description. We discuss a delocalization formalism which,through the introduction of artificial electric fields which emulate bulk dipole fields, allows calculation between a pair of identical molecules (A+A- (R)A-+A) with several open states. Dynamical electron polarization effects can be inserted with TDDFT and are crucial for large separations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗