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Cation-cation contact pairing in water: Guanidinium
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View: Figures


Image of FIG. 1.
FIG. 1.

Measured nitrogen K-absorption edge spectra of aqueous guanidinium chloride solutions for various concentrations. A, B, and C mark the major groups of core level resonances.

Image of FIG. 2.
FIG. 2.

The carbon-carbon radial distribution function calculated from classical MD simulations of 1.8M GdmCl solution. The broad peak at 3.9 Å demonstrates the formation of contact cation pairs. Details are presented in Sec. II .

Image of FIG. 3.
FIG. 3.

(a) Calculated spatial distribution function for a 1.8M GdmCl solution showing carbon (yellow, 0.002 atoms Å) and water (green, 0.025 atoms Å) density maps around the Gdm ion, demonstrating cation-cation stacked pairing and the preferential binding sites of water. (b) Same spatial distribution function showing carbon (yellow, 0.002 atoms Å) and chloride (red, 0.002 atoms Å) distribution.

Image of FIG. 4.
FIG. 4.

Potential of mean force for guanidinium ion pairs (black), compared with that for the structurally similar, but independently solvated nitrate ion pairs (red) in water along the C—C(N—N) distance coordinate.

Image of FIG. 5.
FIG. 5.

Upper: Calculated nitrogen K-edge spectra for free and stacked Gdm pairs. Each spectrum represents the average of 100 individual spectra. Lower: experimental nitrogen K-edge spectra for the lowest and highest concentrations: 0.5M and 6.0M solutions. All spectra have been normalized with the peak heights of the first resonance. Compared to the experimental spectra, the calculated spectra exhibit a smaller spacing between the first resonance group at 401.9 eV and the second broader peak because of the well-known tendency of DFT to underestimate the water bandgap. The higher energy resonances are primarily water-rich states, hence the simulated transition energy is affected by this artifact.

Image of FIG. 6.
FIG. 6.

Single snapshot spectra with stick spectra and associated states (15% isosurface) for free Gdm pair. Peak A comprises two transitions, A1 (1σ*(NH), 401.91 eV) and A2 (1 → π*, 402.04 eV). Lower panel shows the strongest transitions B1 (403.10 eV) and B2 (403.59 eV) in Peak B group.

Image of FIG. 7.
FIG. 7.

Comparison of the unoccupied states for ground state and core-excited Gdm ion.

Image of FIG. 8.
FIG. 8.

Sample snapshot spectrum with corresponding projected Gdm and water density of states.

Image of FIG. 9.
FIG. 9.

Calculated nitrogen K-edge spectra for free Gdm pairs, stacked Gdm pairs, and Gdm–Cl pairs. The carbon-carbon distance was constrained to 10.0–10.5 Å for the free pairs and 3.7–3.9Å for the stacked pairs. The nitrogen-chloride distance was constrained to 3.2–3.4 Å for two nitrogen atoms in Gdm ion for the Gdm−Cl pair. All spectra have been normalized with the peak heights of the first resonance A.

Image of FIG. 10.
FIG. 10.

Arrangement of the guanidinium and chloride ions in the crystal. The three lower guanidinium ions and three chloride ions (green) are in the same plane, the top chloride coordinates with the top two guanidinium ions, which are not in the same plane.

Image of FIG. 11.
FIG. 11.

Calculated spectra due to individual nitrogen atom excitation in crystalline guanidinium chloride, compared with calculated spectra of an isolated guanidinium ion and a solvated guanidinium free pair. All spectra are processed with energy alignment but without intensity normalization.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Cation-cation contact pairing in water: Guanidinium