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.
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 .
(a) Calculated spatial distribution function for a 1.8M GdmCl solution showing carbon (yellow, 0.002 atoms Å−3) and water (green, 0.025 atoms Å−3) 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−3) and chloride (red, 0.002 atoms Å−3) distribution.
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.
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.
Single snapshot spectra with stick spectra and associated states (15% isosurface) for free Gdm+ pair. Peak A comprises two transitions, A1 (1s → sσ*(NH2), 401.91 eV) and A2 (1s → π*, 402.04 eV). Lower panel shows the strongest transitions B1 (403.10 eV) and B2 (403.59 eV) in Peak B group.
Comparison of the unoccupied states for ground state and core-excited Gdm+ ion.
Sample snapshot spectrum with corresponding projected Gdm+ and water density of states.
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.
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.
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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