Molecular Dynamics Simulations of the Squarate Dianion (C₄O₄^2-) in Aqueous Solution

Abstract

A molecular dynamics simulation study of the structure and dynamics of aqueous solutions of the squarate oxocarbon dianion (C 4 O 4 2-) is presented. Analyses of the solute-solvent radial distribution functions and hydrogen (H)-bonding distributions indicate a well-defined hydration shell consisting of approximately 18 water molecules. About 12 of these molecules are tightly H-bonded to the oxocarbon (an average of three molecules per oxygen atom) forming a highly symmetric solute-solvent aggregate, whereas the remaining six water molecules (not bonded to the ion) are more loosely distributed above and below the oxocarbon plane. The mean residence time for molecules that are H-bonded to the solute is estimated to be larger than 20 ps, whereas molecules that are not directly bonded to the ion are frequently exchanged with the bulk and remain within the first solvation shell for times of the order of a few picoseconds. For the dynamics, we find that the translational motions projected along the squarate plane and perpendicular to it are comparable to each other. The rotational diffusion coefficients for the main symmetry axes also indicate that the spinning and tumbling motions of the oxocarbon are roughly isotropic despite the shape of the solute. The squarate is found to perform fast librational motions inside the solvente cage, with a characteristic frequency of approximately 70 cm -1 , in close agreement with recent experimental Raman band shape analysis. These results are discussed in the light of the hydration structure.

The Jahn‐Teller effect of the Cr2+ ion in aqueous solution: Ab initio QM/MM molecular dynamics simulations

“…Computer simulations such as Monte Carlo (MC) and molecular dynamics (MD) simulations have been quite successful in evaluating the structural and dynamical properties of metal ions in solution 13–27. However, the accuracy of results strongly depends on the potential models employed 28–30.…”

Vibrational dephasing of the croconate dianion in different environments

“…This finding was corroborated by MD simulation of C 4 O 4 2 in aqueous solution, in which was found fast librational motion of the oxocarbon inside the solvent cage due to tight hydrogen bonds between the anion and water molecules. 12 By replacing the simple alkali-metal cation for a much larger one, namely, tetra(n-butyl)ammonium croconate, [(n-C 4 H 9 ) 4 N] 2 C 5 O 5 Ð4H 2 O, TBCR, we obtained 13 a very viscous pale yellow liquid with glass transition temperature around C20°C. Raman spectra of TBCR was investigated in the low wavenumber range (0-250 cm 1 ) 13 and in the highwavenumber (intramolecular) range.…”

A review on the dynamics of water

“…Indeed, our recent MD simulation studies of the squarate in water show the existence of a well-defined hydration shell, composed of an average of 12 water molecules Hbonded to the carbonyl oxygens (three water molecules per oxygen atom) which is preserved for more than 20-30 ps [59,60]. The dynamics is isotropic and the anion librates at 70 and 250 cm −1 [60].…”

“…The dynamics is isotropic and the anion librates at 70 and 250 cm −1 [60]. In a separate MD study, we have also investigated more closely the features of the vibrational dephasing of this species in water.…”