A gold cyano complex in nitromethane: MD simulation and X-ray diffraction

Abstract

The solvation structure around the dicyanoaurate(I) anion (Au(CN)₂^-) in a dilute nitromethane (CH₃NO₂) solution is presented from X-ray diffraction measurements and molecular dynamics simulation (NVT ensemble, 460 nitromethane molecules at room temperature). The simulations are based on a new solute-solvent force-field fitted to a training set of quantum-chemically derived interaction energies. Radial distribution functions from experiment and simulation are in good agreement. The solvation structure has been further elucidated from MD data. Several shells can be identified. We obtain a solvation number of 13-17 nitromethane molecules with a strong preference to be oriented with their methyl groups towards the solute.

Graphical abstract

Figure 1

Structures of the Au(CN)₂⁻ ion and the nitromethane (NM) molecule used in the quantum-mechanical calculations. The lines in (b) and (c) indicate the directions of approach of the Au(CN)₂⁻ ion towards the NM molecule for the exploration of the potential energy curves. In (b), the NC–Au–CN axis is oriented along the directions shown. In (c), the NC–Au–CN axis is oriented perpendicular to the directions shown, with X = Au, C or N, yielding the triplet of numbers listed for each direction. For the cases (11, 12, 13) and (17, 18, 19), the NC-Au-CN axis is perpendicular to the NO₂ plane. For the cases (14, 15, 16), (20, 21, 22), (5, 6, 7) and (8, 9, 10), the NC–Au–CN axis is parallel to the NO₂ plane.

Figure 2

(a) NM–Au(CN)₂⁻ potential energy curves from the QC calculations (x) and from the fitted force-field expression (solid lines). (b) Fitting accuracy of the NM–Au(CN)₂⁻ potential energy function.

Figure 3

Comparison of the total radial distribution functions obtained from the X-ray diffraction experiment and from the MD simulation.

Figure 4

Au(CN)₂⁻⋯NM radial distribution functions and running integration numbers from the MD simulation. The distance scale is the same in the three bottom panels and is different from that in the top panel.

Figure 5

Two dimensional projection of the distribution of nitromethane molecules around the central dicyanoaurate(I) ion resulting from the MD simulation The upper half of the plot shows the distribution of the CH₃ groups of the NM molecules, the lower part shows the distribution of the NO₂ groups of the same NM molecules. The plot has been normalized as explained in the text, i.e. each area element (x, r) represents the average population in the corresponding ring around the dicyanoaurate(I) iońs axis.

Figure 6

Angular distribution function of NM molecules in the first shell around the Au(CN)₂⁻ ion. The first shell is here defined as all NM molecules whose N atom or C atom (or both) reside within 5 Å from at least one of the Au, N1 and N2 atoms in the Au(CN)₂⁻ ion. This region is outlined in the inset. Θ Is the angle between the dipole moment vector of the center of NM and the Au(I) atom.

Figure 7

Three selected snapshots from the MD simulation showing the NM coordination around the Au(CN)₂⁻ anion. All solvent molecules within the first solvation shell, defined as the union of the three spheres with radii 5 Å, defined in Figure 6 are displayed. All C⋯N inter- and intra-molecular distances are drawn in the same style.