Modelling of aniline-vermiculite and tetramethylammonium-vermiculite;: Test of force fields

Molecular mechanics simulations in Cerius(2) have been used for modelling vermiculite intercalated with tetramethylammonium and aniline cations. The published structure data obtained for these intercalated structures from X-ray single crystal diffraction have been used to test the force fields and modelling strategy for organo-clays. The strategy of modelling was based on the nonbond host-guest interactions and on rigid silicate layers and rigid guest species. The rigidity of silicate layers requires that the cell parameters a, b and gamma are kept fixed during the energy minimisation. The energy term was set up using the nonbond interaction terms only and the Crystal Packer module in Cerius(2) has been used for the energy minimisation. In Crystal Packer the rigid units, i.e. the silicate layers and guest species can be translated and rotated during energy minimisation and the cell parameters c, alpha, and beta have been varied. Three sets of Van derWaals (VDW) parameters available in Crystal Packer: Tripos, Universal and Dreiding have been used in present molecular simulations. Ab initio MP2 calculations were performed to justify the application of the force field. The best agreement of molecular mechanics simulations with both: experimental and ab initio data was obtained with the Tripos VDW parameters for both intercalates. The results of modelling are in good agreement with the experimental data as to the cell parameters and the interlayer packing. The cell parameters reported by Vahedi-Faridi and Guggenheim (1997) for tetramethylammonium-vermiculite are: c = 13.616 Angstrom, alpha = 90 degrees, beta = 97.68 degrees; from the present modelling we obtained: c = 13.609 Angstrom, alpha = 90.19 degrees, beta = 97.56 degrees. Tetramethylammonium-cations are arranged in one layer in the interlayer space. One C-C edge of NC4 tetrahedra is perpendicular to the silicate layers. The deep immersion of the methyl groups into the ditrigonal cavities suggested by Vahedi-Faridi and Guggenheim was not confirmed by modelling. Slade and Stone (1984) presented the measured cell parameters for aniline vermiculite: c = 14.89 Angstrom, alpha = 90 degrees, beta = 97 degrees; present result is: c = 14.81 Angstrom , alpha = 90.72 degrees, beta = 96.70 degrees for partially exchanged vermiculite and c = 14.84 Angstrom, alpha = 90.53 degrees, beta = 97.17 degrees for fully exchanged vermiculite. The aniline cations are positioned over the ditrigonal cavities alternating in their anchoring to lower and upper silicate layer. The C-N bonds are perpendicular to layers.