SYMPOSIUM MINISYMPOSIA

MINISYMPOSIUM ABSTRACT

We study the interaction of 1,3,5-trinitrobenzene (TNB) with the basal siloxane surface of clay minerals by ab initio computations of molecular models at the SCF, DFT (B3LYP), and MP2 levels of theory. A series of progressively increasing basis sets is used, from the 6-31G up to 6-311+G(d) set of atomic orbitals. We consider two molecular models which correspond to the coplanar orientation of TNB with respect to the siloxane layer at the SiO4 tetrahedra and the hexagonal ring. The calculations show that the basis set superposition error (BSSE) corrected DFT binding energy (Eb) of TNB at these surface sites is well approximated by the Coulombic contribution to this energy (CEb). (CEb is evaluated by the DFT calculations of TNB in the electric field of these sites mimicked via electrostatic-potential derived charges). This result rules out a hypothesis of the electron donor-acceptor mechanism of the adsorption proposed in the literature. The smallest molecular model, the TNB at the SiO4 tetrahedra, is computed in both the DFT and MP2 approximations. Three positions of TNB relative to this site are examined, which can be obtained from each other by the rotation of TNB round the perpendicular to the surface crossing the center of the TNB ring. For this model the DFT method underestimates Eb compared to the MP2 method to a factor of ~3. We infer from this result that the main contribution to the stabilization of TNB at the siloxane sites is given by dispersion interaction, which is included by the MP2 method in our calculations. The averaged difference in MP2 Eb between the considered positions of TNB at the SiO4 tetrahedra is small (7 kJ/mol) in comparison with the averaged MP2 Eb (38 kJ/mol). This result suggests that the interaction of TNB with the basal siloxane surface is substantially non-specific in nature, the energetically optimal arrangement of TNB with respect to the surface being governed by the balance between favorable dispersion and electrostatic, and repulsive exchange forces.

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