The origin of the interaction of 1,3,5-trinitrobenzene with siloxane surface of clay minerals

The results of the application of ONIOM (n-layered integrated molecular orbital and molecular mechanic method) approximation to the modelling of the interaction of 1,3,5-trinitrobenzene (TNB) with the siloxane surface of clay minerals are presented. The target cluster having the chemical formula of Al22Si13O81H44 has been subdivided into the high level part which has been calculated at the MP2/6-31G(d) or HF/6-31G(d) level of theory; the medium level part,which has been calculated at the HF/6-31G(d) level of the theory, and the low level part which has been calculated at the HF/3-21G or HF/STO-3G level. For the aim of the comparison, we have also considered the interaction of TNB with a pure silicon-oxygen cluster having the formula of Si13O37H22. These calculations have been performed at the HF, MP2, and DFT levels of theory using the standard 6-31G(d) basis set. We have found that the attractive component of adsorption energy consists mainly of two commensurable parts. The first one is electrostatic contribution. This interaction is responsible for the virtually planar orientation of absorbed TNB with respect to the silicon-oxygen plane of clay minerals. Another stabilizing interaction originates from dispersion energy. This contribution amounts to 30-50% of the total adsorption energy. (c) 2006 Elsevier B.V. All rights reserved.