Investigation of intermolecular interaction using ab initio calculations

Abstract

This dissertation consists of two parts: the first part discusses the interaction of the 1st row transition metal atoms intercalated in between two benzene molecules, and the second part discusses the effects of an electric field on interaction of aromatic systems. First, structures of neutral metal-dibenzene complexes, M(C6H6)2 (M = Sc - Zn), are investigated by using Møller-Plesset second order perturbation theory (MP2). The benzene molecules change their conformation and shape upon complexation with the transition metals. I find two types of structures; (i) stacked forms for early transition metal complexes and (ii) distorted forms for late transition metal ones. The benzene molecules and the metal atom are bound together by δ-bonds which originate from the interaction of π-MO’s and d orbitals. The binding energy shows a maximum for Cr(C6H6)2, which corresponds to a 18-electron complex. It is noticeable that Mn(C6H6)2, a 19-electron complex, manages to have a stacked structure with an excess electron delocalized. For other late transition metal complexes having more than 19 electrons, the benzene molecules are bent or stray away from each other to reduce the electron density around a metal atom. For the early transition metals, the M(C6H6) complexes are found to be weakly bound than those of late transition metals. This is because the early transition metal complexes do not have enough electrons to fill all the available orbitals of D6h structure, and so these dibenzene-metal complexes generally tend to have tighter binding with a shorter benzene-metal length than the monobenzene-metal complexes, which is quite unusual. Second, the effect of uniform external electric field on the interactions between small aromatic compounds and an argon atom is investigated using post-HF (MP2, SCS-MP2 and CCSD(T)) and density functional (PBE0-D3, PBE0-TS and vdW-DF2) methods. The electric field effect is quantified by the difference of interaction energy calculated in the presence and absence of electric field. All the post-HF methods describe electric field effects accurately, although the interaction energy itself is overestimated by MP2. The electric field effect is explained by classical electrostatic models for distances greater than 4.0 Å, whereas exchange repulsion starts to take effect for closer distances. PBE0-based methods give reasonable interaction energies and electric field response in every case, owing to their ability to reproduce electric polarizability accurately. However, vdW-DF series sometimes give unphysically large polarizabilities of aromatic compounds unless the real space sampling of electron density is large enough. This leads to unphysical destabilization of the X···Ar complex with increment of electric field strength. Therefore, one should be aware of this issue when using the vdW-DF method series under an external field.