INVESTIGATION OF DONOR-ACCEPTOR INTERACTIONS - A CHARGE DECOMPOSITION ANALYSIS USING FRAGMENT MOLECULAR-ORBITALS

A partitioning scheme for analyzing donor-acceptor interactions in a complex is proposed. The charge decomposition analysis (CDA) constructs the wave function of the complex in terms of the linear combination of the donor and acceptor fragment orbitals (LCFO). Three terms are then calculated for each LCFO orbital of the complex: (i) the charge donation d given by the mixing of the occupied orbitals of the donor and the unoccupied orbitals of the acceptor; (ii) the back donation b given by the mixing of the occupied orbitals of the acceptor and the unoccupied orbitals of the donor; (iii) the charge depletion from the overlapping area (charge polarization) r given by the mixing of the occupied orbitals of donor and acceptor. The sum of the three contributions gives the total amount of donation, back donation and charge polarization in the complex. The CDA is performed for canonical orbitals at HF/6-31G(d) and for natural orbitals at MP2/6-31G(d) for the complexes H3BCO, H3BNH3, W(CO)(6), Ag(CO)(+), and Au(CO)(+). To investigate the basis set dependence of the results, additional calculations are carried out using a TZ2P basis set. Strong electron donation OC --> BH3 (0.550 e) and significant back donation H3B --> CO (0.253 e) are calculated at MP2/6-31G(d) for H-3-BCO. Strong electron donation H3N --> BH3 (0.382 e) but no back donation H3B --> NH3 is predicted at MP2/6-31G(d) for H3BNH3. A comparable magnitude of donation (0.315 e) and back donation (0.233 e) is calculated at MP2/6-31G(d) for W(CO)(6). The electron donation is clearly larger (0.330 and 0.282 e) than the back donation (0.016 and 0.089 e) for Ag(CO)(+) and Au(CO)(+), respectively. The calculated charge polarization is quite large. It is -0.272 e for H3BCO, -0.367 e for H3BNH3, -0.278 e for W(CO)(6), -0.062 e for Ag(CO)(+), and -0.190 e for Au(CO)(+) (all values at MP2/6-31G(d)). Very similar values are calculated at MP2/TZ2P. The results of the CDA method are compared with qualitative MO interaction diagrams. The CDA can be used as a quantitative expression of the Dewar-Chatt-Duncanson model.