Population UNI Science Case
Besides the standard natural bond orbital analysis implemented in many quantum chemical codes, the state of the art is represented by the NBO6.0 analysis. This is a standalone programme which needs special input. Hence, after the basic optimisation with NWChem a special input file for the subsequent single point job has to be generated by a script. This input file then goes into the single point calculation using NWChem (sp files). The output files of these calculations are then the input files for NBO6, AOMix and AIM. AOMix and AIM are small but commercial programmes where a working group licence is needed. They provide with different kinds of population analyses which allow for different orbital dissections, electronic analyses, and calculation of bonding parameters and so on.
Orbital analyses depend on the chosen model for the question to the molecule. Hence, various population schemes should be applied to avoid misinterpretations. This study leads to a better electronic understanding of the regarded molecules.
The first input file is an opt.nw file for a basic optimisation simulation. The output of the first basic WF is an opt.out file which is parsed for the geometry by the subsequent script which generates the input files for NWChem jobs which generate the input files for the population analyses of the other codes.
For a series of bis(pyrazolyl)methane transition metal complexes we performed NBO analyses. The NBO analysis yields also the charge transfer energies (by means of second order perturbation theory) for the donation from the pyrazolyl/pyridinyl units to the metal ions.
The NBO calculation shows that the Npy donor atoms possess a more negative charge than the Npz donor atoms independent of coordinating a metal or not. Furthermore, the relative basicities of the donor functions of the ligand HC(Pz)2(Py) have been determined by DFT. The protonation of the pyridinyl donor is 9 kcal/mol more favourable than the protonation of the pyrazolyl donor. This result agrees to the known pKB values of pyrazole (11.5) and pyridine (8.8). So the pyridinyl donor is the stronger base. Basicity and donor strength often correlate strongly, so in complexes, the pyridinyl function might also act as stronger donor. In this complicated situation, the donor competition between pyrazolyl and pyridinyl donors is very close and can be influenced by subtle varieties such as chelate bite, spin state and Jahn-Teller distortion.
By means of a NBO analysis for all complexes and comparative complexes, partial charges, charge transfer energies and hybridisation of the donating atoms have been calculated. Hereby, the donor competition has been elucidated and set in comparison to experimental structural data. In general, with only small special exceptions, we find that the pyrazolyl donors donate more strongly to the metals with shorter bonds although the pyridinyl donors are more basic. This delicate bias towards pyrazolyl as stronger donor can easily be disturbed by the change of coordination geometry, spin state and Jahn-Teller effects.
- A. Hoffmann, U. Flörke, S. Herres-Pawlis, Insights into Different Donor Abilities Within Bis(pyrazolyl)pyridinylmethane Transition Metal Complexes, Eur. J. Inorg. Chem. 2014, 2296-2306.
- A. Hoffmann, R. Grunzke, S. Herres-Pawlis, Insights into the influence of dispersion correction in the theoretical treatment of guanidine-quinoline copper(I) complexes, J. Comp. Chem. 2014, doi: 10.1002/jcc.23706.