JultikaUniversity of Oulu repository

Abstract

Ab initio methods such as the Hartree-Fock method, the Moller and Plesset perturbation method and the Coupled-Cluster method can be used to solve the electronic Schrödinger equation and calculate the total and orbital energies of the system. The total energy can then be used for optimizing the geometry of the system. The binding energies of the molecule can then be approximated from the orbital energies at the optimized geometry with Koopmans’ theorem, which states that the one-electron orbital energy can be taken to be roughly the negative of the ionization energy of the electron from that orbital. The approximations of binding energies can then be displayed as a density of states spectra and compared to experimental binding energy spectra.

By optimizing the geometries of the two rotamers of meta-aminobenzoic acid and the para-aminobenzoic acid molecules with ab initio methods and comparing the results with the experimental binding energy spectra of meta-aminobenzoic acid it is easy to identify most of the peaks from the experimental spectra and analyze the differences between experimental and calculated binding energies.

By comparing the approximated binding energies of the two rotamers of meta-aminobenzoic acid molecules to each other and the experimental meta-aminobenzoic acid molecule binding energy spectra it can be seen that it is difficult to differentiate between rotamers based on the experimental spectra, as most of the differences are small and the larger ones are in binding energy regions where they would be difficult to notice.

The differences between experimental and calculated approximations of binding energies for meta-aminobenzoic acid molecules and the calculated approximations of para-aminobenzoic acid’s binding energies can be used to form a decent approximation of what para-aminobenzoic acid’s true binding energy spectra would look like.