Energy Ordering of Molecular Orbitals

Abstract

Orbitals are invaluable in providing a model of bonding in molecules or between molecules and surfaces. Most present-day methods in computational chemistry begin by calculating the molecular orbitals of the system. To what extent have these mathematical objects analogues in the real world? To shed light on this intriguing question, we employ a photoemission tomography study on monolayers of 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) grown on three Ag surfaces. The characteristic photoelectron angular distribution enables us to assign individual molecular orbitals to the emission features. When comparing the resulting energy positions to density functional calculations, we observe deviations in the energy ordering. By performing complete active space calculations (CASSCF), we can explain the experimentally observed orbital ordering, suggesting the importance of static electron correlation beyond a (semi)local approximation. On the other hand, our results also show reality and robustness of the orbital concept, thereby making molecular orbitals accessible to experimental observations.

Predictive Models for HOMO and LUMO Energies of N‐Donor Heterocycles as Ligands for Lanthanides Separation

“…In the approximation of the Koopmans’ theorem, the highest occupied molecular orbital energy (− E HOMO ) and the lowest unoccupied molecular orbital energy (− E LUMO ) correspond to the first ionization potential ( IP ) and the electron affinity ( EA ) of the molecule, respectively. The orbital energies obtained by different quantum chemical techniques in the frame of different approximations have diverse values . The reasons for such discrepancies have been studied in detail, and several methods have been proposed for calculating E HOMO and E LUMO with an accuracy acceptable for the direct comparison with experimental IP and EA values ,,,,,…”

Influence of Film and Substrate Structure on Photoelectron Momentum Maps of Coronene Thin Films on Ag(111)

“…1−5 By additionally considering band structure and Bloch states, substructures of the main PMM features can also be explained with high precision. 6 Recent angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) measurements of one monolayer (1 ML) coronene thin films on Au(111) demonstrated that the momentum maps are further influenced by vibrational modes, dispersion of the molecular states, and back-folded substrate bands.…”

Orbitals and the Interpretation of Photoelectron Spectroscopy and (e,2e) Ionization Experiments

“…This conclusion undermines claims that these experiments reflect "what actually electrons do in amolecule prior to their knock-out" [7] and therefore that hybrid molecular orbitals (HMOs) or localized molecular orbitals (LMOs) do not represent ap hysically valid orbital picture [7,8] In order to try to prevent these misinterpretations from gaining af oothold, we present some pedagogical background (see also previous articles along these lines [14,15] )t hat we hope will help clarify orbital concepts.T wo key points we emphasize are:1 )One cannot describe an N-electron wave function exactly in terms of N spin-orbitals,e ach containing one electron, and 2) a Dyson orbital is not an orbital of the molecule studied by photoelectron spectroscopy,and thus it is not an orbital in the usually understood sense,but rather it involves ageneralized use of the word "orbital" as an amplitude representation of the difference in the electron distribution of am olecule before and after ionization. Even when the correct interpretation in terms of Dyson orbitals has been cited, as in arecent paper [16] on photoemission tomography,t he authors discuss using the experiment to obtain information about "the molecular orbitals of the system", which can be deceptive if one does not interpret it using the generalized meaning of the word "orbital".…”