Electron densities for the outer valence orbitals of pyridine: comparison of EMS measurements with near Hartree–Fock limit and density functional theory calculations

Abstract

Momentum distributions of the outer valence orbitals of the aromatic molecule pyridine (C 5H 5N), measured by electron momentum spectroscopy (EMS), are compared with Hartree–Fock (HF) and density functional theory (DFT) calculations. The experiment was performed using a multichannel EMS spectrometer at a total energy of 1200 eV plus the binding energy and in symmetric non-coplanar kinematics. Binding energy spectra have been measured in the energy range of 4–45 eV at the azimuthal angles φ=0° and φ=8°, and in the range of 4–24 eV over a range of azimuthal angles from 0° to ±30°. Theoretical momentum profiles are calculated in the plane wave impulse approximation using the target HF approximation, and also with DFT using the target Kohn–Sham approximation with the B3LYP and B3PW91 gradient corrected functionals. Basis sets range from STO-3G to aug-cc-pVQZ. The shapes of the experimental momentum profiles, and thus the frontier orbital electron behaviour, are generally quite well reproduced by both the HF and the DFT calculations of the delocalised (canonical) molecular orbitals when using the larger diffuse basis sets. It is also found that the calculated localised molecular orbitals (i.e. valence bond hybrid orbitals involving resonance forms) completely fail to describe the frontier orbital electron behaviour observed in the EMS experiments for pyridine.