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

Momentum distributions of the outer valence orbitals of the aromatic molecu le pyridine (C5H5N), measured by electron momentum spectroscopy (EMS), are compared with Hartree-Fock (HF) and density functional theory (DFT) calcula tions. The experiment was performed using a multichannel EMS spectrometer a t a total energy of 1200 eV plus the binding energy and in symmetric non-co planar kinematics. Binding energy spectra have been measured in the energy range of 4-45 eV at the azimuthal angles phi = 0 degrees and phi = 8 degree s, and in the range of 4-24 eV over a range of azimuthal angles from 0 degr ees to +/- 30 degrees. Theoretical momentum profiles are calculated in the plane wave impulse approximation using the target HF approximation, and als o with DFT using the target Kohn-Sham approximation with the B3LYP and B3PW 91 gradient corrected functionals. Basis sets range from STO-3G to aug-cc-p VQZ. The shapes of the experimental momentum profiles, and thus the frontie r orbital electron behaviour, are generally quite well reproduced by both t he HF and the DFT calculations of the delocalised (canonical) molecular orb itals when using the larger diffuse basis sets. It is also found that the c alculated localised molecular orbitals (i.e. valence bond hybrid orbitals i nvolving resonance forms) completely fail to describe the frontier orbital electron behaviour observed in the EMS experiments for pyridine. (C) 2001 E lsevier Science B.V. All rights reserved.