Theoretical investigation of UV photoelectron spectra of pyridine compound molecules

Abstract

A set of pyridine compounds are used to illustrate the possibility of assigning complex UV photoelectron spectra (UPS) by means of independent channel calculations that account for both energies and cross-sections. 2-ethynylpyridine (2EP), 4-ethynylpyridine (4EP), 1,4-bis(2-pyridyl)-1,3-butadine (B2PBD), 1,4-bis(4-pyridyl)-1,3-butadine (B4PBD), and their subunits, pyridine and mono-, di- and tri-acetylene, have thus been investigated using the independent channel direct static exchange method. The calculations show that the cross-section of the 4a 1 (n) channel of pyridine at 21.2 eV photon energy is twice those of the π channels (1a 2 and 2b 2 ), which supports the reversed order of the 2b 2 and 4a 1 (n) Hartree-Fock molecular orbitals (MOs) suggested recently by Moghaddam et al. , (Chem. Phys., 207 (1996) 19). Correlations of occupied orbitals between the subunits and their compound molecules, 2EP, 4EP, B2PBD and B4PBD have been predicted by means of population analysis. A simple relation for correcting the calculated binding energies (BEs) for the σ MOs has been found for pyridine and for the compound molecules in the region (12 eV < 20 eV) where the pyridine character is dominant. Previous assignments for the π bands (BE < 12 eV) have been slightly renewed and assignments for the σ bands (BE > 12 eV, up to 20 eV) have been made for the first time. Significant cross-section variations with photoelectron energy are predicted, leading to He I and He II spectra with quite different intensity distributions.