Theoretical investigation of UV photoelectron spectra of pyridine compound molecules

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), 1,3-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 pi 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 at, (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 sigma 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 pi bands (BE < 12 eV) have been slightly renewed and assignments for the sigma 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. (C) 1997 Elsevier Science B.V.