Web Release Date: October 29,
Probing the Shape and Stereochemistry of Molecular Orbitals in Locally Flexible Aromatic Chains: A Penning Ionization Electron Spectroscopy and Green's Function Study of the Electronic Structure of Biphenyl
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
Group of Theoretical Chemistry, Department SBG, Hasselt University, Agoralaan, Gebouw D, B-3590 Diepenbeek, Belgium
Received: July 5, 2005
In Final Form: August 30, 2005
Abstract:
We report on the results of an exhaustive study of the interplay between the valence electronic structure, the
topology and reactivity of orbitals, and the molecular structure of biphenyl by means of Penning ionization
electron spectroscopy in the gas phase upon collision with metastable He*(23S) atoms. The measurements
are compared with one-particle Green's function calculations of one-electron and shake-up valence ionization
spectra employing the third-order algebraic diagrammatic construction scheme [ADC(3)]. Penning ionization
intensities are also analyzed by means of the exterior electron-density model and comparison with photoelectron
spectra: in contrast with the lines originating from 
orbitals, ionization lines belonging to the 
-band system
have large Penning ionization cross sections due to their greater extent outside the molecular van der Waals
surface. The involved chemi-ionization processes are further experimentally investigated using collision-energy-resolved Penning ionization electron spectroscopy. The cross sections of -ionization bands exhibit a
markedly negative collision-energy dependence and indicate that the interaction potential that prevails between
the molecule and the He*(23S) atom is strongly attractive in the -orbital region. On the other hand, the
partial ionization cross sections pertaining to -ionization channels are characterized by more limited collision-energy dependencies, as a consequence of rather repulsive interactions within the -orbital region. A comparison
of ADC(3) simulations with the Penning ionization electron spectra and UV photoelectron spectra measured
by Kubota et al. [Chem. Phys. Lett. 1980, 74, 409] on thin films of biphenyl deposited at 170 and 109 K on
copper demonstrates that biphenyl molecules lying at the surface of polycrystalline layers adopt predominantly
a planar configuration, whereas within an amorphous sample most molecules have twisted structures similar
to those prevailing in the gas phase.
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