An experimental and theoretical study of the valence shell photoelectron spectra of 2-chloropyridine and 3-chloropyridine.

D M P Holland,A W Potts,E V Gromov,J Schirmer,A B Trofimov,T E Moskovskaya,I Powis,I L Badsyuk,L Karlsson,R C Menzies

doi:10.1063/1.4999433

Abstract

The valence shell photoelectron spectra of 2-chloropyridine and 3-chloropyridine have been studied both experimentally and theoretically. Synchrotron radiation has been employed to record angle resolved photoelectron spectra in the photon energy range 20-100 eV, and these have enabled anisotropy parameters and branching ratios to be derived. The experimental results have been compared with theoretical predictions obtained using the continuum multiple scattering Xα approach. This comparison shows that the anisotropy parameter associated with the nominally chlorine lone-pair orbital lying in the molecular plane is strongly affected by the atomic Cooper minimum. In contrast, the photoionization dynamics of the second lone-pair orbital, orientated perpendicular to the molecular plane, seem relatively unaffected by this atomic phenomenon. The outer valence ionization has been studied theoretically using the third-order algebraic-diagrammatic construction (ADC(3)) approximation scheme for the one-particle Green's function, the outer valence Green's function method, and the equation-of-motion (EOM) coupled cluster (CC) theory at the level of the EOM-IP-CCSD and EOM-EE-CC3 models. The convergence of the results to the complete basis set limit has been investigated. The ADC(3) method has been employed to compute the complete valence shell ionization spectra of 2-chloropyridine and 3-chloropyridine. The relaxation mechanism for ionization of the nitrogen σ-type lone-pair orbital (σN LP) has been found to be different to that for the corresponding chlorine lone-pair (σCl LP). For the σN LP orbital, π-π* excitations play the main role in the screening of the lone-pair hole. In contrast, excitations localized at the chlorine site involving the chlorine πCl LP lone-pair and the Cl 4p Rydberg orbital are the most important for the σCl LP orbital. The calculated photoelectron spectra have allowed assignments to be proposed for most of the structure observed in the experimental spectra. The theoretical work also highlights the formation of satellite states, due to the breakdown of the single particle model of ionization, in the inner valence region.

Highlights

Pyridine (C5H5N, Figure 1) is the simplest six-membered nitrogen-containing aromatic molecule which appears as a structural unit in many biological and biologically active compounds
The experimental results have been compared to theoretical predictions obtained using the continuum multiple scattering Xα approach. This comparison shows that the anisotropy parameter associated with the nominally chlorine lone-pair orbital lying in the molecular plane is strongly affected by the atomic Cooper minimum
In order to improve our understanding of the differing extent to which orbital relaxation and electron correlation affect orbitals of alternative type (σ, π, or non-bonding), we have extended our studies to 2- and 3-chloropyridine (C5H4NCl, Figure 1)

Summary

Introduction

Pyridine (C5H5N, Figure 1) is the simplest six-membered nitrogen-containing aromatic molecule which appears as a structural unit in many biological and biologically active compounds. B. Assignment of the photoelectron spectrum of 2-chloropyridine The present HF, OVGF, ADC(3), EOM-IP-CCSD and EOM-IP-CC3 results for the vertical outer valence ionization transitions of 2-chloropyridine, obtained with the cc-pVDZ and the aug-cc-cVTZ basis sets, are listed, together with our experimental values.

Results

Conclusion