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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. [ABSTRACT FROM AUTHOR]

For the purpose of investigation of the electronic structures of functionalized pyrroles with potential biological activity the electronic structures and ionization spectra of S-cis-(Z)-2-(2-formylethenyl)pyrrole (FP) were calculated by the propagator quantum chemical method. The calculations were performed using the third-order algebraic diagrammatic construction method (ADC(3)) for one-particle Green´s function (electronic propagator) and the 6-31G** basis set. Going from FP (possessing the intramolecular hydrogen bond H⋯O) to its conformation FPR (without H⋯O bonding), the O1s-ionization energy and the ionization energy of the σ-type lone electron pair orbital of the O atom decrease by ~0.2 eV, which is a consequence of stopping the electron density transfer from the O atom. A strong electron density transfer through the hydrogen bond from the O atom to the NH group occurs in the nitrogen core level ionization spectrum as evidenced by a lower N1s-ionization energy of FP (by ~0.7 eV ) compared to that of FPR. The valence shell ionization spectra of FP and FPR calculated using the ADC(3) method are characterized by a high density of the satellite states. The results obtained indicate that the electronic structures of the compounds of the considered class are characterized by pronounced effects of electron correlation. [ABSTRACT FROM AUTHOR]

The group of three lowest singlet C 1s-excited states of formaldehyde H[sub 2]CO is studied theoretically. The equilibrium geometries are determined at the restricted open-shell Hartree-Fock (ROHF) level and refined total energies are obtained using the multireference configuration interaction (MRCI) approach. In agreement with an earlier prediction [Chem. Phys. 122, 9 (1988)] the second lowest singlet state, [sup 1]B[sub 2], is characterized by a doubly excited, "two particle-two hole" (2p-2h), configuration C 1s,n→π[sup *2]. Our calculations predict that H[sub 2]CO in the [sup 1]B[sub 2](2p-2h) state has a stable pyramidal equilibrium structure with a barrier to inversion of 0.28 eV, the valence angle being close to 107°. The calculated length of the CO bond is 1.390 Å. The [sup 1]B[sub 2](2p-2h) state is shown to be also bound with respect to all possible dissociation and rearrangement processes. The lowest predicted dissociation energy for the [sup 1]B[sub 2] state (H[sub 2]CO[sup *]→H[sub 2]+CO[sup *] reaction) is 0.29 eV (6.69 kcal/mol). The rationalization of the great stability of the [sup 1]B[sub 2](2p-2h) state is the similarity of its electronic structure to that of the first singly-excited state [sup 2]A[sup ″](n) of nitroxyl radical H[sub 2]NO·. The neighboring states [sup 1]B[sub 1](C 1s→π[sup *]) and [sup 1]A[sub 1](C 1s→3s) are characterized within the same framework. Spectroscopic implications and possibilities for the experimental identification of the [sup 1]B[sub 2](2p-2h) state are discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]