L. Åsbrink

HAM/3, a semi-empirical MO theory. I. The SCF method

Abstract An MO SCF theory for molecules has been developed starting from Slaters study of atomic shielding constants. Use of shielding “constants” which are functions of the nature of the shielded electron gives good total energies for a number of atomic species. The good agreement must mean negligible errors due to correlation and self-repulsion. To extend this treatment to molecules the relation Fμν = ∂E/∂Pμν is used. The procedure has been parametrized for H, C, N, O and F.

RYDBERG SERIES IN SMALL MOLECULES

Abstract New Rydberg series have been identified in the ultraviolet spectrum of furan. The charge exchange mass spectrum has been measured as a function of energy, and the protection spectrum has been measured up to ionization potentials of 25 eV. This formation, together with quantum-chemical calculations, allows a description of the electronic structure of furan to be given.

HAM/3, a semi-empirical MO theory. III. Unoccupied orbitals

Abstract Electron affinities and excitation energies for benzene, pyridine, ethylene, ozone, butadiene and cyclopropane are calculated by use of the MO SCF method HAM/3. Since in this method the self-repulsion seems to be completely eliminated, the unoccupied orbitals have correct energies, and therefore the average excitation energy is obtained directly as the difference of two orbital energies. The electron affinities are calculated using a transition state. The calculations are compared with experiments.

Photoelectron-spectroscopical study of the vibrations of furan, thiophene, pyrrole and cyclopentadiene

Abstract It is shown that photoelectron spectroscopy is of considerable value in the interpretation of Raman and infrared spectra. On the basis of photoelectron-spectroscopical results, new assignments are made for the symmetric “ring distorting” vibrations in furan, pyrrole and cyclo pentadiene. The uncertainty concerning the assignments for the symmetric “hydrogen bending” vibrations of the molecules, furan, pyrrole and cyclo pentadiene is largely eliminated. The assignments for the “ring breathing” vibration of furan and the symmetric “double bond stretching” vibrations of furan, thiophene, pyrrole and cyclo pentadiene are confirmed.

Strong correlation effects in the ionisation of CS2

Abstract The Hell photoelectron spectrum of CS 2 is presented. This spectrum is investigated by two different Greens function calculations. For the outer valence region the origin and assignment of satellite lines of significant intensity is clarified. Strong final state correlation effects are found for the inner valence region showing the so-called breakdown of single-particle picture of ionisation.

Rydberg series in small molecules: XVIII. Photoelectron, UV, mass, and electron impact spectra of s-tetrazine

Abstract The photoelectron spectrum of pyrimidine has been measured up to 25 eV and compared with the spectra of the other azabenzenes, using quantum-chemical calculations. For interpretation of the electronic structure Rydberg transitions in the ultraviolet spectrum have been used. The valence transitions and the mass spectrum are studied. The lowest IP corresponds to ionization of a “lone-pair” electron with bonding properties.

Valence excitation of linear molecules. II. Excitation and UV spectra of C2N2, CO2 and N2O

Abstract Valence excitation energies of C 2 N 2 , CO 2 and N 2 O are calculated by use of the new quantum-chemical method HAM/3. The high-intensity ππ* 1 Σ + transitions have been identified. These results have made possible new interpretations of a number of Rydberg transitions. Photoelectron spectrum and electron impact energy loss spectrum of C 2 N 2 have been studied experimentally.

XIV. Photoelectron, uv, mass and electron impact spectra of s-triazine

Abstract The electronic structure of pyrazine has been studied in a photoelectron spectrometer, an electron spectrometer and a tandem mass spectrometer. The Rydberg transitions from electron spectrometry and ultraviolet spectrometry make possible an interpretation of the photoelectron spectrum. The first and third ionization potentials correspond to “lone-pair” electrons and the second and fourth to π electrons. The “lone-pair” electrons are bonding and the π electrons nearly non-bonding. The mass-spectrometric breakdown is discussed.

Photoelectron Spectrum and Rydberg Transitions of CO

A He II (304 A) photoelectron spectrum of CO is presented and a new interpretation of the Rydberg series going to the higher excited states of CO+ is discussed. This work supports earlier assignments of these excited states.

Rydberg series in small molecules: XIII. Photoelectron spectroscopy and electronic structure of cyclopentadiene

Abstract The photoelectron spectrum of cyclopentadiene has been measured up to 25 eV and its charge exchange mass spectrum has been determined as a function of energy. New Rydberg series have been identified in the ultraviolet spectrum. This information together with quantum-chemical calculations allows a description of the electronic structure of cyclopentadiene to be given.