C. Fridh

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.

A modification of INDO for interpretation of photoelectron spectra

Abstract It is possible to attain improved agreement between photoelectron-spectroscopic ionzation potentials and theoretical orbital energies simultaneously for all electrons of some hydrocarbons by a suitable parametrization of INDO in its modified form, MINDO. The changed parameters concern mainly the resonance integrals Hμν for which the separate cases of interaction are treated separately. The new procedure gives orbital energies in acceptable agreement with the photoelectron spectra of benzene, methane, ethane, and ethylene. As the procedure might be useful for photoelectron-spectroscopic studies of hydrocarbons it will be referred to as SPINDO (Spectroscopic-Potentials-adjusted-INDO).

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.

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.

Valence excitation of linear molecules.I. Excitation and UV spectra of N2, Co, acetylene and HCN

Abstract Valence excitation energies of N 2 , CO, HCCH and HCN are calculated by use of the new quantum-chemical method HAM/3. The average energy thus obtained is then split to give singlet and triplet energies. For ππ* the Recknagel formulas are used. Theoretical and experimental energies are compared. The V-transitions (ππ* 1 σ + ) in CO, HCCH and HCN have been discussed.

Spectra of p-Benzoquinone, studied with ham/3

Abstract Photoelectron spectra of p -benzoquinone and tetrafluoro- p -benzoquinone have been measured by He II up to 25 eV and interpreted using the semi-empirical MO method HAM/3. The order of the four highest occupied MOs has been obtained as n n π π from a HAM study of the methylated compounds. The electron affinities have been calculated. Finally, the accuracy of these studies has been checked by a study of the singlet and triplet excitation of p -benzoquinone.

Interpretation of electron spectra I. Spectra of furan, pyrrole and cyclopentadiene, studied with HAM/3

Abstract The interpretation of spectra of molecules by use of semi-empirical methods, of which SPINDO is an example, is usually possible for only one property of the molecule. If several properties of the molecules are used in the parametrization process the resulting method will have a more general applicability. This possibility is illustrated by the use of HAM/3 to interpret ionization energies, excitation energies and electron affinities of furan, pyrrole and cyclopentadiene.