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586. CNDO-SIGMA*: Energies of SIGMA* Orbitals
by Einar Lindholm, Departments of Physical Chemistry and Physics and Leif sbrink, Department of Physics, Royal Institute of Technology, S-100 44 Stockholm, Sweden Although pi* orbitals in many molecules are well known from experimental and theoretical studies of electron affinities and UV spectra, the sigma* orbitals (especially their energies) have largely been unknown. We have therefore tried to create a semi-empirical SCF method which can be used to estimate electron affinities and UV transitions. We have parameterized it by comparison with the few experimental data which were available when this work started. It has been possible to obtain such a method by including several parameters (including the bond parameter "beta") in the parameterization procedure. We then used the new method to study different properties of many molecules, thereby extending the basis for the parameterization. Our heavily parameterized method can be considered a "rule of thumb" which can be used to relate different properties in different molecules. We have, for example, obtained approximate values for the electron affinities of ethane, mainly by use of an electron affinity of acetylene and UV bands in the spectra of ethylene, butadiene and allene. Previously, sigma* orbitals have been discussed very little, probably because the experimental studies are very difficult and the ab-initio calculations seem to be too difficult. Instead, Rydberg orbitals have been used to explain UV spectra, core-excitation spectra and details in attachment processes. Some of these explanations may need revision. Our work may well be useful as part of the foundation of a future semiempirical MO method. The parameterization should be performed in two steps: the first concerns the orbital energies of occupied and unoccupied orbitals and the second the total energy (heat of formation). It has until now been impossible to carry out the first step in a reliable way due to lack of data for the sigma* orbitals. Another application concerns the theory of chemical reactions. In the SN2 theory and in the frontier- orbital theory, quantitative data for sigma* orbitals would be of value. FORTRAN 77 (VAX) Lines of Code: 1907 |