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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)
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