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Potential Surface Studies of Open Shell Systems

P. Bischof ; Institut fur Organische Chemie der THD, Petersenstrasse 22, D 6100 Darmstadt

Puni tekst: engleski pdf 10.036 Kb

str. 51-68

preuzimanja: 222



The electronic structure of a large number of organic radicals
was examined by using the developed unrestricted version of the
semiempirical MIND0/3 method. A review of the obtained results
is presented. The calculated heats of formation (/I.Hr) are generally
in good agreement with available experimental data. The estimated
/I.Hr values are usually too low - 40 kJ/mol. This is not a serious
drawback and had to be expected from the single determinant
approach. However, it only shifts the origin of the energy scale.
The relative stabilities of radical isomers are in most cases predicted
quite correctly. Theoretical interatomic distances are in
reasonably good accordance with experiment. Particular emphasis
is placed on the discussion of the reactivity of radicals and reaction
paths. Theoretical results are here of utmost importance because
the elementary reactionsteps in radical rearrangements are extremely
difficult to access by conventional experimental techniques.
An original procedure for searching and verifying local minima
and saddle points on the energy hypersurfaces is discussed. This
approach is applied on the series of w-alkenyl radicals and their
routes of cyclisations were throughly investigated. It was found
that the experimental findings can be rationalised by a simple
orbital overlap model which on the other hand leads to a natural
link to the allowed/forbidden concept. These findings are corroborated
by the available pieces of experimental evidence. An
important outcome of our analyses is, that unlike for closed shell
systems, the activated complexes of forbidden reactions for doublet
states can be successfully described by single configurations and
should therefore be accessible by appropriate semiempirical treatments.

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