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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. November 2002, Volume 1, Number 11, 572-582 |
An MO Theoretical Study of Organic Dyes. II. Comparisons of the
Electronic Spectra Calculated by PPP and ab Initio Methods with
Various Levels of Theory
Kichisuke Nishimoto
Internet Electron. J. Mol. Des. 2002, 1, 572-582
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Abstract:
The electronic spectra of some typical aromatic hydrocarbons and
organic colorants have been calculated by PPP-CI and ab initio
methods with various levels of theory. A comparison of calculated
results showed that simple PPP-CI calculation with suitable two-center
electron repulsion integral sets is successful for the calculation of the
π-electronic spectra of organic colorant with high accuracy and also
for understanding the nature of their electronic spectra. One can
compute molecules having the coronene size within a few seconds
using a PC. Thus, this approach is useful for the combinatorial
chemistry and for the design of organic colorants. Furthermore, the
nature of the π-electronic spectra and the characteristic feature of the
lowest excited states, such as ionic, covalent, diradical, etc., are clearly
understood by PPP-CI calculations. On the other hand, in some cases,
ab initio calculations provide erroneous results and the calculated
excited state functions are very complicated. For example, the
HOMO-LUMO transition is dispersed over among many excited states. The
reason why a modified two-center electron repulsion integral set works
satisfactorily is qualitatively discussed.
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