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PM3 geometry optimization and CNDO/S‐CI computation of UV/Vis spectra of large organic structures: Program description and application to poly(triacetylene) hexamer and taxotere
Author(s) -
Baumann Harold,
Martin Rainer E.,
Diederich François
Publication year - 1999
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/(sici)1096-987x(199903)20:4<396::aid-jcc2>3.0.co;2-9
Subject(s) - cndo/2 , random hexamer , excited state , molecule , conjugated system , spectral line , materials science , molecular physics , computational chemistry , chemistry , polymer , crystallography , atomic physics , physics , organic chemistry , quantum mechanics
SIXW.C, a new C version of the computer program CNDUV99 (based on CNDO/S‐CI with inclusion of doubly excited configurations) is described and shown to be useful for the computation of the UV/Vis spectra of fairly large molecules. The geometries of the molecules were obtained by our C version of program PM3. To demonstrate the broad applicability of program SIXW.C we have chosen two representative examples: first, a linearly conjugated oligomer of defined length; and, second, a natural product that currently plays a very significant role in cancer therapy. The first molecule is a recently synthesized linearly conjugated monodisperse hexamer of ∼4.6 nm in length (C 126 H 222 O 12 Si 14 ), with poly(triacetylene) backbone. Despite the 36 conjugated C‐atoms of the framework, it exhibits remarkable thermal and environmental stability, which allows in‐depth investigation of its physical properties. Therefore, this made it a very attractive candidate for comparison of the theoretically calculated and experimentally measured UV/Vis spectra, as there is still considerable interest in predicting linear optical properties of large conjugated organic molecules at the border to polymers. The agreement between experimental and calculated spectra is better with the UHF‐PM3‐ rather than with the RHF‐PM3‐optimized structure of the molecule, showing, for the former, excellent agreement between the experimental and theoretical longest wavelength maximum (λ max ), which differ by only 16 nm (802 cm −1 ). This result is of fundamental interest, because the electronic structure reveals a spin‐polarized character, usually referred to as spin density wave. In addition to the flat poly(triacetylene) oligomer, we have calculated N‐tert ‐butoxycarbonyl‐10‐deacetyl‐ N ‐debenzoyltaxol (C 43 H 53 NO 14 , taxotere) as an example for a three‐dimensional molecule for which, until now, only MM2 force‐field computations were performed. It is closely related to the potent inhibitor of cell division, taxol, which was isolated first in 1971 from the stem bark of the yew Taxus brevifolia Nutt . Taxotere is one of the key weapons in modern cancer therapy. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 396–411, 1999