Ab initio conformational analysis of cyclooctane molecule

✍ Scribed by Rocha, Willian R.; Pliego, Josefredo R.; Resende, Stella M.; Dos Santos, H�lio F.; De Oliveira, Marcos A.; De Almeida, Wagner B.

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 259 KB
• Volume: 19
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/(sici)1096-987x(19980415)19:5<524::aid-jcc5>3.0.co;2-o

✦ Synopsis

The potential energy surface PES for the cyclooctane molecule Ž . was comprehensively investigated at the Hartree᎐Fock HF level of theory employing the 3᎐21G, 6᎐31G, and 6᎐31G U basis sets. Six distinct true minimum Ž . energy structures named B, BB, BC, CROWN, TBC, and TCC , characterized 1 through harmonic frequency analysis, were located on the multidimensional PES. Two transition state structures were also located on the PES for the cyclooctane molecule. Electron correlation effects were accounted for using the Ž . Møller᎐Plesset second-order perturbation theory MP2 approach. The predicted global minimum energy structure on the ab initio PES for the cyclooctane molecule is the BC conformer. A gas phase electron diffraction study at 300 K suggested a conformational mixture while an NMR study in solution at 161.5 K predicted the BC conformer as the predominant form. The equilibrium constants reported in the present study, which were evaluated from the ab initio calculated total Gibbs free energy change values, were in good agreement with both experimental investigations. The ab initio results showed that the low temperature condition significantly favored the BC conformer while above room temperature both BC and CROWN structures can coexist.