title: On the nature of the surprisingly small (red) shift in the halothane...acetone complex.
abstract: The halothaneacetone and fluoroformacetone complexes are studied using the second-order Møller-Plesset (MP2) method with a cc-pVTZ basis set and the density functional theory (DFT) method with a TZVP basis set. Whereas halothane exhibits a small red shift upon complexation, fluoroform shows a pronounced blue shift. To explain this difference in behavior, we perform symmetry-adapted perturbation theory (SAPT) and natural bond orbital (NBO) analyses. Although the composition of the total stabilization energy of each complex is different, that alone does not provide a satisfactory explanation for the difference in the spectral shifts. This difference is interpreted as a result of the interplay of the hyperconjugation and rehybridization mechanisms. The small and surprising red shift of the C-H stretching frequency of halothane, which resulted from the complexation of this species with acetone,is explained by the compensation of the two above-mentioned mechanisms. On the other hand, the fluoroformacetone complex exhibits a blue shift of the C--H stretching frequency upon complexation, the most likely reason for this shift being a concerted occurrence of the hyperconjugation and rehybridization mechanisms. The calculated shift of the C-H stretching vibration frequencies of halothane (+27 cm(-1)) agree with the experimental value of +5 cm(-1).
authors.affiliation: Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic and Centre for Biomolecules and Complex Molecular Systems, Flemingovo nám. 2, 166 10 Prague 6, Czech Republic.
authors.forename: Kristýna
authors.initials: K
authors.lastname: Pluhácková
authors.forename: Pavel
authors.initials: P
authors.lastname: Hobza