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).