Complexes of ketones with SbF5 in the condensed phase. Structural effects on the carbonyl stretching frequencies
The cryochemical matrix isolation method has been used to measure the down frequency shift of CO stretching vibrations (Δν) which originate from ketones complexing with SbF5. Such ketone–SbF5 complexes can serve as good models of analogous carbocations. The weakening of the CO bond in the complex is a consequence of its increased polarization. The influence of the hydrocarbon skeleton on the stabilization of the incipient positively charged carbon atom is qualitatively similar to that in carbocations. Examined ketones 1–23 can be sorted into two categories. Complexes of ketones similar to classical carbocations belong to the first category. Because in this group the inductive donation of electrons to the positively charged carbon atom is the dominant effect, the Δν values correlate well with the number of Cα–Cβ bonds. Ketone complexes in which the Δν values substantially deviate from the linear correlation belong to the second category where the structures of the hydrocarbon skeleton are the same as in carbocations in which other stabilizing effects are operative. The observed deviation from linearity can be rationalized by other effects such as hyperconjugation, bridging, participation and homoaromaticity. This view has been supported by semiempirical and ab initio calculations.