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 C=O stretching vibrations (DELTAnu) which orig inate from ketones complexing with SbF5. Such ketone-SbF5 complexes ca n serve as good models of analogous carbocations. The weakening of the C=O bond in the complex is a consequence of its increased polarizatio n. The influence of the hydrocarbon skeleton on the stabilization of t he incipient positively charged carbon atom is qualitatively similar t o that in carbocations. Examined ketones 1-23 can be sorted into two c ategories. Complexes of ketones similar to classical carbocations belo ng to the first category. Because in this group the inductive donation of electrons to the positively charged carbon atom is the dominant ef fect, the DELTAnu values correlate well with the number of C(alpha)-C( beta) bonds. Ketone complexes in which the DELTAnu values substantiall y deviate from the linear correlation belong to the second category wh ere the structures of the hydrocarbon skeleton are the same as in carb ocations in which other stabilizing effects are operative. The observe d deviation from linearity can be rationalized by other effects such a s hyperconjugation, bridging, participation and homoaromaticity. This view has been supported by semiempirical and ab initio calculations.