Blue-Shifted and Red-Shifted Hydrogen Bonds in Hypervalent Rare-Gas FRg−H···Y Sandwiches

Increased F - [Rg−H] + resonance contribution provides an efficient mechanism for the Rg−H bond repolarization in the hypervalent 3c,4e−F−Rg−H···Y complexes, where Rg is a rare gas atom. This effect which parallels the effect of rehybridization in classic 2c,2e chemical bonds, decreases the population of the σ*(Rg−H) orbitals and increases the covalent Rg−H bond order leading to Rg−H bond contraction. The origin of this electronic reorganization can be ascribed to competition of two hyperconjugative donors for the same σ*(Rg−H) acceptor. As donor Y approaches the H atom of an FRgH moiety, the n(Y) → σ*(H−Rg) interaction becomes stronger, whereas the n(F) → σ*(Rg−H) interaction becomes weaker in an asynchronous process. For weaker donors Y and longer Y···H distances, breaking of the Rg−F covalent bond proceeds faster than formation of the H−Y bond. As a result, both the σ*(Rg−H) population and the Rg−H distance decrease thus accounting for the blue-shift in such H-bonded complexes. However, when Y is a moderately strong donor and/or when σ*(Rg−H) is a very strong acceptor, a further decrease in Y···H distance leads to the formation of a sigma bond with the incoming donor (Y) in an S N 2-like process with concomitant elimination of the fluoride moiety. As a result, the F−Rg−H···Y systems are transformed into dissociated F - ···Rg···[HY] + complexes, and a large Rg−H elongation and red shift are observed.