LONG-RANGE SPIN-DENSITY PROPAGATION IN SATURATED-HYDROCARBONS - 3-[N]STAFFYL RADICALS

The bridgehead radicals derived from the first three [n]staffanes (n = 1-3), oligomers of [1.1.1]propellane, have been generated from the corresponding bromides, and their solution EPR spectra have been recorded. Remarkably long-range hyperfine coupling has been found to epsilon, zeta, and even iota hydrogens, in qualitative agreement with ab initio UHF calculations. The coupling to the bridgehead hydrogen is attenuated by a factor of about 25 per added bicyclo[1.1.1]pentane cage. The long-range propagation of spin density can be attributed to strong interaction between the orbitals used to make the exocyclic bonds in the 1 and 3 positions of each bicyclo[1.1.1]pentane cage. The situation can be understood simply in terms of a linear sigma-hyperconjugated chain of orbitals interacting through resonance integrals whose effective magnitude alternates in an about 1:5 ratio. A more detailed analysis is provided by considering the effect on the spin density of the various types of off-diagonal elements in the UHF Hartree-Fock matrix expressed in terms of maximally spin-paired natural bond orbitals (MSP-NBO). This permits a clean separation of through-space and through-bond interactions as well as further separation of each of these into contributions due to bond delocalization and those due to bond spin polarization.