| Abstract
| - Large long-range indirect nuclear spin coupling constants are of great interest for quantum computers. Butthey are rarely observed and are usually considered very small, unless the coupled nuclear spins are proximatein space. Looking for counterexamples, we have calculated F−F couplings in four different series of acyclichydrocarbons (alkanes, conjugated polyenes, conjugated polyynes, and cumulenes) where the coupled fluorinenuclei are separated by up to 11 bonds or 1.4 nm. The calculations were carried out at the level of thesecond-order polarization propagator approximation using locally dense basis sets. This approach has, inrecent years, been shown to be particularly successful in reproducing indirect nuclear spin−spin couplings inorganic molecules. We find that the F−F couplings in saturated alkanes diminish very quickly with the numberof bonds between the coupled fluorine atoms, whereas in the conjugated polyenes and in particular polyynesthe F−F couplings can be transmitted over much longer distances. We predict that the F−F coupling over 9bonds or 1.1 nm is 12 Hz in (1E,3E,5E,7E)-1,8-difluoroocta-1,3,5,7-tetraene and the coupling over 11 bondsor 1.4 nm is 7 Hz in difluorodecapentayne. Analyzing the four Ramsey contributions, we find that the F−Fcouplings in the polyenes are dominated by the spin−dipolar term, which is known to be favored by π-electronicsystems, whereas in the case of the polyynes the orbital paramagnetic terms make the largest contributions,although the spin−dipolar and the Fermi contact contributions are also significant.
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