Ab initio and experimental study of NMR coupling constants in bicyclo [1.1.1]pentane

The different dependences of NMR spin-spin coupling constants of a medium-sized molecule on the quality of the ground-state wavefunction is studied using three different basis sets of increasing size, namely, 6-31G, 6-31G* and 6-31G**. Calculations of the Fermi contact terms were also carried out using partially uncontracted s and p atomic orbitals with the same exponents as those of the 6-31G basis set. Bicyclo[1.1.1]pentane was chosen as a model compound; it was synthesized for the measurement of several couplings as part of this work in order to compare theoretical results with experimental values. The through-space transmission of couplings via the interaction of rear lobes of orbitals inside the bicyclic cage were studied with the IPPP approach using the above basis sets.