Analysis of the electronic origin of the 1JCH spin-spin coupling trend in 1-X-cyclopropanes: experimental and DFT study.

A conceptual analysis of the CLOPPA (Contributions from Localized Orbitals within the Polarization Propagator Approach) expressions that deconvolute NMR spin-spin coupling constants [Diz A. C.; Giribet C. G.; Ruiz de Azua, M. C.; Contreras, R. H. Int. J. Quantum Chem. 1990, 37, 663.] into orbital contributions can provide an in-depth insight into the features of the electronic molecular structure that originate a given 1JCH experimental trend. In this work, several 1-X-cyclopropane derivatives are taken as model compounds to apply such ideas to rationalize substituent effects on the Fermi contact term of 1JC1,H spin-spin coupling. It is shown that in this type of coupling, its experimental trend, as measured in this work, cannot be accounted for with only the "bond" and the "other bond" contributions, requiring the inclusion of "other antibonding contributions". Such effect is discussed in terms of hyperconjugative interactions.