Natural J-coupling analysis: Interpretation of scalar J-couplings in termsof natural bond orbitals

The natural J-coupling (NJC) method presented here analyzes the Fermi conta ct portion of J-coupling in the framework of finite perturbation theory app lied to ab initio/density function theory (DFT) wave functions, to compute individual and pairwise orbital contributions to the net J-coupling. The ap proach is based on the concepts and formalisms of natural bond orbital (NBO ) methods. Computed coupling contributions can be classified as Lewis (indi vidual orbital contributions corresponding to the natural Lewis structure o f the molecule), delocalization (resulting from pairwise donor-acceptor int eractions), and residual repolarization (corresponding to correlation-like interactions). This approach is illustrated by an analysis of the angular a nd distance dependences of the contributions to vicinal (3)J(HH) couplings in ethane and to the long-range (6)J(HH) couplings in pentane. The results indicate that approximately 70% or more of the net J-coupling is propagated by steric exchange antisymmetry interactions between Lewis orbitals (predo minantly a bonding orbitals). Hyperconjugative sigma to sigma* delocalizati on interactions account for the remainder of the coupling. Calculated pairw ise-steric and hyperconjugative-delocalization energies provide a means for relating coupling mechanisms to molecular energetics. In this way, J-coupl ing contributions can be related directly to the localized features of the molecular electronic structure in order to explain measured J-coupling patt erns and to predict J-coupling trends that have yet to be measured.