NMR spin–spin coupling constants: bond angle dependence of the sign and magnitude of the vicinal 3JHF coupling

Abstract

The dependence of the magnitude and sign of ³J_HFF on the bond angle in fluoro-cycloalkene compounds is evaluated by electronic structure calculations using different levels of theory, viz. DFT, SOPPA(CCSD) and SOPPA(CC2). Localized molecular orbital contributions to ³J_HFF are analyzed to assess which orbitals are responsible for ³J_HFF and which are the most important coupling transmission mechanisms for each compound. Fluoro-ethylene is used as a model system to evaluate the dependence of the ³J_HFF coupling constant on the angle between the σ_Cα–F and σ_Cα′–HF vectors. Through-space and hyperconjugative transmission pathways and ring strain are identified as responsible for the opposite trend between ³J_HFF and bond angle, and for the negative signs obtained for the two molecules, respectively. One of the fluorine lone pairs, σ_Cα′–HF, σ_Cα–F, σ_{Cα′–Cβ′} bonding orbitals and the σ*_Cα–F antibonding orbital are involved in the J-coupling pathways, according to analyses of pairwise-steric and hyperconjugative energies.