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 3JHFF 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 3JHFF are analyzed to assess which orbitals are responsible for 3JHFF 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 3JHFF coupling constant on the angle between the σCα–F and σCα′–HF vectors. Through-space and hyperconjugative transmission pathways and ring strain are identified as responsible for the opposite trend between 3JHFF and bond angle, and for the negative signs obtained for the two molecules, respectively. One of the fluorine lone pairs, σCα′–HF, σCα–F, σCα′–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.