Nmr Spin-spin Coupling Constants: Bond Angle Dependence Of The Sign And Magnitude Of The Vicinal (3)j(hf) Coupling

Abstract

The dependence of the magnitude and sign of (3)J(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 (3)J(HFF) are analyzed to assess which orbitals are responsible for (3)J(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 (3)J(HFF) coupling constant on the angle between the sigma(C alpha-F) and sigma(C alpha)'(-HF) vectors. Through-space and hyperconjugative transmission pathways and ring strain are identified as responsible for the opposite trend between (3)J(HFF) and bond angle, and for the negative signs obtained for the two molecules, respectively. One of the fluorine lone pairs, sigma(C alpha)'(-HF), sigma(C alpha-F), and sigma(C alpha)'(-C beta)' bonding orbitals and the sigma(C alpha-F)* antibonding orbital are involved in the J-coupling pathways, according to analyses of pairwise-steric and hyperconjugative energies.