19F–19F spin–spin coupling constant surfaces for (HF)2 clusters: The orientation and distance dependence of the sign and magnitude of JF–F
Ab initio calculations using the equation-of-motion coupled cluster method have been carried out to investigate 19F–19F spin–spin coupling constants for a pair of HF molecules. The overall features of the JF–F coupling surface with respect to the F–F distance and the orientation of the pair of HF molecules reflect those of the Fermi-contact (FC) surface, although the FC term may not be a good quantitative estimate of JF–F. The hydrogen-bonded HF dimer exhibits unusual behavior compared to other hydrogen-bonded complexes, since both the FC term and 2hJF–F exhibit variations in sign and magnitude as the F–F distance changes and the linearity of the hydrogen bond is destroyed. The FC term for F–F coupling is relative small and negative for the equilibrium dimer. At the dimer F–F distance, the maximum negative value for the FC term is found for the linear arrangement F–H⋯H–F, while the maximum positive value is found for the linear H–F⋯F–H arrangement, despite the fact that neither of these structures is bound. Changes in the sign and magnitude of the FC term are analyzed using the nuclear magnetic resonance triplet wave function model, which relates the orientation of magnetic nuclei to the phases of the wave functions for excited triplet states that couple to the ground state. The FC term for a particular orientation is a result of competing positive and negative contributions from different triplet states, the sign of each contribution being determined by the alignment of the nuclear magnetic moments in that state. Factors are identified which must play a role in determining which types of wave functions dominate.