Ab Initio Study of Ternary Complexes A···NCH···C with A,C = HCN, HF, HCl, ClF, and LiH: Energetics and Spin−Spin Coupling Constants across Intermolecular Bonds

A systematic ab initio study has been carried out to investigate the structures, binding energies, and spin-spin coupling constants of binary complexes A...NCH and NCH...C and ternary complexes A...NCH...C for A,C = HCN, HF, HCl, FCl, and HLi. These complexes are stabilized by some combination of hydrogen bonds, dihydrogen bonds, halogen bonds, and lithium bonds. The binding energies of the ternary complexes are enhanced relative to those of the corresponding binary complexes, suggesting that A...NCH is a stronger proton-donor acid and NCH...C is a stronger base relative to the isolated monomer HCN. The ternary complexes with the largest enhancement energies are FH...NCH...HLi, HLi...NCH...NCH, and HLi...NCH...HLi, each containing molecules with large dipole moments, and each with a linear head-to-tail arrangement of dipole moment vectors. The equation-of-motion coupled-cluster single and doubles method (EOM-CCSD) has been employed to evaluate spin-spin coupling constants across intermolecular bonds for binary complexes A...NCH and NCH...C and for ternary complexes A...NCH...C. These data are used to investigate how the presence of C influences coupling constants associated with the A...NCH interaction, and how the presence of A influences coupling constants for the NCH...C interaction. Changes in coupling constants in the ternary complexes relative to the binary may be related to the binding energies of corresponding binary complexes and the enhanced binding energies of the ternary complexes. Two-bond coupling constants across X-H...Y hydrogen bonds (2h)J(X-Y) are related to changes in intermolecular distances.