To What Extent Do External Fields and Vibrational and Isotopic Effects Influence NMR Coupling Constants Across Hydrogen Bonds? Two-Bond Cl−N Spin−Spin Coupling Constants (2hJCl-N) in Model ClH:NH3 Complexes

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EOM-CCSD calculations are performed to evaluate two-bond 35Cl−15N spin−spin coupling constants (2hJCl-N) for ClH:NH3 complexes. Coupling constants for structures in external electric fields of 0.0000, 0.0055, and 0.0150 au are investigated as models for complexes with traditional, proton-shared, and ion-pair hydrogen bonds. Two-dimensional coupling constant surfaces are constructed at these field strengths in the NH and ClH distances, and expectation values, 〈2hJCl-N〉, are calculated for ground and selected excited vibrational states of the dimer- and proton-stretching modes from the corresponding anharmonic wave functions. Single-point values, 2hJCl-N, are also calculated at the equilibrium geometry for each field strength and at the geometry corresponding to the ground-state expectation values of the NH and ClH bond lengths. Coupling constants evaluated in the presence of the electric field are referred to as explicit 〈2hJCl-N〉 and 2hJCl-N. Implicit 〈2hJCl-N〉 and 2hJCl-N are evaluated from the zero-fiel...