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

EOM-CCSD calculations are performed to evaluate two-bond Cl-35-N-15 spin-spin coupling constants ((2h)J(Cl-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 CIH distances. and expectation values. <(2h)J(Cl-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. (2h)J(Cl-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 <(2h)J(Cl-N)> and <(2h)J(Cl-N)>. Implicit <(2h)J(Cl-N)> and (2h)J(Cl-N) are evaluated from the zero-field coupling constant surface using the geometries and vibrational wave functions (for expectation values) from the 0.0055 and 0.0150 au surfaces. Both <(2h)J(Cl-N)> and (2h)J(Cl-N) are larger when computed in the presence of the external field, and exhibit maximum (absolute) values for proton-shared hydrogen bonds, (2h)J(Cl-N) computed at the equilibrium geometry may be significantly different from <(2h)J(Cl-N)>(0.0) and (2h)J(Cl-N) computed at the ground-state geometry, but whether the equilibrium or the ground-state coupling constant is greater depends on hydrogen bond type. Similarly, isotopic substitution of D for the hydrogen-bonded H also changes both <(2h)J(Cl-N)> an d (2h)J(Cl-N), but which isotopomer has the larger coupling constant also depends on hydrogen bond type. Thermal vibrational averaging of ClH:NH3 and ClD:NH3 Cl-N spin-spin coupling constants at temperatures below 300 K has essentially no effect.