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 (~(2h)J_(Cl-N)) in Model CIH:NH_3 Complexes

摘要

EOM-CCSD calculations are performed to evaluate two-bond ~(35)Cl-~(15)N spin-spin coupling constants (~(2h)J_(Cl-N)) for ClH:NH_3 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, <~(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)> and ~(2h)J_(Cl-N), but which isotopomer has the larger coupling constant also depends on hydrogen bond type. Thermal vibrational averaging of ClH:NH_3 and ClD:NH_3 Cl-N spin-spin coupling constants at temperatures below 300 K has essentially no effect.