Magn. Reson. Chem. 46, 457-463 (2008)

Doi: 10.1002/mrc.2199

Spin-spin coupling across intramolecular N...H+...N hydrogen bonds in models for proton sponges: an ab initio investigation

Ab initio calculations have been performed to obtain structures and coupling constants 1J(N[BOND]H), 1hJ(H[BOND]N), and 2hJ(N[BOND]N) for models of proton sponges with symmetric and asymmetric N[BOND]H+[BOND]N intramolecular hydrogen bonds (IMHBs). For a given model, the asymmetric structure has a lower energy, a longer N[BOND]N distance, and a hydrogen bond which has a greater deviation from linearity. The computed values of 2hJ(N[BOND]N) for the models are significantly less than predicted values based on the distance dependence of 2hJ(N[BOND]N) for complexes with intermolecular N[BOND]H+[BOND]N hydrogen bonds. However, the reduced values of 2hJ(N[BOND]N) cannot be attributed solely to the distortion of the hydrogen bond in the models, but also reflect differences in s electron populations at the nitrogens in both the ground state and the excited states which couple to it through the Fermi-contact (FC) operator. Values of 2hJ(N[BOND]N) for IMHBs can be related quadratically to the N[BOND]N distances in the models, and demonstrate that there is no discrepancy between computed values of2hJ(N[BOND]N) at the short N[BOND]N distances found in these systems and experimental data for proton sponges.