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 1 J (NH), 1h J (HN), and 2h J (NN) for models of proton sponges with symmetric and asymmetric NH + N intramolecular hydrogen bonds (IMHBs). For a given model, the asymmetric structure has a lower energy, a longer NN distance, and a hydrogen bond which has a greater deviation from linearity. The computed values of 2h J (NN) for the models are significantly less than predicted values based on the distance dependence of 2h J (NN) for complexes with intermolecular NH + N hydrogen bonds. However, the reduced values of 2h J (NN) 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 2h J (NN) for IMHBs can be related quadratically to the NN distances in the models, and demonstrate that there is no discrepancy between computed values of 2h J (NN) at the short NN distances found in these systems and experimental data for proton sponges. Copyright © 2008 John Wiley & Sons, Ltd.