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(NH), 1hJ(HN), and 2hJ(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 2hJ(NN) for the models are significantly less than predicted values based on the distance dependence of 2hJ(NN) for complexes with intermolecular NH+N hydrogen bonds. However, the reduced values of 2hJ(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 2hJ(NN) for IMHBs can be related quadratically to the NN distances in the models, and demonstrate that there is no discrepancy between computed values of2hJ(NN) at the short NN distances found in these systems and experimental data for proton sponges.