Journal
MAGNETIC RESONANCE IN CHEMISTRY
Volume 46, Issue 5, Pages 457-463Publisher
WILEY
DOI: 10.1002/mrc.2199
Keywords
N-N and N-H spin-spin coupling; proton sponges; EOM-CCSD
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Ab initio calculations have been performed to obtain structures and coupling constants (1)J(N-H), (1h)J(H-N), and (2h)J(N-N) for models of proton sponges with symmetric and asymmetric N-H+-N intramolecular hydrogen bonds (IMHBs). For a given model, the asymmetric structure has a lower energy,a longer N-N distance, and a hydrogen bond which has a greater deviation from linearity. The computed values of (2h)J(N-N) for the models are significantly less than predicted values based on the distance dependence of (2h)J(N-N) for complexes with intermolecular N-H+-N hydrogen bonds. However, the reduced values of (2h)J(N-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 (2h)J(N-N) for IMHBs can be related quadratically to the N-N distances in the models, and demonstrate that there is no discrepancy between computed values of (2h)J(N-N) at the short N-N distances found in these systems and experimental data for proton sponges. Copyright (C) 2008 John Wiley & Sons, Ltd.
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