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(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.
Del Bene
Janet E
JE
Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555, USA. jedelbene@ysu.edu
Alkorta
Ibon
I
Elguero
José
J
D000042:Absorption; D003198:Computer Simulation; D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D009584:Nitrogen / Q000737:chemistry*; D011522:Protons; D013113:Spin Labels
D011522:Protons; D013113:Spin Labels; D006859:Hydrogen; D009584:Nitrogen