Unusual behaviour of the spin-spin coupling constant ¹J_CH upon formation of CHX hydrogen bond

One-bond coupling constants ¹J_XY are usually used as a measure of the corresponding XY interatomic distances. However, the physical nature of this correlation is not well understood and, in some cases, a counterintuitive behaviour of ¹J_XY upon hydrogen bonded complex formation has been reported. In this work, the behavior of ¹J_CH upon formation and strengthening of complexes with CHX hydrogen bonds and upon a proton transfer process is investigated by means of ¹H NMR spectroscopy and quantum chemical calculations. ¹H NMR spectra of 1,1-dinitroethane solution at room temperature in various solvents (carbon tetrachloride, chloroform, dichloromethane, acetone, dimethylformamide and dimethyl sulfoxide) illustrate the increase of ¹J_CH by several Hz upon an increase of the complex strength. Computational results (MP2/aug-cc-pVDZ) reproduce this observation and allow one to conclude that the increase of ¹J_CH is mainly caused by the change of the carbon hybridization (an increase of s-character), rather than by the change in interatomic distance r_CH. The behavior of ¹J_CH was also examined computationally for a wide range of CHX hydrogen bond energies and geometries. For this purpose, quantum-chemical modeling of the partial proton transfer process for complexes formed by 1,1-dinitroethane and trinitromethane as hydrogen bond donors with acetone, pyridine and fluoride anion as hydrogen bond acceptors was performed. The obtained results have confirmed the above-mentioned idea - for rather weak complexes, the dominant impact on the change of ¹J_CH magnitude is the increase of the s-character of carbon atom hybridization, while for complexes with a significantly transferred proton, the exponential decrease of the Fermi-contact term dominates.