Unusual behaviour of the spin-spin coupling constant upon formation of CH X hydrogen bond

One-bond coupling constants 1 J XY are usually used as a measure of the corresponding X Y interatomic distances. However, the physical nature of this correlation is not well understood and, in some cases, a counterintuitive behaviour of 1 J XY upon hydrogen bonded complex formation has been reported. In this work, the behavior of 1 J CH upon formation and strengthening of complexes with CH X hydrogen bonds and upon a proton transfer process is investigated by means of 1 H NMR spectroscopy and quantum chemical calculations. 1 H 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 1 J 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 1 J 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 1 J CH was also examined computationally for a wide range of CH X 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 1 J 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. One-bond coupling constants 1 J XY are usually used as a measure of the corresponding X Y interatomic distances.