Influence of Hydrogen Bonds in 1:1 Complexes of Phosphinic Acids with Substituted Pyridines on 1H and 31P NMR Chemical Shifts

Two series of 1:1 complexes with strong OHN hydrogen bonds formed by dimethylphosphinic and phenylphosphinic acids with 10 substituted pyridines were studied experimentally by liquid state NMR spectroscopy at 100 K in solution in a low-freezing polar aprotic solvent mixture CDF3/CDClF2. The hydrogen bond geometries were estimated using previously established correlations linking H-1 NMR chemical shifts of bridging protons with the O center dot center dot center dot H and H center dot center dot center dot N interatomic distances. A new correlation is proposed allowing one to estimate the interatomic distance within the OHN bridge from the displacement of P-31 NMR signal upon complexation. We show that the values of P-31 NMR chemical shifts are affected by an additional CH center dot center dot center dot O hydrogen bond formed between the P=O group of the acid and ortho-CH proton of the substituted pyridines. Breaking of this bond in the case of 2,6-disubstituted bases shifts the P-31 NMR signal by ca. 1.5 ppm to the high field.