Sensitivity of P NMR chemical shifts to hydrogen bond geometry and molecular conformation for complexes of phosphinic acids with pyridines.

The results of the quantum-chemical investigation of a series of hydrogen-bonded 1:1 acid-base complexes formed by model phosphinic acids, Me POOH, and PhHPOOH, are reported. A series of substituted pyridines (pK range from 0.5 to 10) was chosen as proton acceptors.

Conformational Mobility and Proton Transfer in Hydrogen-Bonded Dimers and Trimers of Phosphinic and Phosphoric Acids.

The monomers, H-bonded cyclic dimers, and trimers of five acids were studied by density functional theory calculations, such as hypophosphorous acid (HPOOH, ), dimethylphosphinic acid (MePOOH, ), phenylphosphinic acid (PhHPOOH, ), dimethylphosphoric acid ((MeO)POOH, ), and diphenylphosphoric acid ((PhO)POOH, ). Particular attention was paid to the conformational manifold existing due to the internal degrees of freedom: proton transfer (PT), puckering ("twist") within the ring of H-bonds, and mobility of the substituents (namely, -Ph, -OMe, and -OPh rotations). For acid , the number of conformers is additionally increased because of the varying relative orientation of nonequivalent substituents in cyclic complexes.

Influence of Hydrogen Bonds in 1:1 Complexes of Phosphinic Acids with Substituted Pyridines on H and P 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 CDF/CDClF. The hydrogen bond geometries were estimated using previously established correlations linking H NMR chemical shifts of bridging protons with the O···H and H···N interatomic distances. A new correlation is proposed allowing one to estimate the interatomic distance within the OHN bridge from the displacement of P NMR signal upon complexation.