The crystal and molecular structure of dimethylphosphinic acid

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 (H₂POOH, 1), dimethylphosphinic acid (Me₂POOH, 2), phenylphosphinic acid (PhHPOOH, 3), dimethylphosphoric acid ((MeO)₂POOH, 4), and diphenylphosphoric acid ((PhO)₂POOH, 5). 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 3, the number of conformers is additionally increased because of the varying relative orientation of nonequivalent substituents in cyclic complexes. We show that ³¹P NMR chemical shifts (δP) are very sensitive to the details of the conformation, spanning ranges from ca. 1 ppm (for trimers of acids 1 and 2) to ca. 12 ppm (for trimers of 4). The energy barriers for the transitions between conformers are rather low (<6 kcal/mol for PTs, <2.5 kcal/mol for puckerings, and ca. <3 kcal/mol for rotations of substituents), such that the fast exchange regime in the NMR timescale and subsequent δP averaging are expected. Correlations are proposed linking the change of average δP with the H-bond energy, showing the slope of ca. 4 ppm per kcal/mol. The sensitivity of δP to the OPO angle and the OPOH dihedral angle and the geometries of both H-bonds formed by the POOH moiety are analyzed.

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 ¹H 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 ³¹P NMR signal upon complexation. We show that the values of ³¹P NMR chemical shifts are affected by an additional CH···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 ³¹P NMR signal by ca. 1.5 ppm to the high field.