Sensitivity of 31 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, Me2 POOH, and PhHPOOH, are reported. A series of substituted pyridines (pKa range from 0.5 to 10) was chosen as proton acceptors. Gradual changes of isotropic 31 P nuclear magnetic resonance (NMR) chemical shift, δP, were correlated with the bridging proton position in the intermolecular OHN hydrogen bond, namely, r (OH) distance; the proposed correlation could easily be extended to other phosphinic acids as well. For complexes with pyridine and 2,4,6-trimethylpyridine, we have investigated in more detail several factors influencing the δP values: (1) the proton transfer within the OHN hydrogen bond; (2) the rotation of the pyridine ring around the hydrogen bond axis (associated with the formation/breakage of additional weak PO···H-C hydrogen bond); and (3) the rotation of the phenyl substituent in phenylphosphinic acid around the P-C axis. All these factors appeared to be of similar magnitude, thus masking their individual contributions that have to be independently estimated for a reliable spectral interpretation.

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.