Sensitivity of ³¹P 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 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.