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

Abstract

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 3 /CDClF 2 . The hydrogen bond geometries were estimated using previously established correlations linking 1 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 31 P NMR signal upon complexation. We show that the values of 31 P 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 31 P NMR signal by ca. 1.5 ppm to the high field.

Relationships between NMR shifts and interaction energies in biphenyls, alkanes, aza‐alkanes, and oxa‐alkanes with X─H…H─Y and X─H…Z (X, Y = C or N; Z = N or O) hydrogen bonding

“…Correlations between hydrogen‐bond strength and spectroscopic parameters go back to Badger and Bauer's work of 1937 on the vibrational spectra of hydrogen‐bonded complexes between alcohols and various solvents . NMR spectral shift enhancement is recognised as a criterion of hydrogen bonding, and many correlations between shifts and different measures of the hydrogen‐bond strength, not only in complexes but also related to intramolecular interactions, have been proposed. Such correlations have been used to infer hydrogen‐bond strengths or distances from spectroscopic data.…”

Phosphine Oxides as Spectroscopic Halogen Bond Descriptors: IR and NMR Correlations with Interatomic Distances and Complexation Energy

“…Again, the average coefficient refers to the fitting of the entire data set shown in Figure 8. It should be noted that despite the generally high sensitivity of δP to the molecular structure and to non-covalent interactions [71], in the literature there are only few attempts to use δP for the solution of the reverse spectroscopic problem for non-covalent complexes, i.e., for the finding of the complex's energy and structure based on the phosphorous chemical shift value [72][73][74][75]. Partially the reason for this might be in the high sensitivity itself, because contributions to δP from various weak secondary non-covalent interactions might "smudge" the effect of the halogen bonding, thus strongly reducing the diagnostic value of the spectroscopic marker.…”

“…value [72][73][74][75]. Partially the reason for this might be in the high sensitivity itself, because contributions to P from various weak secondary non-covalent interactions might "smudge" the effect of the halogen bonding, thus strongly reducing the diagnostic value of the spectroscopic marker.…”

Sensitivity of 31P NMR chemical shifts to hydrogen bond geometry and molecular conformation for complexes of phosphinic acids with pyridines

“…The chemical non‐equivalence of two ortho ‐protons due to formation of a single CHO bond was previously reported for complexes of pyridine with diols. [ 31 ] We note, however, that in experiment [ 16 ] for A3 – A12 , only one broad signal of ortho ‐protons was observed, because even at low temperature, the rotation of pyridine ring is fast in the NMR time scale (more on this in the “Pyridine ring rotation” subsection below). The principal components of 31 P NMR chemical shift anisotropy tensor (CSA tensor; δ ii P, i = 1, 2, 3) are listed in Table S3.…”

“…[ 18 ] The energies, optimized geometries, and vibrational frequencies were obtained by means of density functional theory (DFT) at the B3LYP/6‐311++G(d,p) level for complexes A1 – A12 and B1 – B12 . In computations of equilibrium structures in vacuum, our main goal was not to achieve a quantitative match with the previously published experimental results [ 16 ] —which would be naïve to expect considering the absence of solvent and dynamic effects—but to qualitatively reproduce general trends and to rationalize the effects on observed NMR parameters that occur due to the changes of several internal degrees of freedom. Thus, the selected functional/basis set was chosen for its accessibility and reasonably good performance for the description of hydrogen‐bonded systems [ 19 ] at a low computational cost.…”