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

Abstract

The results of the quantum‐chemical investigation of a series of hydrogen‐bonded 1:1 acid–base complexes formed by model phosphinic acids, Me2POOH, 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 31P 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.

NMR Quantification of Hydrogen-Bond-Accepting Ability for Organic Molecules

“…Factors contributing to selection of an NMR probe included commercial availability, cost, ease of/safe handling, solubility in CH 2 Cl 2 /CD 2 Cl 2 , and sufficiently strong complexation to detect interactions and correlation of 19 F or 31 P NMR data with the extent of interaction. For 31 P NMR spectroscopy, phenylphosphinic acid (PPA, p K a = 1.75) was selected as a probe (Figure A). , For 19 F NMR spectroscopy, pentafluorobenzoic acid (PFBA, p K a = 1.48) was selected as a probe (Figure B) and also compared with 4-fluorophenol (4-FP, p K a = 9.89), inspired by Taft . NMR measurements were conducted in CH 2 Cl 2 due to superior solubility properties for a broad range of organic molecules of interest, with 27% (v/v) CD 2 Cl 2 used for NMR locking/shimming.…”

Self-Assembly of Hydrogen-Bonded Cage Tetramers of Phosphonic Acid

“…The formation of hydrogen-bonded tetramers is a rather rare type of association since, as a rule, it is not favorable in terms of entropy. The entropy loss can be small if the molecules retain high conformational mobility relative to each other within the complex [15], or the entropy loss could be compensated by the high complexation enthalpy. Previously, hydrogen-bound cyclic (ring) tetramers were found in crystals [1,[65][66][67] or in solution [33,68], and tetramers were also considered experimentally (IR) [69] and theoretically [70] for methanol, but in the latter case, the structure with tetrahedral symmetry (with the equivalence of all four molecules) was obtained only dynamically, as a result of successive proton transfers and rearrangements of hydrogen bonds in the cluster.…”

“…The calculated 1 H NMR shieldings were converted into chemical shifts in tetramethylsilane (TMS) scale δH = σH(TMS) − σH. The calculated 31 P NMR shieldings were converted to chemical shift by using the monomeric H3PO4 molecule in vacuum as a standard: δP = σP(H3PO4) − σP This approach has inherent imperfections, as outlined in [54], but it seems to be one of th most common approaches used in the literature and it serves the purpose of this work sufficiently well [15]. The visualization of obtained structures was carried out using GaussView 5.0 [55] and Chemcraft.…”

“…An interesting case is presented by phosphorus-containing acids (phosphinic, phosphoric, and phosphonic), which have the POH and P=O groups acting as proton donors and acceptors, respectively, thus being able to form hydrogen-bonded self-associates. Previously, we determined the stoichiometry and structure of self-associates of dimethylphosphoric [12], diphenylphosphoric, dimethylphosphinic, phenylphosphinic, and bis(2,4,4-trimethylpentyl)phosphinic acids in polar aprotic solutions using low temperature NMR [13,14] and density functional theory (DFT) calculations [15,16]. We have shown experimentally that R 2 POOH and (RO) 2 POOH acids predominantly form cyclic trimers in CDF 3 /CDF 2 Cl mixtures.…”

Influence of intermolecular interactions on the infrared complex indices of refraction for binary liquid mixtures

“…S6 in the ESI †) data support the notion that the OH and PQO groups are interacting and causing electron density perturbations as the 1 H NMR data indicate an upfield chemical shift for the OH proton 58 with increasing TBP concentration, and, in contrast, the 31 P NMR data show a downfield chemical shift for the 31 P with increasing TBP concentration. 59,60…”