Investigating Hydrogen-Bonded Phosphonic Acids with Proton Ultrafast MAS NMR and DFT Calculations

Hydrogen-bonding plays a key role in the structure and dynamics of a wide range of materials from small molecules to complex biomolecules. ¹H NMR has emerged as a powerful tool for studying hydrogen-bonding because the proton isotropic chemical shift exhibits a dependence on the interatomic distances associated with the hydrogen bond. In the present work, we illustrate the use of ultrafast magic angle spinning at high magnetic field (800 MHz) for resolving multiple hydrogen-bonding sites in a set of crystalline phosphonic acids that contain various functional groups (−COOH, −PO₃H₂, and −NH₃⁺). Trends are observed between the proton chemical shift of the hydrogen-bonded proton and the associated hydrogen-bonding distances (O–H···X) from X-ray crystallography. Density functional theory calculations conducted on the phosphonic acid structures illustrate that the experimental proton chemical shift dependence on hydrogen-bond distance agrees with the expected theoretical trends. Further, it is shown that the chemical shift trend varies considerably depending on the functional group participating in the hydrogen bonding, albeit a −COOH, −PO₃H₂, or −NH₃⁺ moiety. An improved understanding of these trends for various functional groups should be useful for determining accurate hydrogen-bond strengths from the proton chemical shift in an array of systems

High-Resolution Zero-Field NMR <i>J</i>‑Spectroscopy of Aromatic Compounds

We report the acquisition and interpretation of nuclear magnetic resonance (NMR) <i>J</i>-spectra at zero magnetic field for a series of benzene derivatives, demonstrating the analytical capabilities of zero-field NMR. The zeroth-order spectral patterns do not overlap, which allows for straightforward determination of the spin interactions of substituent functional groups. Higher-order effects cause additional line splittings, revealing additional molecular information. We demonstrate resonance linewidths as narrow as 11 mHz, permitting resolution of minute frequency differences and precise determination of long-range <i>J</i>-couplings. The measurement of <i>J</i>-couplings with the high precision offered by zero-field NMR may allow further refinements in the determination of molecular structure and conformation

High-Resolution Zero-Field NMR <i>J</i>‑Spectroscopy of Aromatic Compounds

We report the acquisition and interpretation of nuclear magnetic resonance (NMR) <i>J</i>-spectra at zero magnetic field for a series of benzene derivatives, demonstrating the analytical capabilities of zero-field NMR. The zeroth-order spectral patterns do not overlap, which allows for straightforward determination of the spin interactions of substituent functional groups. Higher-order effects cause additional line splittings, revealing additional molecular information. We demonstrate resonance linewidths as narrow as 11 mHz, permitting resolution of minute frequency differences and precise determination of long-range <i>J</i>-couplings. The measurement of <i>J</i>-couplings with the high precision offered by zero-field NMR may allow further refinements in the determination of molecular structure and conformation

High-Resolution Zero-Field NMR <i>J</i>‑Spectroscopy of Aromatic Compounds

We report the acquisition and interpretation of nuclear magnetic resonance (NMR) <i>J</i>-spectra at zero magnetic field for a series of benzene derivatives, demonstrating the analytical capabilities of zero-field NMR. The zeroth-order spectral patterns do not overlap, which allows for straightforward determination of the spin interactions of substituent functional groups. Higher-order effects cause additional line splittings, revealing additional molecular information. We demonstrate resonance linewidths as narrow as 11 mHz, permitting resolution of minute frequency differences and precise determination of long-range <i>J</i>-couplings. The measurement of <i>J</i>-couplings with the high precision offered by zero-field NMR may allow further refinements in the determination of molecular structure and conformation

Zero-Field NMR Enhanced by Parahydrogen in Reversible Exchange

We have recently demonstrated that sensitive and chemically specific NMR spectra can be recorded in the absence of a magnetic field using hydrogenative parahydrogen induced polarization (PHIP)− and detection with an optical atomic magnetometer. Here, we show that non-hydrogenative parahydrogen-induced polarization− (NH-PHIP) can also dramatically enhance the sensitivity of zero-field NMR. We demonstrate the detection of pyridine, at concentrations as low as 6 mM in a sample volume of 250 μL, with sufficient sensitivity to resolve all identifying spectral features, as supported by numerical simulations. Because the NH-PHIP mechanism is nonreactive, operates in situ, and eliminates the need for a prepolarizing magnet, its combination with optical atomic magnetometry will greatly broaden the analytical capabilities of zero-field and low-field NMR