Deuterated Solvents with Nuclear Magnetic Resonance (NMR) spectroscopy
Background
Nuclear Magnetic Resonance (NMR) Spectroscopy is a non-destructive analytical technique used to study the nature and characteristics of molecules with atomic-level precision. It is widely applied in synthetic chemistry, biological and biochemical research, and drug development. NMR can determine the composition of samples and elucidate the structure of molecules, ranging from small organic compounds to large biomolecules and solid materials. Furthermore, advanced NMR experiments enable the study of protein structures and dynamics, providing insights into their functions and interactions. This versatility makes NMR a cornerstone tool in both academic research and industrial applications.
Deuterated Solvents
In proton NMR spectroscopy, the majority of molecules in a solution are solvent molecules. Most common solvents are hydrocarbons, containing NMR-active hydrogen-1 (H) nuclei. To prevent the solvent hydrogen signals from overwhelming the spectra and interfering with the analysis of the dissolved analyte, deuterated solvents are typically used. These solvents have over 99% of their hydrogen atoms replaced with deuterium (H), which is less sensitive to the proton NMR spectroscopy.
The choice of solvent depends on factors such as the solubility of the analyte, the need to control hydrogen bonding, and the solvent’s physical properties like melting or boiling points.
Chemical shifts of analytes can vary slightly between different solvents due to solvent-solute interactions, which is why the solvent used is almost always reported alongside the chemical shifts in NMR studies. Residual solvent proton peaks, which arise from the small amount of non-deuterated solvent present, are often used as internal standards for calibration.
Working Principle
The underlying principle of NMR depends on the intrinsic spin of the nucleus involved. When strong external magnetic field is applied to a sample the interaction of the spin states is observed. The transition of the different spin states are unique for the different nuclei and their chemical environment. This enables characterization of their molecular structure. The energy E of the magnetic dipole μ under in a magnetic field B is
E = −μ · B
B is usually chosen to align with the z-axis so the energy is
E = −μz · B = −γmℏB
where m is the spin state, γ is a positive number, and ℏ is Planck’s constant divided by 2π. The isotopes and have specific frequencies of which they will resonate. The magnetic field around the nucleus will also change with electron density that is determined by the chemical structure of the isotope. This phenomenon is referred to as the chemical shift and enables NMR to differentiate between different molecules. The chemical shift δ is defined as
where Href is the resonance frequency of a reference, Hsub is the resonance frequency of a substance, and Hmachine is the operating frequency of the spectrometer. The chemical shift is found relative to a reference, that is added to the sample.