Desirable Characteristics of Solvents used for NMR Studies

NMR grade solvents
NMR grade solvents

NMR (Nuclear Magnetic Resonance) Spectroscopy is a technique that helps in analyzing organic compounds’ molecules by scanning their various nuclei. Through this method, different peaks in the spectra can be interpreted that ultimately helps in discovering the compound’s structure. However, it is only possible to attain signals from the nuclei if it has Spin quantum no.=½ and multiple states (ground and excited). Before picking the NMR solvents, let’s understand how the procedure follows. 

NMR Spectroscopy

Firstly, it is crucial to understand a few terms:

  • Spin Quantum Number (I): This defines the particle’s intrinsic angular momentum. It helps in describing the particle’s quantum state. 
  • Spin Angular Momentum (S):  This can be calculated with the help of the formula S = h/2π *(I). 
  • Magnetic Moment(µ): This shows the particle’s tendency of getting arranged when passed through a magnetic field. It is calculated with the formula µ = y * S, where y is the gyromagnetic ratio. 
  • Magnetic Energy: This is the energy possessed by spinning nuclei while they are in a magnetic field. It is calculated with the formula E = µ * Bo, where Bo is the strength of the magnetic field. 

The process starts with supplying a specific amount of energy to the nuclei. This energy transits the nuclei from its ground state to the excited state. After that, signals in NMR spectra can be observed easily. But it is necessary to understand here that the procedure is not as straightforward as it seems. Some challenges are found in it due to the presence of electrons around the nucleus. 

These electrons form a magnetic field of their own. As this new field opposes the applied one, a shielding effect gets created that reduces the strength of the field and the NMR signal peak makes a shift. This is termed as the “Chemical Shift in NMR.” There are only two solutions to this issue:

  • Increase the strength of the magnetic field
  • Reduce the frequency of the radiation to get resonance

The chemical shift is calculated in parts per million (ppm) by keeping a standard peak as reference. 

Solvents In NMR Spectroscopy

Now that everything is clear about the NMR procedure, let’s move on to its solvents. All the components are first dissolved in a solvent and then moved for the research.

This is essential for the homogenized distribution of sample molecules throughout the observation volume. But again, it is not easy to select just any solvent. The chosen alternative must not interrupt with the general study of the component.  That is why these characteristics are checked before choosing the NMR solvent. 

Solubility

Higher solubility of sample in the chosen solvent contributes to higher sensitivity. This property assumes significance in cases where sample availability is limited.

Purity

Degree of solvent purity is one of the major selection criteria. Chemical impurities or water content can lead to interference signals in the NMR spectrum.

Hence, the NMR solvent peaks will be observed at higher chemical shifts. Solvent purity is of even higher concern as the proportion of the isotope nuclei is much less than the major isotope.

High purity solvents are available in sealed glass bottles. After use, the bottle should be immediately closed to prevent intake of moisture or other impurities from the atmosphere. Even a little moisture can incorporate water’s protons into the solvent. This will lead to problems during the experiment. 

Solvent blanks should be run in advance so as to determine that the NMR solvent peaks do not interfere with the sample peaks.

Solvent viscosity

Lower the sample viscosity the better will be the spectral resolution due to better homogenization of the sample.

Moisture Content

As mentioned earlier, the presence of traces of water will contribute to spectral interference as water itself contains two protons. This degrades the NMR spectra quality. But the fact that almost all solvents have some water content can not be denied.

They tend to absorb this water from the atmosphere around them. In order to avoid this, the bottle of the solvent can be sealed properly. In case the water content has already developed in the bottle, it has to be reduced by filtration of solvent through a drying agent or storing the solvent over molecular sieves.

Price

The solvent price is of concern especially when a large number of samples need to be analysed. The price goes up further as the degree of deuteration increases. So one must opt for solvents that perform well and do not prove to be heavy on the budgets. 

By keeping all these factors in mind, one can easily pick a suitable NMR solvent.

Deuterated Solvents

NMR solvents are distinctly different from other spectroscopic solvents as the majority of hydrogen nuclei are replaced with deuterium so as to minimize the interference due to protons.

Though deuterium also has a nuclear spin, it does not operate on the same frequency as protons in the given magnetic field. Therefore, it serves the purpose of NMR spectroscopy. 

It is crucial to remember that the price increases with the degree of deuteration. Deuterated chloroform, 𝐶𝐷𝐶𝑙3 is most commonly used because of its low price. Plus, the 𝐶𝐷𝐶𝑙3 NMR peak is observed with the minimal chemical shift. A reference standard such as tetramethylsilane is commonly added (around 0.03%) to most commercially available solvent grades to serve as zero ppm chemical shift reference for NMR studies.

Common spectroscopic solvents are available commercially in different degrees of deuteration. Apart from CDCl3 other deuterated solvents in common use are:

  • Deuterated water-\(D_2O\)
  • \(DMSO-d_6\) (There is no 𝐷𝑀𝑆𝑂 NMR peak, making this one of the best solvents)
  • \(Methanol- d_4\)
  • \(Methylene chloride-d_2\)
  • \(Pyridine-d_5\)
  • \(THF-d_8\)
  • \(Acetic  acid- d_4\)
  • \(Acetone-d_6\)
  • \(CD_3 CN\)

However, it is vital to compare all the aspects of these solvents before picking one. Each one of them has its own benefits and drawbacks. So a comparison will serve as the best method for selecting the solvent. 

Remember to flush the sample drawing syringe with dry nitrogen before use. After use inject the sample bottle with the same volume of dry nitrogen and reseal. This will keep the sample environment dry and moisture free to provide interference free spectra.

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  1. can i get the references of these criteria of NMR solvent? please

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