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Review
. 2023 Feb 3;28(3):1499.
doi: 10.3390/molecules28031499.

Searching for the Best Values of NMR Shielding and Spin-Spin Coupling Parameters: CH4-nFn Series of Molecules as the Example

Karol Jackowski  1 , Mateusz A Słowiński  1
Affiliations
Review

Searching for the Best Values of NMR Shielding and Spin-Spin Coupling Parameters: CH4-nFn Series of Molecules as the Example

Karol Jackowski et al. Molecules. .

Abstract

Attempts at the theoretical interpretation of NMR spectra have a very long and fascinating history. Present quantum chemical calculations of shielding and indirect spin-spin couplings permit modeling NMR spectra when small, isolated molecules are studied. Similar data are also available from NMR experiments if investigations are performed in the gas phase. An interesting set of molecules is formed when a methane molecule is sequentially substituted by fluorine atoms-CH4-nFn, where n = 0, 1, 2, 3, or 4. The small molecules contain up to three magnetic nuclei, each with a one-half spin number. The spectral parameters of CH4-nFn can be easily observed in the gas phase and calculated with high accuracy using the most advanced ab initio methods of quantum chemistry. However, the presence of fluorine atoms makes the calculations of shielding and spin-spin coupling constants extremely demanding. Appropriate experimental 19F NMR parameters are good but also require some further improvements. Therefore, there is a real need for the comparison of existing NMR measurements with available state-of-the-art theoretical results for a better understanding of actual limits in the determination of the best shielding and spin-spin coupling values, and CH4-nFn molecules are used here as the exceptionally important case.

Keywords: NMR spectroscopy; fluoromethanes; nuclear magnetic shielding; spin-spin coupling.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Comparison of experimental and calculated NMR spectral parameters σ and nJ. The thickness of Levels illustrates the increasing uncertainty of determining parameters from −2 to 0 for experiments and from 2 to 0 for calculations.
Figure 2
Figure 2
Experimental 19F, 13C, and 1H NMR shielding values in isolated molecules of fluoromethanes. Each substitution of an electronegative fluorine atom leads to significant deshielding effects for all the nuclei in fluoromethane molecules. Note: The scale of shielding is different for 19F, 13C, and 1H nuclei for better visualization.
Figure 3
Figure 3
Spin-spin couplings of fluoromethanes without the influence of intermolecular interactions as seen in 19F, 13C, and 1H NMR measurements. The opposite signs of 1JCH and 1JCF coupling constants were first found in double-resonance experiments [104].
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