19F NMR Prediction

About

¹⁹F Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to study the structure, composition, and environment of fluorine-containing compounds, such as Per- and Polyfluoroalkyl Substances (PFAS). PFAS are a group of man-made chemicals that have been widely used in various industrial applications due to their resistance to water, oil, and heat. The ¹⁹F nucleus is highly sensitive to NMR analysis, making it an ideal tool for investigating PFAS.

1. Basics of ¹⁹F NMR

• Nucleus: Fluorine-19 (¹⁹F) is a naturally occurring isotope of fluorine and is 100% abundant, making it the primary target in fluorine NMR.

• Magnetic Properties: The ¹⁹F nucleus has a high magnetic moment and a spin of 1/2, which provides high sensitivity in NMR experiments. It is approximately 83% as sensitive as proton (¹H) NMR.

• Chemical Shift Range: ¹⁹F NMR spectra have a wide chemical shift range (up to several hundred ppm), which allows for the differentiation of fluorine atoms in various chemical environments.

2. Application of ¹⁹F NMR in PFAS Analysis

Structural Identification: ¹⁹F NMR is used to identify the structure of PFAS by examining the chemical shifts and splitting patterns of fluorine atoms in the molecule. Each fluorine atom in a different chemical environment will resonate at a different frequency, providing detailed structural information.

Quantification: The area under the peaks in an NMR spectrum is proportional to the number of fluorine atoms contributing to that signal, allowing for the quantification of different PFAS components in a mixture.

Functional Group Identification: ¹⁹F NMR can be used to identify functional groups in PFAS. For example, the chemical shifts of fluorine atoms attached to different carbon atoms (e.g., CF₃, CF₂, and CF) can help identify whether the molecule contains perfluorinated or polyfluorinated groups.

3. Interpretation of ¹⁹F NMR Spectra for PFAS

• Chemical Shift: The chemical shift (δ) in ¹⁹F NMR is influenced by the electronic environment around the fluorine nucleus. For PFAS, fluorine atoms in different positions (e.g., terminal CF₃ groups, internal CF₂ groups) will have distinct chemical shifts. The chemical shifts can help distinguish between various types of PFAS (e.g., PFOA, PFOS, etc.).

  • CF₃ groups: Typically resonate upfield (lower ppm).

  • CF₂ groups: Resonate downfield relative to CF₃ groups, depending on their position in the chain.

• Multiplicity (Splitting Patterns): The splitting pattern in ¹⁹F NMR spectra arises from the coupling between fluorine atoms and neighboring nuclei (such as other fluorines or protons). For PFAS, the coupling is often between adjacent fluorine atoms, leading to characteristic multiplets.

  • J-Coupling: The coupling constant (J) gives information about the number of bonds between coupled nuclei. For example, a CF₃ group adjacent to a CF₂ group may show a triplet (from CF₃) coupled with a quartet (from CF₂) pattern.

• Integration: The integration of peaks in ¹⁹F NMR gives the relative number of fluorine atoms corresponding to each signal. This is particularly useful for analyzing mixtures of PFAS compounds, where different chain lengths or branching may be present.

4. Challenges and Considerations in ¹⁹F NMR for PFAS

• Complexity of Spectra: PFAS molecules can have long fluorinated chains, leading to complex spectra with many overlapping peaks. Advanced NMR techniques, such as two-dimensional (2D) NMR, may be necessary to resolve and interpret these complex spectra.

• Solvent Effects: The choice of solvent can affect the chemical shifts in ¹⁹F NMR. Non-fluorinated solvents, such as deuterated chloroform (CDCl₃), are commonly used to avoid interference with the fluorine signals.

• Sensitivity to Trace Impurities: ¹⁹F NMR is highly sensitive, and even small amounts of impurities can produce detectable signals. Careful sample preparation and purification are required to obtain accurate results.

5. Advantages of ¹⁹F NMR for PFAS Analysis

• High Sensitivity: ¹⁹F NMR provides high sensitivity for detecting fluorine atoms, making it suitable for analyzing trace amounts of PFAS.

• Non-Destructive: NMR is a non-destructive technique, allowing the same sample to be analyzed by other methods if needed.

• Detailed Structural Information: ¹⁹F NMR provides detailed information on the electronic environment, connectivity, and conformation of PFAS molecules.

6. Complementary Techniques

While ¹⁹F NMR is powerful, it is often used in conjunction with other analytical techniques to obtain a comprehensive understanding of PFAS:

• Mass Spectrometry (MS): Provides molecular weight and fragmentation patterns.

• Infrared (IR) Spectroscopy: Helps identify functional groups.

• Chromatography: Separates complex mixtures of PFAS before NMR analysis.

Summary:

¹⁹F NMR is an essential tool for analyzing PFAS, providing detailed insights into the structure, composition, and environment of fluorine atoms in these molecules. Its sensitivity and specificity make it particularly valuable for studying these environmentally persistent and bioaccumulative substances.

Method

The ¹⁹F NMR was predicted with https://fluobase.cstspace.cn/fnmr

Li Y, Huang WS, Zhang L, Su D, Xu H, Xue XS. Prediction of 19F NMR chemical shift by machine learning. Artificial Intelligence Chemistry. 2024 Jun 1;2(1):100043.

Find

The predicted ¹⁹F NMR chemical shifts can be found under the Spectra category in the property tree