2‑Fluoropyridine: A Cross-Sequence NMR Internal Standard for Detecting Tar Nitrogen Addition Reaction

Area of Science:

• Analytical Chemistry
• Sustainable Resources
• Materials Science

Background:

• Nitrogen-containing compounds in coal tar are crucial for sustainable energy and chemical industries.
• Accurate molecular-level characterization and quantification of these compounds are challenging.
• Existing two-dimensional ¹H-¹⁵N HMBC methods lack sensitivity and internal standard compatibility for quantitative analysis.

Purpose of the Study:

• To screen and validate a small-molecule internal standard for quantitative NMR analysis of coal tar nitrogen compounds.
• To investigate the linear response of the internal standard across various NMR experiments.
• To apply the internal standard for analyzing nitrogen-doped coal tar and understanding structural evolution.

Main Methods:

• Screening and validation of a small-molecule internal standard.
• Systematic investigation of linear response characteristics in ¹H NMR, ¹⁹F NMR, ¹³C NMR, ¹H-¹³C HMBC, ¹H-¹³C HSQC, and ¹H-¹⁵N HMBC experiments.
• Application of the internal standard to raw and nitrogen-doped coal tar samples.

Main Results:

• The internal standard demonstrated excellent linear relationships (R ² ≥ 0.984) across all tested NMR pulse sequences.
• A strong linear correlation (R ² ≥ 0.957) was observed between the internal standard and quinoline compounds in ¹H-¹⁵N HMBC.
• Successful relative quantification of nitrogen-containing compounds in nitrogen-doped tar and revealed structural evolution during pyrolysis.

Conclusions:

• The developed cross-sequence linear internal standard effectively addresses limitations in quantitative NMR analysis of complex matrices like coal tar.
• This method reduces systematic errors, enables cross-validation, and simplifies experimental design and sample preparation.
• Provides a novel strategy and reliable data for the quantitative application of ¹H-¹⁵N HMBC in energy and materials science.