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

The nitrogen-containing compounds in coal tar have always been an essential focus in the field of sustainable resources. They play a crucial role in the full utilization of coal-derived energy substances. The molecular-level structural characterization and quantitative analysis of nitrogen-containing compounds in coal tar represent key challenges for the energy and chemical industries. However, the two-dimensional ¹H-¹⁵N HMBC method suffers from limitations in quantitative applications owing to low signal sensitivity and poor compatibility with internal standards. This study screened and validated a small-molecule internal standard and systematically investigated its linear response characteristics across various nuclear magnetic resonance experiments (¹H NMR, ¹⁹F NMR, ¹³C NMR, ¹H-¹³C HMBC, ¹H-¹³C HSQC, and ¹H-¹⁵N HMBC). The results showed that the internal standard exhibited a good linear relationship (R ² ≥ 0.984) under all tested pulse sequences within a specified concentration range. In the ¹H-¹⁵N HMBC experiment, the peak integral values of the internal standard signal showed excellent linear correlation with the characteristic signal integral values of various quinoline compounds (R ² ≥ 0.957). The internal standard was applied to the analysis of raw and nitrogen-doped coal tar samples, successfully enabling signal identification and relative quantification of specific nitrogen-containing compounds in nitrogen-doped tar, and clearly revealing the structural evolution patterns in the transformation of particular nitrogen functional groups during pyrolysis. The cross-sequence linear internal standard developed in this study addresses the poor internal standard compatibility of internal standards for quantitative analysis using multipulse NMR sequences in complex matrices, which helps reduce intersequence systematic errors, achieve cross-validation, and streamline experimental design and sample preparation in future applications, and provides a novel strategy and reliable data support for the quantitative application of ¹H-¹⁵N HMBC technology in the fields of energy and materials.