Chlorinated molecules are widely used in making plastics, solvents, pesticides, and medicines.[1]  However, they can also be harmful pollutants that pose risks to health and the environment.[2] Despite their relevance, understanding the structure of these chlorine-containing compounds is challenging, especially because chlorine atoms (which have a nuclear spin of 3/2) produce weak and broad signals in standard solid-state NMR, making them hard to detect. To overcome this, we use a method called ³⁵Cl Nuclear Quadrupole Resonance (NQR), which is highly sensitive to chlorine’s local chemical environment.[3] In this study, we use DFT (Density Functional Theory) to model a variety of chlorine-containing organic compounds and predict their NQR signatures, defined by two key parameters: C_Q (quadrupolar coupling constant) and η_Q (asymmetry parameter). Our goal is to see how these parameters can help identify chlorine in different chemical environments. This information can then be used to better detect and understand chlorine pollution in the environment.

The predicted v_Q frequencies of 80 chlorine-containing organic compounds (Fig. 1A) representing a large range of possible chlorine-carbon environments. We noticed that acyl chlorides, sp², sp³ and  sp-bound chlorine can indeed be distinguished from either their v_Q or from the quadrupolar parameters. We then used the predicted values to measure NQR signals for 16 signals of reference molecules (Fig. 1B) and found a clear link between the amount of chlorine and the observed frequency. Finally, we tested this approach on a prototypical example, namely PVC (polyvinyl chloride, Fig. 1C). We could distinguish terminal from central chlorine atoms. We also saw that isomers, defects, and intermolecular interactions affected the signal’s position.

In this work, we have explored the potential application of ³⁵Cl NQR for characterizing chlorinated compounds, common environmental pollutants with significant health and ecological risks. We first established a protocol for predicting and measuring the quadrupolar constants of different organic chlorides. These parameters allow for differentiation of Cl functionalities based on their hybridization, functional group, and neighboring Cl. These findings are then applied to PVC to differentiate isomers, defects, and intermolecular interactions. These results represent a step toward the detection of chlorine pollutants and the understanding of PVC depolymerization processes.

[1] R. Lin, A. P. Amrute, J. Pérez-Ramírez, Chem. Rev. 2017, 117, 4182-4247. [2] Y. Gu, J. Meng, J. Duo, J. S. Khim, T. Wang, G. Su, Q. Li, B. Shi, B. Sun, Y. Zhang, K. Ouyang, Journal of Hazardous Materials 2024, 480, 136329. [3] R. P. Chapman, C. M. Widdifield, D. L. Bryce, Progress in Nuclear Magnetic Resonance Spectroscopy 2009, 55, 215-237.