Chlorinated Byproducts in Diisocyanates Synthesis from 35Cl Nuclear Quadrupole Resonance

<jats:p>Chlorinated impurities in industrial diisocyanates production significantly impact product quality and stability, yet their speciation remains an analytical challenge. This study demonstrates that 35Cl NQR spectroscopy, supported by DFT calculations, provides a powerful framework for identifying byproducts in these processes. We show that NQR frequencies effectively categorize chlorinated species—such as amino hydrochlorides and carbamoyl chlorides—into distinct chemical families, with signal shifts driven by local electronic structure, intermolecular interactions, and conformation. Applied to commercial 4,4'-MDI, this analytic ruler successfully identifies the presence of specific byproducts and establishes 35Cl NQR as a uniquely sensitive tool for monitoring byproducts and thereby the quality of industrial diisocyanate streams.</jats:p>