Applications and Advances of Liquid-State 13C–13C 2D INADEQUATE NMR in Structural Elucidation: From Small Molecules to Polymers

Liquid-state 13C–13C 2D INADEQUATE NMR spectroscopy provides unparalleled direct carbon–carbon connectivity mapping via scalar couplings, delivering unambiguous structural evidence that conventional HSQC and HMBC methods cannot, particularly for quaternary carbons, fully substituted aromatics, and overlapping polymer resonances. Despite sensitivity challenges arising from low 13C natural abundance (~1.1%), this review examines the technique’s evolution from its 1981 introduction to contemporary innovations, compiling over 100 application entries across natural products, synthetic molecules, mixtures, fullerenes, metabolomics, oligomers and polymers. Significant methodological advancements—including cryogenic probe technology (up to 5.5-fold sensitivity gain), J-compensated sequences, composite refocusing (INADEQUATE CR), hybrid INEPT-INADEQUATE approaches, adiabatic pulses, and Overhauser DNP-INADEQUATE—are critically assessed alongside computerized analysis algorithms (CCBond, FRED) and network-based metabolomics tools (INETA, PyINETA). Practical guidance covering optimal JCC coupling selection, relaxation management, and a decision tree for experiment selection is consolidated. Looking forward, the convergence of artificial intelligence, non-uniform sampling, and integration with density functional theory promises to transform 2D INADEQUATE from a specialist technique of last resort into a routinely accessible tool for structural elucidation.