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Yes—but usually not because the sample contained no molecule at all. In a misassigned natural product, the compound may be real while the published structural story is wrong: atoms are connected in the wrong order, a functional group is placed incorrectly, or the stereochemistry is reversed. Modern instruments can measure tiny samples with extraordinary precision, yet scientists still have to interpret those measurements as a molecular model. That interpretation is where a plausible structure can survive longer than it should.
The reliable answer is not one “perfect” instrument. It is a chain of tests that fail in different ways: multidimensional nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HRMS), X-ray crystallography when crystals can be obtained, quantum-chemical prediction of NMR parameters, and independent synthesis or another orthogonal experiment. Agreement across those methods turns a drawing into a defensible structure; disagreement tells researchers to stop chasing a phantom.
What “a molecule that was never there” really means
The phrase is shorthand for a wrong identity, not necessarily an imaginary compound. A natural-product extract can contain a genuine, biologically active substance while the structure assigned to it is incorrect. The isolation, activity assay and analytical signals may all be real. The error is the mapping from those signals to a particular molecular drawing.
Several kinds of mistake recur:
- Constitutional error: the atoms are joined in a different sequence or a substituent is attached at another position.
- Regiochemical error: the right functional groups are present but placed on the wrong ring, chain or heteroatom.
- Stereochemical error: a chiral center, alkene geometry or ring junction has the wrong configuration.
- Conformational or protonation error: a flexible molecule, tautomer or salt state is interpreted as a single fixed form that the sample does not actually adopt.
- Mixture error: signals from closely related compounds, impurities or degradation products are combined into one apparent structure.
Consequences are practical. A wrong structure can derail total synthesis, mislead mechanism-of-action studies, produce false claims of a new (“novel”) scaffold and send later researchers searching for a compound whose assigned form does not exist in nature.
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No reliable field-wide percentage for misassigned natural products has been established in the sources available for this topic. The problem is documented through individual reassignments and methodological reviews, not a validated prevalence statistic.
Why better instruments do not eliminate the problem
Analytical technology has changed the scale of structural work. Older investigations often relied on degradation and derivatization, which could require gram quantities and years of effort. Nicolaou and Snyder’s 2005 review describes the later shift to multidimensional NMR, HRMS and X-ray crystallography, methods that can often work with milligram or even sub-milligram quantities.
More sensitivity and resolution reduce uncertainty in the measurements; they do not choose the molecular model for you. An NMR spectrum is a set of chemical shifts, coupling patterns, relaxation behavior and through-space correlations. A researcher must decide which connectivity and stereochemical arrangement best explains them. Overlapping resonances, conformational averaging, exchange, accidental similarity and incomplete assignments can make two different structures appear comparably plausible.
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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesHRMS is powerful for establishing an elemental composition and detecting isotopic patterns. It usually cannot, by itself, distinguish constitutional isomers that have the same formula, nor can it settle most stereochemical questions. NMR supplies richer connectivity and spatial information, but a misleading peak assignment can propagate through an entire structure. X-ray diffraction can be decisive, yet it depends on obtaining a suitable crystal and on modeling disorder, solvent and absolute configuration correctly.
What the landmark reviews changed
Nicolaou and Scott A. Snyder’s review in Angewandte Chemie International Edition (first published 1 February 2005, volume 44, pages 1012–1044) surveys natural products whose structures were later revised. Its central message is that advanced spectroscopy did not make synthesis obsolete. The authors wrote: “This Review makes the case that imaginative detective work and chemical synthesis still have important roles to play in the process of solving nature’s most intriguing molecular puzzles.”
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The review discusses cases including azaspiracid-1 and diazonamide A, using revised structures and total syntheses to show how a proposed drawing can be tested rather than merely admired for fitting a spectrum. The lesson is methodological: a synthesis that reproduces the natural material’s analytical and physical behavior is an independent check on the original interpretation, especially when several isomers fit the initial data.
A 2024 review reframed the question as “Are we still chasing molecules that were never there?” Its focus is quantum-chemical simulation of NMR parameters for structural reassignment. Instead of asking only whether a candidate structure is chemically plausible, researchers can calculate expected chemical shifts and related observables, then compare those predictions with the experimental spectrum. A recent review of false novelty in antibacterial natural-product discovery makes a related point: dereplication, early verification checkpoints, computational NMR reassessment and synthesis should be connected as one workflow, not treated as optional rescue steps after publication.
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The following sequence is designed to catch the cheapest, most consequential errors early while preserving enough evidence for a later audit.
1. Preserve provenance and sample integrity
Record the organism, collection location and date, extraction solvent, fractionation steps, storage conditions and every purification operation. Keep aliquots of the original fraction and purified material. Recheck purity with chromatography and inspect whether the spectrum changes with concentration, solvent, temperature or time. A structure cannot be more reliable than the sample history behind it.
2. Establish the formula with HRMS
Use accurate mass, isotope patterns and—where useful—tandem MS to constrain elemental composition and likely substructures. Treat the result as a formula constraint, not a complete structure. Isomeric candidates can share an exact mass, and adducts or in-source fragments can be mistaken for the molecular ion. Report ion type, calibration, mass error and sample conditions so another laboratory can reproduce the assignment.
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3. Acquire complementary one- and two-dimensional NMR data
Collect a coherent set of experiments rather than relying on a single attractive correlation. One-dimensional 1H and 13C spectra provide shifts and multiplicities; COSY and TOCSY reveal proton spin systems; HSQC links hydrogen to directly attached carbon; HMBC supplies longer-range connectivity; and NOESY or ROESY can constrain spatial proximity. Measure at more than one field or temperature when overlap, exchange or conformational averaging is suspected.
Assign peaks systematically, including negative evidence. The absence of an expected correlation can be as informative as its presence, provided sensitivity and relaxation limitations are documented. Keep raw data, processing parameters and an assignment table rather than only a polished spectrum.
4. Enumerate realistic candidate structures
Separate what the data require from what is merely plausible biosynthetically. Draw constitutional isomers, stereoisomers, tautomers and likely protonation states that survive the formula and NMR constraints. This prevents “confirmation by familiarity,” in which a known scaffold is selected before alternatives are tested.
5. Use quantum-chemical NMR calculations to rank candidates
For each serious candidate, generate conformers, optimize their geometries and calculate predicted NMR parameters with an appropriate quantum-chemical method. Boltzmann-weight the conformers when the molecule is flexible, then compare calculated and experimental shifts using transparent statistical measures and atom-by-atom inspection.
This step is not a magic oracle. Results depend on conformer sampling, solvent modeling, the level of theory and the quality of the experimental assignment. Its value is comparative: if one candidate consistently predicts the observed data better than close alternatives, confidence rises; if all candidates perform poorly, the structural model or the peak assignments need revision. Save input structures, conformers, methods and scripts so the comparison has a reproducible audit trail.
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6. Seek an X-ray structure when crystals are available
Single-crystal X-ray diffraction can determine connectivity and relative stereochemistry directly and may establish absolute configuration when anomalous-scattering information or a known reference is adequate. Crystal growth is often the limiting factor, and a crystal can represent a particular solvate, polymorph or conformer. Report the crystallization conditions and refinement quality, and confirm that the crystallized material is the same chemical entity as the solution sample.
7. Test the proposal by independent synthesis
Plan a total or partial synthesis of the proposed structure, prioritizing a route that distinguishes the disputed feature. Compare the synthetic product with authentic natural material by co-injection chromatography, NMR, HRMS, optical rotation, circular dichroism or other appropriate measurements. A match across several independent observables is strong evidence; a mismatch is valuable even when the synthesis itself is sound, because it identifies a structural feature that needs reassignment.
Synthesis remains especially important when crystals cannot be grown and NMR differences between stereoisomers are subtle. It also exposes hidden assumptions: a route may reveal that a proposed ring closure, oxidation state or stereochemical relationship is chemically implausible, or that the published structure would have properties inconsistent with the isolated material.
8. Add an orthogonal biological or physical test
Choose a measurement that does not simply repeat the original evidence. Examples include derivatization followed by chiral analysis, enzymatic or receptor binding with a stereochemically defined standard, ion-mobility separation, vibrational spectroscopy, or an isotope-labeling experiment. The goal is not to accumulate redundant spectra but to combine methods with different failure modes.
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Show the original proposal, the alternatives considered, decisive discrepancies, computational settings, raw-data availability and the evidence supporting the final structure. State what remains uncertain—for example, relative configuration established but absolute configuration unresolved. A transparent uncertainty statement is stronger than a falsely absolute drawing.
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How the main verification tools compare
| Approach | Independence from the original assignment | What it can distinguish well | Material and time | Interpretation and audit trail |
|---|---|---|---|---|
| Multidimensional NMR | Partial; it is often the source of the initial model | Connectivity, spin systems and some relative stereochemistry | Milligram to sub-milligram quantities are often possible; acquisition and analysis can be extensive | Expert assignment is essential; raw spectra and peak lists provide a reproducible record |
| HRMS and MS/MS | Moderate for formula and fragments, but not a full structural check | Elemental composition, isotope pattern and selected substructures | Very little material; rapid once the instrument is available | Mass accuracy is reproducible, while fragment interpretation can remain model-dependent |
| Quantum-chemical NMR | High relative to a hand-assigned spectrum, because predictions are generated from candidate models | Close constitutional and stereochemical alternatives when calculations are well converged | Small sample requirement; computation ranges from hours to substantial workflows | Highly auditable if conformers, methods and comparison metrics are archived |
| Single-crystal X-ray diffraction | High, provided the crystal is representative | Connectivity and relative stereochemistry; sometimes absolute configuration | Requires a suitable crystal and often significant crystallization effort | Refinement statistics and deposited coordinates support independent inspection |
| Independent synthesis | Very high: it tests whether a defined structure can reproduce the natural material | Constitution, stereochemistry and disputed functional-group placement | Often the greatest material, skill and time investment | Strong when multiple physical and analytical properties match; route details make the test repeatable |
Where NMR misassignments usually begin
Overlapping and weak signals
Dense proton spectra can hide small couplings or make a cross-peak appear to belong to a neighboring atom. A higher field instrument, selective decoupling, isotope labeling or a different solvent may resolve the ambiguity; simply increasing confidence in the same crowded spectrum does not.
Long-range correlations treated as proofs
HMBC cross-peaks are correlations over a range of bonds, not photographs of a single bond. Their intensities depend on coupling pathways and pulse-sequence settings. A proposed connection should be supported by a network of correlations and by the absence of contradictions elsewhere.
NOE and ROE proximity mistaken for configuration
Through-space signals report that nuclei are close in a particular conformation. Flexible molecules can produce or lose such contacts as conformers exchange. Relative stereochemistry should be tested against conformational analysis and, where possible, calculation or synthesis.
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Impurities, degradation and exchange
A trace contaminant can contribute a biologically active signal or a misleading peak. Repeating the experiment with freshly purified material, monitoring stability and comparing chromatographic fractions can reveal whether one “compound” is actually a mixture or a time-dependent product.
How to avoid false novelty in natural-product discovery
Dereplication compares an extract’s measured features with known compounds early, reducing the chance that a familiar substance receives a new name because its structure was drawn differently. That check should be paired with an explicit novelty claim: a new bioactivity is not automatically a new molecule, and a new molecular formula is not automatically a new scaffold.
For antibacterial programs, an efficient sequence is to dereplicate active fractions, verify the proposed structure before scaling isolation, run computational NMR on close alternatives, and reserve synthesis for the features that remain uncertain. This front-loads decisions that are cheap compared with building an entire medicinal-chemistry campaign around the wrong isomer.
A practical decision tree for researchers
- Do formula and chromatography agree? If not, resolve adducts, purity and sample stability before assigning a structure.
- Are all key NMR signals assigned? If not, acquire the experiment that addresses the missing connection instead of guessing.
- Do two or more candidate structures fit? Calculate NMR parameters for each and compare them quantitatively.
- Is stereochemistry still unresolved? Try X-ray crystallography, derivatization, chiral methods or a stereochemically diagnostic synthesis.
- Does an independent test disagree? Treat the disagreement as evidence for reassignment, not as an inconvenience to be explained away.
- Can another laboratory reproduce the conclusion? Archive spectra, computational inputs, samples where possible and a clear chain of reasoning.
What readers should take away
The modern answer to “Are we still chasing molecules that were never there?” is yes, the risk remains—but it is manageable. Spectrometers have made structural analysis faster and more sensitive, not self-interpreting. The strongest assignments combine complementary measurements, explicit comparison of competing models and at least one check independent of the original NMR interpretation.
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When a structure matters for synthesis, biology or a claim of novelty, the defensible standard is convergence: HRMS constrains composition; multidimensional NMR maps relationships; quantum calculations test candidate models; X-ray diffraction can provide a direct solid-state view; and synthesis or another orthogonal experiment asks whether the proposed molecule behaves like the natural material. That is how researchers stop pursuing a compelling drawing and start identifying the compound that was actually present.
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