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DEPT, COSY, HSQC, HMBC, NOESY, and integrated structure elucidation from combined IR/MS/NMR data
Learn step-by-step with practice exercises built right in.
Move beyond 1D ¹H/¹³C: solve real natural-product structures by integrating 2D experiments.
DEPT-135 — distinguishes CH₃ (up), CH (up), CH₂ (down), and quaternary C (absent) in ¹³C.
COSY (¹H–¹H correlation) — diagonal + cross-peaks identify J-coupled protons (³J most prominent). Walk along a spin system to map a chain.
HSQC (¹H–¹³C, one-bond) — each cross-peak ties a proton to its directly attached carbon. Resolves overlapping ¹H signals via the ¹³C dimension.
HMBC (¹H–¹³C, multi-bond) — 2- and 3-bond correlations across heteroatoms and quaternary carbons; the key tool for connecting fragments through carbonyls and across ring junctions.
NOESY — through-space (~5 Å) interactions; assigns relative stereochemistry and conformation.
Workflow — molecular formula from MS + degree of unsaturation → IR for functional groups → ¹H integration/multiplicity for fragments → COSY/HSQC to wire fragments → HMBC to bridge across heteroatoms → NOESY for stereochem.
A natural product has molecular formula C₉H₁₀O₂ and shows IR ν 1715 cm⁻¹, ¹H NMR signals at δ 7.30 (m, 5H), 4.15 (q, J = 7 Hz, 2H), 3.65 (s, 2H), 1.25 (t, J = 7 Hz, 3H), and a single COSY cross-peak between δ 4.15 and δ 1.25. Deduce the structure.
Degrees of unsaturation = (2·9 + 2 − 10)/2 = 5 → consistent with a benzene ring (4) + one additional π or ring (likely C=O, supported by IR 1715 cm⁻¹ for an ester carbonyl).
Fragment analysis:
Avoid these 3 frequent errors
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mol of reacts with mol of . How many grams of water are produced? Which is the limiting reagent? ()
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Structure: ethyl phenylacetate (PhCH₂CO₂Et) — fits the formula, IR, and all NMR / COSY data.
Outline a 2D-NMR strategy (which experiments and what you look for) to fully assign the structure of a C₁₀ natural product containing two quaternary aromatic carbons, one carbonyl, two methyl groups, and one CH–CH₃ stereocenter. Assume only ¹H, ¹³C, DEPT, COSY, HSQC, HMBC, and NOESY are available.
Strategy:
Together these experiments give a complete structure including stereochemistry without needing X-ray diffraction.