Interpret NMR Spectrum

Analyze 1D + 2D NMR → assign peaks, determine coupling, propose structural fragments consistent w/ all data.

Use When

• Structure of unknown organic compound from NMR
• Confirm identity + purity of synthesized product
• Assign peaks in complex spectra w/ overlap
• Correlate multi-exp (1H, 13C, DEPT, COSY, HSQC, HMBC) → unified picture
• Distinguish regioisomers / stereoisomers / conformational

In

• Req: NMR data (min 1H w/ shifts, multiplicities, integration)
• Req: Mol formula / MW (from MS, EA)
• Opt: 13C + DEPT (shifts + multiplicities)
• Opt: 2D (COSY, HSQC, HMBC, NOESY/ROESY correlation tables)
• Opt: Solvent + field strength
• Opt: Known constraints (rxn starting material, IR confirmed groups)

Do

Step 1: Spectrum Type + Acquisition

Establish what data + quality before interpret:

1. ID exp types: Catalog which avail (1H, 13C, DEPT-135, DEPT-90, COSY, HSQC, HMBC, NOESY, ROESY, TOCSY). Note nucleus + dimensionality.
2. Acquisition params: Spectrometer freq (400 MHz, 600 MHz), solvent, temp, ref standard.
3. Solvent + ref peaks: Locate + exclude.

4. Quality: Baseline flatness, multiplet res, S/N. Flag artifacts (spinning sidebands, 13C satellites, solvent impurity H2O ~1.56 ppm CDCl3).

→ Inventory of exps, solvent/ref peaks excluded, quality assessed.

If err: Poor S/N / severe baseline distortion → note limitation + cautious. Flag peaks indistinguishable from noise.

Step 2: 1H Chemical Shifts

Assign each 1H → environment using shift ranges:

1. Tabulate: Per peak → shift (ppm), multiplicity, J (Hz), rel int.
2. Classify by shift:

3. Count H: Integration ratios rel to formula → # protons per signal. Normalize simplest whole-# ratio.
4. Exchangeable protons: Signals disappear on D2O shake (OH, NH, COOH) = exchangeable. Record presence + shift.

→ Table of 1H signals w/ shift, multiplicity, J, integration (# H), prelim env assignment.

If err: Integration doesn't sum to expected → check overlapping, broad peaks hidden in baseline, wrong formula.

Step 3: Coupling Patterns + J-Values

Extract connectivity from splitting:

1. Multiplicities: s, d, t, q, dd, etc. Complex m → estimate # coupling partners.
2. Measure J: Extract Hz. Match reciprocal (if H_A ↔ H_B J = 7.2, H_B shows same J to H_A).
3. Classify J:

4. Map coupling networks: Group mutually coupled protons → spin systems. Each = connected frag.
5. Roof effect: AB-type → inner lines of doublets more intense → chemical shift proximity.

→ All J measured + matched reciprocally, spin systems ID'd, coupling types classified.

If err: Multiplets too complex for first-order → note higher-order pattern. Overlapping / strongly coupled (δν/J < 10) → non-first-order requires simulation.

Step 4: 13C + DEPT

Determine C types + count:

1. Count distinct 13C signals: Compare # peaks vs formula. Fewer → symmetry.
2. Classify by shift:

3. DEPT editing: DEPT-135 (CH + CH3 up, CH2 down, quaternary absent) + DEPT-90 (CH only) → # attached H per C.
4. DBE: DBE = (2C + 2 + N - H - X) / 2. Compare # π bonds + rings implied.

→ Every 13C signal classified by type (CH3, CH2, CH, C) + env, DBE consistent w/ observed groups.

If err: No DEPT → infer H attachment from HSQC (Step 5). C count ≠ formula → coincident signals / quaternary Cs in noise.

Step 5: 2D NMR

Build connectivity using 2D exps:

1. COSY (1H-1H): Which H 2-3 bonds apart. Map cross-peaks → confirm+extend spin systems Step 3.
2. HSQC (1H-13C 1-bond): Assign each H → directly bonded C. Links 1H + 13C unambiguously.
3. HMBC (1H-13C long-range): 2-3 bond H-C. Critical for connecting frags across quaternary C, heteroatoms, carbonyls w/o direct H-C.
4. NOESY/ROESY (through-space): H's spatially close (<5 Å) regardless bonding. → Stereochem + conformational.
5. Build frag connectivity: HMBC → connect COSY spin systems → larger frags. Each HMBC cross-peak = 2-3 bond H-C path.

→ Connectivity map linking spin systems into coherent framework + stereochem from NOE where avail.

If err: 2D incomplete / ambiguous → note tentative connections. Multiple proposals poss. Prioritize HMBC → bridges gaps COSY can't.

Step 6: Propose + Validate Structure

Assemble frags → complete proposal:

1. Assemble: Connect frags Steps 2-5 using HMBC + DBE constraints.
2. Check formula: Proposed matches formula exactly (atom count, DBE).
3. Back-predict shifts: For proposed → predict 1H + 13C shifts. Compare observed; deviations > 0.3 ppm (1H) / > 5 ppm (13C) → re-examine.
4. Verify all correlations: Every COSY, HSQC, HMBC explained. Unexplained → error / impurity.
5. Alternatives: Multiple structures fit → list distinguishing exps / correlations.
6. Stereochem: NOE + J analysis (Karplus for dihedral) + known conformational prefs → relative + (where poss) absolute.

→ Single best-fit proposal w/ all NMR accounted, or ranked candidates + plan to distinguish.

If err: No single structure → check: mixture (extra peaks non-integer int), dynamic processes (broad peaks from conformational exchange), paramagnetic impurities (anomalous broadening). Re-examine formula if multiple equally viable.

Check

• Solvent + ref peaks ID'd + excluded
• Every 1H signal → shift region, multiplicity, J, integration
• J reciprocal (matched between partners)
• 13C classified by DEPT multiplicity + shift
• DBE calc + consistent w/ proposed
• 2D (COSY, HSQC, HMBC) all explained
• Proposed matches formula exactly
• Back-predicted shifts agree w/ observed within tolerance
• Stereochem via NOE / J where applicable

Traps

• Ignore solvent peaks: Common solvents → signals overlap analyte. Always ID + exclude residuals, water, grease.
• Force 1st-order on 2nd-order: Strongly coupled nuclei (small Δshift rel J) → distorted multiplets, can't interpret w/ simple n+1. Roof effects + non-binomial intensity → indicators.
• Overlook exchangeable: OH + NH may be broad, shift w/ conc/temp, absent in protic solvents. D2O shake → clarifies.
• Assume all 13C visible: Quaternary Cs → long relax times + low int. May be absent short-acquisition. HMBC often only way to detect.
• Misinterpret HMBC artifacts: HMBC → 1-bond artifacts (mis-assigned long-range) + weak 4-bond. Cross-check w/ HSQC → filter 1-bond leakthrough.
• Neglect symmetry: Fewer 13C peaks than formula → symmetry element. Account before proposing.

→

• interpret-ir-spectrum — func groups → constrain NMR structure
• interpret-mass-spectrum — formula + frag for cross-val
• interpret-uv-vis-spectrum — chromophores + conjugation extent
• interpret-raman-spectrum — complementary vibrational → symmetric modes
• plan-spectroscopic-analysis — select + sequence techniques pre-acquisition