Appendix D — Spectroscopy Reference
Comprehensive reference for IR, MS, NMR (¹H, ¹³C, 2D), and UV-Vis. Use alongside Chs 6 (structure determination overview) and 9 (NMR in depth). The final section gives a structure-determination workflow.
1. Infrared (IR) Spectroscopy
Range — 4000-400 cm⁻¹. Diagnostic region — 4000-1500 cm⁻¹ (functional groups). Fingerprint region — 1500-400 cm⁻¹ (overall pattern, usually for comparison rather than direct assignment).
Intensity codes — s = strong, m = medium, w = weak, br = broad, sh = sharp.
O-H, N-H, C-H region (3700-2500 cm⁻¹)
| Bond | Wavenumber | Intensity / shape |
|---|---|---|
| O-H, alcohol/phenol, free (dilute) | 3580-3650 | sharp, m |
| O-H, alcohol/phenol, H-bonded | 3200-3550 | broad, s |
| O-H, COOH (dimeric) | 2500-3300 | very broad, s, often a "shoulder" |
| N-H, 1° amine (two bands) | 3300-3500 | m, two peaks |
| N-H, 2° amine (one band) | 3300-3500 | m, one peak |
| N-H, 3° amine | — | none (no N-H) |
| N-H, amide (1° two bands; 2° one) | 3180-3500 | m |
| N-H, amide (also free N-H sharp) | 3470 (free), 3350 (H-bond) | m |
| =C-H, vinyl sp² | 3000-3100 | m |
| =C-H, aromatic sp² | 3000-3100 | m (often weak) |
| ≡C-H, terminal alkyne | 3260-3330 | s, sharp |
| -C-H, sp³ | 2840-3000 | s |
| -CHO (aldehyde C-H) | 2700-2900 (often two bands ~2720 and 2820) | m |
Triple-bond and cumulated region (2500-2000 cm⁻¹)
| Bond | Wavenumber | Intensity |
|---|---|---|
| C≡C, alkyne | 2100-2260 | w (stronger if terminal) |
| C≡N, nitrile | 2210-2260 | m, sharp |
| -N=C=O (isocyanate) | 2250-2275 | s |
| -N=C=N- (carbodiimide) | 2130 | s |
| -N=N=N (azide) | 2100-2270 | m |
| -N=C=S (isothiocyanate) | 2050-2150 | s |
| -C=C=C (allene) | 1950 (asym) | w |
Double-bond region (2000-1500 cm⁻¹)
| Bond | Wavenumber | Intensity / note |
|---|---|---|
| C=O, acid chloride | 1770-1815 | s |
| C=O, anhydride | two bands: 1750 and 1820 | s, distinctive |
| C=O, ester | 1735-1750 | s |
| C=O, aldehyde | 1720-1740 | s |
| C=O, ketone | 1705-1725 | s |
| C=O, COOH | 1700-1725 | s, with broad OH |
| C=O, amide (1°/2°/3°) | 1630-1690 | s (lowest of the C=O group) |
| C=O, cyclic ketone (ring strain raises ν): cyclobutanone | 1780 | s |
| Cyclopentanone | 1745 | s |
| Cyclohexanone | 1715 | s |
| α,β-unsat C=O (conjugation lowers ν by ~30 cm⁻¹) | 1670-1700 | s |
| Aryl ketone | 1680-1700 | s |
| C=C, alkene | 1620-1680 | w-m |
| C=C, aromatic | 1450-1600 | m (often two bands ~1500 and 1600) |
| C=N, imine | 1640-1690 | m |
| N=O, nitro | 1500-1570 (asym) and 1300-1370 (sym) | s, two bands diagnostic |
| N=O, nitroso | 1500-1600 | m |
| C=S, thione | 1050-1200 | m |
Single-bond / fingerprint region (1500-400 cm⁻¹)
| Bond | Wavenumber | Notes |
|---|---|---|
| C-H bending (CH₃ asym/sym, CH₂ scissor) | 1350-1480 | m, diagnostic for alkyl |
| C-N, amine | 1020-1250 | m |
| C-O, ether/alcohol/ester | 1050-1300 | s, often broad complex |
| C-O, aryl ether | 1200-1275 (Ar-O-R asym) | s |
| C-Cl | 600-800 | s |
| C-Br | 500-600 | s |
| C-I | 500-600 | s |
| Aromatic C-H out-of-plane bend (substitution pattern) | 690-900 | s — useful for mono/di/tri substitution |
Aromatic substitution pattern (out-of-plane C-H bending)
| Pattern | Bands (cm⁻¹) |
|---|---|
| Monosubstituted | 690-710 and 730-770 |
| 1,2-disubstituted (ortho) | 735-770 |
| 1,3-disubstituted (meta) | 690-710, 750-810, 880-900 |
| 1,4-disubstituted (para) | 800-840 |
Common IR artifacts
| Artifact | Cause |
|---|---|
| Sharp band ~2350 | atmospheric CO₂ (background) |
| Broad ~3400 | H₂O absorbed on KBr plate or in solvent |
| Sharp 1080 | Si-O (silicone grease) |
| Broad 1620-1680 + 3300 | residual H₂O |
| Doubled C=O | Fermi resonance (aldehyde C-H/C=O); rotamers |
2. Mass Spectrometry (MS)
M⁺• = molecular ion (radical cation). Base peak = tallest peak in spectrum. m/z = mass-to-charge.
Ionization techniques — when to use
| Technique | Sample type | Energy | Output |
|---|---|---|---|
| EI (electron ionization, 70 eV) | small, volatile, neutral | hard | extensive fragmentation; M⁺• may be weak |
| CI (chemical ionization) | small, neutral | soft | [M+H]⁺, less fragmentation |
| ESI (electrospray) | polar, large, soluble | very soft | [M+H]⁺, [M+Na]⁺, multiply charged for proteins |
| MALDI | large biomolecules, polymers | soft | [M+H]⁺, mostly singly charged |
| APCI | medium polarity small molecules | soft | [M+H]⁺ |
Exact masses (monoisotopic, for high-res MS)
| Element | Exact mass |
|---|---|
| ¹H | 1.00783 |
| ¹²C | 12.00000 (definition) |
| ¹⁴N | 14.00307 |
| ¹⁶O | 15.99491 |
| ³²S | 31.97207 |
| ³⁵Cl | 34.96885 |
| ⁷⁹Br | 78.91834 |
| ¹²⁷I | 126.90447 |
| ³¹P | 30.97376 |
| ¹⁹F | 18.99840 |
Use — high-resolution mass spectrometry distinguishes isobars (e.g., C₂H₄ at 28.0313 vs N₂ at 28.0061 vs CO at 27.9949). Match to ~5 ppm.
Common neutral losses
| Loss (Δm/z) | Neutral | Source / mechanism |
|---|---|---|
| 1 | H | alpha-H loss |
| 15 | CH₃ | α-cleavage of methyl |
| 17 | OH | alcohols, carboxylic acids |
| 18 | H₂O | alcohols (dehydration), aldehydes |
| 19 | F | aryl fluoride loss (rare) |
| 26 | C₂H₂ | aromatic loss |
| 27 | HCN | nitriles, amines, N-heterocycles |
| 28 | CO or C₂H₄ or N₂ | many sources — context required |
| 29 | CHO or C₂H₅ | aldehyde α-cleavage; ethyl loss |
| 30 | CH₂O or NO | aldehyde + H rearr; nitroso/nitro |
| 31 | OCH₃ | methyl ester |
| 32 | CH₃OH or S | methyl ester / dehydration |
| 33 | SH or CH₃ + H₂O | thiols |
| 35 | Cl | aryl chloride |
| 36 | HCl | dehydrohalogenation |
| 41 | C₃H₅ (allyl) | allylic systems |
| 42 | C₃H₆ or CH₂=C=O (ketene) | propyl loss; McLafferty acyl |
| 43 | CH₃CO (acetyl) or C₃H₇ | α-cleavage of methyl ketone; propyl |
| 44 | CO₂ or C₂H₄O | decarboxylation; acetaldehyde elim |
| 45 | COOH or OC₂H₅ | acid; ethyl ester |
| 46 | NO₂ | nitro arene |
| 57 | C₄H₉ (t-Bu, n-Bu) | branched alkyl |
| 60 | CH₃COOH | acetate ester rearr |
| 73 | TMS (Si(CH₃)₃) | silyl ethers |
| 77 | C₆H₅ (phenyl) | aryl loss |
| 91 | C₇H₇⁺ (tropylium) | benzyl — extremely stable cation |
| 105 | C₆H₅CO (benzoyl) | aryl ketone α-cleavage |
Isotope patterns
| Element | Isotopes (% natural) | M+2 pattern |
|---|---|---|
| C | ¹²C (98.9), ¹³C (1.1) | small M+1 (~1.1% per C); no M+2 |
| H | ¹H (99.99), ²H (0.01) | negligible |
| N | ¹⁴N (99.6), ¹⁵N (0.4) | negligible |
| O | ¹⁶O (99.76), ¹⁸O (0.20) | M+2 ~0.2% per O |
| F | ¹⁹F (100) | none |
| Si | ²⁸Si (92), ²⁹Si (5), ³⁰Si (3) | clear M+1, M+2 |
| S | ³²S (95), ³⁴S (4.25) | M+2 ~4% per S |
| Cl | ³⁵Cl (75.8), ³⁷Cl (24.2) | M:M+2 = 3:1 per Cl |
| Br | ⁷⁹Br (50.7), ⁸¹Br (49.3) | M:M+2 = 1:1 per Br |
| I | ¹²⁷I (100) | none, but heavy mass |
Patterns to recognize instantly:
| Pattern | Interpretation |
|---|---|
| M:M+2 = 3:1, ~equal | one Cl |
| M:M+2 = 1:1, ~equal | one Br |
| M:M+2:M+4 = 9:6:1 | two Cl |
| M:M+2:M+4 = 1:2:1 | two Br |
| M:M+2:M+4 = 3:4:1 | one Cl + one Br |
| Strong M+1 (~1% per C) | counts carbons (RDB calc) |
Nitrogen rule
For neutral compounds with C, H, N, O, S, halogens — odd-mass M⁺• ⇒ odd number of N atoms (1, 3, 5...). Even-mass M⁺• ⇒ 0 or even N. Catches most assignment mistakes.
Degree of unsaturation (DBE)
DBE = (2C + 2 + N − H − X) / 2
| DBE | Common interpretation |
|---|---|
| 0 | fully saturated, acyclic |
| 1 | one ring OR one π bond |
| 4 | aromatic ring (3 π + 1 ring) |
| 5 | aromatic ring + C=O OR substituted phenyl |
| 7 | naphthalene |
Important fragmentation rules
| Rule | Description |
|---|---|
| α-cleavage | Break C-C next to heteroatom or carbonyl; charge stays on heteroatom-stabilized fragment |
| Loss of largest substituent | At branched carbons, largest R leaves preferentially as radical |
| McLafferty | Carbonyl with γ-H undergoes [1,5]-H shift through 6-membered TS, expelling alkene; charge on enol fragment (mass 58 for methyl ketone, 60 for acid, 74 for methyl ester etc.) |
| Retro-Diels-Alder | Cyclohexenes expel a stable diene + dienophile (often after EI ionization) |
| Tropylium | Benzyl cations rearrange to tropylium (C₇H₇⁺, m/z 91) — exceptionally stable |
| Acylium | RC(=O)⁺ (m/z = R + 28); CH₃CO⁺ at 43, PhCO⁺ at 105 |
3. ¹H NMR
Cross-reference Ch 9 for theory.
Chemical shift table (δ, ppm — referenced to TMS = 0)
Alkyl region (0-3 ppm)
| Proton environment | δ (ppm) | Notes |
|---|---|---|
| TMS reference | 0.0 | by definition |
| Cyclopropane CH | 0.0-0.5 | unusual upfield |
| -CH₃ (alkane) | 0.8-1.0 | |
| -CH₂- (alkane) | 1.2-1.5 | |
| -CH< (alkane) | 1.4-1.8 | |
| -CH₂- (cyclohexane axial vs eq) | 1.0-1.7 | varies |
| -CH₃ on sp² C (allylic, vinyl methyl) | 1.6-2.0 | |
| -CH₃ on sp C (propargyl-like) | 1.7-2.0 | |
| -C≡C-H | 2.0-3.0 | sharp |
| -CH₂- α to C=O (ketone, aldehyde) | 2.0-2.5 | |
| -CH₃ α to C=O | 2.0-2.4 | "methyl ketone" CH₃CO |
| -CH₂- α to COOR (ester) | 2.0-2.4 | |
| -CH₂- α to COOH | 2.0-2.5 | |
| -CH₂- α to C≡N (nitrile) | 2.2-2.5 | |
| -CH₂- α to NO₂ | 4.2-4.6 | (way downfield) |
| -CH₂- α to Ar | 2.3-2.8 | benzylic |
| -CH₂- α to amine N (sp³) | 2.2-2.9 |
Heteroatom-α region (3-5 ppm)
| Proton environment | δ (ppm) |
|---|---|
| -CH₃-O- (methyl ether) | 3.2-3.4 |
| -OCH₂- (any) | 3.3-4.0 |
| -CH-O- | 3.5-4.5 |
| -CH₂-Cl | 3.4-3.8 |
| -CH₂-Br | 3.3-3.6 |
| -CH₂-I | 3.0-3.3 |
| -CH₂-F | 4.3-4.5 |
| -CH-OH | 3.4-4.0 |
| -OCH₂-O- (acetal CH₂) | 4.5-5.5 |
| -O-CH(-)-O- (acetal CH) | 5.0-5.8 |
| -O-CHO (formate) | 8.0-8.1 |
Alkene / aromatic region (4.5-9 ppm)
| Proton environment | δ (ppm) |
|---|---|
| Vinyl =CH₂ | 4.6-5.0 |
| Vinyl =CHR | 5.0-5.5 |
| Vinyl =CHR (conjugated) | 5.5-6.5 |
| Vinyl α to EWG | 5.5-7.5 |
| Aromatic Ar-H (benzene) | 7.27 |
| Aromatic Ar-H, EDG ring | 6.5-7.2 |
| Aromatic Ar-H, EWG ring | 7.5-8.4 |
| Heteroaromatic (pyridine 2,6-H) | 8.5-8.7 |
| Heteroaromatic (furan, thiophene) | 6.0-7.5 |
Aldehyde, COOH, exchangeable region (9-15 ppm)
| Proton environment | δ (ppm) | Notes |
|---|---|---|
| Aldehyde -CHO | 9.5-10.0 | sharp singlet often |
| Aryl aldehyde -CHO | 9.7-10.5 | |
| Carboxylic acid -COOH | 10.5-13 | very broad |
| Phenol -OH | 4-12 | variable, broad |
| Alcohol -OH | 0.5-5 | variable, broad, exchangeable |
| Amine -NH | 0.5-5 | variable, exchangeable |
| Amide -NH | 5-9 | broad |
| Enol -OH (β-dicarbonyl) | 15-17 | very downfield, H-bonded |
Coupling constants (J, Hz)
| Coupling | J (Hz) | Notes |
|---|---|---|
| Vicinal sp³-sp³, free rotation | 6-8 | classic "n+1" |
| Vicinal sp³-sp³, fixed dihedral 60° (gauche) | 2-4 | |
| Vicinal sp³-sp³, fixed dihedral 180° (anti) | 8-13 | |
| Vicinal sp³-sp³, axial-axial (cyclohexane) | 8-12 | |
| Vicinal sp³-sp³, ax-eq or eq-eq | 2-4 | |
| Geminal sp³ (H-C-H) | 10-18 (negative, but reported positive) | |
| Geminal sp² (H₂C=) | 0-3 | small |
| Vicinal sp²-sp² cis (Z) | 6-12 | |
| Vicinal sp²-sp² trans (E) | 12-18 | |
| Vinyl cross-coupling (H-C=C-CH-) | 4-10 | |
| Allylic (H-C-C=C-H, 4-bond) | 0-3 | |
| Homoallylic (5-bond, through 2 sp²) | 0-1 | barely visible |
| Aromatic ortho (3-bond) | 6-10 | |
| Aromatic meta (4-bond) | 1-3 | |
| Aromatic para (5-bond) | 0-1 | |
| α,β to carbonyl (enone Hα-Hβ) | 10-17 | E-enone trans |
| Heteroaromatic (pyridine 2-3) | 4-6 | |
| H-F (vicinal) | 5-30 | very large, depends |
| H-P (P=O esters) | 5-20 |
Splitting interpretation
| Pattern | Coupling partners | Example |
|---|---|---|
| singlet (s) | 0 neighbors | -OCH₃, -CHO (sometimes), isolated -CH₂- |
| doublet (d) | 1 neighbor | -CH(-)-CH₃ if asymmetric |
| triplet (t) | 2 equivalent neighbors | -CH₂-CH₃ ethyl |
| quartet (q) | 3 equivalent neighbors | -CH₂-CH₃ for the CH₂ |
| heptet | 6 equivalent neighbors | (CH₃)₂CH- isopropyl CH |
| doublet of doublets (dd) | 2 non-equivalent neighbors | typical for vinyl H in trisubstituted alkene |
| broad singlet (br s) | exchangeable, or 14N-coupled | -OH, -NH, -COOH |
Integration
Integrals are ratios. Set the smallest integral to a small integer (often 1, 2, 3) and scale others. A 3-H peak at 2.1 ppm singlet ⇒ CH₃-CO- (methyl ketone). A 9-H singlet at 1.4 ppm ⇒ t-Bu.
Solvent residual peaks (¹H δ, ppm)
| Solvent | Residual ¹H | Water peak |
|---|---|---|
| CDCl₃ | 7.26 | 1.56 |
| DMSO-d₆ | 2.50 | 3.33 |
| CD₃OD | 3.31 | 4.87 |
| D₂O | (none) | 4.79 |
| Acetone-d₆ | 2.05 | 2.84 |
| Benzene-d₆ | 7.16 | 0.40 |
| CD₃CN | 1.94 | 2.13 |
| Pyridine-d₅ | 8.74, 7.58, 7.22 | 4.97 |
| Toluene-d₈ | 7.09, 7.00, 6.98, 2.09 | 0.43 |
| THF-d₈ | 3.58, 1.72 | 2.46 |
| DMF-d₇ | 8.03, 2.92, 2.75 | 3.50 |
Pick deuterated solvent based on solubility of analyte and clarity in region of interest. CDCl₃ is the default; switch to DMSO-d₆ for OH/NH peaks (slow exchange) or polar substrates.
4. ¹³C NMR
Chemical shift table (δ, ppm)
| Carbon environment | δ (ppm) |
|---|---|
| TMS reference | 0 |
| -CH₃, -CH₂-, -CH-, -C- (alkane sp³) | 5-50 |
| -C(CH₃)₃ (quaternary sp³, t-Bu central C) | 28-35 |
| -CH₂-X (X = Cl, Br, I) | 25-50 |
| -CH₂-N (amine α-C) | 30-55 |
| -CH₂-O (alcohol, ether α-C) | 50-90 |
| -O-CH₃ (methoxy) | 50-60 |
| Acetal -O-C(-)-O- | 90-110 |
| -C≡C- (alkyne sp) | 60-90 |
| -C=C- (alkene sp²) | 100-150 |
| Aromatic C (ipso, ortho, meta, para) | 110-170 |
| C-O (aromatic, attached to OMe/OH) | 145-165 |
| -C≡N (nitrile C) | 115-125 |
| -C=N- (imine) | 150-170 |
| -COOR (ester C=O) | 165-175 |
| -COOH (acid C=O) | 175-185 |
| -CONR₂ (amide C=O) | 165-180 |
| -COCl (acid chloride C=O) | 165-175 |
| -CONHR (amide) | 165-180 |
| -CHO (aldehyde C=O) | 190-205 |
| -CO- (ketone C=O) | 195-220 |
| Cyclopentanone C=O | 220 |
| α,β-unsat ketone C=O (conjugation lowers) | 195-205 |
DEPT interpretation
DEPT-135 phases CH and CH₃ up, CH₂ down, and quaternary C invisible. DEPT-90 shows only CH.
| DEPT-135 phase | CHₙ |
|---|---|
| Up | CH, CH₃ |
| Down | CH₂ |
| Absent | C (quaternary), C=O carbon, ipso aromatic |
| DEPT-90 phase | CHₙ |
|---|---|
| Up | CH only |
| Absent | CH₂, CH₃, C |
Combine with ¹³C to assign every peak as 0, 1, 2, or 3 attached H's.
Solvent residual peaks (¹³C δ, ppm — septet for CDCl₃)
| Solvent | Peak (center) |
|---|---|
| CDCl₃ | 77.16 |
| DMSO-d₆ | 39.52 |
| CD₃OD | 49.00 |
| D₂O | (none — reference externally) |
| Acetone-d₆ | 29.84, 206.26 |
| Benzene-d₆ | 128.06 |
| CD₃CN | 1.32, 118.26 |
| Pyridine-d₅ | 150.35, 135.91, 123.87 |
| THF-d₈ | 67.21, 25.31 |
Other ¹³C techniques
| Experiment | Use |
|---|---|
| ¹H-decoupled ¹³C | Default — singlets only |
| Gated decoupling | Quantitative ¹³C (preserves NOE) |
| Inverse-gated decoupling | Quantitative ¹³C (suppresses NOE) — for integration |
| APT (Attached Proton Test) | Like DEPT — CH/CH₃ vs CH₂/C |
5. 2D NMR cheat sheet
| Experiment | Correlation | What it shows | When to use |
|---|---|---|---|
| ¹H-¹H COSY | through-bond (²J, ³J, ⁴J) | ¹H neighbors via J-coupling | mapping spin systems |
| TOCSY | full spin system | all coupled H's via mixing | overlapping multiplets, sugars, peptides |
| HSQC | ¹H-¹³C, 1-bond (¹J) | which H is on which C | assigning CHₙ |
| HMQC | ¹H-¹³C, 1-bond (older, similar to HSQC) | same as HSQC | legacy use |
| HMBC | ¹H-¹³C, 2-3 bond (²J, ³J) | long-range H-C correlations | finding quaternary C's, connectivity across heteroatoms |
| NOESY | through-space (< ~5 Å) | spatial proximity | stereochemistry, conformation |
| ROESY | through-space (small molecules) | spatial proximity (for medium MW) | when NOE is zero |
| INADEQUATE | ¹³C-¹³C 1-bond | direct C-C connectivity | rare, requires concentration |
Strategy — get HSQC first (assign CHₙ types); HMBC for piecing chains together across O/N; COSY for H-spin chains; NOESY for relative stereochem.
6. UV-Vis
Range — 200-800 nm. Beer's Law — A = ε c l, ε in M⁻¹cm⁻¹.
Common chromophores
| Chromophore | λmax (nm) | ε (M⁻¹cm⁻¹) | Transition |
|---|---|---|---|
| Isolated C=C | ~170 | 10⁴ | π→π* (below typical scan) |
| Isolated C=O | ~280 | 10-30 | n→π* (weak) |
| C=C conjugated to C=O (enone) | 220, 320 | 10⁴, 30 | π→π, n→π |
| Benzene | 254 | 200 | π→π* (B band) |
| Naphthalene | 312 | 200 | |
| Anthracene | 380 | 8000 | |
| Tetracene | 480 | "violet" — visible color start | |
| -N=N- (azo) | 350 | 10-50 | n→π* |
| -NO₂ | 270 | 10 |
Woodward-Fieser rules (dienes)
Base values (per parent chromophore):
| Parent | Base λ (nm) |
|---|---|
| Acyclic conjugated diene | 217 |
| Heteroannular diene | 215 |
| Homoannular diene | 253 |
Add increments:
| Substituent | Δλ (nm) |
|---|---|
| Each additional conjugated C=C | +30 |
| Each alkyl group / ring residue | +5 |
| Each -OR | +6 |
| Each -SR | +30 |
| Each -NR₂ | +60 |
| Each Cl, Br | +5 |
| Each exocyclic C=C | +5 |
Woodward-Fieser (enones)
| Parent (α,β-unsat carbonyl) | Base λ (nm) |
|---|---|
| Acyclic / 6-ring enone | 215 |
| 5-ring enone | 202 |
| Acyclic α,β-unsat aldehyde | 207 |
| α,β-unsat acid/ester | 195 |
Add:
| Substituent | Δλ (nm) |
|---|---|
| Each extra conjugated C=C | +30 |
| α-substituent (alkyl) | +10 |
| β-substituent (alkyl) | +12 |
| γ, δ, etc. (alkyl) | +18 |
| α-OH | +35 |
| β-OH | +30 |
| Each Cl on α | +15 |
| Solvent (EtOH ref); +H₂O | −8; +MeOH |
7. Combined structure determination workflow
When given a spectrum set for an unknown:
- Mass spec — molecular formula. - From M⁺• (or [M+H]⁺), find molecular weight. - Use HRMS exact mass and isotope pattern to nail down formula. Check N rule. - Calculate DBE = (2C + 2 + N − H − X)/2.
- DBE interpretation. - DBE = 0: saturated, acyclic. - DBE = 1: one ring or one C=C/C=O. - DBE ≥ 4: probably aromatic ring (4 DBE = benzene).
- IR — functional groups. - Look for C=O first (1650-1800) — very diagnostic. Position narrows it (acid chloride vs amide). - Then O-H, N-H (broad 3200-3500). - Then C≡C/C≡N (2100-2260). - Then aromatic markers (1450-1600, ~3000+, fingerprint sub. pattern).
- ¹H NMR — fragments and counts. - Note the integral ratios → relative H counts; scale to match molecular formula. - Identify each multiplet's chemical shift, integration, multiplicity, J-values. - Map shifts onto fragment guesses (e.g., 0.9 ppm 3H triplet ⇒ CH₃ end of an ethyl-or-longer chain; 3.7 ppm 3H singlet ⇒ -OCH₃ on ester or ether; 9.7 ppm 1H singlet ⇒ aldehyde -CHO). - Add up the H's accounted for; what's missing?
- ¹³C + DEPT — count distinct carbons by type. - Count peaks → distinct C environments. - Use DEPT to label CH₃, CH₂, CH, and quaternary. - Watch for symmetry: if formula says 8 C's but you see 4 peaks, the molecule has symmetry.
- Sketch fragments. Combine evidence into building blocks. Sum the formulas of the fragments; verify they add to the molecular formula (account for atoms used in linkages).
- Connect fragments. Use coupling (J-values) and 2D NMR (HSQC, HMBC, COSY) if needed.
- Stereochemistry. Use vicinal J-values (alkene cis/trans; cyclohexane ax/eq), then NOESY for proximity.
- Sanity check. Does the structure account for every signal and every atom? Does it match all the spectra and the DBE?
Worked-style heuristics:
- Triplet 3H near 0.9 + quartet 2H near 2.4 + aromatic ring multiplet ⇒ propiophenone (Ph-CO-CH₂CH₃).
- 9H singlet ~1.4 ⇒ t-Bu.
- 2H singlet near 5.1 + 5H aromatic ⇒ -OCH₂Ph (benzyl ether).
- 6H doublet ~1.2 + 1H septet ~4 ⇒ isopropyl on heteroatom.
- 2H doublet ~7.5 + 2H doublet ~6.8 ⇒ para-disubstituted aromatic with EDG/EWG.
Combine the IR/MS/NMR/UV tables. The spectra are not mysteries — they are puzzles with all the rules in plain sight. Use Chs 6 and 9 for theory and worked examples. Bring this appendix to every problem set.