Appendix D: Spectroscopy Reference
Chapters 18–20 deliberately teach spectroscopy as a process of reasoning — narrowing down structural possibilities by combining evidence — rather than a set of numbers to memorize. This appendix is the numeric companion to that process: representative IR absorptions, NMR chemical shifts, splitting patterns, and mass spectrometry fragmentation patterns, organized for lookup once a spectrum is at hand.
How to use this appendix: treat every value as a typical range, not an exact cutoff — real spectra vary with the rest of the molecule. The Chapter 18 Common Mistake applies here too: the goal is to recognize a handful of diagnostic signals, not to memorize the entire table.
Infrared (IR) Absorption Table
| Functional Group | Bond | Wavenumber Range (cm⁻¹) | Intensity / Shape |
|---|---|---|---|
| Alcohol, phenol | O–H | 3200–3550 | Strong, broad |
| Carboxylic acid | O–H | 2500–3300 | Strong, very broad (often overlaps with C–H stretches) |
| Primary amine | N–H | 3300–3500 | Medium; two bands (asymmetric + symmetric stretch) |
| Secondary amine | N–H | 3300–3500 | Medium; one band |
| Alkane | C–H (sp³) | 2850–2960 | Medium |
| Alkene, arene | C–H (sp²) | 3000–3100 | Medium |
| Alkyne | C–H (sp) | ~3300 | Sharp, medium–strong |
| Aldehyde | C–H (of C(=O)H) | 2720 and 2820 | Two weak bands, diagnostic alongside the C=O stretch |
| Alkyne | C≡C | 2100–2260 | Weak (can be absent if the alkyne is symmetric) |
| Ether | C–O | 1050–1150 | Strong |
| Alkene, arene | C=C | 1450–1680 | Medium; aromatic rings often show several bands in this range |
Carbonyl (C=O) Stretch by Compound Class
The carbonyl stretch is one of the most diagnostic IR signals, and its exact position tracks the same reactivity/resonance trend developed in Chapters 12–13 and Appendix C: the more a heteroatom lone pair donates into the carbonyl (lowering its double-bond character), the lower the stretching frequency.
| Compound Class | C=O Range (cm⁻¹) | Note |
|---|---|---|
| Acid chloride | 1790–1815 | Highest frequency — no resonance donation into the carbonyl, most electrophilic (matches its position at the top of the reactivity order in Appendix C) |
| Anhydride | 1750 and 1820 | Two bands (asymmetric + symmetric stretch of the two carbonyls) |
| Ester | 1735–1750 | — |
| Aldehyde | 1720–1740 | Paired with the diagnostic 2720/2820 C–H doublet above |
| Ketone | 1705–1725 | — |
| Carboxylic acid | 1710–1760 | Broadened/shifted by hydrogen bonding (acids often exist as dimers) |
| Amide | 1630–1695 | Lowest frequency — nitrogen’s lone pair donates most effectively into the carbonyl, reducing its double-bond character (same resonance argument as amide’s low reactivity and low nitrogen basicity, Appendix A) |
¹H NMR Chemical Shift Table
All values in δ (ppm), referenced to tetramethylsilane (TMS, δ = 0).
| Proton Environment | Typical δ Range | Note |
|---|---|---|
| Alkyl C–H, not adjacent to a functional group | 0.9–1.5 | Baseline aliphatic region |
| C–H adjacent to C=O, C=C, or aromatic ring (allylic/benzylic) | 2.0–2.5 | Slightly deshielded by the adjacent π system |
| C–H adjacent to a halogen | 3.0–4.0 | Deshielded by the electronegative halogen |
| O–CH₂/O–CH₃ (ether or ester alkyl group) | 3.3–4.5 | Deshielded by the adjacent oxygen |
| Alcohol O–H | 1–5 (variable) | Broad; position shifts with concentration and hydrogen bonding; exchangeable |
| Vinyl C–H (alkene) | 4.5–6.5 | — |
| Aromatic C–H | 6.5–8.5 | Deshielded by the aromatic ring current |
| Amine N–H | 1–5 (variable) | Broad; exchangeable |
| Amide N–H | 5–8 (variable) | Broad; exchangeable |
| Aldehyde C–H | 9.5–10.0 | Highly diagnostic — appears in no other common environment |
| Carboxylic acid O–H | 10–13 | Broad; exchangeable; one of the most downfield common signals |
Exchangeable protons (O–H, N–H) vary in position and often appear broadened; they can be confirmed by shaking the sample with D₂O, which causes the signal to disappear (Chapter 19).
¹³C NMR Chemical Shift Table
| Carbon Environment | Typical δ Range | Note |
|---|---|---|
| Alkyl carbon (sp³), not adjacent to a heteroatom | 0–40 | Baseline aliphatic region |
| Carbon attached to a halogen | 0–70 | Range depends heavily on which halogen |
| Carbon attached to oxygen or nitrogen (C–O, C–N) | 50–90 | — |
| Alkyne carbon (sp) | 65–90 | — |
| Alkene and aromatic carbon (sp²) | 100–150 | — |
| Carboxylic acid, ester, and amide carbonyl carbon | 160–185 | Shifted upfield relative to ketones/aldehydes by resonance donation from the attached heteroatom (same reasoning as the IR C=O trend above) |
| Aldehyde carbonyl carbon | 190–205 | — |
| Ketone carbonyl carbon | 205–220 | Furthest downfield — no adjacent heteroatom lone pair to donate into the carbonyl and reduce its electron deficiency |
Reading the carbonyl region as a shortcut: a signal above 190 ppm signals an aldehyde or ketone; a signal in the 160–185 range signals an acyl derivative with an attached heteroatom (acid, ester, or amide) — the same addition-vs-substitution distinction from Appendix C shows up directly in the carbon shift.
Splitting Patterns (¹H NMR)
Splitting follows the n + 1 rule: a proton with n chemically non-equivalent neighboring protons is split into n + 1 peaks.
| Neighboring Protons (n) | Multiplicity | Peaks |
|---|---|---|
| 0 | Singlet | 1 |
| 1 | Doublet | 2 |
| 2 | Triplet | 3 |
| 3 | Quartet | 4 |
| 4 | Quintet | 5 |
Representative Coupling Constants (J, in Hz)
| Relationship | Typical J (Hz) |
|---|---|
| Vicinal, freely rotating sp³–sp³ (³J) | 6–8 |
| Vicinal, alkene, cis | 6–12 |
| Vicinal, alkene, trans | 12–18 |
| Geminal, alkene (same carbon) | 0–3 |
| Aromatic, ortho | 7–10 |
| Aromatic, meta | 1–3 |
| Aromatic, para | 0–1 |
Why trans coupling exceeds cis coupling on an alkene: this is a geometric/orbital overlap effect, not something to derive from first principles at this stage — but it is worth recognizing as a diagnostic tool: a large vicinal J (12–18 Hz) across a double bond indicates a trans (E) relationship, while a smaller one (6–12 Hz) indicates cis (Z), directly connecting an NMR spectrum to the E/Z nomenclature in Appendix E.
Mass Spectrometry: Common Fragment Losses
| Mass Lost | Fragment | Suggests |
|---|---|---|
| 15 | •CH₃ | Methyl group present |
| 17 | •OH | Alcohol |
| 18 | H₂O | Alcohol (dehydration in the mass spectrometer) |
| 28 | CO or C₂H₄ | Carbonyl compound (loss of CO) or an ethyl/alkene fragment |
| 29 | •CHO or •C₂H₅ | Aldehyde (loss of CHO) or an ethyl group |
| 43 | C₃H₇⁺ or CH₃CO⁺ (acylium) | Propyl fragment, or a methyl ketone (acylium ion is often a strong, diagnostic peak) |
| 45 | •COOH | Carboxylic acid |
| 57 | C₄H₉⁺ or C₂H₅CO⁺ (acylium) | Butyl fragment, or an ethyl ketone |
| 77 | C₆H₅⁺ (phenyl cation) | Monosubstituted benzene ring |
Acylium ions (R–C≡O⁺, the same cationic species that drives Friedel-Crafts acylation in Appendix C) are common, stabilized fragments from ketones and aldehydes, which is why losses corresponding to R–CO⁺ (43, 57, and similar) are frequently prominent peaks.
The molecular ion (M⁺) — the unfragmented, ionized starting molecule — gives the molecular weight directly and is the anchor point for interpreting every fragment loss above it (Chapter 20).
Cross-References
- Chapter 18 (Infrared Spectroscopy) — the conceptual role of IR as a first-pass functional group screen.
- Chapter 19 (NMR Spectroscopy) — chemical shift, integration, and splitting as three separate, complementary questions.
- Chapter 20 (Mass Spectrometry) — combining IR, NMR, and MS evidence into a single structural proposal.
- Appendix A (Functional Group Atlas) — polarity and structural notes that explain many of the IR and NMR trends above.
- Appendix C (Reaction Summary Tables) — the reactivity order (acid chloride → anhydride → ester → amide) that the carbonyl IR/¹³C trends directly parallel.
- Appendix E (IUPAC Nomenclature Reference) — E/Z geometry, referenced above in the coupling constant discussion.