Chapter 9 — Quiz
Twenty-five questions on NMR spectroscopy: chemical shift, integration, multiplicity, coupling, ¹³C, 2D, applications.
1. NMR probes: (a) bond vibrations (b) nuclear spin transitions in a magnetic field (c) molecular mass (d) electron transitions
2. The reference standard for ¹H NMR: (a) water (b) CDCl₃ (c) TMS (tetramethylsilane) (d) methanol
3. Chemical shift is measured in: (a) MHz (b) Hz (c) ppm (parts per million) (d) cm⁻¹
4. The n+1 rule says: (a) a proton with n neighbors shows n multiplets (b) n+1 peaks with n non-equivalent neighbors (c) n+1 total peaks for all (d) does not apply to aromatic
5. A CH₃ next to CH₂ shows: (a) singlet (b) triplet (split by 2 H neighbors) (c) quartet (d) doublet
6. An aldehyde H has δ: (a) 0-1 (b) 2-3 (c) 5-7 (d) 9-10 (very distinctive)
7. An aromatic H has δ: (a) 1-2 (b) 3-4 (c) 6.5-8.5 (d) 10-12
8. In ¹H NMR, integration gives: (a) number of environments (b) relative proton count per environment (c) chemical shift (d) coupling constants
9. ¹³C NMR chemical shift range: (a) 0-12 ppm (b) 0-220 ppm (c) 50-200 (d) 100-500
10. Carbonyl carbon of aldehyde has δ¹³C: (a) 10-50 (b) 40-80 (c) 100-160 (d) 190-220
11. Cis alkene HH coupling: (a) 0-2 Hz (b) 6-10 Hz (c) 14-18 Hz (d) 20+ Hz
12. Trans alkene HH coupling: (a) 0-2 Hz (b) 6-10 Hz (c) 14-18 Hz (d) 20+ Hz
13. DEPT experiments: (a) classify C by number of attached H's (CH₃, CH₂, CH, quaternary) (b) measure H coupling (c) proton decoupling (d) magnetic sensitivity
14. A -CH₂- group adjacent to a chiral center: (a) appears as a single peak (the two H's are equivalent) (b) appears as two separate peaks (diastereotopic; AB pattern) (c) appears as a triplet (d) does not show in NMR
15. NOESY shows: (a) through-bond couplings (b) through-space proton-proton correlations (within ~5 Å) (c) chemical shift differences (d) coupling constants only
16. A typical 500 MHz NMR uses a magnetic field of: (a) 1.4 T (b) 7.0 T (c) 11.7 T (for ¹H at 500 MHz) (d) 50 T
17. Why is CDCl₃ used as NMR solvent (instead of CHCl₃)? (a) CDCl₃ has no ¹H signal (deuterated; ¹H signal is from impurity at 7.26 only) (b) CDCl₃ is cheaper (c) CDCl₃ does not interact with the analyte (d) CDCl₃ is easier to evaporate
18. A 1H NMR shows 3H triplet at 1.2 + 2H quartet at 3.7. The pattern suggests: (a) an isopropyl group (b) an ethyl group adjacent to an electronegative atom (ester, etc.) (c) a tert-butyl group (d) a methyl ketone
19. Karplus equation describes: (a) the relationship between dihedral angle and ³J coupling (b) the chemical shift of carbonyls (c) the coupling between cis alkene protons (d) ring current shielding
20. A protein NMR study uses: (a) ¹H NMR only (b) ¹H, ¹³C, and ¹⁵N NMR (with isotope-enriched protein) (c) ¹⁹F NMR exclusively (d) ³¹P NMR for the phosphate backbone of DNA
21. A chiral shift reagent (e.g., Eu(hfc)₃): (a) shifts the two enantiomers' signals to different chemical shifts, allowing ee measurement (b) doesn't affect chiral compounds (c) increases the magnetic field (d) makes the spectrum unreadable
22. A compound with 5 H's at δ 7.2-7.4 (aromatic), 2 H quartet at 2.7, 3 H triplet at 1.2 is most likely: (a) ethylbenzene (PhCH₂CH₃) (b) toluene (c) ethanol (d) acetone
23. A compound's ¹³C shows peaks at δ 14, 22, 35, 207. Likely structure: (a) propyl methyl ether (b) methyl ethyl ketone (2-butanone): C=O at 207; 3 distinct alkyl C's (c) ethanol (d) methyl propanoate (would have peak at 170, not 207)
24. A 2D COSY spectrum shows: (a) ¹H-¹H J-couplings (which protons are coupled to which) (b) ¹H chemical shifts only (c) ¹³C only (d) molecular mass
25. A 2D HSQC spectrum shows: (a) ¹H-¹³C one-bond correlations (which proton is attached to which carbon) (b) only ¹H NMR (c) only ¹³C NMR (d) only the molecular ion
Answer Key
| # | Answer | Explanation |
|---|---|---|
| 1 | b | NMR = nuclear spin transitions |
| 2 | c | TMS = standard at δ 0 |
| 3 | c | ppm |
| 4 | b | n neighbors → n+1 peaks |
| 5 | b | -CH₃ split by 2 H's = triplet |
| 6 | d | aldehyde H is at 9-10 ppm |
| 7 | c | aromatic at 6.5-8.5 |
| 8 | b | integration = relative proton count |
| 9 | b | 0-220 ppm range |
| 10 | d | aldehyde/ketone C=O at 190-220 |
| 11 | b | cis ³J = 6-10 Hz |
| 12 | c | trans ³J = 14-18 Hz |
| 13 | a | DEPT classifies by H count |
| 14 | b | diastereotopic = separate peaks |
| 15 | b | NOESY = through-space, ≤5 Å |
| 16 | c | 500 MHz at 11.7 T |
| 17 | a | CDCl₃ deuterated; no ¹H |
| 18 | b | -CH₂CH₃ adjacent to electronegative atom |
| 19 | a | Karplus: J vs dihedral angle |
| 20 | b | proteins use ¹H, ¹³C, ¹⁵N |
| 21 | a | shift reagent splits enantiomers |
| 22 | a | ethylbenzene fits the pattern |
| 23 | b | 207 = ketone; 3 alkyl C's = 2-butanone |
| 24 | a | COSY = ¹H-¹H couplings |
| 25 | a | HSQC = ¹H-¹³C one-bond |
Concept connections
- NMR is the most powerful single tool for structure determination.
- Four pieces of info per ¹H peak: shift, integration, multiplicity, coupling constant.
- ¹³C extends ¹H NMR: broader range, classifies carbons.
- Coupling constants reveal geometry: cis vs trans alkenes; dihedral angles via Karplus.
- 2D NMR (COSY, HSQC, HMBC, NOESY) is essential for complex structures.
- Combined IR + MS + NMR = standard structure determination.
Scoring: 22+ = ready to use NMR for structure determination. 17–21 = practice with simple unknowns. Below 17 = re-read the chapter and practice with SDBS spectra.