Interpreting NMR Spectra CHEM 318
Introduction • You should read the assigned pages in your text (either Pavia or Solomons) for a detailed description of NMR theory and instrumentation. • This presentation will focus on interpreting 1H and 13C NMR spectra to deduce the structure of organic compounds. • Here is a reminder of some important points and vocabulary:
Introduction • Atomic isotopes with either an odd number of protons or neutrons possess a spin angular momentum (1H, 13C, and 15N but not 12C and 16O). • The positively charged nuclei generate a small electric current and thus also generate an associated magnetic field. • When the nuclei are placed in an external magnetic field, their magnetic spins align either with the external field (lower energy) or against it (higher energy).
Introduction • When irradiated with electromagnetic radiation (radio frequency range), the lower energy nuclei absorb the energy and become aligned against the field in the higher energy state. The magnetic "spin-flip" transition is in "resonance" with the applied radiation. • For nuclei in different bonding environments, the resonance energy is different. • The position of the energy absorption signal is known as the chemical shift, in units of ppm (parts per million) relative to a standard reference compound.
Introduction • Most carbons are the 12C6 isotope – 6 protons and 6 neutrons in the nucleus. ~ 1.1% of carbons are the 13C6 isotope with an extra neutron. With this odd number of nucleons, the 13C6 atoms can give rise to an NMR signal. • Most hydrogens are the 1H1 isotope – 1 proton in the nucleus and no neutrons. The hydrogen atom thus has an odd number of nucleons and can produce an NMR signal.
13C NMR Spectroscopy • Information obtained from a 13C NMR spectrum: • the number of resonance signals denotes the number of different carbon atoms present • the chemical shift reveals the electronic bonding environment of the carbons • signal splitting gives the number of hydrogens bonded to each carbon
13C NMR Spectroscopy • The range of chemical shifts is about 0-220 ppm. • Saturated carbon atoms absorb upfield (lower ppm) relative to unsaturated carbons (downfield at higher ppm). • Electron-withdrawing atoms cause a downfield shift • compare shifts of –CH2– with –CH-O– and C=C with C=O
Carbon-Hydrogen Spin Splitting • The spectrum just shown has 1 resonance signal for each different carbon atom. • A spectrum can be taken that “couples” the spins of a carbon and the hydrogens that are bonded directly to it. • The resulting “splitting” pattern (n+1 peaks) shows the number of hydrogens (n) bonded to the carbon that is undergoing resonance. Quartet(4) (q) 3 H’s -CH3 methyl Triplet (3) (t) 2 H’s -CH2- methylene Doublet (2) (d) 1 H >CH- methine Singlet (1) (s) 0 H
Hexane ppm 14.16 q 22.89 t 31.87 t
2,2-Dimethylbutane ppm 8.88 q 28.96 q 30.42 s 36.49 t
Ethyl propionate ppm 174.40 s 60.26 t 27.71 t 14.32 q 9.19 q
1-Butyne ppm 85.96 d 71.94 s 13.81 q 12.27 t
1H NMR Spectroscopy • Information obtained from a 1H NMR spectrum: • the number of resonance signals denotes the number of different hydrogen atoms present • the chemical shift reveals the electronic bonding environment of the hydrogens • integration of the peaks gives the relative number of hydrogens causing the resonance • signal splitting gives the number of hydrogens bonded to the adjacent atom(s)
1H NMR Spectroscopy • The range of chemical shifts is about 0-12 ppm. • Hydrogens bonded to saturated carbons absorb upfield relative to hydrogens on unsaturated carbons (downfield). • Electron-withdrawing atoms cause a downfield shift. • compare >CH – with >CH-O– and compare C=CH with HC=O
1H NMR Spectroscopy Chemical shift: There are two resonance peaks, indicating two different kinds of H’s. The upfield signal is in the saturated C region; the downfield signal is in the unsaturated C region.
1H NMR Spectroscopy Integration: The area under the peak is proportional to the number of hydrogens causing the absorbance. After integration and rounding to small whole numbers, the peak area ratio is 5:3.
1H NMR Spectroscopy Spin-spin splitting: A resonance peak is split into n+1 peaks by n H atoms on adjacent atoms. The CH3 resonance (1.8 ppm) is split by the neighboring CH2. There are n=2 neighboring H’s and so the methyl resonance is split into n+1=3 peaks – a triplet. Integration: 2:3
1H NMR Spectroscopy Spin-spin splitting: A resonance peak is split into n+1 peaks by n H atoms on adjacent atoms. The CH2 resonance (3.2 ppm) is split by the neighboring CH3. There are n=3 neighboring H’s and so the methylene resonance is split into n+1=4 peaks – a quartet. Integration: 2:3
1H NMR Spectroscopy Spin-spin splitting: A resonance peak is split into n+1 peaks by n H atoms on adjacent atoms. These are some simple, common splitting patterns: Ethyl group is a quartet-triplet, integration 2:3 Isopropyl group is a septet (hard to see) and a doublet, integration 6:1 Para-disubstituted benzenes give this symmetrical splitting; other substituted benzenes are not well-resolved. Relative integrations are shown.