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 1 H and 13 C 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 ( 1 H, 13 C, and 15 N but not 12 C and 16 O). 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 12 C 6 isotope – 6 protons and 6 neutrons in the nucleus. ~ 1.1% of carbons are the 13 C 6 isotope with an extra neutron. With this odd number of nucleons, the 13 C 6 atoms can give rise to an NMR signal. Most hydrogens are the 1 H 1 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.

13 C NMR Spectroscopy Information obtained from a 13 C 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

13 C NMR Spectroscopy The range of chemical shifts is about 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 –CH 2 – with –CH-O– and C=C with C=O

Hexane

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-CH 3 methyl Triplet (3)(t)2 H’s-CH 2 -methylene Doublet (2)(d)1 H>CH-methine Singlet (1)(s)0 H

Hexane ppm q t t

2,2-Dimethylbutane

ppm 8.88 q q s t

Acetaldehyde

Ethyl propionate ppm s t t q 9.19 q

ortho-Xylene

1-Butyne ppm d s q t

1 H NMR Spectroscopy Information obtained from a 1 H 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)

1 H 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

1 H 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.

1 H 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.

1 H NMR Spectroscopy Spin-spin splitting: A resonance peak is split into n+1 peaks by n H atoms on adjacent atoms. Integration: 2:3 The CH 3 resonance (1.8 ppm) is split by the neighboring CH 2. There are n=2 neighboring H’s and so the methyl resonance is split into n+1=3 peaks – a triplet.

1 H NMR Spectroscopy Spin-spin splitting: A resonance peak is split into n+1 peaks by n H atoms on adjacent atoms. Integration: 2:3 The CH 2 resonance (3.2 ppm) is split by the neighboring CH 3. There are n=3 neighboring H’s and so the methylene resonance is split into n+1=4 peaks – a quartet.

1 H 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.

Isopropylbenzene (cumene)

Propanoic acid 25

Acetaldehyde (ethanal) 26

3-Hydroxy-2-butanone (acetoin)