Common 1 H NMR Patterns 1. triplet (3H) + quartet (2H) -CH 2 CH 3 2. doublet (1H) + doublet (1H) -CH-CH- 3. large singlet (9H) t-butyl group 4. singlet 3.5 ppm (3H) -OCH 3 group 5. large double (6H) + muliplet (1H) isopropyl 6. singlet 2.1 ppm (3H) methyl ketone

Common 1 H NMR Patterns 7. multiplet ~7.2 ppm (5H) aromatic ring, monosubstituted 8. multiplet ~7.2 ppm (4H) aromatic ring, disubstituted 9. broad singlet, variable -OH or –NH chemical shift (H on heteratom)

Solving NMR Problems 1. Check the molecular formula and degree of unsaturation. How many rings/double bonds? 2. Make sure that the integration adds up to the total number of H’s in the formula. 3. Are there any signals in the double bond region? 4. Check each signal and write down a possible sub-structure for each one. 5. Try to put the sub-structures together to find the structure of the compound.

Proton NMR Spectrum: C 9 H 12 D. of Unsat = 4 aromatic, disubst.

1 H NMR Spectrum: C 4 H 7 O 2 Br t 2H t 2H s 3H

Electronegative Substituents: Shift Left H 3 C—CH 2 —CH 3 O 2 N—CH 2 —CH 2 —CH 3  0.9  1.3  1.0  4.3  2.0 –CH 3 Cl 3.1 (one Cl) –CH 2 Cl (two Cl’s) –CHCl (three Cl’s) Effect is cumulative Propane: heteroatom region small effect ~no effect

Hydrogens on Heteroatoms Type of proton Chemical shift (ppm) 1-3HNR0.5-5HOR6-8HOAr10-13 COHO Chemical shifts for protons on heteroatoms are variable, and signals are often broad (not generally useful). may be useful far left

13 C NMR Spectroscopy Carbon-13: only carbon isotope with a nuclear spin natural abundance of 13 C is only 1.1% (99% of carbon atoms are 12 C, with no NMR signal) All signals are obtained simultaneously using a broad pulse of energy. The resulting “mass signal” changed into an NMR spectrum mathematically using the operation of Fourier transform (FT-NMR) Frequent repeated pulses give many data sets that are averaged to eliminate noise

13 C signals go from 0 to 240 ppm. 13 C signals: always sharp singlets. (wider range than in 1 H NMR)( 1 H signals: broad multiplets) These two facts mean that in carbon-13 NMR, each separate signal is usually visible, and you can accurately count the number of different carbons in the molecule. Chemical shift affected by electronegativity of nearby atoms: alkane-like range: 0 – 40 ppm (R-CH 2 -R) heteroatom range: 50 – 100 ppm (O-CH 2 -R) double bond range:100 – 220 ppm (sp 2 carbons) No signal overlap! 13 C NMR Spectroscopy

NMR: Scanning for All Nuclei An instrument can only examine one area at a time. To see both proton and C-13 nuclei, a very wide region would have to be scanned. 1 H area is small 13 C area is much wider

Why does 13 C NMR give singlets? 13 C is only 1.1% natural abundant, so most carbons are 12 C, and give no NMR signal. No splitting seen with carbon, because carbons next to the 13 C are likely to be carbon-12: Sample of 1-Propanol: 12 CH CH CH 2 -OH 12 CH CH CH 2 -OH 12 CH CH CH 2 -OH 13 CH CH CH 2 -OH 12 CH CH CH 2 -OH 12 CH CH CH 2 -OH 12 CH CH CH 2 -OH

NMR: Number of Signals for 13 C NMR How many signals should appear in the carbon-13 NMR spectrum for these compounds? In theory: 10 4 Signals actually resolved: 10 4

13 C NMR Example Note the wide spectral width and the sharp singlets in the spectrum below. Also note that there is no integration with 13 C NMR.

13 C NMR: smaller signal to noise ratio Noise

13 C NMR Spectrum: C 5 H 11 Cl D. of Unsat = 0 five 13 C signals

13 C NMR Spectrum: C 4 H 7 O 2 Br CDCl 3 double bond region D. of Unsat = 1