Chapter 13 NMR Spectroscopy

1. Chapter 13 NMR Spectroscopy NMR - Nuclear Magnetic Resonance NMR is a form of spectroscopy that uses an instrument with a powerful magnet to analyze organic compounds. Invented by physicists (1950’s), then used by chemists (1960’s).

2. Why is it called NMR? Nuclear Magnetic Resonance Nuclear – because it looks at the nucleus of an atom, most commonly a hydrogen atom. A hydrogen atom nucleus consists of one proton with a +1 charge and “spin” of ½. It acts like a tiny bar magnet. generatesmagneticfield

3. NMR – Effect of Magnetic Field No external magnetic field applied to sample Random orientation of nuclear spins Sample placed in an external magnetic field

4. NMR: Absorption of Energy Initial State – nucleus at low energy level

5. NMR: Information Obtained from a Spectrum • An NMR Spectrum will generally provide three types of information: • Chemical Shift – indicates the electronic environment of the nucleus (shielded or deshielded) • Integration – gives the relative number of nuclei producing a given signal • Spin-Spin Coupling – describes the connectivity

6. 1H NMR Spectrum – H2O A sample of water is placed in an NMR instrument, and a proton spectrum is recorded (scanned from left to right).

7. When does nucleus absorb energy? 3. 2, External Field (Ho)from magnet

8. NMR: Simple 1H NMR Spectrum Showing Chemical Shift Two types of protons (a CH2 and a CH3) give two separate signals at two different chemical shifts.

9. NMR: Chemical Shift Practice EN Group -O-CH3 -Si-CH3 -C-CH3 Cl3C-H Assign the four groups shown to the four NMR singals, based on each element’s electronegativity.

10. NMR: Chemical Shift Regions Chemical shift zero is set to TMS (tetramethylsilane), a reference compound Chemical shift measured in ppm.

11. NMR: Chemical Shift Regions Alkane Region (high electron density): Heteroatom Region: Double Bond Region:

12. NMR: Chemical Equivalence and Number of Signals How many signals will the following compounds show in their 1H NMR Spectrum? (Hint: check for symmetry)

13. NMR: Chemical Equivalence and Number of Signals How many signals should appear in the proton NMR spectrum for these compounds? In theory: Signals actually resolved:

14. NMR: Overlapping Proton Signals Protons b, c, and d are all nearly the same, and their signals are not resolved in this spectrum.

15. Review: How Many NMR Signals? How many signals will the following compounds show in their 1H NMR Spectrum? (Hint: check for symmetry) No rotation about double bonds

16. NMR: Chloroethane Fast rotation around single bonds gives an “averaged” spectrum for the three methyl hydrogens.

17. NMR: A Second Proton Spectrum Note: the signal for the nine methyl H’s is larger than the CH2 signal

18. NMR: Integration Indicates Relative Number of Nuclei The height of the integration line (“integral”) gives you the relative number of nuclei producing each signal.

19. NMR: Splitting into a Doublet doublet Note that the signal at 1.6 ppm for the methyl group is split into two peaks. Remember that this is one signal, composed of two separate peaks.

20. NMR: Signal Splitting, n+1 Rule • A signal is often split into multiple peaks due to interactions with protons on carbons next door. Called spin-spin splitting • The splitting is into one more peak than the number of H’s on adjacent carbons (“n+1 rule”) • Splitting of a signal can give doublets (two peaks), triplets (three peaks), quartets (4 peaks), ect. • The relative intensities given by Pascal’s Triangle: doublet 1 : 1 triplet 1 : 2 : 1 quartet 1 : 3 : 3 : 1 pentet: 1 : 4 : 6 : 4 : 1

21. NMR: Signal Splitting, n+1 Rule n+1 Rule: A signal in the proton NMR spectrum will be split into n+1 peaks, where n is the number of protons on adjacent carbons. Example: CH3-CH2-Br For the Methyl Group – There are two protons ‘next door’ (n=2), so the methyl signal will be split into three peaks (2+1), which is called a triplet. For the -CH2- Group: Three protons next door means the CH2 signal will be split into 4 (3+1) peaks, called a quartet.

22. 1H NMR Spectrum for Bromoethane integration: 2 H 3 H Note the expansions printed above

23. NMR: Signal Splitting, n+1 Rule

24. NMR: Signal Splitting, n+1 Rule H

25. NMR: Origin of Spin-Spin Splitting Net result:

26. NMR: Doublets and Triplets Triplet: for the two protons next door,there are four combinations possible: αααβ βββ α Doublet: the one proton next doorcan be either up or down (α or β)

27. NMR: Signal Splitting, n+1 Rule

28. NMR: Using the n+1 Rule Using the n+1 rule, predict the 1H NMR spectrum of 2-iodopropane.Give splitting pattern, integration, and approximate chemical shift. Note that the methyl groups are equivalent, so they will give one signal in the NMR spectrum.

29. doublet Seven line pattern NMR: Spectrum of 2-iodopropane

30. NMR: Rules for Spin-Spin Splitting • The signal of a proton with n equivalent neighboring H’s is split into n + 1 peaks • Protons farther than two carbon atoms apart do not split each other • Equivalent protons do not split each other

31. Common 1H NMR Patterns 1. triplet (3H) + quartet (2H) 2. doublet (1H) + doublet (1H) 3. large singlet (9H) 4. singlet 3.5 ppm (3H) 5. large double (6H) + muliplet (1H) 6. singlet 2.1 ppm (3H)

32. Common 1H NMR Patterns 7. multiplet ~7.2 ppm (5H) 8. multiplet ~7.2 ppm (4H) 9. broad singlet, variable chemical shift