NMR

NMR A spinning charge generates a magnetic field, as shown by the animation The resulting spin-magnet has a magnetic moment (μ) proportional to the spin.

In the presence of an external magnetic field (B0), two spin states exist, +1/2 and -1/2. The magnetic moment of the lower energy +1/2 state is aligned with the external field, but that of the higher energy -1/2 spin state is opposed to the external field. Note that the arrow representing the external field points North.

Nuclear Magnetic Resonance (NMR) Radio waves are used as the source of electromagnetic radiation to flip the nuclear spins. The chemical shifts are relative value, they are usually compared to TMS (tetramethyl silane) = 0 ppm. The compounds with the highest chemical shift are those that are lightly shielded, near to electronegative atoms.

Nuclei of isotopes which possess an odd number of protons, an odd number of neutrons, or both, exhibit mechanical spin phenomena which are associated with angular momentum When a spinning nucleus is placed in a magnetic field, the nuclear magnet experiences a torque which tends to align it with the external field.; parallel to the field (low energy) and against the field (high energy). Since the parallel orientation is lower in energy, this state is slightly more populated than the anti-parallel, high energy state. If the oriented nuclei are now irradiated with electromagnetic radiation, the lower energy state will absorb a quantum of energy and spin-flip to the high energy state. When this spin transition occurs, the nuclei are said to be in resonance with the applied radiation, hence the name nuclear magnetic resonance.

NMR Hydrogen nuclei also behave as little magnets and a hydrogen nucleus can also be aligned with an external magnetic field or opposed to it. Again, the alignment where it is opposed to the field is less stable (at a higher energy). It is possible to make it flip from the more stable alignment to the less stable one by supplying exactly the right amount of energy

A Simple NMR Spectra. Low resolution spectra of ethanoic acid.

Low Resolution NMR Spectra Each peak represents a different environment for hydrogen atoms in the molecule. In the methyl propanoate spectrum above, there are three peaks because there are three different environments for the hydrogens. Remember that methyl propanoate is CH3CH2COOCH3. The hydrogens in the CH2 group are obviously in a different environment from those in the CH3 groups. The two CH3 groups aren't in the same environment either. One is attached to a CH2 group, the other to an oxygen

Low Resolution NMR Spectra

Example 1. An organic compound was known to be one of the following. Use its low resolution NMR spectrum to decide which it is.

Answer. Checking the positions of the various hydrogens in the two possible compounds against the chemical shift table gives you this pattern of shifts: Comparing these with the actual spectrum means that the substance was propanoic acid, CH3CH2COOH.

Example 2 How would you use low resolution NMR to distinguish between the isomers propanone and propanal?                                                              Answer. The propanone would only give one peak in its NMR spectrum because both CH3 groups are in an identical environment - both are attached to -COCH3. The propanal would give three peaks with the areas underneath in the ratio 3:2:1.

Using the areas under the peaks The ratio of the areas under the peaks tell you the ratio of the numbers of hydrogens in the various environments. In the methyl propanoate case, the areas were in the ratio of 3:2:3, which is exactly what you want for the two differently placed CH3 groups and the CH2 group. NMR spectrometers have a device which draws another line on the spectrum called an integrator trace (or integration trace). You can measure the relative areas from this trace.

Chemical Shift. (δ) The chemical shift given by a particular proton is the frequency difference between its absorption and the absorbtion from tetramethylsilane, TMS, Si(CH3)4. The value of δ is quoted in parts per million. A peak at a chemical shift of 2.0 is said to be downfield of TMS. The further to the left a peak is, the more downfield it is

Chemical Shift You might predict that all nuclei of a given type would undergo the spin-flip transition at exactly the same applied frequency in a given magnetic field, this is not the case. This is because the electrons in the molecule have small magnetic fields which tend to oppose the applied field, screening the nuclei from the full strength of the applied field. The greater the electron density, the greater this 'shielding' will be, hence nuclei which are in electron rich environments will undergo transition at a higher applied field than nuclei in electron poor environments. The resulting shift in the NMR signal for a given nuclei is referred to as the chemical shift, and, in general, protons or carbons adjacent to electronegative atoms will be shielded and moved to a higher chemical shift (undergo transition at a lower applied field). The scale utilized for measuring chemical shifts is defined by the equation shown below

Chemical Shift. (δ) Suppose that you attached the hydrogen to something more electronegative. The electrons in the bond would be further away from the hydrogen nucleus, and so would have less effect on the magnetic field around the hydrogen.

1H NMR of Ethanoic Acid

1H NMR of Ethanal.

Chemical Shifts

Typical 1H chemical shifts relative to TMS = 0 RCOOH carboxylic acids +9 to +13 Can be broad and solvent- dependent RCONH2 amide +5 to +12 Broad and solvent- dependent RCHO aldehyde +8 to +10 Sharp C6H6 aromatics +6 to +10 R2C=CHR alkene +4 to +8 RNH2 amines +1 to +6 Broad and solvent- dependent ROH alcohols +0.5 to +8 Broad and solvent- dependent RCH2R methylene +1.5 to +4.5 RCH3 methyl 0 to +4

Low Resolution Spectra of Benzene. Benzene has only one sort of hydrogen atom, so that the NMR spectrum shows a single peak (the TMS peak is omitted):

Low Resolution Spectra of Ethanal. Ethanal has two sorts of hydrogen atom, those on the methyl group and that on the aldehyde group. It therefore has two peaks in its spectrum (the TMS peak is omitted).

Low Resolution Spectra of Propanal.

Spectra of Propanone

Example 1. 6 4 Structure: IUPAC Name: (pentan-3-one)

Example 2 IUPAC Name: ethyl ethanoate The proton NMR has three peaks; a quartet at 4.1 (2H), a triplet at 1.2 (3H) and a singlet at 1.97 (3H). The quartet and triplet suggest a CH2 coupled to a CH3 in an ethyl group. The peak at 4.1 is in the area generally observed for CH groups adjacent to electronegative groups, i.e., oxygen. The peak at 2 is in the region for a methyl group adjacent a carbonyl IUPAC Name: ethyl ethanoate

Common Functional Groups.

Chemical Signals.

Splitting. (n+1 Rule).                                                                                                                                                                                                 J = the coupling constant.

High Resolution Spectra- Ethanol http://www.aist.go.jp/RIODB/SDBS/cgi-bin/direct_frame_top.cgi?lang=eng Assign. Shift(ppm) A 3.687 B 2.61 C 1.226 CH3CH2OH CAB

Butan-2-one

NMR Spectroscopy Draw the skeletal and displayed formula of ethyl ethanoate. Sketch a low resolution proton NMR spectrum of ethyl ethanoate. Label each of the peaks with the environment that caused them.