NMR Spectroscopy Dr. Joshua Osbourn – Dept. of Chemistry, West Virginia University 1. Theory of NMR Spectroscopy 2. Spectrum Basics 3. Chemically Equivalent and Distinct Hydrogen 4. Chemical Shift 5. Integration (Peak Height) 6. Coupling (Splitting) 7. Complex Splitting 8. Special Features to Lookout For 9. Examples – 1 H NMR 10. Carbon-13 NMR NMR Theory NMR = Nuclear Magnetic Resonance Spectroscopy Spectroscopy is the interaction of matter with energy. NMR uses low energy radiation from the radio frequency (RF) region of the electromagnetic spectrum.
NMR Theory Only certain nuclei are NMR active. Only nuclei containing odd mass numbers or odd atomic numbers will give NMR signals. These nuclei possess a property known as nuclear spin . The proton ( 1 H) for example: 1 proton, 1 electron, 0 neutrons Nuclear Spin = + ½ and – ½ In the absence of any external stimuli, there is an equal probability of a proton being in either the a or the b state. NMR Theory H H H H H H B ext H H H H H H H β -state RF Radiation Relaxation H H H H H H H H H H H α -state B ext B ext B ext + NMR Signal
NMR Theory Every distinct proton in a molecule exists in a distinct chemical environment and thus requires a different RF frequency to cause excitation from the a to the b state. This results in each distinct proton providing a distinct signal in the NMR spectrum. NMR Instrument
Spectrum Basics O HO H 12 10 8 6 4 2 0 PPM Chemical Shift ( d ) Increasing ν Chemically Equivalent/Distinct Hydrogen Protons that exist in different chemical environments give rise to different NMR signals. Equivalent protons correspond to the same NMR signal. H H C H HO H
Chemically Equivalent/Distinct Hydrogen H 2 H 3 C C O CH 3 H 2 H 3 C C C CH 3 H 2 Chemically Equivalent/Distinct Hydrogen O H 3 C C C CH 3 H 2 CH 3 O CH C H 3 C C NH 2 H 2
Chemically Equivalent/Distinct Hydrogen H 3 C H H H Chemically Equivalent/Distinct Hydrogen Cl Cl Cl Cl
Chemically Equivalent/Distinct Hydrogen Enantiotopic and Diastereotopic Protons The Chemical Equivalence Test (X Test): When the equivalency of two protons is in question, draw the compound twice. In one, replace one H with X. In the second, replace the other H with X. • If the two are identical – the protons are identical. • If the two are enantiomers – the protons are enantiotopic , but are still equivalent and thus result in the same NMR signal. • If the two are diastereomers – the protons are diastereotopic and are not equivalent. These protons result in two different NMR signals. Chemically Equivalent/Distinct Hydrogen Enantiotopic and Diastereotopic Protons Identical Protons: H H H
Chemically Equivalent/Distinct Hydrogen Enantiotopic and Diastereotopic Protons Enantiotopic Protons: H H Chemically Equivalent/Distinct Hydrogen Enantiotopic and Diastereotopic Protons Diastereotopic Protons: Cl H H
Chemically Equivalent/Distinct Hydrogen Enantiotopic and Diastereotopic Protons One Last Example: OH Chemical Shift Downfield Upfield Deshielded Shielded O HO H 12 10 8 6 4 2 0 PPM Increasing Chemical Shift
Chemical Shift A decrease in electron density around a nucleus deshields that nucleus and moves the signal downfield Increasing d F CH 3 Cl CH 3 Br CH 3 I CH 3 H CH 3 Chemical Shift A decrease in electron density around a nucleus deshields that nucleus and moves the signal downfield Increasing d Cl Cl Cl CH 3 H CH 3 Cl CH Cl CH 2 Cl
Chemical Shift Deshielding effects are diminished with distance. H 2 C CH 3 F C H 2 Chemical Shift Attached electron withdrawing groups deshield a proton. H 2 C H 3 C CH 3 O H 3 C O CH 3
Chemical Shift Increasing the # of alkyl groups deshields a proton. Increasing d CH 3 CH 3 CH 3 H 3 C CH H CH 3 H 3 C C H H 2 C H H CH 3 Chemical Shift The chemical shift of protons attached to sp 2 hybridized carbon atoms are unusually high. sp 2 sp 3 sp 2 sp H H H H 7.2 ppm 2.5 ppm 0.9 ppm 5.4 ppm This is due to a phenomenon known as magnetic anisotropy where the loosely held p electrons move in a circular path in the presence of a magnetic field. This induced magnetic field reinforces the magnetic field applied by the instrument resulting in a larger deshielding effect.
Chemical Shift The chemical shift of protons attached to sp 2 hybridized carbon atoms are unusually high. sp 2 sp 3 sp 2 sp H H H H 7.2 ppm 2.5 ppm 0.9 ppm 5.4 ppm This is due to a phenomenon known as magnetic anisotropy where the loosely held p electrons move in a circular path in the presence of a magnetic field. This induced magnetic field reinforces the magnetic field applied by the instrument resulting in a larger deshielding effect. Chemical Shift The chemical shift of protons attached to sp 2 hybridized carbon atoms are unusually high. H H H
Chemical Shift General Regions in a 1 H NMR Spectrum Some Basic Chemical Shift Regions: Z H O O H H Z= O,C OH H H H carboxylic aldehyde aromatic vinyl X acid saturated X = O, N, Halogen 12 10 8 6 4 2 0 PPM Integration (Peak Height) The area under the signal is proportional to the number of protons that signal corresponds to.
Integration (Peak Height) O 3H 3H 2H 3 2 1 0 PPM 6H 4H This image cannot currently be displayed. 4 3 2 1 0 PPM Integration (Peak Height) The area under an NMR signal is proportional to the number of protons that signal corresponds to. 3H O Cl O 2H C CH 3 H 2 H Cl 1H 7 6 5 4 3 2 1 0 PPM
Integration (Peak Height) Case 1: Integration values accurately reflect the number of H. 3H O Cl O 2H C CH 3 H 2 H Cl 1H 7 6 5 4 3 2 1 0 PPM Integration (Peak Height) Case 2: Whole number integrations values that do not integrate for enough total H. 3H C 6 H 12 O H 3 C CH 3 H 3 C CH 3 O 1H 3 2 1 0 PPM
Integration (Peak Height) Case 3: One or more integration values are less than 1. 0.230 C 4 H 8 O 0.230 0.153 5 4 3 2 1 0 PPM Integration (Peak Height) Case 4: Integration values appear as very large numbers 4430 C 6 H 12 O 8840 2950 1475 3 2 1 0 PPM O
Integration (Peak Height) Case 5: The height of integral symbols printed on the spectrum reflect the area under the curve. C 3 H 7 Cl 4 3 2 1 0 PPM Integration (Peak Height) Case 5: The height of integral symbols printed on the spectrum reflect the area under the curve. C 3 H 7 Cl 4 3 2 1 0 PPM 40 mm 40 mm 60 mm Peak Ratio – 1 : 1 : 1.5 x 2 2 : 2 : 3
Integration (Peak Height) In some cases the integrals may all be connected together. Br 4 3 2 1 0 PPM NMR Spectroscopy Dr. Joshua Osbourn – Dept. of Chemistry, West Virginia University 1. Theory of NMR Spectroscopy 2. Spectrum Basics 3. Chemically Equivalent and Distinct Hydrogen 4. Chemical Shift 5. Integration (Peak Height) 6. Coupling (Splitting) 7. Complex Splitting 8. Special Features to Lookout For 9. Examples – 1 H NMR 10. Carbon-13 NMR
Coupling (Splitting) The signal corresponding to a particular proton will split due to the protons on adjacent atoms. Coupling (Splitting) The most common coupling that is observed is between protons on adjacent carbon atoms. Simple coupling follows the n+1 rule. n = # of protons on the adjacent carbon. H H H H Cl Cl Cl CH 3 H H H H
Coupling (Splitting) H H Cl Br Cl H 7 6 5 4 3 PPM Coupling (Splitting) Many names describing the same phenomenon…. • Splitting or Spin-Spin-Splitting – refers to a proton signal being split due to neighboring protons. • Coupling – refers to one type of proton being “coupled” to neighboring proton(s). This results in splitting of the signal. • Multiplicity – this refers to the type of splitting observed. If the signal is split into two peaks, the multiplicity is said to be a doublet.
Coupling (Splitting) What gives rise to signal splitting? H H H H H Coupling (Splitting) Predicting connectivity using signal splitting 9H 3H tert -Butyl Ethyl 2H R R 6H Isopropyl R 1H
Coupling (Splitting) Common Features: 1. The most common coupling occurs from non- equivalent proton that are separated by 3-bonds. H H O Cl Cl H Br Cl H H H H Br H Br H H Coupling (Splitting) Common Features: 2. Coupling will be observed between protons separated by two bonds if the two protons are chemically distinct. H H Cl
Coupling (Splitting) Common Features: 3. Coupling between protons separated by 4 or more bonds is not generally observed. O H Cl H H H H Coupling (Splitting) Common Features: 4. Coupling is usually not observed through oxygen and nitrogen. H H H H H H N CH 3 O Cl H Alcohol and amine proton NMR signals usually show up as a singlet.
Coupling (Splitting) Common Features: 5. Chemically equivalent protons do not couple! H H Cl H 3 C Cl Cl H H Coupling (Splitting) Example: O HO 5 4 3 2 1 0 PPM
The Coupling Constant For every split signal in an NMR spectrum, a coupling constant can be calculated. Essentially, a coupling constant is a measure of the interaction between coupled protons. Coupling constants are also known as J-values and reported in Hz. The Coupling Constant H H J = 8.1 Hz J = 8.1 Hz Cl CH 3 4 3 2 1 0 PPM
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