Nuclear Magnetic Resonance Spectroscopy (NMR)
Nuclear Magnetic Resonance Spectroscopy (NMR)
1
If irradiated with the right amount of EM radiation, nuclei spin states can flip The absorption of EM radiation by a spinning nucleus and resulting flip is called resonance
Analysed against the reference compound tetramethylsilane (TMS)
H-NMR spectroscopy analyses H, and 13C-NMR-spectrocopy analyses C A nucleus has a spin quantum number of ½, and only certain isotopes are NMR active When 1H and 13C are put into a magnetic field, their orientations align
𝛿=
𝑠ℎ𝑖𝑓𝑡 𝑖𝑛 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑜𝑓 𝑎 𝑠𝑖𝑔𝑛𝑎𝑙 𝑓𝑟𝑜𝑚 𝑇𝑀𝑆 (𝐻𝑧) 𝑜𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑝𝑒𝑐𝑡𝑟𝑜𝑚𝑒𝑡𝑒𝑟 (𝑀𝐻𝑧)
Shielding
If 1H nuclei were isolated from other atoms, they would all give the same signal However, being surrounded by other atoms exposes them to local magnetic fields Thus the 1H atom experienced less of the applied magnetic field This is shielding; weaker resonance signals are shifted upward Protons can also be deshielded by reduced electron density
Equivalent Protons and Shift
Protons experiencing the same local magnetic fields are equivalent, and give the same 1H-NMR signal Test for equivalence − Replace the proton with a test atom (e.g. Cl); if the resultant compounds are the same, it is equivalent set of protons The number of signals tells us the minimum number of sets of equivalent H atoms
Signal Areas / Integration
The intensity of 1H-NMR signals is directly proportional to the number of 1H atoms giving the signal
The ratio of the above is 3:1 → abundance of equivalent protons Aromatic protons have very similar chemical shifts, and can appear as one peak Magnetic resonance imaging (MRIs) analyses water in tissues to detect tumours
Chemical Shift
The position of the signal can indicate the surrounding environment
Multiplicity
A proton’s resonance frequency can be affected by other H atoms close by; these fields cause the signal to be split into 2+ peaks (doublets, triplets, multiplets) The multiplicity of a 1H signal is determined by the number of adjacent 1H spins 𝑠𝑖𝑔𝑛𝑎𝑙 𝑚𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑐𝑖𝑡𝑦 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑎𝑑𝑗𝑎𝑐𝑒𝑛𝑡 𝑠𝑝𝑖𝑛𝑠 + 1 (𝑛 + 1)
Different intensities of each peak in a multiplet follow the pattern of Pascal’s trangle
Note: Adjacent 1H spins include those on all carbon atoms around it, not only one 13
C-NMR Spectroscopy
Nuclei of 13C have nuclear spin whereas 12C does not They appear as singlets in the 13C NMR spectrum if the molecule is unsymmetrical Since all are singlets, the number of 13C signals = # different equivalent sets
1
If irradiated with the right amount of EM radiation, nuclei spin states can flip The absorption of EM radiation by a spinning nucleus and resulting flip is called resonance
Analysed against the reference compound tetramethylsilane (TMS)
H-NMR spectroscopy analyses H, and 13C-NMR-spectrocopy analyses C A nucleus has a spin quantum number of ½, and only certain isotopes are NMR active When 1H and 13C are put into a magnetic field, their orientations align
𝛿=
𝑠ℎ𝑖𝑓𝑡 𝑖𝑛 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑜𝑓 𝑎 𝑠𝑖𝑔𝑛𝑎𝑙 𝑓𝑟𝑜𝑚 𝑇𝑀𝑆 (𝐻𝑧) 𝑜𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑝𝑒𝑐𝑡𝑟𝑜𝑚𝑒𝑡𝑒𝑟 (𝑀𝐻𝑧)
Shielding
If 1H nuclei were isolated from other atoms, they would all give the same signal However, being surrounded by other atoms exposes them to local magnetic fields Thus the 1H atom experienced less of the applied magnetic field This is shielding; weaker resonance signals are shifted upward Protons can also be deshielded by reduced electron density
Equivalent Protons and Shift
Protons experiencing the same local magnetic fields are equivalent, and give the same 1H-NMR signal Test for equivalence − Replace the proton with a test atom (e.g. Cl); if the resultant compounds are the same, it is equivalent set of protons The number of signals tells us the minimum number of sets of equivalent H atoms
Signal Areas / Integration
The intensity of 1H-NMR signals is directly proportional to the number of 1H atoms giving the signal
The ratio of the above is 3:1 → abundance of equivalent protons Aromatic protons have very similar chemical shifts, and can appear as one peak Magnetic resonance imaging (MRIs) analyses water in tissues to detect tumours
Chemical Shift
The position of the signal can indicate the surrounding environment
Multiplicity
A proton’s resonance frequency can be affected by other H atoms close by; these fields cause the signal to be split into 2+ peaks (doublets, triplets, multiplets) The multiplicity of a 1H signal is determined by the number of adjacent 1H spins 𝑠𝑖𝑔𝑛𝑎𝑙 𝑚𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑐𝑖𝑡𝑦 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑎𝑑𝑗𝑎𝑐𝑒𝑛𝑡 𝑠𝑝𝑖𝑛𝑠 + 1 (𝑛 + 1)
Different intensities of each peak in a multiplet follow the pattern of Pascal’s trangle
Note: Adjacent 1H spins include those on all carbon atoms around it, not only one 13
C-NMR Spectroscopy
Nuclei of 13C have nuclear spin whereas 12C does not They appear as singlets in the 13C NMR spectrum if the molecule is unsymmetrical Since all are singlets, the number of 13C signals = # different equivalent sets
