Presentation on theme: "Outcomes State that NMR spectroscopy involves interaction of materials with low-energy radio-frequency radiation. State the need for deuterated solvents."— Presentation transcript:
Outcomes State that NMR spectroscopy involves interaction of materials with low-energy radio-frequency radiation. State the need for deuterated solvents. Describe the use of TMS (tetramethylsilane) as the standard for chemical shift measurements. Analyse carbon-13 NMR spectra to make predictions about the different types of carbon atoms present. Predict the chemical shifts of carbons within a given molecule.
Carbon-13 NMR Spectroscopy 12 C atoms do not have nuclear spin, but 1.1% of carbon atoms are 13 C and these do have nuclear spin and so produce NMR spectra. NMR spectra give a lot of valuable information about the chemical environment of C atoms (e.g. the difference between C atoms in C=O, C-N, C≡N, C-C, C=C, etc.). Samples carried out in solution – to prevent the C/H atoms in the solvent giving a signal Deuterium (isotope of H) is used as it has an even number of nucleons so will produce no NMR signal.
– this makes them behave like tiny magnets. They will match or oppose an external field. – this makes them behave like tiny magnets. Both Protons Neutrons & Protons possess spin NMR Spectrometry: Spin states Magnetic field Proton B Proton A E Proton B Proton A Radio waves are required to ‘flip’ the nuclei so that all nuclei are no longer aligned with the magnetic field. Energy N S N S Nuclei that oppose the magnetic field have a higher energy level than those aligned with the field.
A low-energy nuclei (aligned with the applied field) will jump to a high energy spin state (opposing the applied field) when given a pulse of RF. (Excitation) NMR Spectrometry: Excitation & Relaxation Against field With field When the magnetic field is removed, the nuclei revert back to their original state releasing the energy that was just given to them in the form of radiation. (Relaxation) The cycle of excitation and relaxation of the nucleus is called resonance – hence the name NMR...nuclear magnetic resonance. RF signal coil detector coil can be the same coil! Induced signal
Chemical shift The chemical shift (δ) is measured relative to TMS TMS is chemically unreactive and is very volatile so is easily removed from the sample after running the NMR spectrum. TMS = 0ppm δ (ppm)
The carbon in the CH3 group is attached to 3 hydrogens and a carbon. The carbon in the CH2 group is attached to 2 hydrogens, a carbon and an oxygen. But which is which?
A table of typical chemical shifts in C-13 NMR spectra carbon environment chemical shift (ppm) C=O (in ketones)205 - 220 C=O (in aldehydes)190 - 200 C=O (in acids and esters)170 - 185 C in aromatic rings125 - 150 C=C (in alkenes)115 - 140 RCH 2 OH50 - 65 RCH 2 Cl40 - 45 RCH 2 NH 2 37 - 45 R3CHR3CH25 - 35 CH 3 CO-20 - 30 R2CH2R2CH2 16 - 25 RCH3RCH3 10 - 15
CH3 CH2 The external magnetic field experienced by the carbon nuclei is affected by the electronegativity of the atoms attached to them. The effect of this is that the chemical shift of the carbon increases if you attach an atom like oxygen to it.