Nmr
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Isomer X has three singlets with integration ratio 1:3:6 in its 1H NMR spectrum. Deduce the structure of X. Explain why the peaks in the 1H NMR spectrum are singlets.
Adjacent C has no (non-equivalent) H attached (so no splitting/spin- spin coupling takes place)
What is spin
Interactions with magnetic fields
Common with 13C and 1H
Number of peaks Chemical shift
Only H
Integration number (area of peak) and splitting pattern (shape of peak)
Number of peaks
Tells us how many non equivalent environments there are. Because each different environment absorbs a specific amount of energy so gives out a particular frequency of energy.
How many peaks for benzene for both carbon nmr and proton nmr
1 peak for both carbon nmr and hydorgen nmr. Only one environemnt
Chlroobenzene peaks
4 diff environments for carbon atoms 3 environemnts for chlorobenzene in proton nmr. This is because the carbon attatched to chlorine has no hydrogen bonded to it.
Solvent nmr
• Must not interfere with the specturm. Must not contain any protons as would create a peak and add to spectrum for the sample. • Must disolve the substance
Polar sample
• dissolve in a polar solvent. Since attrcations can form between s+ and s- regions of the molecules. • example is CDCl3 No spin so no interfere with spectrum Uneven distribution of electrons so there will be a dipole moment.
Non polar sample
• dissolve in a nonpolar solvent. Examples include Tetrachloromethane and ccl4 Tetrachloromethane has polar bonds, symetrical shape so no diple moment. So non polar
Chemical shift
Needs a standard reference substance Energy absorbed by sample is compared to the energy absorbed by standard Chemical shift is the measure of the energy difference
What is the standard for carbon and hydrogen MMR?
Tetramethylsilene. Tms Only gives one single peak. Peak is a long way away from the peaks produced by sample. Given a value of 0 so everything compared to that
Tms
It is inert which won’t react with the sample It is non-toxic so less risk of harm Volatile easy to separate from the sample
What does spin depend on
Depends what type of atom is nearby within molecule which affects the chemical shift value
What do electronegatuve atoms do
Highly electronegative atoms the shield a nucleus which gives a larger chemical shifts value. The larger the chemical shift means closer to an electronegative atom
Integration ration
Area of the peak is given a ratio. Shows number of hydrogen atoms contributing to each peak. Usually, the ratio adds up to the total number of hydrogen atoms in a molecule.
Splitting pattern
Only seen in high resolution proton NMR Depends on the interaction of magnetic fields. H can affect eachother depending on how close they are. Peaks split once for each adjacent H N+1 where n is number of non equivalent H on adjacent carbon atoms
Only H on carbon atoms *are split * cause splitting So there is no splitting in NH2 or OH
What does two triplets suggest
-ch2ch2- is present as common way.
A compound is usually mixed with Si(CH3)4 and either CCl4 or CDCl3 before recording the compound’s 1H NMR spectrum. State why Si(CH3)4, CCl4 and CDCl3 are used in 1H NMR spectroscopy. Explain how their properties make them suitable for use in 1H NMR spectroscopy.
Stage 1: 1a CDCl3 or CCl4 solvent 1b TMS as reference / calibration / standard / peak at 0 (ppm) 1c Inert (so unlikely to react with the sample allow if inert tied to either TMS or CDCl3 or CCl4) Stage 2 CCl4 or CDCl3 as solvent: 2a (Both) have no H (atoms so give no signals in spectrum) tied to either CDCl3 or CCl4 2b CCl4 non polar (- good solvent for non-polar organic molecules) 2c CDCl3 polar covalent molecule (– good solvent for polar organic compounds) Stage 3 TMS as reference: 3a (Lots (12) of equivalent H to) give one signal / single environment 3b Signal in an area away from other typical H signals / peak upfield from others OR (Low electronegativity of Si shifts) signal right 3c Easy to remove / volatile / low bp