Chapter 37 Analytical Techniques (HNMR) Flashcards
What does HNMR use?
radiowaves
How does HNMR work?
- Protons are positively-charged & generate a magnetic field by spinning.
- When an external magnetic field is applied, one proton aligns & one opposes the magnetic field.
Which hydrogen has higher or lower energy?
- Hydrogen aligned with external magnetic field = lower energy
- Hydrogen aligned opposed to external magnetic field - higher energy
How do electrons affect HNMR?
- Electrons also spin & generate their own magnetic field.
- Electrons provide shielding effect. Placing an electron between the external magnetic field & the proton, shields the hydrogen.
O-H VS C-H
- Electrons in O-H further from hydrogen atom than electrons in C-H.
- The closer the electrons to the hydrogen atom, the more the hydrogen is shielded.
- Energy gap between the electrons is different.
What is the environment of a hydrogen atom?
- Distance of electrons from the Hydrogen atom.
- E.g. O-H, C-H, CH2, NH2 are all different environments.
What does number of peaks mean in NMR spectroscopy?
- Number of environments
What do the heights of peaks mean in NMR spectroscopy?
- Ratio of hydrogen environments in the compound
Ethanol HNMR:
- Number of peaks
- Ratio
- 3 peaks
- 3:2:1
Cyclopentane HNMR:
- Number of peaks
- Ratio
- 1 peak
- 10
Butanol HNMR:
- Number of peaks
- Ratio
- 5 peaks
- 1:2:2:2:3
Pentan-2-one HNMR:
- Number of peaks
- Ratio
- 4 peaks
- 3:2:2:3
3-chloropentane HNMR:
- Number of peaks
- Ratio
- 3 peaks
- 6:4:1
Cyclohexanol HNMR:
- Number of peaks
- Ratio
- 5 peaks
- 1:1:4:4:2
What do chemical shifts mean in NMR spectroscopy?
- Information about the type of environment the hydrogen atoms are in / what the hydrogen atoms are bonded to
Tetramethylsilane
- (CH3)4-Si
- produces a strong singlet peak (because it has 12 hydrogen atoms)
- maximum shielding
- a reference point
- inert, volatile
LHS & RHS of NMR graph?
- LHS: electrons furthest from H atom
- RHS: electrons closest to H atom
- Deshielding increases as you go further left of the graph, chemical shift in ppm increases.
1.6 pm
alkyl / alkane
2 - 3 ppm
CH3-CO (aldehyde) / CH2
3 - 4 ppm
CH2-O / CH3
6 - 9 ppm
benzene
9 to 10 ppm
aldehydes
11 to 12 ppm
COOH
How does High Resolution NMR work?
-Think of it as zooming in to the peaks on a low resolution NMR graph
- Each peak on an LNMR graph appears as a cluster of peaks on a HNMR graph
Number of peaks at a certain position on a HNMR graph?
- Number of Hydrogens on an adjacent C atom
Heights of peaks on a HNMR graph?
- Chemical shift
The n+1 rule?
- Number of sub-peaks in a cluster is one more than the number of hydrogens attached to the next door carbon(s)
Sample answer.
at 1.0 ppm, CH3-CH2, triplet peak suggests 2H on adjacent C
9701/42/M/J/23:
Suggest why CDCl3 is used as a solvent instead of CHCl3 for the HNMR spectrum.
CDCl3 does not give a peak.
Why is CDCl3 needed when obtaining a proton NMR spectrum? (3)
- When samples are analysed through NMR spectroscopy, they must be dissolved in a solvent.
- Unlike TMS (which shows one sharp reference peak), a solvent (CDCl3) containing Deuterium (an isotope of hydrogen) can be used instead.
- Deuterium nuclei absorb radio waves in a different region to the protons analysed in organic compounds. Thus, the reference solvent peak will not interfere with those of the sample.
How is D2O used to identify O-H & N-H protons by proton exchange?
- Adding D2O ‘removes’ N-H & O-H peaks from the spectrum
- The -OH proton & the -NH proton undergo an exchanging process with a deuterium atom on D2O.
- E.g.: -OH + D2O –> -OD + HOD
- E.g.: -NH-CO- + D2O –> -ND-CO- + HOD
