Chromatography And Spectroscopy Chp 29 Flashcards
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Why is TMS used as a standard reference chemical?
It produces a strong single peak because:
- All Hs and Cs in the same environment
- All Hs and Cs are highly shielded (not affected by magnetic field)
- Inert, non-toxic, easy to remove (volatile)
Why are deuterated solvents used?
Deuterated solvents do not produce signals in the frequency ranges used in 1H and 13C NMR spectroscopy because D atoms have an even number of nucleons.
What are examples of deuterated solvents?
CDCL3 or Heavy water
What is the chemical shift?
The shift in frequency compared to TMS, required for a nucleus to undergo nuclear magnetic resonance.
What does the chemical shift depend on? What is it especially caused by?
The shift depends on the chemical environment, especially caused by the proximity of electronegative atoms or pi bonds.
What is Nuclear Magnetic Resonance (NMR) used for
To help determine the structure of a molecule
Resonance
When the nucleus absorbs energy and rapidly flips between the two spin states.
Carbon-13 NMR Spectroscopy
- Tell us the number of different carbon environments- from the number of peaks
- Tells us the types of carbon environments present, from the chemical shift
Proton NMR
- Tells us the number of different hydrogen environment
Spin-spin coupling/splitting
Provides info about the number of hydrogens bonded to the adjacent carbon atom.
Why are OH and NH peaks difficult to identify?
-They sit in a broad area of the spectrum
-Normally found unsplit
-Large range
How do we identify OH and NH samples? (Proton transfer)
1) Run proton NMR with no D2O
2) Add D2O and shake vigorously
3) Run proton NMR again and compare 2 spectra to see if a peak has disappeared (OH or NH was present)
n + 1
ānā protons attached to adjacent carbon atoms
splitting patterns
-singlet
-doublet
-triplet
-quartet
Chromatography Stage 1: Preparing plate and applying solution
- Apply solution of sample to pencil line using a capillary tube
- Pencil line must be 1cm from bottom
Chromatography Stage 2: Running and developing plate
- Place solvent in chromatography tank, level solvent below line and allow solvent to reach within 1cm of the top
Chromatography Stage 3: Developing and analysing
- Remove plate from tank
- Use UV to make spots visible
- Calculate RF value and compare to data books
RF Value equation
distance moved by component/distance moved by solvent front
Integration value
Relative area under the peak is proportional the number of hydrogens causing the signal
How does gas chromatography separate the compounds in a mixture if the stationary phase is liquid?
Relative solubility in the liquid polymer/stationary phase
What happens to the RF value if you use a very polar solvent?
RF value would be larger
Retention Time
The time taken for a component to travel through the capillary column in gas chromatography
