Spectroscopy Flashcards
(19 cards)
What is spectrometry? What is spectroscopy?
Spectrometry: A technique used to record a spectrum
Spectroscopy: The study of interaction of electromagnetic radiation with matter
How does NMR spectroscopy work overall?
Nuclear Magnetic Resonance Spectroscopy
- nuclei with an odd number of nucleons possess spin which can have magnetic moments C13, 1H
- In a magnetic field, frequency needed to cause resonance absorption for C13 100MHz, in the radio range
- at this freq, exactly the amount of energy can be absorbed to bridge the gap between high and low energy spin states, aligned switch between states
- when they spin states relax again, energy released, detected by NMR
How does NMR spectroscopy tell us about the structure of a compound?
A singular type of nuclei can be studied e.g 1H, 19F, 13C
Chemical shifts are seen as the different carbon environments cause different absorption frequencies
Why do chemical shifts occur?
Electrons also have magnetic moments
Electron spins surround the nucleus, with their magnetic fields shielding the nuclei from the external field
Differences in shielding produce different chemical shifts
How has the chemical shift scale been calculated?
Chemical Shift= (freq of peak - freq peak reference ) / freq peak reference
Dimensionless, and relative so independent on magnetic field
Parts per million
What is the reference material for 13C NMR and why?
TMS, tetramethylsilane
Set at 0.0ppm
- 4 equivalent carbons for a strong signal
- chemical inert and volatile
- resonates at a very low freq, as very shielded by silicon, so does not overlap with other peaks and unlikely to be below
What are the features of a C13 NMR spectrum?
Stick-like appearance, 0-220ppm
Each unique carbon environment is a peak
What are the overall regions of a C13 spectrum?
0-100 ppm saturated
100-150ppm unsaturated
150-220 ppm carbonyl
How does hybridisation affect C13 chemical shift?
sp3> 90ppm
sp2<110ppm
sp: 80-110 ppm
How do EWGs affect chemical shift? Are there exceptions?
Generally, more EW, increase the chemical shift via inductive effects, deshielding
However, iodine has an unusually large electron cloud, so actually ends up shielding the electrons more, below alkyls (but follows trend that down the groups halogen chemical shifts becoming smaller)
How do mesomeric effects affect chemical shift?
EDGs via resonance can decrease chemical shift as electron density increases to increase shielding e.g phenols for C=C
But EWGs like conjugated carbonyls will increase chemical shift as decreasing electron density most by carboxylic acid derivatives
How can you predict the number of carbon peaks of a molecule?
Each chemical environment has a unique peak
Look for symmetry, if the same environment, same peak
What is spin-spin coupling?
Magnetic interactions between carbon atoms and its NMR active neighbours such as H
Interaction between nuclei relayed by electron spins in intervening bonds
Why do C13 peaks sometimes have splitting?
Coupling between C-Hs results in splitting
This is because the nuclei of C and H , each C-H bond can be either up or down
Both of these variations produce a peak, so splitting
How can the N+1 rule be explained? And intensities?
Each C-H bond can be up or down
The total combinations possible= N+1
And the number of combinations from the nCr function
See image below
Why do most C13 NMR spectra not show coupling and why it desirable? When is it not fully successful ?
Broadband proton decoupled spectra
All proton spin coupling turned off when carbon data collected
To simplify carbon spectra, little overlap between peaks, and to reduce noise/increase sensitivity
Still splits if a non hydrogen NMR active atom e.g F
Why do we not see carbon-carbon coupling?
Need both atoms to be C13
Already a low chance in one molecule, so adjacent and to be large enough in numbers is unlikely, too low to be seen
What solvents are used in 13C NMR ?
Deuterated solvents e.g CDCl3 or D2O
- protonated solvents not helped for proton nmr
Solvent peaks not decoupled and Dspin=1 so a triplet for CDCl3
Compare benchtop NMR and high field NMR?
Benchtop: 15MHz, simple to use but needs lots of material
High field: 100MHz C13, greater signal dispersion and higher detection sensitivity
