3. Mass Spectrometry
Basic function
To measure the mass of a molecule
Only separate and detect charged ions
Mass spectrum of
ethanol
4. Mass Spectrometry
Isotopic signatures
Corresponding to % abundance of the elements
Elements Isotope % abundance
H 1H 99.985
C 12C 98.89
13C 1.11
N 14C 99.634
O 16O 99.762
Cl 35Cl 75.77
37Cl 24.23
Br 79Br 50.69
81Br 49.31
6. What is spectroscopy?
The interaction between light and molecules and
what we can learn from it.
Structural determination
7. What is spectroscopy?
Spectroscopy techniques in organic chemistry:
Mass spectrometry
Infrared (IR) spectroscopy
Nuclear magnetic resonance (NMR) spectroscopy
8. IR Spectroscopy
Related to the vibrational energy
levels
Strength of an IR absorption
depends on dipole moment
changes during vibration
Larger the change of dipole, the
stronger absorption will be
9. Only one stretch frequency of CO2: antisymmetric stretch
antisymmetric stretch
symmetric stretch
10. IR is used to assign functional group in the molecules
11. NMR Spectroscopy
Exploiting the spin states of
nucleus of an the atom.
Common nuclei in for
organic chemistry:
1H, 13C
Operating in radiowave
frequency
13. How to read NMR spectrum
The position of signal (chemical shift, δ)
Relative to standard (TMS, tetramethylsilane)
Related to the environment of the nucleus
‘shielding’ of the nucleus by electron
15. How to read NMR spectrum
Number of peaks (signal)
Indicating the non-equivalent environment.
Related to the symmetry of the molecules
16. How to read NMR spectrum
Integration of peaks
Mainly in 1H-NMR
Peak integration: area under the peak
Corresponding to the ratio of H atom in the
molecule.
18. How to read NMR spectrum
Peak multiplicity in 1H-NMR
Interaction with the neighbouring nuclei
The spins can couple with one another
The splitting pattern: (2nI + 1)
n: equivalent nuclei
I: spin nuclei (for 1H = ½)
No splittings are seen between equivalent nuclei
21. More information and practice questions can be
found here:
https://chem-is-
you.blogspot.co.id/search/label/Characterisati
on%20of%20Organic%20Compounds