SPECTROSCOPY

How to identify organic compounds from the synergistic Information of spectroscopy:
1.Mass Spectrometry (MS)
2.Infra Red (IR)
3.Nuclear Magnetic Resonance (NMR)
4.Ultraviolet/Visible Spectroscopy


Electromagnetic Spectrum




Mass Spectrometry (MS)
- An analytical technique for measuring the mass-to-charge ratio (m/z) of ions, most commonly positive ions, in the gas phase
- Mass spectrometry is the most valuable analytical tool for the determination of precise molecular weights


A Mass Spectrometer
a) A mass spectrometer is designed to do three things
1. Convert neutral atoms or molecules into a beam of positive (or negative) ions
2. Separate the ions on the basis of their mass-to-charge ratio (m/z)
3. measure the relative abundance of each ion

b) Electron Ionization
1. in the ionization chamber, the sample is bombarded with a beam of high-energy electrons
2. collisions between these electrons and the sample result loss of electrons from sample molecules and formation of positive ions





Resolution
A measure of how well a mass spectrometer separates ions of different mass
-Low resolution - capable of distinguishing among ions of different nominal mass, that is ions that differ by at least one or more mass units
-High resolution - capable of distinguishing among ions that differ in mass by as little as 0.0001 mass unit
-C3H6O and C3H8O have nominal masses of 58 and 60, and can be distinguished by low-resolution MS
-these two compounds each have nominal mass of 60. Distinguish between them by high-resolution MS


M+2 and M+1 Peaks
1. The most common elements giving rise to M + 2 peaks are chlorine and bromine
2. Chlorine in nature is 75.77% 35Cl and 24.23% 37Cl
a ratio of M to M + 2 of approximately 3:1 indicates the presence of a single chlorine in a compound
3. Bromine in nature is 50.7% 79Br and 49.3% 81Br
a ratio of M to M + 2 of approximately 1:1 indicates the presence of a single bromine in a compound


Interpreting MS
1. The only elements to give significant M + 2 peaks are Cl and Br. If no large M + 2 peak is present, these elements are absent
2. Is the mass of the molecular ion odd or even?
3. Nitrogen Rule: if a compound has
-zero or an even number of nitrogen atoms, its molecular ion will appear as a even m/z value
-an odd number of nitrogen atoms, its molecular ion will appear as an odd m/z value



MS of methylcyclopentane




Mass Molecular Ion and Isotope peaks
The unit mass of the molecular ion of C7H7NO is m/z 121 that is
7 x 12C = 84
7 x 1H = 7
1 x 14N = 14
1 x 16O = 16

Molecular species exist that contain the less abundant isotope at
And these give use to the “isotope peaks” at M + 1, M + 1, etc.

Contributing to the M + 1 peak are the isotope 13C, 2H, 15N, and 18O





















REFERENCE :
Sukeki. 2008. SPECTROSCOPY . University of Lampung. Bandarlampung