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Infrared Spectroscopy Gives information about the functional groups in a molecule.

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Presentation on theme: "Infrared Spectroscopy Gives information about the functional groups in a molecule."— Presentation transcript:

1 Infrared Spectroscopy Gives information about the functional groups in a molecule

2 region of infrared that is most useful lies between m ( cm -1 ) depends on transitions between vibrational energy states stretchingbending Infrared Spectroscopy

3 Fig

4 Fig IR Spectrum of Hexane

5 Fig

6 Fig

7 Fig

8 Fig

9 Fig

10 Ultraviolet-Visible (UV-VIS) Spectroscopy Gives information about conjugated electron systems

11 gaps between electron energy levels are greater than those between vibrational levels gap corresponds to wavelengths between 200 and 800 nm Transitions between electron energy states E = h E = h

12 X-axis is wavelength in nm Y axis is a measure of absorption of electromagnetic radiation expressed as molar absorptivity ( ) max is the wavelength of maximum absorption and is related to electronic makeup of molecule,especially electrons max is the wavelength of maximum absorption and is related to electronic makeup of molecule,especially electrons Conventions in UV-VIS

13 max 230 nm max 230 nm max 2630 max 2630 UV Spectrum of cis,trans-1,3-cyclooctadiene

14 * Transition in Alkenes * Transition in Alkenes HOMO-LUMO energy gap is affected by substituents on double bond as HOMO-LUMO energy difference decreases (smaller E), max shifts to longer wavelengths

15 Methyl groups on double bond cause max to shift to longer wavelengths C C H H H H C C H H CH 3 max 170 nm max 170 nm CH 3 max 188 nm max 188 nm

16 Extending conjugation has a larger effect on max ; shift is again to longer wavelengths C C H H H H C C H H max 170 nm max 170 nm max 217 nm max 217 nm H C C H H H

17 max 217 nm (conjugated diene) max 217 nm (conjugated diene) H C CHH C C H H H C C H CH 3 H H C C H3CH3CH3CH3CH C C H H max 263 nm conjugated triene plus two methyl groups max 263 nm conjugated triene plus two methyl groups

18 Lycopene max 505 nm max 505 nm orange-red pigment in tomatoes

19 Mass Spectrometry mass spec is different because it is not related to electromagnetic radiation

20 Atom or molecule is hit by high-energy electron from an electron beam at 10ev e – e –beam Principles of Electron-Impact Mass Spectrometry forming a positively charged, odd-electron species called the molecular ion e–e–e–e– +

21 Molecular ion passes between poles of a magnet and is deflected by magnetic field deflection depends on mass-to-charge ratio highest m/z deflected least lowest m/z deflected most Principles of Electron-Impact Mass Spectrometry +

22 If the only ion that is present is the molecular ion, mass spectrometry provides a way to measure the molecular weight of a compound and is often used for this purpose. However, the molecular ion often fragments to a mixture of species of lower m/z.

23 The molecular ion dissociates to a cation and a radical. Principles of Electron-Impact Mass Spectrometry +

24 The molecular ion dissociates to a cation and a radical. Principles of Electron-Impact Mass Spectrometry + Usually several fragmentation pathways are available and a mixture of ions is produced.

25 mixture of ions of different mass gives separate peak for each m/z intensity of peak proportional to percentage of each ion of different mass in mixture separation of peaks depends on relative mass Principles of Electron-Impact Mass Spectrometry

26 mixture of ions of different mass gives separate peak for each m/z intensity of peak proportional to percentage of each ion of different mass separation of peaks depends on relative mass Principles of Electron-Impact Mass Spectrometry

27 Fig

28 m/z = 78 Some molecules undergo very little fragmentation Benzene is an example. The major peak corresponds to the molecular ion. The largest peak is called the base peak and is reference to 100 to give relative intensity.

29 Isotopes in Chlorobenzene 35 Cl 37 Cl

30 Isotopic Clusters in Chlorobenzene ion responsible for m/z 77 peak does not contain Cl H H H H H+

31 Alkanes undergo extensive fragmentation Decane, C 10 H 22 The largest peak may not be the parent ion, or may not be visible at all.

32 Propylbenzene fragments mostly at the benzylic position 91 CH 2 CH 2 CH 3 120

33 Molecular Formula as a Clue to Structure

34 Molecular Weights One of the first pieces of information we try to obtain when determining a molecular structure is the molecular formula. We can gain some information about molecular formula from the molecular weight. Mass spectrometry makes it relatively easy to determine molecular weights.

35 The Nitrogen Rule A molecule with an odd number of nitrogens has an odd molecular weight. A molecule that contains only C, H, and O or which has an even number of nitrogens has an even molecular weight. NH2NH2NH2NH NH2NH2NH2NH2 O2NO2NO2NO2N 183 NH2NH2NH2NH2 O2NO2NO2NO2N NO2NO2NO2NO2

36 Exact Molecular Weights CH 3 (CH 2 ) 5 CH 3 Heptane CH 3 CO O Cyclopropyl acetate Molecular formula Molecular weight C 7 H 16 C5H8O2C5H8O2C5H8O2C5H8O Exact mass Mass spectrometry can measure exact masses. Therefore, mass spectrometry can be used to distinguish between molecular formulas.

37 Index of Hydrogen Deficiency Degree of Unsaturation relates molecular formulas to multiple bonds and rings For a molecular formula, C c H h N n O o X x For a molecular formula, C c H h N n O o X x, the degree of unsaturation can be calculated by: Degree = ½ (2c h - x + n)

38 Rings versus Multiple Bonds Index of hydrogen deficiency tells us the sum of rings plus multiple bonds. Using catalytic hydrogenation, the number of multiple bonds can be determined.


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