Infrared Spectroscopy

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

2. Infrared Spectroscopy region of infrared that is most useful lies between2.5-16 m (4000-625 cm-1) depends on transitions between vibrational energy states stretching bending

3. Fig. 13.30

4. Fig. 13.31 IR Spectrum of Hexane

5. Fig. 13.32

6. Fig. 13.33

7. Fig. 13.34

8. Fig. 13.35

9. Fig. 13.36

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

11. Transitions between electron energy states gaps between electron energy levels are greater than thosebetween vibrational levels gap corresponds to wavelengthsbetween 200 and 800 nm E = h

12. Conventions in UV-VIS 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

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

14. * 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. C C C C Methyl groups on double bond cause max to shift to longer wavelengths H H CH3 H H CH3 H H max 170 nm max 188 nm

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

17. H H C C H H C C H H H3C H C C H H H C C H C C CH3 H max 217 nm(conjugated diene) max 263 nmconjugated triene plus two methyl groups

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

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

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

21. +• Principles of Electron-Impact Mass Spectrometry 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

22. Principles of Electron-Impact Mass Spectrometry 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. Principles of Electron-Impact Mass Spectrometry The molecular ion dissociates to a cationand a radical. +•

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

25. Principles of Electron-Impact Mass Spectrometry 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 + + + + + +

26. Principles of Electron-Impact Mass Spectrometry 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 + + + + + +

27. Fig. 13.39

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

29. 35Cl 37Cl Isotopesin Chlorobenzene

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

31. Alkanes undergo extensive fragmentation Decane, C10H22 The largest peak may not be the parent ion, or may not be visible at all.

32. CH2—CH2CH3 Propylbenzene fragments mostlyat the benzylic position 91 120

33. Molecular Formulaas aClue 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. 138 NO2 NH2 NH2 NH2 O2N O2N 183 The Nitrogen Rule 93 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.

36. O CH3CO Exact Molecular Weights CH3(CH2)5CH3 Mass spectrometry can measure exact masses. Therefore, mass spectrometry can be used to distinguish between molecular formulas. Heptane Cyclopropyl acetate C7H16 C5H8O2 Molecular formula 100 100 Molecular weight Exact mass 100.1253 100.0524

37. Index of Hydrogen DeficiencyDegree of Unsaturation relates molecular formulas to multiple bonds and rings For a molecular formula, CcHhNnOoXx, the degree of unsaturation can be calculated by: Degree = ½ (2c + 2 - h - x + n)

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