1. INFRA RED SPECTROSCOPY A guide for A level students KNOCKHARDY PUBLISHING

2. Uses: Monitoring the degree of unsaturation in polymers. Quality control in perfume manufacture Drug analysis Testing the breath of suspected drunk drivers INFRA RED SPECTROSCOPY

3. Different covalent bonds have different strengths due to the masses of different atoms at either end of the bond. As a result, the bonds vibrate at different frequencies The frequency of vibration can be found by detecting when the molecules absorb electro-magnetic radiation. Various types of vibration are possible. INFRA RED SPECTROSCOPY

4. Different covalent bonds have different strengths due to the masses of different atoms at either end of the bond. As a result, the bonds vibrate at different frequencies The frequency of vibration can be found by detecting when the molecules absorb electro-magnetic radiation. Various types of vibration are possible. Examples include... STRETCHING and BENDING INFRA RED SPECTROSCOPY SYMMETRIC BENDING ASYMMETRIC STRETCHING STRETCH

5. SYMMETRIC STRETCHING BENDING AND STRETCHING IN WATER MOLECULES

6. ASYMMETRIC STRETCHING BENDING AND STRETCHING IN WATER MOLECULES

7. BENDING

8. a beam of infra red radiation is passed through the sample a similar beam is passed through the reference cell the frequency of radiation is varied bonds vibrating with a similar frequency absorb the radiation the amount of radiation absorbed by the sample is compared with the reference the results are collected, stored and plotted The Infra-red Spectrophotometer

9. A bond will absorb radiation of a frequency similar to its vibration(s) The Infra-red Spectrophotometer normal vibrationvibration having absorbed energy

10. IDENTIFICATION OF PARTICULAR BONDS IN A MOLECULE INFRA RED SPECTRA - USES The presence of bonds such as O-H and C=O within a molecule can be confirmed because they have characteristic peaks in identifiable parts of the spectrum.

11. IDENTIFICATION OF PARTICULAR BONDS IN A MOLECULE INFRA RED SPECTRA - USES The presence of bonds such as O-H and C=O within a molecule can be confirmed because they have characteristic peaks in identifiable parts of the spectrum. IDENTIFICATION OF COMPOUNDS BY DIRECT COMPARISON OF SPECTRA The only way to completely identify a compound using IR is to compare its spectrum with a known sample. The part of the spectrum known as the ‘Fingerprint Region’ is unique to each compound.

12. Infra-red spectra are complex due to the many different vibrations taking place in each molecule. INFRA RED SPECTRA - INTERPRETATION

13. Infra-red spectra are complex due to the many different vibrations taking place in each molecule. Total characterisation of a substance based only on its IR spectrum is almost impossible unless one has computerised data handling facilities for comparison of the obtained spectrum with one in memory. INFRA RED SPECTRA - INTERPRETATION

14. Infra-red spectra are complex due to the many different vibrations taking place in each molecule. Total characterisation of a substance based only on its IR spectrum is almost impossible unless one has computerised data handling facilities for comparison of the obtained spectrum with one in memory. The technique is useful when used in conjunction with other methods - nuclear magnetic resonance spectroscopy and mass spectroscopy. INFRA RED SPECTRA - INTERPRETATION

15. Infra-red spectra are complex due to the many different vibrations taking place in each molecule. Total characterisation of a substance based only on its IR spectrum is almost impossible unless one has computerised data handling facilities for comparison of the obtained spectrum with one in memory. The technique is useful when used in conjunction with other methods - nuclear magnetic resonance spectroscopy and mass spectroscopy. Peak position depends on bond strength masses of the atoms joined by the bond strong bonds and light atomsabsorb at lower wavenumbers weak bonds and heavy atoms absorb at high wavenumbers INFRA RED SPECTRA - INTERPRETATION

16. Vertical axisAbsorbance the stronger the absorbance the larger the peak Horizontal axisFrequencywavenumber (waves per centimetre) / cm -1 Wavelengthmicrons (m); 1 micron = 1000 nanometres INFRA RED SPECTRA - INTERPRETATION

17. FINGERPRINT REGION organic molecules have a lot of C-C and C-H bonds within their structure spectra obtained will have peaks in the 1400 cm -1 to 800 cm -1 range this is referred to as the “fingerprint” region the pattern obtained is characteristic of a particular compound the frequency of any absorption is also affected by adjoining atoms or groups.

18. IR SPECTRUM OF A CARBONYL COMPOUND carbonyl compounds show a sharp, strong absorption between 1700 and 1760 cm -1 this is due to the presence of the C=O bond

19. IR SPECTRUM OF AN ALCOHOL alcohols show a broad absorption between 3200 and 3600 cm -1 this is due to the presence of the O-H bond

20. IR SPECTRUM OF A CARBOXYLIC ACID carboxylic acids show a broad absorption between 3200 and 3600 cm -1 this is due to the presence of the O-H bond they also show a strong absorption around 1700 cm -1 this is due to the presence of the C=O bond

21. IR SPECTRUM OF AN ESTER esters show a strong absorption between 1750 cm -1 and 1730 cm -1 this is due to the presence of the C=O bond

22. WHAT IS IT! O-H STRETCH C=O STRETCH O-H STRETCH C=O STRETCH AND ALCOHOL ALDEHYDE CARBOXYLIC ACID One can tell the difference between alcohols, aldehydes and carboxylic acids by comparison of their spectra.

23. O-HC=OC-O N-H Aromatic C-CC-H C=CC-C alkanes CNCN C-Cl CHARACTERISTIC FREQUENCIES

24. BondClass of compoundRange / cm -1 Intensity C-HAlkane2965 - 2850strong C-CAlkane1200 - 700weak C=CAlkene1680 - 1620variable C=OKetone1725 - 1705strong Aldehyde1740 - 1720strong Carboxylic acid1725 - 1700strong Ester1750 - 1730strong Amide1700 - 1630strong C-OAlcohol, ester, acid, ether1300 - 1000strong O-HAlcohol (monomer)3650 - 3590variable, sharp Alcohol (H-bonded)3420 - 3200strong, broad Carboxylic acid (H-bonded)3300 - 3250variable, broad N-HAmine, Amide3500 (approx)medium C  NNitrile 2260 - 2240medium C-XChloride800 - 600strong Bromide600 - 500strong Iodide500 (approx)strong CHARACTERISTIC ABSORPTION FREQUENCIES

25. Bonds in a molecule vibrate, twist and bend, but only at certain fixed ________. When ir radiation is passed through a molecular compound, the molecules absorb radiation with just the right ________to cause the bonds to vibrate, twist or bend more ________. The energy required depends in each case on the atoms involved and the nature of the bond, eg. Bonds between light atoms vibrate more _______ than bonds between ________ atoms. Bonds between double and triple covalent bonds vibrate at higher frequencies than ________ covalent bonds. When the full spectrum of ir radiation is passed through a compound, an absorption spectrum is obtained that is characteristic for that compound only. INFRA RED SPECTROSCOPY