Ir

INFRARED Spectroscopy
A brief Introduction and spectrum analysis
Analytics

1. INFRARED SPECTROSCOPY KUMARI NEHA Ph.D. (RESEARCH SCHOLAR) DEPARTMENT OF PHARMACEUTICAL CHEMISTRY 1

2. CONTENTS Introduction Vibrational mode Principle Instrumentation IR Bands Interpretation Examples Uses and applications Advantages & Disadvantages References 2

3. INTRODUCTION Infrared (IR) spectroscopy or vibrational spectroscopy is an analytical technique that takes advantage of the vibrational transitions of a molecule. It is one of the most common and widely used spectroscopic techniques employed mainly by inorganic and organic chemists due to its usefulness in determining functional groups of compounds and identifying them. It is a method of measurement of the extent, at a various wave numbers, of absorption of infrared radiation when it passes through a layer of substances. Intensity and frequency of samples absorption are depicted in a two-dimensional plot called a spectrum. Intensity is generally reported in terms of absorbance - the amount of light absorbed by a sample, or percent transmittance – i.e. the amount of light, which passes through it. 3

4. VIBRATIONAL MODE It induces stronger molecular vibrations in covalent bonds, which can be viewed as springs holding together two masses, or atoms. Covalent bonds can vibrate in several modes, including stretching, rocking, and scissoring. 4

5. principle When the energy in the form of IR is applied and if the applied IR frequency = Natural frequency of vibrations, the absorption of IR take place and a peak is observed. Molecules are excited to the higher energy state from the ground state when they absorb IR radiation. When a compound is exposed to IR radiation, it selectively absorbs the radiation resulting in vibrations of the molecules of the compound, giving rise to closely packed absorption bands, known as IR absorption spectrum . The band correspond to the characteristic functional groups and the bonds present in a chemical substance. Thus, an IR spectrum of the compound is considered as the fingerprint for its chemical identification. 5

6. INSTRUMENTATION The method or technique of infrared spectroscopy is conducted with an instrument called an infrared spectrometer (or spectrophotometer) to produce an infrared spectrum. Infrared (IR) spectrometers measure the interaction of IR radiation with samples. The FTIR spectrometer measures the frequencies at which the samples absorb the radiation, and the intensities of the absorptions. The main parts of IR spectrometer are as follows: Radiation source Sample cells and sampling of substances Monochromators Detectors Recorder 6

7. Fourier-Transform Infrared Spectrometer 7

8. IR BANDS IR bands can be classified as strong (s), medium (m), or weak (w) , depending on their relative intensities in the infrared spectrum. A strong band covers most of the y-axis. A medium band falls to about half of the y-axis, and a weak band falls to about one third or less of the y-axis. 8

9. interpretation The typical IR absorption range for polar covalent bonds is 600 - 4000 cm-1. The graph shows the regions of the spectrum where the following types of bonds normally absorb. For example a sharp band around 2200-2400 cm-1 would indicate the possible presence of a C-N or a C-C triple bond. 9

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11. FINGERPRINT REGION Although the entire IR spectrum can be used as a fingerprint for the purposes of comparing molecules, the 600 - 1400 cm-1 range is called the fingerprint region. This is normally a complex area showing many bands, frequently overlapping each other. This complexity limits its use to that of a fingerprint, and should be ignored by beginners when analyzing the spectrum. As a student, you should focus your analysis on the rest of the spectrum, that is the region to the left of 1400 cm-1. 11

12. IR Spectra OF ALKANES, ALKENES & alkynes Alkanes have no functional groups. Their IR spectrum displays only C-C and C-H bond vibrations. Of these the most useful are the C-H bands, which appear around 3000 cm-1. Alkenes also show sharp, medium bands corresponding to the C=C bond stretching vibration at about 1600-1700 cm-1. Some alkenes might also show a band for the =C-H bond stretch, appearing around 3080 cm-1 as shown below. 12

13. Alkynes displays as a sharp, weak band at about 2100 cm-1. The reason it’s weak is because the triple bond is not very polar. Terminal alkynes, that is to say those where the triple bond is at the end of a carbon chain, have C-H bonds involving the sp carbon (the carbon that forms part of the triple bond). Therefore they may also show a sharp, weak band at about 3300 cm-1 corresponding to the C-H stretch. 13

14. IR SPECTRUM OF NITRILE In a manner very similar to alkynes, nitriles show a prominent band around 2250 cm-1 caused by the CN triple bond. This band has a sharp, pointed shape just like the alkyne C-C triple bond, but because the CN triple bond is more polar, this band is stronger than in alkynes. 14

15. IR SPECTRUM OF ALCOHOL The most prominent band in alcohols is due to the O-H bond, and it appears as a strong, broad band covering the range of about 3000 - 3700 cm-1. The sheer size and broad shape of the band dominate the IR spectrum and make it hard to miss. 15

16. IR SPECTRA OF ALDEHYDES AND KETONES Carbonyl compounds are those that contain the C=O functional group. In aldehydes, this group is at the end of a carbon chain, whereas in ketones it’s in the middle of the chain. Aldehydes and ketones show a strong, prominent, stake-shaped band around 1710 - 1720 cm-1 (due to the highly polar C=O bond). Aldehydes also contain a C-H bond to the sp2 carbon of the C=O bond, they also show a pair of medium strength bands positioned about 2700 and 2800 cm-1. These bands are missing in the spectrum of a ketone because the sp2 carbon of the ketone lacks the C-H bond. 16

17. IR SPECTRUM OF CARBOXYLIC ACID A carboxylic acid functional group combines the features of alcohols and ketones because it has both the O-H bond and the C=O bond. Therefore carboxylic acids show a very strong and broad band covering a wide range between 2800 and 3500 cm-1 for the O-H stretch. At the same time they also show the stake-shaped band in the middle of the spectrum around 1710 cm-1 corresponding to the C=O stretch. 17

18. IR SPECTRA OF AMINES The most characteristic band in amines is due to the N-H bond stretch, and it appears as a weak to medium, somewhat broad band (but not as broad as the O-H band of alcohols). This band is positioned at the left end of the spectrum, in the range of about 3200 - 3600 cm-1. Primary amines have two N-H bonds, therefore they typically show two spikes that make this band resemble a molar tooth. Secondary amines have only one N-H bond, which makes them show only one spike, resembling a canine tooth. Finally, tertiary amines have no N-H bonds, and therefore this band is absent from the IR spectrum altogether. 18

19. IR Spectrum OF Amide The amide functional group combines the features of amines and ketones because it has both the N-H bond and the C=O bond. Therefore amides show a very strong, somewhat broad band at the left end of the spectrum, in the range between 3100 and 3500 cm-1 for the N-H stretch. At the same time they also show the stake-shaped band in the middle of the spectrum around 1710 cm-1 for the C=O stretch. As with amines, primary amides show two spikes, whereas secondary amides show only one spike. 19

20. examples 20

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23. Dibuty l ether 23

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29. Uses and application Chemical Analysis: Testing Pill Quality (Near-infrared spectroscopy (NIR) to make a prediction regarding quick dissolution of pills inside the body. Protein characterization Nanoscale semiconductor analysis Space exploration. Analysis of gaseous, liquid or solid samples Identification of compounds .(Discovery of new chemical compounds ) Information regarding functional groups of molecules and constitution of molecules can be deduced from IR spectrum It is also used in forensic analysis in both criminal and civil cases, for example in identifying polymer degradation. 29

30. Advantages & disadvantages Advantages It's cheap and fast compared to things like NMR. It also works for a wide variety of samples and can detect things very strongly, whereas similar techniques like Raman spectroscopy are weaker. Disadvantages Sample preparation is time consuming and that it can't give information as detailed as other techniques such as NMR. It's also a destructive analysis method and therefore precious or scarce sample should be analyzed by a non-destructive method such as Raman. It's also qualitative rather than quantitative and there are a lot of compounds which are not IR active and therefore can't be detected. 30

31. CONCLUSION IR identifies the components of a sample (liquid, solid or gas). Infrared (IR) spectrometers measure the interaction of IR radiation with samples. The FTIR spectrometer measures the frequencies at which the samples absorb the radiation, and the intensities of the absorptions. Intensity and frequency of samples absorption are depicted in a two-dimensional plot called a spectrum. Intensity is generally reported in terms of absorbance - the amount of light absorbed by a sample, or percent transmittance – i.e. the amount of light, which passes through it. What makes up an unknown sample, and how much of each component is present in that sample, can be valuable information supplied by this technique. Its many applications include research and development of new products. 31

32. references https://commons.wikimedia.org/wiki/File:IR-spectroscopy-sample.svg https://ruby-sapphire.com/index.php/about/10-articles/810-ftir-intrigue https://www.dummies.com/education/science/chemistry/how-to-identify-carbonyls-alkenes-alkynes-and-aromatics-in-the-ir-spectrum/ https://orgspectroscopyint.blogspot.com/2014/12/infrared-spectroscopy.html 32

33. Thank you 33