INFRARED(IR) SPECTROSCOPY OF NATURAL PRODUCTS

DESCRIPTION OF PRINCIPLE, INSTRUMENTATION AND SPECTRAL CHARECTERIZATION BY IR SPECROSCOPY

1. infrared Spectroscopy (IR) It is the analysis of infrared light interacting with a molecule. This can be analyzed in three ways by measuring absorption, emission and reflection . The main use of this technique is in organic and inorganic chemistry.  infrared region most useful for analysis of organic compounds is wavelength range from 2,500 to 16,000 nm , with a corresponding frequency range from 1.9*10 13  to 1.2*10 14  Hz.

2. Photon energies associated with this part of the infrared (from 1 to 15 kcal/mole) are not large enough to excite electrons, but may induce vibrational excitation of covalently bonded atoms and groups. covalent bonds in molecules are more like stiff springs that can be stretched and bent .  all organic compounds will absorb infrared radiation that corresponds in energy to these vibrations. Thus a sample that did not absorb at all would record a horizontal line at 100% transmittance (top of the chart).

3. The frequency scale at the bottom of the chart is given in units of  reciprocal centimeters (cm -1 )  rather than Hz, because the numbers are more manageable. The reciprocal centimeter is the number of wave cycles in one centimeter ; whereas, frequency in cycles per second or Hz is equal to the number of wave cycles in 3*10 10  cm (the distance covered by light in one second).  Infrared spectra may be obtained from samples in all phases ( liquid, solid and gaseous ). Liquids are usually examined as a thin film sandwiched between two polished salt plates (note that glass absorbs infrared radiation, whereas NaCl is transparent).

4. If solvents are used to dissolve solids, care must be taken to avoid obscuring important spectral regions by solvent absorption. Perchlorinated solvents such as carbon tetrachloride , chloroform and tetrachloroethene are commonly used . Alternatively, solids may either be incorporated in a thin KBr disk , prepared under high pressure, or mixed with a little non-volatile liquid and ground to a paste (or mull) that is smeared between salt plates.

5. IR radiation causes the excitation of the vibrations of covalent bonds within that molecule . These vibrations include the stretching and bending modes. Number of vibrational modes- In order for a vibrational mode in a sample to be "IR active", it must be associated with changes in the dipole moment . A permanent dipole is not necessary , as the rule requires only a change in dipole moment . A molecule can vibrate in many ways , and each way is called a  vibrational mode .  

6. For molecules with N number of atoms , linear molecules have 3N – 5 degrees of vibrational modes , whereas nonlinear molecules have 3N – 6 degrees of vibrational modes (also called vibrational degrees of freedom). As an example H2O, a non-linear molecule , will have 3 × 3 – 6 = 3 degrees of vib rational freedom, or modes

7. Simple  diatomic molecules  have only one bond and only one vibrational band . If the molecule is symmetrical , e.g. N 2 , the band is not observed in the IR spectrum , but only in the Raman spectrum. Asymmetrical diatomic molecules , e.g.  CO, absorb in the IR spectrum . More complex molecules have many bonds , and their vibrational spectra are correspondingly more complex, i.e. big molecules have many peaks in their IR spectra. atom, can vibrate in nine different ways.

8. symmetrical stretching Asymmetrical stretching Scissoring rocking Wagging Twisting

9. in water, the rocking, wagging, and twisting modes do not exist because these types of motions of the H represent simple rotation of the whole molecule rather than vibrations within it. An IR spectrum show the energy absorptions as one 'scans' the IR region of the EM spectrum.  As an example, the IR spectrum of butanal is shown below. In general terms it is convenient to split an IR spectrum into two approximate regions: 4000-1000 cm -1  known as the  functional group region , and < 1000 cm -1  known as the  fingerprint region

10. Most of the information that is used to interpret an IR spectrum is obtained from the  functional group region . In practice, it is the  polar  covalent bonds than are IR "active" and whose excitation can be observed in an IR spectrum. In organic molecules these polar covalent bonds represent the  functional groups. Hence,  the most useful information obtained from an IR spectrum is what functional groups are present within the molecul

11. some functional groups can be "viewed" as combinations of different bond types.  For example, an ester, CO 2 R contains both C=O and C-O bonds, and both are typically seen in an IR spectrum of an ester. In the  fingerprint regio n , the spectra tend to be more complex and much harder to assign.

12. IR spectroscopy the infrared spectrum of a sample is recorded by passing a beam of infrared light through the sample. When the frequency of the IR is the same as the vibrational frequency of a bond or collection of bonds, absorption occurs. Examination of the transmitted light reveals how much energy was absorbed at each frequency (or wavelength). This measurement can be achieved by scanning the wavelength range using a monochromator. Alternatively, the entire wavelength range is measured using a Fourier transform instrument and then a transmittance or absorbance spectrum is generated using a dedicated procedure. This technique is commonly used for analyzing samples with covalent bonds. Simple spectra are obtained from samples with few IR active bonds and high levels of purity. More complex molecular structures lead to more absorption bands and more complex spectra.

13. Sample preparation- Gaseous samples require a sample cell with a long path length to compensate for the diluteness. The path length of the sample cell depends on the concentration of the compound of interest. A simple glass tube with length of 5 to 10 cm equipped with infrared-transparent windows at the both ends of the tube can be used for concentrations down to several hundred ppm. Liquid samples can be sandwiched between two plates of a salt (commonly sodium chloride, or common salt, although a number of other salts such as potassium bromide or calcium fluoride are also used). The plates are transparent to the infrared light and do not introduce any lines onto the spectra.

14. Solid samples can be prepared in a variety of ways. One common method is to crush the sample with an oily mulling agent (usually mineral oil Nujol ). A thin film of the mull is applied onto salt plates and measured. The second method is to grind a quantity of the sample with a specially purified salt (usually potassium bromide) finely (to remove scattering effects from large crystals). This powder mixture is then pressed in a mechanical press to form a translucent pellet through which the beam of the spectrometer can pass

15. Comparing to a reference It is typical to record spectrum of both the sample and a "reference". This step controls for a number of variables, e.g.IR detector, which may affect the spectrum. The reference measurement makes it possible to eliminate the instrument influence. The simplest reference measurement is to simply remove the sample (replacing it by air). However, sometimes a different reference is more useful. For example, if the sample is a dilute solute dissolved in water in a beaker, then a good reference measurement might be to measure pure water in the same beaker. 

16. Absorption bands IR spectroscopy is often used to identify structures because functional groups give rise to characteristic bands both in terms of intensity and position (frequency). The positions of these bands are summarized in correlation tables as shown below.

17. IR  Instrumentation The main parts of IR spectrometer are as follows: radiation source sample cells and sampling of substances Monochromators detectors Recorder IR radiation sources-- IR instruments require a source of radiant energy which emit IR radiation which must be steady, intense enough for detection and extend over the desired wavelength. Various sources of IR radiations are as follows. a)             Nernst glower b)             Incandescent lamp c)             Mercury arc d)             Tungsten lamp e)              Glober source f)               Nichrome wire

18. sample cells and sampling of substances- IR spectroscopy has been used for the characterization of solid, liquid or gas samples. Monochromators  –  Various types of monochromators are prism, gratings and filters. Prisms are made of Potassium bromide, Sodium chloride or Caesium iodide. Filters are made up of Lithium Fluoride and Diffraction gratings are made up of alkali halides. Detectors –  Detectors   are used to measure the intensity of unabsorbed infrared radiation. Detectors like thermocouples, Bolometers , thermisters , Golay cell, and pyro -electric detectors are used.

19. APPLICATIONS OF IR SPECTROSCOPY Identification of functional group and structure elucidation 2. Identification of substances IR spectroscopy is used to establish whether a given sample of an organic substance is identical with another or not. This is because large number of absorption bands is observed in the IR spectra of organic molecules and the probability that any two compounds will produce identical spectra is almost zero. So if two compounds have identical IR spectra then both of them must be samples of the same substances.

20. 3. Studying the progress of the reaction Progress of chemical reaction can be determined by examining the small portion of the reaction mixture withdrawn from time to time. The rate of disappearance of a characteristic absorption band of the reactant group and/or the rate of appearance of the characteristic absorption band of the product group due to formation of product is observed. 4. Detection of impurities IR spectrum of the test sample to be determined is compared with the standard compound. If any additional peaks are observed in the IR spectrum, then it is due to impurities present in the compound.

21. 5. Quantitative analysis The quantity of the substance can be determined either in pure form or as a mixture of two or more compounds. In this, characteristic peak corresponding to the drug substance is chosen and log I0/It of peaks for standard and test sample is compared. This is called base line technique to determine the quantity of the substance.