Raman spectroscopy (1)

ADVANCED SPECTRAL ANALYSIS

1. RAMAN SPECTROSCOPY Presented by Madhura D 1 st Mpharm Pharmaceutical Chemistry Government college of Pharmacy, Bengaluru.

2. Contents RAMAN SPECTROSCOPY INTRODUCTION PRINCIPLE INSTRUMENTATION APPLICATIONS

3. INTRODUCTION Raman spectroscopy deals with the scattering of light . Homonuclear diatomic molecules such as H 2 , N 2, etc. don’t show IR spectra. They do show Raman spectra There will be a change in the induced dipole moment at the vibrational frequency. All modes are either IR active or Raman active.

4. PRINCIPLE Rayleigh scattering: It was found that the frequency of the scattered light was same as the frequency of the incident radiations. In 1923, Smekel predicted that a substance on irradiation of monochromatic light, the scattered light contain radiation with different frequencies than the frequency of incident light.

5. In 1928, Sir C.V Raman discovered that when a beam of monochromatic light irradiated on a substance, the scattered light contains some additional frequencies over and above that of incident frequency. The scattered lines by Raman effect are known as Raman lines. The lines having wavelengths greater than that of the incident wavelength are called Stoke’s lines. lines with shorter wavelength are known as Anti- Stoke’s lines.

6. If v i is the frequency of incident radiation and v s radiation scattered by the given molecular species then the Raman shift Δv , is defined by the following equation. Δ v = v i - v s

7. CHARACTERISTICK PROPERTIES OF RAMAN LINES The intensity of Stoke’s lines is always greater than the corresponding Anti- Stoke’s lines. Raman shift Δv generally lies within the far and near infrared regions of the spectrum. Raman lines are symmetrically displaced about the parent lines. When the temperature rises their individual separation from the parent lines decrease.

8. DIFFERENCES BETWEEN RAMAN SPECTRA AND INFRARED SPECTRA RAMAN SPECTRA Its due to the scattering of light by the vibrating molecules. It is the result of absorption of light by vibrating molecules. Polarizability of the molecule will decide whether the Raman spectra will be observed or not. The presence of permanent dipole moment in a molecule may be regarded as a criterion of infrared spectra. It can be recorded only in one exposure. It requires at least 2 separate runs with different prisms to cover the whole region of infrared. Water can be used as a solvent Water cannot be used as a solvent because it is opaque to infrared radiation. The method is accurate but not sensitive. The method is accurate and sensitive. Optical systems are made of glass or quartz. Optical systems are made up of special crystals such as CaF 2, NaBr , etc. INFRARED SPECTRA

9. MECHANISM OF RAMAN EFFECT Classical theory of Raman effect: when an electric field is applied to a molecule, its electron and nuclei are displaced. Thus an induced dipole moment is produced in the molecule due to the displacement of the electrons and the nuclei, and the molecule gets polarized. Suppose E is the strength of electronic field and µ is the magnitude of the induced dipole moment µ=αE where α is the polarizability of the molecule .

10. The electric field experienced by each molecule varies according to the following equation. E=E O Sin 2π vt (1) And therefore, the induced dipole undergoes oscillation of frequency v i.e µ =αE=αE Sin 2 π vt (2)

11. Effect of vibration : if a molecule undergoes vibratory motion, this changes the polarizability periodically then the oscillating dipole will have superimposed upon it the vibrational oscillation. Suppose a vibration of frequency v vib changes the polarizabilty . Then one can write. α = α +β sin 2π v vib t (3) α = the equilibrium polarizability and β = the rate of changes of polarizability with the vibration Substituting equation (3) in (1) we get µ =αE=(α o + Sin 2 π v ib t ) E sin 2 π vt

12. The induced oscillating dipole has frequency components v+v vib or v- v vib as well as the exciting frequency v. Thus Raman shift= ( v+v vib ) –v = v vib We conclude that for a molecular vibration or rotation to be active in Raman spectrum, it must cause a change in the Raman spectrum. Effect of rotation : when a diatomic molecule rotates the orientation of a molecule varies with respect to the electric field of rotation. If the molecule is not optically isotropic,the polarization will vary with time.

13. The Quantum theory of Raman effect : Raman effect can be regarded as the outcome of the collisions between the light photons and molecules of substance. consider a molecule of mass ‘m’ in the energy state E p is moving with a velocity v and is colliding with a light photon hv . Suppose this molecule undergoes a change in its energy state as well as in its velocity. Let this new energy state be E q and the velocity be v’ after suffering a collision. If we apply the principle of conservation of energy, we can write E p +1/2 mv 2 +hv=E q +1/2mv’ 2 +hv’

14. Change in velocity of molecule is negligible. Thus equation can be written as Ep+Hv = Eq+Hv ’ or V’=V+ΔV From the above equation three cases may arise If E p = E q , the frequency difference ΔV ( raman shift) is zero. E p > E q then v’>v which refers to the anti- stoke’s lines. If E p < E q then v’<v this corresponds to stoke’s lines

15. INSTRUMENTATION The basic instrument for obtaining a Raman spectrum is shown in figure.

16. Source of light :

17. Welsh and Craford devised a lamp known as Toronto lamp which consists of a tube water cooled low pressure mercury lamp. In this the sample tube is kept in the central axis of the spiral. Before the development of the laser as an excitation source. Raman spectroscopy suffered various disadvantages. Samples should to clear, colorless , non fluorescent liquids. The low intensity of the Raman effect required relatively concentrated solutions. Much larger volumes of sample solutions were needed than for infrared spectroscopy.

18. Helium- Neon Laser The tube is filled with a 7:1 mixture of helium neon gas for optimum of 6328 A laser line. To start the laser 5 to 10k V dc is used thereafter the beam can be maintained on the lower operating voltage.

19. Filters: For getting monochromatic radiation filters are used. They may be made of nickel oxide, glass or quartz glass. Sometimes a sutaible coloured solution such as an aqueous solution of ferricyanide or iodine in carbon tetrachloride may be used as a monochromator .

20. Sample holder The type of sample holder to be used depends upon the intensity of source, and the nature and the availability of the sample. The study of Raman spectra of gases requires sample holders which are generally bigger in size than those for liquids.

21. Spectrograph : The spectrograph used for the study of Raman spectrum should possess the following characteristics. It should have large gathering power. Special prisms of high resolving power should be employed. A short focus camera should be employed.

22. WORKING Light from the Helium-Neon laser beam is allowed to enter the sample compartment horizontally. Then the Raman scattering from the sample cell is focused on the monochromator entrance slit. If depolarization measurements are to be made, the Raman emission is first allowed to pass through an analyser prism before entering a monochromator which is a double pass Littrow - mounted grating type. A 13Hz chopper is used between the first and second passes and the detector is made to respond only to this signal. The primary function of Raman spectrometer is clean rejection of the intense Rayleigh scattering and detection of the weak raman shited components.

23. laser Raman spectrometer

24. A laser raman spectrometer has been shown in fig. which consist of 2 parts the laser excitation unit and the spectrometer unit and after passing through this unit, it illuminates the sample. The raman scattering collected at 90 to the exciting laser beam, is focused on the entance slit of a 0.5 m focal length, Czerny- turner grating double mono chromator . A polarization analyser is placed between the condenser lens and the polarization scrambler when measuring the polarized raman spectrum of single crystals. The spectrometer arranged in back to back configuration. The Raman scattered light is dispersed using gratings with 1200 grooves/mm and finally passes through the monochromator exit slit and onto the photocathode of the photomultiplier tube.

25. These signals are displayed on the two pen digitals X-Y recorder which is linked to the monochromator driven by the master pulse clock. Intensity of Raman peaks The intensity power of a Raman peak has been found to depend upon the following factors Polarizability of the molecule Intensity of the source and Concentration of the active group.

26. APPLICATIONS Nowadays Raman spectra are obtained from miligrams or micrograms of sample. Spectra of crystal powders, single crystals, polymers and colored substances may be recorded. Its an useful tool for solving the intricate research problems concerning the constitution of compounds.

27. Applications in Inorganic Chemistry Structure of CO 2 : if it is assumed to be a symmetrical linear molecule O-C-O, one should expect fundamental lines (V 2 and V 3 ) in infrared and one (V 1 ) in the raman . This has been experimentally confirmed and following assignments are made V 1 =1340 cm -1 , V 2 =667 cm -1 , V 3 ­=2349cm­­ -1 Structure of N 2 O: If it is unsymmetrical i.e N-N-O, v 1 should become active in the infrared as well as in Raman. This additional fundamental has been observed in both IR and Raman spectra. This proves that the structure of N 2 O is N-N-O.

28. Structure of mercurous salts: when a Raman spectrum of an aqueous solution of mercurous nitrate is recorded, it shows a line which is absent in the spectra of other metal nitrates. This line may be attributed to the vibration of Hg-Hg covalent bond in the diatomic molecule Hg 2 . Nature of bonding: when we record the infrared spectra of tetrahedral complexes ML 4 (ZnCl 4 , CdCl 4 ) and octahedral complexes ML 6 (SiF 6 ), no bands will be observed because only the bond length changes during vibration.But intense lines are observed in Raman spectra. from the M-L bond stretching force constant it becomes possible to obtain useful information about the strength of metal ligand bond

29. Hydrogen cyanide : its Raman spectrum exhibits two lines at 2062 cm -1 and 2094 cm -1 . these may be due to two isomers in dynamic equilibrium. Applications in Physical chemistry The amorphous state of a substance gives rise to broad and diffused bands while crystalline state gives sharp lines. In the case of the phenomena of electrolyte disassociation, the intensity of raman lines enables us to determine the number and nature of ions produced . The degree of hydrolysis of a salt can be determined by measuring the relative intensities of a set off lines characteristics of the base, the acid or the salt.

30. Ionic equilibria in solution. Consider the dissociation process. HNO 3 + H 2 O ionised into H 3 O = +NO 3 - By monitoring the intensity of the nitrate ion and the nitric acid in the Raman spectrum it is possible to calculate the dissociation constant of nitric acid .

31. Applications in organic chemistry A Raman spectrum provides information about the following The presence or absence of specific linkages in a molecule. The structure of simple compounds Study of isomers The presence of impurities in dyes Classification of compounds It has been observed that Raman lines generally lie in the region 500-3500cm -1 .

32. Benzene : its Raman spectrum shows two strong Raman lines at 995 cm -1 and 1050 cm -1 which are due to C-C and C≡C linakages respectively Determination of location of groups in benzene: Meta substituted benzene exhibit intense strongly polarised line at 995 cm-1 which is not present in ortho and para compounds. Determination of Cis and trans isomers: determination of cis and trans isomers is possible.C 2 H 2 Cl 2 mixture of cis and trans isomers can be separated. Because only trans isomer have center of symmetry. Presence of tautomers . Raman effect can be used to show the presence of tautomers . For example the equilibrium mixture of acetic acid shows raman effect due to both C-C and C=O groups.

33. Reference Gurudeep R Chatwal, Sham K Anand . Instrumental methods of chemical analysis;2013; 2.83-2.101