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Instrumental Techniques

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Published in: Instrumentation | Mechanical
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Presentation on Instrumental Techniques.

Trinity A / Chandigarh

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  1. Instrumental Techniques
  2. Syllabus of CU Fundamentals of spectroscopy, electromagnetic spectrum, absorption and emission spectra, atomic and molecular spectroscopy. Electronic (UV-visible) Spectroscopy —Introduction chromophores & Auxochrome concept, absorption and intensity shifts; Applications of electronic spectroscopy. Infrared spectroscopy-Introduction, Principle of IR spectroscopy Fundamental vibrations selection rules and Applications to simple organic molecules (effects of masses of atoms, Bond strength, Nature of substituent, Hydrogen bonding on IR frequencies). Theory of Raman spectra, Conditions for Raman spectroscopy, Characteristics of Raman lines, Applications of Raman spectroscopy
  3. Instrumental technique Instrumental techniques deals with the experimental techniques for analyzing the compounds qualitatively or quantitatively with maximum precession or accuracy. The branch of chemistry which deals with instrumental techniques is called ANALYTICAL CHEMISTRY. Some important needs of instrumental techniques 1. 2. 3. 4. 5. Useful in the determination of structure of compounds Useful in the study of intermediates during reactions. Useful in the determination of quality of the products. Useful in checking air pollution/soil pollution/water pollution. Useful in medical science for diagnosing diseases.
  4. Spectroscopy •Spectroscopy: It is the branch of science that deals with the study of interaction of matter with light or electromagnetic radiation with matter.
  5. Fundamentals of Spectroscopy >Spectroscopy may be defined as study of interaction of electromagnetic radiation with molecules. > The principle is based on the measurement of spectrum of a sample containing atoms / molecules. >Spectrum is a graph of intensity of absorbed or emitted radiation by sample verses frequency (v) or wavelength (X). >Spectrometer is an instrument design to measure the spectrum of a compound.
  6. Fundamentals of Spectroscopy (contd) 0000 The energy changes within a molecule during emission or absorption of electromagnetic radiation are quantized. Energy change (AE) = hy =h. c/X= h! Where, Y frequency X = wavelength h = Planck's constant c = velocity of light = wave number There are two types of spectra 1. Absorption spectra 2. Emmision spectra
  7. Absorption Spectroscopy: When a substance is irradiated with electromagnetic radiation, the energy of the incident photons may be transferred to the molecules, raising them from the ground state to excited state. This process is called absorption and the resultant spectra is called absorption spectrum. Energy absorption occurs only when the energy difference between the ground and excited state is exactly matched with the energy of the incident electromagnetic radiation. The energy absorbed (AE) by a molecule may bring changes in one or more of its energy levels such as rotational, vibrational and electronic. Example of absorption spectra: UV (185 - 400 nm) / Visible (400 - 800 nm) Spectroscopy, IR Spectroscopy (0.76 - 15 um)
  8. Emission Spectroscopy: Molecule give emission spectra when subjected to heat or electric discharge. The molecule obtain the necessary energy to become excited. On returning to their lower energy state, molecules may emit radiation which is a result of transition of a molecule from excited state to lower energy state, usually the ground state. This excess energy is emitted as a photon and the corresponding frequency is recorded as the emission spectrum. If the transition is from upper energy state (El) to lower energy state (E2), the frequency (y) of the emmision spectrum is given by AE/h Example of emission spectrum: Mass Spectroscopy
  9. Excited state Ground state Absorption Chemistry Energy losses
  10. ElectromagneticRadiatio Electromagnetic radiation is a simple harmonic wave with the property of a sine wave. It travels in a straight line unless it is refracted or reflected. It has both electrical and magnetic components at right angles to each other as shown in the figure given below. Direction af propagation of radiation Magnetic field component Wa Electric field component nght eledromagneticwave
  11. Absorption Spec roscopy: Absorption spectra can be of different types on the basis of radiation absorbed. 1 .UV and Visible Spectroscopy 2. IR Spectroscopy 3. Microwave spectra Absorption spectrometer is used to record the spectra of molecules. It consists of A. A suitable source of electromagnetic radiation B. A system to analyze the radiation or monochromator C. An appropriate detector to detect the intensity of the radiation absorbed. The final absorption spectra is displayed on a computer screen and can be recorded on a chart paper.
  12. nteraction of EMR with ma Electronic Energy Levels: er At room temperature the molecules are in the lowest energy levels Eo. When the molecules absorb UV-visible light from EMR, one of the outermost bond / lone pair electron is promoted to higher energy state such as E E .. .En, etc is called as electronic transition and the difference is as: AE -EO where (n = 1, 2, 3, ... etc) AE = 35 to 71 kcal/mole
  13. nteractlon o Vibrational Energy Levels: R with matt r These are less energy level than electronic energy levels. The spacing between energy levels are relatively small i.e. 0.01 to 10 kcal/mole. For e.g. when IR radiation is absorbed, molecules are excited from one vibration level to another or it vibrates with higher
  14. action o Rotational Energy Levels: ' h-matter These energy levels are quantized & discrete. The spacing between energy levels are even smaller than vibrational energy levels. AE < AE rotational vibrational electronic
  15. High energy y rays x rays Spectroscopy ELECTROMAGNETIC SPECTRUM Microwaves Radiowaves Low energy uv vis ir 10—8 10-6 esr nmr 10-4 10—2 102 Wavelength (m) on a logarithmic scale VISIBLE SPECTRUM Ultraviolet Violet Blue Green 500 Yellow Orange 600 Red 700 104 Infrared 800 400 Fig. 10.1 Wavelength (nm) on a linear scale nm = cm Wavelength regions of the electromagnetic spectrum.
  16. UV-Visible Spectroscopy: >Ultraviolet-visible spectroscopy involves the absorption of ultraviolet/visible light by a molecule causing the promotion of an electron from a ground electronic state to an excited electronic state. >Ultraviolet/Visible light: wavelengths (l) between 190 and 800 nm
  17. U V-visible Spectroscopy The two main properties of an absorbance peak are: 1. 2. Absorption wavelength Absorption intensity max o max Wavelength
  18. Principle According to Born Openheimer Approximation the Toatal Internal Energy of Molecules i.e. E -E +E +Evib+Erot+E total — elec nucl trans Where : electronic transitions (U V, X-ray) • vibrational transitions (Infrared) E vib Erot: rotational transitions (Microwave) : nucleus spin (nuclear magnetic resonance) or (MRI: magnetic resonance imaging)
  19. Vibrational electronic levels Lid Rotational electronic levels
  20. Absorption and intensity shift Chromophores: functional groups that give electronic transitions. For e.g. : Ethene, Ethylene Auxochromes: substituents with unshared pair e's like OH, NH, SH when attached to chromophore they generally move the absorption max. to longer X. For e.g. : Nitro group
  21. 1. Absorption and in ensn y s t Hypsochromic shift (Blue shift): It leads to shift of absorption maximum towards shorter wavelength. It happens either the removal of conjugation or change of polarity of the solvent. For example absorption maximum of aniline shifts from 280 to 200 micrometer in acidic solution, which results in formation of C6H5NH + from C6H5NH2. This happens because the lone pair of electron on nitrogen is no longer present in —NH2 group, hence there is a blue shift. 2. Bathochromic shift (Red shift): It leads to shift of absorption maximum towards longer wavelength due to presence of auxochrome or by change of solvent. For example: n to transition for carbonyl compounds experience bathochromic shift when polarity of solvent is lowered.
  22. Spectral nomenclature of shifts hypsochromic hyperchromic hypochromic 400 blue red bathochromic 800 X (nm) 22
  23. Types of Electronic Transition UV/VIS z Vacuum UV or Farw nm o (anti-bonding) (anti-bonding) n (non-bonding) (bonding) o (bonding)
  24. Infrared spectroscopy (IR) Anfrared spectroscopy (IR) measures the bond vibration frequencies in a molecule and is used to determine the functional groups. > The infrared region of the spectrum encompasses radiation with wave numbers ranging from about 12,500 to 50cm-1 (or) wave lengths from 0.8 to 200P. Anfrared region lies between visible and microwave region.
  25. Applications of UV-Visible Spectroscopy: Detection of Impurities Structure elucidation of organic compounds Quantitative analysis Molecular weight determination Distinction between Cis & Trans isomerism Effect of Conjugation
  26. Infrared spectroscopy (JR) contd > The infrared region constitutes 3 parts a) The near IR (0.8 -2.5gm) (12,500-4000cm-1) b) The middle IR (2.5 -15 gm) (4000-667cm-1) i) Group frequency Region (4000-1500cm-1) ii) Finger print Region (1500-667cm-1) c) The far IR (15-200gm) (667-50cm-1) >Most of the analytical applications are confined to the middle IR region because absorption of organic molecules are high in this region. > Wave number is mostly used measure in IR absorption because wave numbers are larger values & easy to handle than wave length which are measured in gm. E = hv = hc/X = hcv At gives sufficient information about the structure of a compound
  27. Principle of spectroscopy (LR) In any molecule it is known that atoms or groups of atoms are connected by bonds. These bonds are analogous to springs and not rigid in nature. As a covalent bond oscillates — due to the oscillation of the dipole of the molecule — a varying electromagnetic field is produced. The greater the dipole moment change through the vibration, the more intense the EM field that is generated. When a wave of infrared light encounters this oscillating EM field generated by the oscillating dipole of the same frequency, the two waves couple, and IR light is absorbed. The coupled wave now vibrates with twice the amplitude IR beam from spectrometer
  28. There are 2 types of vibrations. 1) Stretching vibrations 2)Bending vibrations 1)Stretching vibrations: in this bond length is altered. They are of 2 types . a) symmetrical stretching: 2 bonds increase or decrease in length. b) Asymmetrical stretching: in this one bond length is increased and other is decreased. out ot rcAqe 2)Bending vibrations: These are also called as deformations. In this bond angle is altered. These are of 2 types a) in plane bending— scissoring, rocking b) out plane bending— wagging, twisting
  29. Stretching vibrations Molecular Vibration Bending vibrations Near Near Out-of-plane wagging Near Far Out-of-plane twisting Symmetric plane scissoring Asymmetric
  30. NUMBER OF VIBRATIONAL MODES A molecule can vibrate in many ways, and each way is called a vibrational mode. If a molecule contains 'N' atoms, total number of vibrational modes >For linear molecule it is (3N-5) >For non linear molecule it is (3N-6) >E.g: H20, a non-linear molecule, will have 3 x 3 degrees of vibrational freedom, or modes.
  31. Condition of active mo ecu e During the vibration of a molecule, if there occurs a change in dipole moment then the molecule will be IR active. Homo nuclear diatomic molecules like 02, H2, N2 etc are IR inactive because during vibration of these molecules there is no change in the dipole moment. However hetero nuclear molecules like HCI, H20, C02 are IR active because during vibration of these molecules there is a change in the dipole moment. Because of this reason IR absoprtion spectra can be used to detect H20, C02 in air.
  32. Table 1 Absorpton frequencies of some comrnon bonds (shown in bold typo) C-N bond -11 -CEC-.H -CEN (stretch) (stretch) (stretch) (stretch) (stretch) (stretch) (stretch) (stretch) (stretch) (stretch) (stretch) (stretch) (stretch) (bending) (bending) (bending) type of compound alkanes alkenes, aromatics alkynes alcohols. phenols carboxylic acids amines aldehvdes alkenes aromatics alkynes aldehyde. ketones, carboxylic acids nitriles amines alkanes alkanes alkanes frequctrcv 3000-3100 (free) 32m--3500 (H-lx»nded) (broad) 2500-33(X) 3300-350k) (doublet for NH2) 2720 and 2820 2100-2270 1680-1740 2220-2260 1180-1360 1375 1460 (methyl and methylene) 1370 and 1385 (isopropyl split)
  33. Effect of E-boncling on spectra Intra and intermolecular H-bonding can be distinguished by IR spectra. As concentration is decreased (by increasing dilution) the IR absorption band, due to intermolecular H-bonding shifted towards higher value. On the other hand, the absorption band due to intramolecular H-bonding remain unchanged by change in concentration of the solvent. Normally, O-H bond of any alcohol exhibits H-bonding. Due to presence of H-bonding, O-H bond became weaker and consequently it's stretching frequency decreases and absorption band shifted towards lower value and peak shape is broad.
  34. Applications of 'Pharmaceutical research 'Forensic investigations 'Detection of hydrogen bonding 'Detection of functional group 'Structure eludication of Organic compounds 'Environmental and water quality analysis methods 'Biochemical and biomedical research 'Coatings and surfactants a ysis
  35. Raman Spectroscopy When a monochromatic radiation of frequency is assed through a non absorbing medium,it is found that most of it is transmitted without any change, and sorne of it is scattered. If the scattered energy is analyzed by means of a spectrometer, the bulk of the energy is found at the frequency of the incident beam o C but a small portion of the scattered energy will be found at frequencies —o C] VM. The scattering of radiation with change of frequency is called Raman scattering.
  36. Raman Spectroscopy In Raman spectroscopy, by varying the frequency of the radiation, a spectrum can be produced, showing the intensity of the exiting radiation for each frequency. This spectrum will show which frequencies of radiation have been absorbed by the molecule to raise it to higher vibrational energy states.
  37. Raman Spectroscopy Stokes vs. Anti Stokes Anti- Stokes Stokes Aton•s are at a certain energy level at any given tirne. As a laser light hits the atorn, it is excited and reaches a higher level of energy, and then is brought back down. If an atorn is at a given energy level, it can be excited then fall below the original level. Anti-stokes spectrurn are rnirror spectrurns of Stokes Rarnan Spectrurns
  38. Energy Scheme for Photon Scattering IR Absorption Virtual State hvo hvo Rayleigh Scattering (elastic) hvo hvo— hvm Stokes Scattering hvo+hv Eo+hvm Anti-Stokes Scattering Raman (inelastic) The Raman effect comprises a very small fraction, about 1 in 107 of the incident photons.
  39. Raman spectrum A Raman spectrum is a plot of the intensity of Raman scattered radiation as a function of its frequency difference from the incident radiation (usually in units of wavenumbers, cm-I). This difference is called the Raman shift. Rayloigh Stokes o Raman Spectrum of CC14
  40. Mutual Exclusion Principle For molecules with a center of symmetry, no IR active transitions are Raman active and vice versa Symmetric molecules IR-active vibrations are not Raman-active. Raman-active vibrations are not IR-active. Raman active IR inactive Raman inactive IR active
  41. Advantages of Raman over IR Simpler and cheaper instrumentation. Less instrument dependent than Raman spectra because IR based on measurement of intensity ratio. Lower detection limit than (normal) Raman. Background fluorescence can overwhelm Raman, spectra are More suitable for vibrations of bonds with very low polarizability (e.g. C F),
  42. 1. 2. 3. 4. References : Izake, E. , Forensic and homeland security applications of modern portable Raman spectroscopy. Forensic Science International. (2010), vol S.E.J. Bell, DOT. Burns, A.C. Dennnis, L.J. Matchett, JOS. Speers, Composition and profiling of seized ecstasy tablets by Raman spectroscopy, Analyst 125 (10) (2000) 541 — 544 Sharma Y. R. , A Text book of Elementary Organic Spectroscopy,2006, S.Chand Publication, New Delhi. Jain P C and Jain M: Engineering Chemistry (15th Edition) 2006 Dhanpat Rai Publishing Company, New Delhi.

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