O slideshow foi denunciado.
Utilizamos seu perfil e dados de atividades no LinkedIn para personalizar e exibir anúncios mais relevantes. Altere suas preferências de anúncios quando desejar.

Ultraviolet (uv) and visible spectroscopy ppt

140 visualizações

Publicada em

Instrumentation and principle of Ultraviolet (uv) and visible spectroscopy

Publicada em: Educação
  • Seja o primeiro a comentar

Ultraviolet (uv) and visible spectroscopy ppt

  1. 1. Ultraviolet (UV) and Visible Spectroscopy HemaT MSc Biochemistry Bharathiar University 2018-2020
  2. 2. What is Spectroscopy? ■ Spectroscopy is a necessary tool for structure determination. ■ Organic chemists use spectroscopy as a necessary tool. ■ The first spectroscope was invented in 1859 by the German chemist Robert Wilhelm Bunsen and the German physicist Gustav Robert Kirchhoff. ■ Spectroscopy may be defined as the study of the quantized interaction of electromagnetic radiations with matter.
  3. 3. Difference between spectrometer and spectrophotometer: ■ SPECTROMETER: An optical spectrometer ( spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. ■ SEPCTROPHOTOMETER: A spectrophotometer is an analytical instrument used to quantitatively measure the transmission or reflection of visible light, UV light or infrared light.
  4. 4. What is electromagnetic spectrum? ■ Electromagnetic spectrum covers a very wide range of electromagnetic radiations from cosmic rays to radio waves at the other end. ■ The arrangement of all types of electromagnetic radiations in order of their wavelengths or frequencies is known as complete electromagnetic spectrum.
  5. 5. Ultraviolet (UV) andVisible Spectroscopy ■ Ultraviolet (UV) andVisible Spectroscopy deals with the recording of the absorption of radiations in the ultraviolet and visible regions of the electromagnetic spectrum. ■ The ultraviolet region extends from 10 to 400 nm. ■ Near ultraviolet (quartz) region (200- 400 nm) ■ Far or vacuum ultraviolet region (10- 200nm) ■ The visible region extends from 400 to 800 nm.
  6. 6. Ultraviolet (UV) and Visible Spectroscopy
  7. 7. Principle of UV-Vis spectroscopy ■ UV-Visible spectroscopy follows the Beer- Lambert Law. ■ Beer’s Law The intensity of a beam of monochromatic light decreases exponentially with the increase in concentration of the absorbing substance. ■ Lambert’s Law When a beam of light is allowed to pass through a medium, the rate of decrease of intensity with the thickness of medium is directly proportional to the intensity of the light. A= Ecl
  8. 8. Instrumentation ■ Radiation source: Hydrogen- discharge lamp is the most commonly used source of radiation in the UV region. A deuterium- discharge lamp is used in its place when more intensity is desired. ■ Monochromator: It disperses the radiations obtained from the source into their separate wavelengths. – Prism – Grating made up of quartz ■ Detectors: These have photocells or photo multiplier tubes which generate voltage proportional to the radiation energy that strikes them.
  9. 9. ■ Amplifier : The spectrometer has balancing electronic amplifier which subtracts the absorption of the solvent from that of the solution electronically. ■ Recorder : It automatically records the spectrum as a plot of wavelengths of absorbed radiations against absorbance or molar absorptivity.
  10. 10. Sample handling ■ UV- visible spectra are usually recorded either in very dilute solutions or in the vapour phase. ■ The sample is dissolved in some suitable solvent which does not itself absorb radiation in the region under investigation. ■ Commonly used solvents are cyclohexane, 1, 4- dioxane, water and 95% ethanol. ■ The chosen solvent should be inert to the sample.
  11. 11. Theory of U-VVisible Spectroscopy ■ U-V visible absorption spectra originate from electronic transitions within a molecule. ■ These transitions involving promotion of valence electrons from the ground state to the higher-energy state(excited state) are called electronic excitations and are caused by the absorption of radiation energy in the UV-visible regions of the electromagnetic spectrum. ■ Since various energy levels of molecules are quantized, a particular electronic excitation occurs only by the absorption of specific wavelength of radiation corresponding to the required quantum of energy.
  12. 12. ElectronicTransitions ■ According to molecular orbital theory, the excitation of a molecule by the absorption of radiation in the UV-visible regions involves promotion of its electrons from a bonding, or non bonding (n)orbital to an antibonding orbital. ■ σ - σ * transition The transition or promotion of an electron from a bonding sigma orbital to the associated antibonding sigma orbital is σ - σ * transition. It is a high energy process because σ bonds are generally very strong. ■ n - σ * transition Transition or promotion of an electron from a non-bonding orbital to an antibonding sigma orbital is designated as n - σ * transition. Compounds containing non bonding electrons on a heteroatom are capable of absorption due to n - σ *Transitions.These transitions require lower energy than σ- σ* transitions
  13. 13. ■ π- π* transition The transition or promotion of an electron from a π bonding orbital to a π antibonding orbital is designated π- π* transition.These type of transitions occur in compounds containing one or more covalently unsaturated groups like C=C,C=O,NO2 etc., π- π*Transitions require lower energy than n - σ * transitions. ■ n - π* transition The transition or promotion of an electron from a non-bonding orbital to a π antibonding orbital is designated n - π*.This transition reqires lowest energy.
  14. 14. Relative energies of electronic transitions
  15. 15. Formation of Absorption Bands ■ Since the energy required for each electronic transition is quantized, the UV-visible spectrum is expected to exhibit a single, discrete line corresponding to each electronic transition. ■ Broad absorption bands are usually absorbed.
  16. 16. Designation of Absorption Bands ■ UV-visible absorption bands may be designated by the type of electronic transition from which they originate. ■ K-Bands: These bands originate from π- π* transitions in compounds having π- π* conjugated system. ■ R-Bands:These bands originate from n - π* transitions of a single chromophoric group. ■ B-Bands:These bands originate from π- π* transitions in aromatic or heteroaromatic compounds. ■ E-Bands: Similar to B-Bands , these are characteristic of aromatic and heteroaromatic compounds and originate from π- π* transitions of the ethylenic bonds present in the aromatic ring.
  17. 17. Absorption and Intensity Shifts ■ Bathochromic Shift or Effect. The shift of an absorption maximum to a longer wavelength due to the presence of an auxochrome or solvent effect is called a bathochromic shift or red shift. ■ Hypsochromic Shift or Effect. The shift of an absorption maximum to a shorter wavelength is called hypsochromic or blue shift.This is caused by the removal of conjunction or change in the solvent polarity. ■ Hyperchromic Effect: An effect which leads to an increase in absorption intensity Emax is called hyperchromic effect.The introduction of an auxochrome usually causes hyperchromic shift. ■ Hypochromic Effect: An effect which leads to a decrease in absorption intensity Emax is called hypochromic effect.This is caused by the introduction of a group which distorts the chromophore.
  18. 18. Applications of Ultraviolet andVisible Spectroscopy ■ Detection of a functional group (Chromophore). ■ Detection of Conjugation and Elucidation of its nature. ■ Study of Extent of Conjugation. ■ Distinction between Conjugated and Unconjugated Compounds. ■ Study of Strain. ■ Determination of Configurations of Geometrical isomers. ■ Study ofTautomerism. ■ Confirmation of Suspected Phenols and Aromatic Amines. ■ Study of Structural Features in Different Solvents.
  19. 19. Thank You

×