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UV VISIBLE SPECTROSCOPY.pptx

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UV VISIBLE SPECTROSCOPY.pptx

  1. 1. UV VISIBLE SPECTROSCOPY S.PRAVALIKA 170417881067 SNVPMV
  2. 2. CONTENTS:  Introduction  Electronic transitions  Chromophores  Auxochromes  Spectral shifts  Solvent effect on absorption spectra
  3. 3. INTRODUCTION  Spectroscopy is the branch of science that deals with the study of interaction of electromagnetic radiation with matter.  It is the most powerful tool available for the study of atomic & molecular structure and is used in the analysis of a wide range of samples. The two main types are:  Atomic Spectroscopy; This Spectroscopy is concerned with the interaction of electromagnetic radiation with atoms which are commonly in the lowest energy state called as ground state.
  4. 4.  Molecular Spectroscopy ; This Spectroscopy deals with the interaction of electromagnetic radiation with molecule.  • Electromagnetic radiation consist of discrete packets of energy which are called as photons.  • A photon consists of an oscillating electric field (E) & an oscillating magnetic field (M) which are perpendicular to each other.
  5. 5. Wavelength (, lambda): Distance from one wave peak to the next. Units: m, cm, m, nm Frequency (, nu): Number of peaks that pass through a given point per second. Units: Cycles/second or s-1 or Hertz (Hz) Wave number Number of waves per cm.
  6. 6. ELECTRONIC TRANSITIONS
  7. 7. 1)σ → σ* transition.  σ electron from orbital is excited to corresponding anti-bonding orbital σ*.  The energy required is large for this transition.  e.g. Methane (CH4) has C-H bond only and can undergo σ → σ* transition and shows absorbance maxima at 125 nm. 2) π → π* transition • π electron in a bonding orbital is excited to corresponding anti- bonding orbital π*. • Compounds containing multiple bonds like alkenes, alkynes, carbonyl, nitriles, aromatic compounds, etc undergo π → π* transitions. • e.g. Alkenes generally absorb in the region 170 to 205 nm.
  8. 8. 3) n → σ* transition • Saturated compounds containing atoms with lone pair of electrons like O, N, S and halogens are capable of n → σ* transition. • These transitions usually requires less energy than σ → σ* transitions. • The number of organic functional groups with n → σ* peaks in UV region is small (150 – 250 nm). 4) n → π* transition • An electron from non-bonding orbital is promoted to anti- bonding π* orbital. • Compounds containing double bond involving hetero atoms (C=O, C≡N, N=O) undergo such transitions. • n → π* transitions require minimum energy and show absorption at longer wavelength around 300 nm.
  9. 9. 5) & 6) σ → π* transition & π → σ* transition .  These electronic transitions are forbidden transitions & are only theoretically possible.  Thus, n → π* & π → π* electronic transitions show absorption in region above 200 nm which is accessible to UV-visible spectrophotometer.
  10. 10. CHROMOPHORES  Defined as any isolated covalently bonded group that shows a characteristic absorption of Electromagnetic radiation in the UV or visible region.  It is a Greek word. Chroma = “color” & phoros = “bearer” Compound containing chromophore is CHROMOGEN  Eg: C=C, C=O, NO2 TYPES OF CHROMOPHORES  1. INDEPENDENT CHROMOPHORES: If one chromophore is required to impart colour Eg: Azo group –N=N-, Nitroso group – NO-  2. DEPENDENT CHROMOPHORES: If more than one chromophore is required to impart colour Eg: Acetone having one ketone group is colorless whereas diacetyl having two ketone groups is yellow.
  11. 11.  Identification of a chromophore depends on a number of factors as follows;  i) Spectrum consisting of a band near 300 mµ may contain two or three conjugated units.  ii) Absorption bands near 270-350 mµ with very low intensity εmax 10-100 are due to n → π*transitions of the carbonyl group.  iii) Simple conjugated chromophores such as dienes or α, β- unsaturated ketones have high εmax values, i.e., 10,000 to 20,000.
  12. 12. AUXOCHROMES  Auxochrome: A saturated/ unsaturated group with non bonding electrons when attached to chromophore altering both wavelength as well as intensity of absorption. Eg: OH, NH2, NHR, COOH, CN, Cl etc.. Two types: 1) Basic/positive auxochromic groups Effective in acid solutions  Eg: OH, OR, NHR etc. 2) Acidic/negative auxochromic groups Effective in alkaline solutions  Eg: NO, CO, CN etc.
  13. 13. SPECTRAL SHIFTS
  14. 14.  Substituents may have any of four effects on a chromophore : i. Bathochromic shift (red shift) – a shift to longer wavelength ; lower energy ii. Hypsochromic shift (blue shift) – shift to shorter wavelength ; higher energy iii. Hyperchromic effect – an increase in intensity. iv. Hypochromic effect – a decrease in intensity.
  15. 15.  Bathochromic shift: - Absorption shifted towards longer wavelength - Change of solvent/ auxochrome -Red shift/ bathochromic shift - n to * transition for carbonyl compounds experiences bathochromic shift when the polarity of the solvent is decreased.  Hypsochromic shift : - Shift towards shorter wavelength -(Blue shift) - Change of solvent towards higher polarity or removal of conjugation - Aniline – 280 nm (conjugation of pair of electrons of nitrogen with benzene ring) In acidic solution it will form - NH+ 3 , due to the removal of conjugation or removal of lone pair of electrons, the absorption takes place at lower wavelength 203nm.
  16. 16.  Hyperchromic shift: - Shift due to increase in intensity- εmax increase - Due to the introduction of auxochrome Ex: Pyridine - 257 nm and εmax is 2750; 2 – methyl pyridine 262 nm and εmax is 3560  Hypochromic shift: - Inverse of hyperchromic shift i.e., decrease of intensity - introduction of any group to the compounds which is going to alter the molecular pattern of the compound. ex: biphenyl absorption is at 250 nm and 19000 εmax - Whereas 2 –methyl biphenyl has an absorption of 237 nm and 10250 εmax
  17. 17. SOLVENT EFFECT Solvent is the one in which solute is dissolved completely. Solvent is the important factor for u.v./visible spectroscopy. Ideal solvent :  It should be cheaper.  It should be easily available.  It should be transparent & less polar Should not possess any kind of absorption when it is exposed to radiation.  A most suitable solvent which does not absorb the radiation.  Most commonly used solvent is 95% ethanol, it is cheap and is transparent down to 210m μ .  Commercial ethanol is not used because it is having benzene which absorbs strongly in u.v.region.
  18. 18.  The position as well as the intensity of absorption maximum get shifted for particular chromophore by change in the polarity of solvent.  The wavelenth for the non-polar compounds is usually shifted by change in polarity of solvent.  α, β - unsaturated carbonyl compounds show 2 different shifts  n -π* transition, absorption band moves to shorter wavelength. Due to H-bonding with solvent molecules occurs to lesser extent with the carbonyl group in the excited state.  E.g. A max of acetone is 279m μ in hexane, while in water 264m μ . 13
  19. 19.  π → π* transition, absorption band moves longer wavelength.  The dipole-dipole interactions with the solvent molecules lower the energy of the energy of the excited state more than that of the ground state. Value of A max in ethanol will be greater than that in hexane.  П* orbital gets more stabilised by H-bonding with the polar solvent like water & ethanol. Because of greater polarity of П* orbital. Thus small energy is needed for transition and absorption shows a red shift.

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