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Pranita kore
Pranita kore
Pranita kore
Pranita kore
Pranita kore
Pranita kore
Pranita kore
Pranita kore
Pranita kore
Pranita kore
Pranita kore
Pranita kore
Pranita kore
Pranita kore
Pranita kore
Pranita kore
Pranita kore
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Pranita kore

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  • 1. JSPM’s Charak College of Pharmacy & Research,GAT No.720/1&2,WAGHOLI, Pune-NAGAR ROAD,PUNE-412 207 Theory of U.V Spectrophotometry Presented By, Guided By, Miss. Pranita Kore Dr. Rajesh J Oswal Prof.Sandip Kshirsagar Department Of Pharmaceutical Chemistry 6/14/2012 1
  • 2.  Introdution Electronic Transition’s Chromophores Auxochromes Beer’s-Lambert Law 6/14/2012 2
  • 3. I.Introduction A.UV radiation and Electronic Excitations: 1.The difference in energy between molecular bonding, non-bonding and anti-bonding orbitals ranges from 125-650 kJ/mole 2.This energy corresponds to EM radiation in the ultraviolet (UV) region, 100-350 nm, and visible (VIS) regions 350-700 nm of the spectrum 3.For comparison, recall the EM spectrum: -rays X-rays UV IR Microwave Radio Visible 4.Using IR we observed vibrational transitions with energies of 8-40kJ/mol at wavelengths of 2500-15,000 nm 5.For purposes of our discussion, we will refer to UV and VIS spectroscopyas UV 6/14/2012 3
  • 4. Introduction B. Band Structure 6. When these energy levels are superimposed, the effect can be readily explained – any transition has the possibility of being observed Disassociation R1 - Rn V4 R1 - Rn V3 R1 - Rn V2 E1 Vo V1 R1 - Rn R1 - Rn Energy Disassociation R1 - Rn V4 R1 - Rn V3 R1 - Rn V2 V1 R1 - Rn E0 Vo R1 - Rn 6/14/2012 4
  • 5. II. Electronic transitionsThe absorption of UV or visible radiation corresponds to the excitation of outerelectrons.There are two types of electronic transition which can be considered;1.Transitions involving p, s, and n electrons2.Transitions involving charge-transfer electronsWhen an atom or molecule absorbs energy, electrons are promoted fromtheir ground state to an excited state.In a molecule, the atoms can rotate and vibrate with respect to each other.These vibrations and rotations also have discrete energy levels, which can beconsidered as being packed on top of each electronic level. 6/14/2012 5
  • 6. Absorbing species containing p, s, and n electronsAbsorption of ultraviolet and visible radiation in organic molecules isrestricted to certain functional groups (chromophores) that containvalence electrons of low excitation energy.The spectrum of a molecule containing these chromophores iscomplex.This is because the superposition of rotational and vibrationaltransitions on the electronic transitions gives a combination ofoverlapping lines.This appears as a continuous absorption band. 6/14/2012 6
  • 7. Possible electronic transitios of , , and n electrons are; 6/14/2012 7
  • 8. s -s* TransitionsAn electron in a bonding s orbital is excited to the correspondingantibonding orbital. The energy required is large.For example, methane (which has only C-H bonds, and can onlyundergo s -s* transitions) shows an absorbance maximum at 125 nm.Absorption maxima due to s - s* transitions are not seen in typical UV-Vis. spectra (200 - 700 nm)n -s* TransitionsSaturated compounds containing atoms with lone pairs (non-bondingelectrons) are capable of n - s* transitions.These transitions usually need less energy than s - s * transitions. Theycan be initiated by light whose wavelength is in the range 150 - 250nm. The number of organic functional groups with n - s* peaks in theUV region is small. 6/14/2012 8
  • 9. n - p* and p - p* TransitionsMost absorption spectroscopy of organic compounds is based ontransitions of n or p electrons to the p* excited state.This is because the absorption peaks for these transitions fall in anexperimentally convenient region of the spectrum (200 - 700 nm).These transitions need an unsaturated group in the molecule toprovide the p electrons. 6/14/2012 9
  • 10. III.Chromophores A. Definition 1. Remember the electrons present in organic molecules are involved in covalent bonds or lone pairs of electrons on atoms such as O or N 2. Since similar functional groups will have electrons capable of discrete classes of transitions, the characteristic energy of these energies is more representative of the functional group than the electrons themselves 3. A functional group capable of having characteristic electronic transitions is called a chromophore (color loving) 4. Structural or electronic changes in the chromophore can be quantified and used to predict shifts in the observed electronic transitions 6/14/2012 10
  • 11. III.Chromophores B. Substituent Effects: General – from our brief study of these general chromophores, only the weak n  * transition occurs in the routinely observed UV The attachment of substituent groups (other than H) can shift the energy of the transition Substituents that increase the intensity and often wavelength of an absorption are called auxochromes Common auxochromes include alkyl, hydroxyl, alkoxy and amino groups and the halogens 6/14/2012 11
  • 12. III.Chromophores C. Substituent Effects General – Substituents may have any of four effects on a chromophore i. Bathochromic shift (red shift) – a shift to longer ; lower energy ii. Hypsochromic shift (blue shift) – shift to shorter ; higher energy iii. Hyperchromic effect – an increase in intensity iv. Hypochromic effect – a decrease in intensity 6/14/2012 12
  • 13. Hyperchromic Hypsochromic Bathochromic Hypochromic200 nm 700 nm Fig: Absorption & intensity shifts 6/14/2012 13
  • 14. IV. AuxochromesThe color of a molecule may be intensified by groups called auxochromes which generally donot absorb significantly in the 200-800nm region, but will affect the spectrum of thechromophore to which it is attached.The most important auxochromic groups are OH, NH2, CH3 and NO2 and their properties areacidic (phenolic) or basic.The actual effect of an auxochrome on a chromophore depends on the polarity of theauxochrome, e.g. groups like CH3-,CH3CH2- and Cl- have very little effect, usually a smallred shift of 5-10nm. Other groups such as -NH2 and -NO2 6/14/2012 14
  • 15. V. Beer-Lambert law cuvette slit source detector 6/14/2012 15
  • 16. The method is most often used in a quantitative way to determineconcentrations of an absorbing species in solution, using the Beer-Lambertlaw: A=log10 (I0/I)=ε.C.LWhere,A is the measured absorbance,I0 is the intensity of the incident light at a given wavelength,I is the transmitted intensity, L the pathlength through the sample, andc the concentration of the absorbing species.For each species and wavelength, ε is a constant known as the molarabsorptivity or extinction coefficient. 6/14/2012 16
  • 17. 11 11 11 11 11 116/14/2012 17

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