Downloaded 13 times
Uploaded byDeepanshu Sharma
Ultravoilet spectroscopy
UV spectroscopy involves promoting electrons from the ground state to excited states of molecules using ultraviolet radiation between 200-400 nm. The absorption of this radiation can be quantified using Beer's law and plotted in an absorption spectrum showing the wavelength of maximum absorption (λmax) and intensity (εmax). Chromophores are functional groups that absorb UV radiation, while auxochromes modify the absorption by inducing bathochromic, hypsochromic, hyperchromic, or hypochromic shifts.
More Related Content
Similar to Ultravoilet spectroscopy
Ultravoilet spectroscopy
- 1.
- 2. TYPES: 1) Absorption Spectroscopy: the study of absorbed radiation by molecule , in form of spectra Eg: UV , IR 2) Emission Spectroscopy: the radiation emitted by molecule can also be studied to reveal the structure of molecule Eg : flourimetry , flame photometry SPECTROSCOPY “The study of interaction of electromagnetic radiation with molecule /atom”
- 3. Wavelength rangeof UV radiation starts at blue end of visible light (about 4000Å) and ends at 2000Å UV REGION INTRODUCTION TO UV SPECTROSCOPY BETWEEN 2000Å -4000Å (Near ultravoilet region ) BELOW 2000Å ( Far or Vaccum ultravoilet region ) Alternate title for this technique is electronic spectroscopy since it involve promotion of electrons from ground state to the higher energy state
- 4.
- 5. Lambert’s law Beer’s law Beer– lambert’s law ABSORPTION LAWS:
- 6. “When a beamof monochromatic light is passed through a homogeneous absorbing medium , the rate of decrease of intensity of radiation with thickness of absorbing medium is propotinal to the intensity of the incident light (radiation) “ dI/dt =KI I = intensity of incident light of wavelength λ t= thickness of medium On integrating the eqaution ang putting I= 𝐼 𝑜 we Get , 𝐼 𝑜/ 𝐼𝑡= kt 𝐼𝑡 = 𝐼 𝑜 𝑒−𝑘𝑡 𝐼 𝑜=intensity of incident light 𝐼𝑡=intensity of transmitted light k=constant which depends upon absorbing medium & λ Converting equation to natural log 𝐼𝑡 = 𝐼 𝑜10−0.4343𝑘𝑡 = 𝐼 𝑜10−𝐾𝑡 LAMBERT’S LAW
- 7. “intensity ofbeam of monochromatic light decreases exponentially with increase in concentration of absorbing substance airthmetically “ 𝐼𝑡 = 𝐼 𝑜 𝑒−𝑘𝑐 Taking natural log 𝐼𝑡 = 𝐼 𝑜10−0.4343𝑘𝑐 = 𝐼 𝑜10−𝐾𝑐 BEER’S LAW:
- 8. On combiningthe two laws , the beer –Lambert law can be formulated as : logI₀/I = €.c.t = A = log1/T = -log T € = molar exctintion coefficient C = concentration of solution A = absorbance T = transmittance i.e, their exist a relation between absorbance and transmittance BEER – LAMBERT’S LAW:
- 9. UV spectroscopy measuresthe response of a sample to a ultraviolet range of EMW Molecules have either n , or σ electrons . These electrons absorb UV radiation & undergoes transitions from ground state to excited state PRINCIPLE:
- 10. When thepromotion of electron from bonding to anti bonding orbital the wavelength of radiation change from minimum to maximum in the given range , and absorbance is recorded . Then a plot of energy absorbed Vs wavelength is called absorption spectrum. Features of spectrum : λmax (wavelength at which max absorption ) €max (intensity of max absorption ) ABSORPTION SPECTRUM:
- 11.
- 12. * * n * n * * * DIFFERENT TYPES OF TRANSITIONS
- 13. Energy requiredfor this transition is large i.e, occur at far UVregion (126 – 135nm ) Example : methane λmax 121.9nm , saturated hydrocarbons *
- 14. This transitioncan in principle occur in molecule having electron system (unsaturated hydrocarbons) Require smaller energy i.e, occur at longer wavelength Example: alkenes , alkynes, carbonyl compound , cyanides , azo compounds Alkenes absorb in the region 170 to 205 nm *
- 15. Saturated compoundscontaining atoms with lone pair of electrons like O , N , S and halogens are capable of these transitions These transition usually require less energy than * Example : water absorb at 167nm n *
- 16. Compounds containingdouble or triple bond involving hetero atoms ( C=O, C≡N, N=O) undergoes such transitions These transitions require minimum energy and show absorption at longer wavelength around 300nm n *
- 17. These areforbidden transitions & are only theoretically possible Thus, these transition show absorption in region above 200nm which is not possible in UVspectrophotometer * & *
- 18. UV radiationmay be absorbed by organic compounds that contain N , O , S, halogen atom or unsaturated hydrocarbons ( such as olefins ) . Functional group that contain these groups and absorb radiation in UV region are called chromophores CHROMPHORES
- 19. The functionalgroup attached to a chromophore which modifies the ability of the chromphore to absorb light , altering the wavelength or intensity of absorption Example : Benzene λmax = 255nm Phenol λmax = 270nm (-OH) Aniline λmax = 280nm (-N𝐻2) AUXOCHROME
- 20.
- 21.
- 22.
- 23. When absorptionmaxima shift toward longer wavelength due to presence of an auxocrome or by the change of solvent Example : n * transition for carbonyl compound experience shift when polarity of solvent decreases groups like (–OH -OCH3) causes absorption at longer wavelength BATHOCHROMIC SHIFT(RED SHIFT)
- 24. When absorptionmaxima shifts toward shorter wavelength Occurs due to removal of conjugation and also by change in polarity of solvent Example :Aniline show shift in acidic medium , loses conjugation HYPSOCHROMIC SHIFT (BLUE SHIFT) Aniline λmax =280nm In acidic medium λmax =265nm Hypsochromic shift
- 25. In thisabsorption intensity of absorption maxima increases i.e, €max increases Introduction of auxochrome usually increases intensity of absorption Example: HYPERCHROMIC SHIFT Pyridine λmax =257nm 2- methyl pyridine λmax =260nm HYPERCHROMIC SHIFT
- 26. In thisabsorption intensity of absorption maxima decreases i.e, €max decreases Introduction of group which distort the geometry of the molecule causes hypochromic shift Example: HYPOCHROMIC SHIFT Biphenyl λmax =250nm 2-methyl biphenyl λmax =237nm HYPOCHROMIC SHIFT Due to the distortion caused by the methyl group in 2- methyl biphenyl
- 27.