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SPECTROSCOPY ABSORPTION EMISSION Absorption Spectroscopy helps us to study the energy levels of the atoms, molecules and solids. Absorption spectrum is absorption of light as a function of wavelength. This is again dependent on the energy level of the absorber. This is dependent on the structure of absorber hence absorption spectroscopy is very helpful to get an idea of the structure of the absorber.

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I0 I l Lambert – Beer law An empirical relationship relating absorption of light and properties of absorbing material log10 Io/I = A = lc c: Concentration in mole Lit-1 l; Cell length (path length) in cm A: Absorbance (this is the measurable quantity)  : Molar extinction coefficient, Absorptivity, a molecular property characteristic of the substance, independent of concentration and given as M-1cm-1

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200-400 UV - 400 Violet Yellow-Green 425 Indigo-Blue Yellow 450 Blue Orange 490 Blue-Green Red Green Purple 530 Yellow-Green Violet 550 Yellow Indigo-Blue 590 Orange Blue 640 Red Blue-Green 730 Purple Green

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SPECTROSCOPY ABSORPTION EMISSION Absorption spectrum is absorption of light as a function of wavelength. This is again dependent on the energy level of the absorber. The energy levelis dependent on the structure of absorber Hence absorption spectroscopy is very helpful to get an idea of the structure of the absorber.

Electron transitions : 

Electron transitions

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 * Energy requirement is very large. Methane absorbance maxima at 125 nm. Not observed in typical UV-Vis spectrometer. n * Energy requirement is less than the earlier case. Saturated organic compounds containing atoms with lone pair of electrons – Range 150-250 nm. Molecules are few in nos. n * and  * Most common transitions of organic compounds. Range 200 – 700 nm. Needs presence of unsaturated group. The basic differences between these two:  for the former is low – 10 to 100 M-1 cm-1 and for later 1000 to 10,000 M-1 cm-1 Polar solvents shift the former to shorter WL (blue shift), often the later shows red shift in polar solvents

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A UV-VIS spectrometer records the wavelength versus absorbance of a molecule after it absorbs the light. n, σ* transitions are of high energy but lower than σ, σ* transitions hence appear at lower side of UV wavelengths. Since it is a symmetry forbidden transition,  values are less, Methyl alcohol: 183 nm,  150 Methyl chloride 173 nm,  100 Methyl Iodide 258 nm,  378 Trimethyl amine 227 nm,  900 [in water TEA does not show this absorption] The molar extinction coefficient  is associated with the intensity of absorption. More value of  denotes more probability of the transition to occur

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π, π* transitions are of less energy than n, σ* transition and of high ε. Associated only with unsaturated compounds. Unsaturated compounds with available non-bonding electrons will also show n, π* transition. Low energy Chromophore An isolated functional group if it exhibits absorption of a characteristic nature in UV or Visible region C=C ~180 nm (10,000 -15,000) C=O ~180 nm (~10,000) and ~280-290 nm (10- 100) CO2H ~200 nm (~30) COCl ~220 nm (~100) CONH2 ~ 180 nm (~10,000), ~220 (~60) CO2R ~210 nm (~50) NO2 ~200 nm (~5000), ~275 (~15) ONO2 ~270 nm (~15)

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Identical functional groups in different molecules may not absorb at same wavelength Absorption WL depends on Transition energy Structural environment will decide this transition energy. Electronic transition results in redistribution of electrons inside the molecule. This changes the characteristics of the molecule in its excited state, C = O C+ - O- C+ - O- n-electrons are more polar in GS and are stabilized by H-bonding or, by electrostatic interaction with a polar solvent. WL shifts to shorter values for n, * transition. If excited states are more polar they will then be stabilized by polar solvents. That happens with -electrons. Polar solvents stabilizes * states and WL shift to higher value.

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4-methyl-3-penten-2-one (Mesityl Oxide) Hexane max: 230 nm (!2,600); 329 nm (41) Water 243 nm (10,000); 305 nm (60) The n,* transition is also effected by steric effects. Ring size and presence of bulky groups: Acetone: 279 nm t-butyl methyl ketone: 289 nm values are also enhanced Energy requirement less, transition probability more.

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Auxochrome: Groups that do not in themselves show selective absorption above 200 nm but which, when attached to a given chromophoric system ususlly cause longer WL absorption. OH, NH2, Halogen

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Absorbs at 65 nm higher value and twice as intense 255 nm (230) 280 nm (1430)

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