• Document: UV-Visible Spectroscopy
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UV-Visible Spectroscopy UV-Visible Spectroscopy What is UV-Visible Spectroscopy ? Molecular spectroscopy that involves study of the interaction of Ultra violet (UV)-Visible radiation with molecules What is the result of interaction of UV-Vis light and the molecule? Ultraviolet light and visible light have just the right energy to cause an electronic Transition of electron from one filled orbital to another of higher Energy unfilled orbital When a molecule absorbs light of an appropriate wavelength and an electron is promoted to a higher energy molecular orbital, the molecule is then in an excited state What is the information you get from UV spectroscopy? The Wave length of absorbed light will provide the information on the energy gap which is related to functional group UV-Spectroscopy: Possible Transitions Organic Molecules s* alkanes s s* p* s p* carbonyls Energy p p* unsaturated compounds n n s* O, N, S, halogens containing n p* carbonyls p s UV spectra of organic compounds are generally collected from 200-700 nm s s* alkanes 150 nm s p* carbonyls 170 nm p p* unsaturated cmpds. 180 nm √ - if conjugated! n s* O, N, S, halogens 190 nm n p* carbonyls 300 nm √ UV-Spectrometer: Measurement of Absorbed Radiation UV-Spectroscopy: Absorption Law Beer–Lambert law: At a given wavelength, absorption is proportional to the concentration of absorbing molecules and the path length of the light through the sample A = εcl A = absorbance of the sample = log10 (I0/I) I0 = intensity of the radiation entering the sample I = intensity of the radiation emerging from the sample l = length of the light path through the sample, in centimeters c = concentration of the sample, in moles/liter ε = molar absorptivity (liter mol-1 cm-1 or M-1 cm-1 ) where M = mol L-1 The molar absorptivity (formerly called the extinction coefficient) of a compound is a constant that is characteristic of the compound at a particular wavelength Molar absorptivities may be very large for strongly absorbing compounds ( >10,000) and very small if absorption is weak ( = 10 to 100) No absorption gives e = 0! UV-Spectroscopy: Absorption Law Guanosine has a maximum absorbance at 275 nm. ε275 = 8400 M-1 Cm-1 and the path length is 1 cm. Using a spectrophotometer, you find the that A275 = 0.7 . What is the concentration of guanosine? A = εcl There is a substance in a solution (4 moles/liter). The length of cuvette is 2 cm and only 50% of the certain light beam is transmitted. What is the absorption coefficient of the substance? M-1 cm-1 PRESENTATION OF SPECTRA The ultraviolet–visible spectrum is generally recorded as a plot of absorbance versus wavelength The choice of the solvent to be used in ultraviolet spectroscopy is quite important Usually solvents that do not contain conjugated systems are most suitable for this purpose λmax is the wavelength where maximum absorption occurred Chromophore A chromophore is that part of a molecule that absorbs UV or visible light Alkanes: molecules contain single bonds and the only possible transitions are σ to σ* Absorb ultraviolet energy at very short wavelengths below 200 nm, shorter than the wavelengths that are experimentally accessible (200-700 nm) Alcohols, Ethers, Amines, and Sulfur Compounds: In saturated molecules that contain atoms bearing nonbonding pairs of electrons, possible transitions of the n to σ* They are also high-energy transitions Alcohols and amines absorb in the range from 175 to 200 nm; Organic thiols and sulfides absorb between 200 and 220 nm Most of the absorptions are below the cutoff points for the common solvents, so they are not observed in solution spectra. Alkenes and Alkynes: Possible transitions are π to π*. These transitions are of rather high energy (170 nm) as well, but their positions are sensitive to the presence of substitution Carbonyl Compounds: Unsaturated molecules that contain atoms such as oxygen or nitrogen may also undergo n to π*transitions (280 to 290 nm). Carbonyl compounds als

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