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UV-Visible spectra can be interpreted to help determine molecular structure, but this is presently confined to the analysis of electron behavior in known compounds. Information from other techniques (NMR, MS, IR) is usually far more useful for structural analysis However, UV-Vis evidence should not be ignored! | a W VILLANOVA UNIVERSITY Optical Electronic Spectroscopy 2 Lecture Date January 28th 2008 Molecular UV-Visible Spectroscopy Molecular UV-Visible spectroscopy is driven by electronic absorption of UV-Vis radiation. Molecular UV-Visible spectroscopy can - Enable structural analysis - Detect molecular chromophore - Analyze light-absorbing properties e.g. for photochemistry . Visible Spectrum . Higher Lower F re q u e n cy Fre q u e n cy uv IR 1 r I I T 400 500 600 700 800 Wavelength in nanometers Basic UV-Vis spectrophotometers acquire data in the 190800 nm range and can be designed as flow systems. Figures from http www.cem.msu.edU reusch VirtualText Spectrpy UV-Vis uvspec.htm uv1 1 Molecular UV-Vis Spectroscopy Terminology UV-Vis Terminology - Chromophore a UV-Visible absorbing functional group - Bathochromic shift red shift to longer wavelengths - Auxochrome a substituent on a chromophore that causes a red shift - Hypsochromic shift blue shift to shorter wavelengths - Hyperchromic shift to greater absorbance - Hypochromic shift to lesser absorbance Molecular UV-Vis Spectroscopy Transitions Classes of Electron transitions - HOMO highest occupied molecular orbital - LUMO lowest unoccupied molecular orbital - Types of electron transitions 1 Ơ A and n electrons mostly organics 2 d and f electrons inorganics organometallics 3 charge-transfer CT electrons 2 Molecular UV-Vis Spectroscopy Theory Molecular energy levels and absorbance wavelength - and - n transitions high-energy accessible in vacuum UV Àmax 150 nm . Not usually observed in molecular UV-Vis. n Ơ and n c transitions non-bonding electrons lone pairs wavelength Àmax in the 150-250 nm region. n n and n n transitions most common transitions observed in organic molecular UV-Vis observed in compounds with lone pairs and multiple bonds with Àmax 200-600 nm. anti-bonding non-bonding bonding bonding Figure from http www.cem.msu.edu reusch VirtualText Spectrpy UV-Vis spectrum.htm Molecular UV-Vis Spectroscopy Theory d
