212Lec8Sp09UV - Todays agenda UV/Vis Spectroscopy (14.7 -...

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Today’s agenda… UV/Vis Spectroscopy (14.7 - 14.9) Spectral Problem Last time… Mass Spectrometry (12.1-12.3) Lecture 8
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In UV/Vis spectroscopy, organic compounds are exposed to UV/Vis radiation. UV: 200 to 400 nm Visible: 400 to 800 nm UV/Vis spectroscopy is used to detect the presence of unsaturation and/or conjugation in a compound. 1. Irradiate sample with UV/Vis radiation 2. Cause electronic transitions (absorption of radiation) 3. Spectrum shows wavelengths of radiation absorbed
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Radiation that is absorbed is recorded as a λ max peak (in nm). Here’s the UV/Vis spectrum of acetone; two λ max peaks are seen. C O CH 3 H 3 C λ max λ max
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Absorption of UV/Vis radiation causes electronic excitations at specific wavelengths ( λ max ). Transitions observed by UV/Vis: π to π * (strong) n to π * (weak, often not observed) σ π n π∗ σ∗ Energy occupied orbitals unoccupied orbitals
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The two λ max peaks in acetone correspond to two electronic transitions. π --> π * Shorter wavelength, stronger absorption n --> π * Longer wavelength, weaker absorption C CH 3 O CH 3 non-bonding electrons π electrons π * are excited π electrons
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wavelength ( λ max value). These values are the π to π * transitions. H
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This note was uploaded on 02/25/2009 for the course CHEM 210 taught by Professor Maslak,przemyslaw during the Spring '08 term at Pennsylvania State University, University Park.

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212Lec8Sp09UV - Todays agenda UV/Vis Spectroscopy (14.7 -...

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