FIU CHM 4130 - CHAPTER 14_Xiao_Molecular_Spectrometry_2018 (56 pages)

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CHAPTER 14_Xiao_Molecular_Spectrometry_2018



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Applications of UV VIS Molecular Spectroscopy Chapter 14 Molar absorptivities 8 7 x 10 19 P A A capture cross section of molecule in cm2 10 15 cm2 molecule P allowed transition probability 0 1 1 104 to 105 M 1cm 1 P 0 01 10 3 forbidden transitions Molecular Absorption Absorption of light is a two step process Absorption n h absorption 10 8 to 10 9 sec Relaxation n heat relaxation process n M A B C photochemical decomposition n h emission of fluorescence and phosphorescence Visible Absorption Spectra Environment has a HUGE influence Vapor electronic vibrational rotational transitions hexane electronic vibrational rotational transitions aq electronic vibrational rotational transitions smooth The absorption of UV visible radiation generally results from excitation of bonding electrons Can identify functional groups in a molecule Can be used for quantitative and qualitative analysis Molecular orbital is the nonlocalized fields between atoms that are occupied by bonding electrons when two atom orbitals combine either a low energy bonding molecular orbital or a high energy antibonding molecular orbital results Sigma orbital The molecular orbital associated with single bonds in organic compounds Pi orbital The molecular orbital associated with parallel overlap of atomic P orbital n electrons No bonding electrons MO theory to examine the simple diatomic molecule H2 The two 2p atomic orbitals overlap Interaction in end on Interaction in parallel http www youtube com watch feature player embedded v VfBcfYR1VQo UV VIS electronic transitions Typical UV Absorption Spectra Organic molecules n 100 3000 1000 10 000 Inorganic ions Transi1ons in the d electrons nonbonding electrons low Charge transfer complexes Typically large 10 000 Absorption by organic compounds Absorption of saturated organic compounds LOW Absorption of unsaturated organic compounds Unsaturated functional group orbital Effects of Multiple Chromophores Rule of thumb for conjugation Transitions for organic



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