1 Fred J. Grieman Benzene Molecular Orbital Theory Delocalized MOs Molecular Motion Energy Spectroscopy 2 Benzene Resonance Structures delocalized p bonding Localized bonding Delocalized bonding 3 6 delocalized molecular orbitals from 6 p atomic orbitals 6 electrons in orbitals 4 Complete MO model CH4 Valence bond theory 4 bonds from C (sp3) H (1s) all of the same energy each localized btwn C and a H Molecular orbital theory - 4 delocalized bonds over whole molecule 1 lower in energy than 3 degenerate orbitals E Molecular Orbital Energies M. O. Theory agrees with experimental measurement of orbital energies M. O.s SPARTAN software (you will use in laboratory) Look at CH4 and formate HCO2- CH4 Photoelectron Spectrum 5 Energy Levels Spectroscopy Molecular Properties Quantized Energy Levels Atoms 1s22s2p2 ______ E absorption emission 1s22s22p ______ 2p ___ ___ ___ 2s ___ 1s ___ E Atomic Line Spectra Molecules also have electronic energy levels and spectra Excited state (s)2 (s)2 ()4 (p) () _______ e.g. N2 (s)2 (s)2 ()4 (p)2 _______ Gound state Molecular Band Spectra Why several bands and not one line 6 Molecules electronic motion and nuclear motion Vibration Rotation Vibrational and Rotational Energies are Quantized Also !!!!! 7 Visible/Ultraviolet (Electronic) Infrared (Vibration) Microwave (Rotation) 8 Colorimetric Determination of Manganese Keep c and b constant Vary gives spectrum Keep e(wavelength) and b constant Vary concentration gives Absorbance vs. c straight line result Absorption Spectrum of MnO4- RedBlue Violet e() molar absorbtivity A e b c c concentration b path length 9 Information from Spectroscopy Identification E.g. vibration of bonds similar from molecule to molecule Infrared Spectrum of CH2Cl2 Spartan vibrations 10 What else What do vibrational and rotational energies depend on Demonstrations Rotation structure (bond distance and bond angles) mass Vibration strength of bond (bond energy) masses From Spectroscopy If you can obtain the energies you can determine structure of a molecule and the strengths of bonds
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