Benzene

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Fred J. Grieman
Benzene Molecular Orbital Theory Delocalized
MOs Molecular Motion Energy Spectroscopy
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Benzene
Resonance Structures delocalized p bonding
Localized bonding
Delocalized bonding
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6 delocalized molecular orbitals from 6 p
atomic orbitals
6 electrons in orbitals
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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
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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
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Molecules electronic motion and nuclear motion
Vibration
Rotation
Vibrational and Rotational Energies are Quantized
Also !!!!!
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Visible/Ultraviolet (Electronic)
Infrared (Vibration)
Microwave (Rotation)
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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
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Information from Spectroscopy
Identification E.g. vibration of bonds similar
from molecule to molecule
Infrared Spectrum of CH2Cl2
Spartan vibrations
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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