genchem lec 28
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CHEM F111 General ChemistryLecture 28Molecular Spectroscopy - Electronic
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01-November-2013 1
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Only this small range is visible to the human eye
Different experiments use different colors, or wavelengths, of light
Depending on how the electrons are moved to create the lightfor each beamline, different colors of light can be produced.
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Electromagnetic Radiations
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E 2
E 2
D E = h n
E 2
E 1
D E = h n
Absorbance Emission
3
Absorption : A transition from a lower level to a higher level withtransfer of energy from the radiation field to an absorber, atom,molecule, or solid.Emission : A transition from a higher level to a lower level withtransfer of energy from the emitter to the radiation field.
The 2 types of transitions are:Radiative - photon is absorbed/emitted
Nonradiative - energy transferred between molecule to thesurroundings
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Internal Energy of Molecules
http://www.chemistry.vt.edu/chem-ed/spec/absorption.htmlhttp://www.chemistry.vt.edu/chem-ed/spec/emission.htmlhttp://www.chemistry.vt.edu/chem-ed/spec/emission.htmlhttp://www.chemistry.vt.edu/chem-ed/spec/absorption.html -
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Internal Energy of Molecules
E total = E trans + E elec + E vib + E rot + E nucl E elec : electronic transitions (UV, X-ray) E vib: vibrational transitions (Infrared) E rot : rotational transitions (Microwave)
E nucl : nucleus spin (nuclear magnetic resonance) or (MRI:magnetic resonance imaging)
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DE = h n = hc/
Wavelength = c/ n , Wavenumber, v = n c
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Population of Energy Levels At any finite T, molecules will be distributed among
available E levels due to thermal agitation The exact distribution among energy levels will
depend upon the temperature and separation betweenenergy levels according to Boltzmann statistics
kT E nn
lower
upper Dexp
k = 1.38 10 -23 JK -1 (Boltzmanns constant) DE = separation in energy level
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Electronic Spectroscopy
Ultraviolet (UV) and visible (VIS) spectroscopyThis is the earliest method of molecular spectroscopy.A phenomenon of interaction of molecules with ultravioletand visible lights.Absorption of photon results inelectronic transition of amolecule, and electrons are
promoted from ground state tohigher electronic states usuallyfrom a molecular orbital calledHOMO to LUMO.
E
s*
p
s
p*
n Atomic orbital
Molecular orbitals
Occupied levels
Unoccupied levels
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Electronic transition
Can be assigned to different transition typesaccording to the molecular orbital involved,such as -> * (in alkenes or benzene), n ->*(in keto group).
Due to their symmetry property in MOs, suchtransition can be allowed (high intensity) orforbidden (low intensity).
Absorptions with high are allowed transitions,and low absorptions are forbidden transition.
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Molecular Energy Levels
RotationalEnergy Levels
VibrationalEnergy Levels
GroundElectronicState
ExcitedElectronicState
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RotationalTransition
1-20 cm -1
VibrationalTransition
2000-4000 cm -1
ElectronicTransition
10000-50000 cm -1
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Molecular Energy Levels
Radiation can be absorbed or emitted if the molecule
changes any of its energy states
RotationalEnergy Levels
VibrationalEnergy Levels
GroundElectronicState
ExcitedElectronicState
RotationalTransition
Vibrational
Transition
Electronic
Transition
10Microwave Infrared UV-Visible BITS Pilani, Pilani Campus
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Frank-Condon Principle
The nuclear motion (10 -13 s) is much slower as compared with
electronic motion in transition (10 -16 s), so it is negligibleduring the time required for an electronic excitation.
Since the nucleus does not move
during the excitation, theinternuclear distance keeps the same,and the most probable componentof a electronic transition involves
only the vertical transitions wherethe position and momentum of thenuclei dont change .
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The excitation going from = 0 (GS) to = 3 (ES) isthe most probable one for vertical transition becauseit falls on the highest point in the electron probabilitycurve for = 3 in ES.
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Molecular Energy Levels
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Absorption and emission pathways
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Jablonski diagram
http://www.google.co.in/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=eLq6NraglYjgpM&tbnid=c0qDHKDSIDJxtM:&ved=0CAUQjRw&url=http://blog.reseapro.com/tag/jablonski-diagram/&ei=9BdzUuqTGcKSrgfs6IHwCA&bvm=bv.55819444,d.bmk&psig=AFQjCNEMe3mNecQwBlJ-klberAHLnHzZ9A&ust=1383360623633092 -
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Selection rules of electronic transition
Spin selection rule : there should be no change in spin orientation orno spin inversion during these transitions. Thus, SS, TT, areallowed, but ST, TS, are forbidden. ( S = 0 transition allowed ).Angular momentum rule : the change in angular momentum should
be within one unit (0 or 1).
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For an electron to transition, certain quantum mechanical constraints apply these are called selection rules . Thus all transitions that are possible are not observed.Gross Selection Rules: Specifies the general features a molecule musthave if it is to have a spectrum of a given kind.
e.g. a vibration is ctive only if there is a change in dipole moment.Specif ic Selection Rules: A detailed study of the transition moment leadsto the specific selection rules that express the allowed transitions interms of the changes in quantum numbers.
Selection Rules