genchem lec 28

8/13/2019 GenChem Lec 28

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BITS Pilani, Pilani Campus

CHEM F111 General ChemistryLecture 28Molecular Spectroscopy - Electronic

BITS PilaniPilani Campus

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.


Electromagnetic Radiations


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E 2

E 2

D E = h n

E 2

E 1

D E = h n

Absorbance Emission

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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


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)


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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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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Absorption and emission pathways


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

genchem lec 28

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