Molecular Spectroscopy PPT

download Molecular Spectroscopy PPT

If you can't read please download the document

  • date post

    02-May-2017
  • Category

    Documents

  • view

    274
  • download

    1

Embed Size (px)

Transcript of Molecular Spectroscopy PPT

Molecular Spectroscopy The Diatomic Rigid Rotator

Molecular SpectroscopyThe Diatomic Rigid Rotator

The rigid diatomic Molecule

m1m2r1r2

Cm1r1m2r2=r0THE MOMENT OF INERTIA IS I = r02 Reduced mass

Rotational Energy LevelEJ = h2

In terms of wavenumber J = EJ/hc cm-1

J = BJ(J+1), where B = h2 cm-1

B is called as the rotational constant.

8 2 IJ (J+1) joules, where J = 0, 1, 2,.. 8 2 I c

Intensities of Spectral LinesIntensities depends on number of molecules in a particular energy level Boltzmann Distribution NJ/NO = EXP (-EJ/kT) = EXP(-Bhc J(J+1)/ kT)

NJ/NO 0.98

There are as many molecules in the J=1 state, at equilibrium as in the J=0, at T=300K (room temperature). Further, there is more rapid decrease of NJ/NO with increasing J and with larger B.

JEXP(-Bhc J(J+1)/ kT)

1100B = 10 cm-1B = 5 cm-1

Degeneracy of the energy statesThe energy and angular momentum of a rigid rotator are E = I 2 P = I

P = (2EI)1/2 = [J(J+1)]1/2 units eg., for J=1, P=(2)1/2

11Spacing between adjacent rotational levels j and j-1,

12Rotational Spectroscopy(1) Bohr postulate

(2) Selection Rule

Isotopic Shift12CO13COJ654321

TYPICALLY ROTATIONAL SPECTRUM SHOWS LINES CORRESPONDING TO 1-100 cm-1

The Vibrating Diatomic Molecule (eg., HCl)

ClCl

reqHHHHBOND LENGTH

APPROX. POTENTIALREAL/ EFFECTIVE POTENTIAL

Morse Potential

THE PARAMETERSE = k (r req)

osc = 1/2 (k/)1/2 Hz osc = osc/c cm-1

Allowed vibrational EnergiesE = ( + 1/2) osc

Selection rule : = 1

Vibrations in Rigid rotator !!

req = 0123ENERGYN1/NO 0.008

The molecular population Dies of very fast

Ignore all possible Vibrational states

21Vibrational SpectroscopyVibrational selection rule

J = -1PJ = +1RJ = +2SJ = -2OJ = 0Q

26Vibration-Rotation Spectra

Infrared spectrumJ = 1Raman spectrumJ = 0 , 2

27Vibration-Rotation Infrared Spectrum of HClvib is different for H35Cl and H37Cl molecules due to the slight difference in their reduced masses.

au

au

28Vibration-Rotation Infrared Spectrum of HClThe lines due to H35Cl transitions are more intense because the isotopic abundance ration of H35Cl to H37Cl molecules is 3:1.

29Vibration-Rotation Infrared Spectrum of HCl

Band centerH35Cl

Band centerH37Cl

30Vibration-Rotation Infrared Spectrum of HClThe rotational constant B slightly decreases as going to higher vibrational levels. This results in decrease of the gaps between transition lines as one goes to higher frequencies.

31Vibration-Rotation Infrared Spectrum of HClThe rotational constant B slightly decreases as going to higher vibrational levels. This results in decrease of the gaps between transition lines as one goes to higher frequencies.

32Vibration-Rotation Infrared Spectrum of HCl

Approximation of B values

RAMAN SPECTROSCOPYNobel in 1930

Discovery of Raman Effect (Raman Scattering)

(7 November 1888 21 November 1970)

WHAT IS RAMAN SPECTROSCOPY ?

Raman spectroscopy is the measurement of the wavelength and Intensity inelastically scattered light from molecules.

May be used to understand chemical composition and molecular structure.

Used in CMP and chemistry to understand the vibrational, rotational and other low-frequency modes of a system.

ScatteringIn addition to being absorbed and emitted by atoms and molecules, photons may also be scattered (approx. 1 in 107 in a transparent medium). This is a molecular effect, which provides another way to study energy levels.

ELASTICINELASTICSYSTEM REMAINS IN THE SAME QUANTUM STATEResulting in CHANGE IN THE QUANTUM STATE

Scatteringin = out Rayleigh scattering Ein = Eout = h

in out Raman scatteringEin Eout

VIRTUAL STATESEXCITED STATESGROUND STATE

RAMAN SCATTERING SELECTION RULES

RAMAN SCATTERING SELECTION RULES

43

Rotational Raman

Some fine pointsInelastic scattering can be in analogy with a ball bearing hitting a drum so that it starts to oscillate at its natural frequency. Similarly if the drum is already oscillating and the ball bearing hits it at the right phase, it may get reflected at a higher energy.In order to be Raman active, a molecular rotation or vibration must cause some change in the component of molecular polarizibility.Strokes lines (those scattered with a lower frequency than the incident radiation) are generally more intense than the anti-stroke lines, because the former is accompanied by an increase in molecular energy.

IRRAMAN

Why is Raman Different to IR?Selection rules are therefore different and can be exclusive for centrosymmetric molecules

IR IR --Change in Dipole Moment Change in Dipole Moment

Raman Raman --Change inChange inPolarizability Polarizability

MM--

MM--

MM++

MM++

MM--

MM--

MM++

MM--

MM--

++

__

++

__

++

__

++

__

++

__

++

__

++

__

++

__

++

__

++

__

++

__

++

__

++

__

++

__

++

__

++

__

++

__

++

__

++

__

++

__

General Aspects of Molecular Spectroscopy