MOLECULAR SPECTROSCOPY

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MOLECULAR SPECTROSCOPY

• SPECTROSCOPY IS THAT BRANCH OF SCIENCE WHICH
  DEALS WITH THE STUDY OF INTERACTION OF
  ELECTROMAGNETIC RADIATION WITH MATTER.
• ELECTROMAGNETIC RADIATION -
• THE TERM ELECTROMAGNETIC RADIATION REPRESENTS
  THE RADIANT ENERGY EMITTED FROM ANY SOURCE IN THE
  FORM OF LIGHT,HEAT ETC. SOME IMPORTANT
  CHARACTERSTICS OF THESE ELECTROMAGNETIC RADIATION
  ARE GIVEN BELOW
• THEY HAVE DUAL CHARACTER.
• THESE WAVES ARE ASSOCIATED WITH ELECTRIC AND
  MAGNETIC FIELDS.

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• ALL ELECTROMAGNETIC RADIATIONS TRAVEL WITH THE
  VELOCITY OF LIGHT.
• c??.
• EACH PHOTON IS A PACKET OF ENERGY WHOSE ENERGY IS
  GIVEN BY PLANCKS EQUATION Eh?.WHERE h IS
  PLANCKS CONSTANT.

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REGION OF ELECTROMAGNETIC SPECTRUM
WHEN THE DIFFERENT TYPES OF ELECTROMAGNETIC
RADIATIONS ARE ARRANGED IN ORDER OF THEIR
INCREASING WAVELENGTHS OR DECREASING
FREQUENCIES, THE COMPLETE ARRANGEMENT IS CALLED
ELECTROMAGNETIC SPECTRUM.
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INTRODUCTION TO MOLECULAR SPECTRO SCOPY AND ITS
DIFFERENCE FROM ATOMIC SPECTROSCOPY

• IN CASE OF MOLECULES,WHEN THE ENERGY IS
  ABSORBED,IT MAY RESULT INTO ROTATION,VIBRATION OR
  ELECTRONIC TRANSITION.JUST AS ELECTRONIC ENERGY
  IS QUANTIZED,THE ROTATIONAL AND VIBRATIONAL
  ENERGIES ARE ALSO QUANTIZED.
• THE ROTATIONAL,VIBRATIONAL AND ELECTRONIC ENERGY
  LEVELS OF A MOLECULE ARE COLLECTIVELY CALLED
  MOLECULER ENERGY LEVELS.THE TRANSITIONS OF ENERGY
  CAN TAKE PLACE ONLY BETWEEN THESE LEVELS.THE
  RESULT IS A MOLECULER SPECTRUM

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ABSORPTION AND EMISSION SPECTROSCOPY

• THE TRANSITION MAY TAKE PLACE FROM LOWER ENERGY
  LEVEL TO HIGHER ENERGY LEVEL BY ABSORBING
  ENERGY.IT IS THEN CALLED ABSORPTION SPECTROSCOPY
  AND THE RESULT OBTAINED AS A RESULT OF A NUMBER
  OF SUCH TRANSITION IS CALLED ABSSORPTION
  SPECTRUM.
• THE TRANSITION MAY TAKE PLACE FROM HIGHER ENERGY
  LEVEL TO A LOWER ENERGY LEVEL THEREBY EMITTING
  THE EXCESS ENERGY AS A PHOTON.IT IS THEN CALLED
  EMISSION SPECTROSCOPY AND THE RESULT OBTAINED AS
  A RESULT OF NO. OF SUCH TRANSITIONS IS CALLED
  EMISSION SPECTRUM.

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E2
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TYPES OF MOLECULAR ENERGIES AND BORN-OPPENHEIMER
APPROXIMATION

• A MOLECULE USUALLY POSSESSES FOUR DIFFERENT
  TYPES OF ENERGIES. THESE ARE
• TRANSLATIONAL ENERGY
• ROTATIONAL ENERGY
• VIBRATIONAL ANERGY
• ELECTRONIC ENERGY
• ACCORDING TO BORN-OPPENHEIMER APPROXIMATION
  ,THE TOTAL ENERGY OF A MOLECULE IS THE SUM OF
  TRANSLATIONAL,ROTATIONA AND ELECTRONIC ENERGIES.

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TYPES OF MOLECULAR SPECTRA

• THE ENERGY ABSORBED FOR ANY TRANSITION IS EQUAL
  TO THE DIFFERENCE IN THE ENERGIES OF THE TWO
  LEVELS INVOLVED.IT IS FOUND THAT THESE ENERGIES
  FOR TRANSITION ARE IN ORDER
• TRANSLATIONAL ENERGY IS CONSIDERED AS CONTINOUS
  AND WE DONOT OBSERVE ANY TRANSLATIONAL SPECTRUM.

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• PURE ROTATIONAL (MICROWAVE) SPECTRA- THESE
  SPECTRA ARE OBSERVED IN FAR INFRA-RED REGION OR
  IN THE MICROWAVE REGION.
• VIBRATIONAL ROTATIONAL SPCTRA- SUCH ENERGIES ARE
  AVAILABLE IN THE NEAR INFRA-RED REGION.
• ELECTRONIC BAND SPECTRA- FOR A GIVEN ELECTRONIC
  TRANSTION, A SET OF BANDS OBSERVED.THIS SET OF
  BANDS IS CALLED A BAND GROUP OR A BAND
  SYSTEM.THUS WHEREAS ATOMS GIVE LINE SPECTRA,
  MOLECULES GIVE BAND SPECTRA.THESE SPECTRA ARE
  OBSERVED IN THE VISIBLE REGION AND ULTRA VIOLET
  REGION.

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RAMAN SPECTRA-

• THIS IS ALSO A TYPE OF VIBRATIONAL-ROTATIONAL
  SPECTRUM.IT IS BASED ON SCATTERING OF RADIATION
  BY THE SAMPLE.RAMAN SPECTRA IS OBSERVED IN
  VISIBLE REGION.

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• RAMAN SPECTRA- THIS IS ALSO A TYPE OF
  VIBRATIONAL-ROTATIONAL SPECTRUM.IT IS BASED ON
  SCATTERING OF RADIATION AND NOT ON THE ABSORPTION
  OF RADIATION BY THE SAMPLE.RAMAN SPECTRA IS
  OBSERVED IN THE VISIBLE REGION.
• NUCLEAR MAGNETIC RESONANCE(NMR) SPECTRA- THIS
  TYPE OF SPECTRA ARISES FROM THE TRANSITIONBETWEEN
  THE NUCLEAR SPIN ENERGY LEVELS OF THE MOLECULE IN
  THE RADIO FREQUENCY REGION.
• ELECTRON SPIN RESONANCE (ESR) SPECTRA-THIS TYPE
  OF SPECTRUM ARISES FROM THE TRANSITIONS BETWEEN
  THE ELECTRON SPIN ENERGY LEVELS OF THE MOLECULE
  WHEN AN EXTERNAL MAGNETIC FIELD IS APPLIED ON IT.

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DEGREE OF FREEDOM OF MOTION

• CONSIDER A MOLECULE MADE UP OF N ATOMS.THE ATOMS
  MAY BE CONSIDERED AS MASS POINTS.
• THE NUMBER OF CO-ORDINATES REQUIRED TO SPECIFY
  THE POSITION OF ALL THE MASS POINTS i.e. ATOMS IN
  A MOLECULE IS CALLED THE NUMBER OF DEGREES OF
  FREEDOM.THUS A MOLECULE MADE UP OF N ATOMS HAS 3N
  DEGREE OF FREEDOM.

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• WHEN THE THERMAL ENERGY IS ABSORBED BY A
  MOLECULE,IT IS STORED WITH IN THE MOLECULE IN THE
  FORM OF
• (1) TRANSLATIONAL MOTION OF THE MOLECULE.
• (2) INTERNAL MOVEMENT OF THE ATOMS OF THE
  MOLECULE, i.e. ROTATIONAL MOTION AND VIBRATIONAL
  MOTION.
• THERE ARE THREE TRANSLATIONAL DEGREE OF
  FREEDOM.THE REMAINING (3N-3) CO-ORDINATES
  REPRESENT THE INTERNAL DEGREE OF FREEDOM.
• THE INTERNAL DEGREE OF FREEDOM MAY BE SUBDIVIDED
  INTO-
• (1) ROTATIONAL DEGREE OF FREEDOM.
• (2) VIBRATIONAL DEGREE OF FREEDOM.
• FOR A ROTATIONAL MOTION, THERE ARE 2 DEGREE OF
  FREEDOM FOR A LINEAR MOLECULE AND 3 FOR A NON
  LINEAR MOLECULE.

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• VIBRATIONAL DEGREES OF FREEDOM OF A LINEAR
  MOLECULE CONTAINING N ATOMS 3N-5
• VIBRATIONAL DEGREES OF FREEDOM OF NON-LINEAR
  MOLECULE 3N-6

FOR A MOLECULE MADE UP OF N ATOMS. TOTAL DEGREE
OF FREEDOM 3N FURTHER SPLIT UP THESE IS AS
FOLLOWS
TRANSLATIONAL ROTATIONAL VIBRATIONAL
FOR LINEAR MOLECULE 3 2 3N-5
FOR NON-LINEAR MOLECULE 3 3 3N-6
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ROTATIONAL SELECTION RULES

• ACCORDING TO SELECTION RULES,THE TRANSITION TAKE
  PLACE ONLY BETWEEN THOSE ROTATIONAL LEVELS FOR
  WHICH ?J 1
• i.e. THE CHANGE IN ROTATIONAL QUANTUM NUMBER IS
  UNITY.
• THE TRANSTION ?J 1 CORRESPONDS TO ABSORPTION.
• THE TRANSITION ?J -1 CORRESPONDS TO EMISSION.

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ROTATIONAL SPECTRA OF DIATOMIC MOLECULES

• THE ALLOWED ROTATIONAL ENERGIES ARE GIVEN BY THE
  EXPRESSION