Download MOLECULAR SPECTROSCOPY

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.

ALL ELECTROMAGNETIC RADIATIONS TRAVEL
WITH THE VELOCITY OF LIGHT.
c=νλ.
EACH PHOTON IS A PACKET OF ENERGY
WHOSE ENERGY IS GIVEN BY PLANCK’S
EQUATION E=hν.WHERE h IS PLANCK’S
CONSTANT.
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.
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
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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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.
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.
• PURE ROTATIONAL (MICROWAVE) SPECTRATHESE SPECTRA ARE OBSERVED IN FAR INFRARED REGION OR IN THE MICROWAVE REGION.
• VIBRATIONAL ROTATIONAL SPCTRA- SUCH
ENERGIES ARE AVAILABLE IN THE NEAR INFRARED 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.
RAMAN SPECTRATHIS IS ALSO A TYPE OF VIBRATIONALROTATIONAL SPECTRUM.IT IS BASED ON
SCATTERING OF RADIATION BY THE
SAMPLE.RAMAN SPECTRA IS OBSERVED
IN VISIBLE REGION.
• RAMAN SPECTRA- THIS IS ALSO A TYPE OF VIBRATIONALROTATIONAL 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.
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.
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.
•VIBRATIONAL DEGREES OF FREEDOM OF A LINEAR
MOLECULE CONTAINING N ATOMS= 3N-5
•VIBRATIONAL DEGREES OF FREEDOM OF NONLINEAR 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
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.
ROTATIONAL SPECTRA OF
DIATOMIC MOLECULES
• THE ALLOWED ROTATIONAL
ENERGIES ARE GIVEN BY THE
EXPRESSION