MOLECULAR SPECTROSCOPY

1. 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.

2. 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.

3. 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.

4. 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

5. 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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7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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:

15. 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.

16. ROTATIONAL SPECTRA OF DIATOMIC MOLECULES • THE ALLOWED ROTATIONAL ENERGIES ARE GIVEN BY THE EXPRESSION