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.
REGION OF 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 MOLECULAR ENERGY
LEVELS.THE TRANSITIONS OF ENERGIES CAN
TAKE PLACE ONLY BETWEEN THESE LEVELS.THE
RESULT IS A MOLECULAR SPECTRUM.
MOLECULAR ENERGY
LEVELS
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 TRANSITIONS IS CALLED ABSORPTION 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.
THE ENERGY OF PHOTON hv EMITTED OR
ABSORBED IS GIVEN BY BOHR’S
FREQUENCY FORMULA …
TYPES OF MOLECULAR ENERGIES AND BORNOPPENHEIMER APPROXIMATION
•
•
•
•
A MOLECULE USUALLY POSSESSES FOUR DIFFERENT
TYPES OF ENERGIES. THESE ARE…
TRANSLATIONAL ENERGY
ROTATIONAL ENERGY
VIBRATIONAL ENERGY
ELECTRONIC ENERGY
• ACCORDING TO BORN-OPPENHEIMER
APPROXIMATION,THE TOTAL ENERGY OF A MOLECULE
IS THE SUM OF TRANSLATIONAL , ROTATIONAL,
VIBRATIONAL AND ELECTRONIC ENERGIES.
E = Et + Er + Ev + Ee
IT IS FOUND THAT THE TRANSLATIONAL ENERGY IS
NEGLEGIBLY SMALL.SO
E = Ev + Er + Ee
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…
• Et << Er << Ev << Ee
• 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 SPECTRA- SUCH
ENERGIES ARE AVAILABLE IN THE NEAR INFRARED REGION.
• ELECTRONIC BAND SPECTRA- FOR A GIVEN
ELECTRONIC TRANSITION, 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.
REGION OF ELECTROMAGNETIC
SPECTRUM AND DIFFERENT TYPES OF
MOLECULAR SPECTRA
• 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 TRANSITION BETWEEN
THE NUCLEAR SPIN ENERGY LEVELS OF THE MOLECULE
WHEN AN EXTERNAL FIELD IS APPLIED ON IT.THE ENERGIES
INVOLVED IN THESE TRANSITIONS ARE VERY HIGH WHICH
LIE IN THE RADIO FREQUENCY REGIONS.
• ELECTRON SPIN RESONANCE (ESR) SPECTRA-THIS TYPE OF
SPECTRUM ARISE 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.
(1). ROTATIONS OF A LINEAR MOLECULE
ABOUT TWO MUTUALLY PERPENDICULAR
AXES.
(2). ROTATIONS OF A NON LINEAR
MOLECULE ABOUT THREE MUTUALLY
PERPENDICULAR AXES.
•VIBRATIONAL DEGREES OF FREEDOM OF A
LINEAR MOLECULE CONTAINING N ATOMS = 3N-5
•VIBRATIONAL DEGREES OF FREEDOM OF NONLINEAR MOLECULE CONTAINING N ATOMS = 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
PURE ROTATIONAL (MICROWAVE)
SPECTRA OF DIATOMIC MOLECULES
ENERGY LEVELS OF A RIGID ROTOR-ON THE BASIS OF
WAVE MECHANICS, IF DIATOMIC MOLECULE IS
CONSIDERED TO BE A RIGID ROTATOR.THEN..
------- (1)
WHERE J IS ROTATIONAL QUANTUM NUMBER THAT
CAN HAVE VALUES 0,1,2,3 etc.
I IS THE MOMENT OF INERTIA OF THE
MOLECULE ABOUT THE AXIS OF
ROTATION, i.e
WHERE m1 AND m2 ARE THE ATOMIC
MASSES OF THE TWO ATOMS OF THE
DIATOMIC MOLECULES.
ROTATIONAL ENERGY LEVELS OF A DIATOMIC
MOLECULE TREATING IT AS A RIGID ROTATOR
PUTTING J = 0, 1, 2, 3 etc. IN EQUATION (1), PATTERN OF
THE ROTATIONAL ENERGY LEVELS OBTAINED.
THE SPACING BETWEEN THE ENERGY LEVELS INCREASES
AS J INCREASES.
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 TRANSITION ∆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
-------------------- (1)
AS
E = hv
--------------------(2)
EQUATION (2) BECOMES…
---------------------(3)
--------------------(4)
WHERE
IS CALLED ROTATIONAL
CONSTANT.PUTTING J = 0, 1, 2, 3 etc. IN EQUATION (4)
THE WAVE NUMBERS OF THE DIFFERENT ROTATIONAL
LEVELS WILL BE…
0, 2B, 6B, 12B, 20B, 30B,… AND SO ON.
WHEN A TRANSITION TAKES PLACE FROM A LOWER
ROTATIONAL LEVEL WITH ROTATIONAL QUANTUM NUMBER J
TO A HIGHER ROTATIONAL LEVEL WITH ROTATIONAL
QUANTUM NUMBER J’ ,THE ENERGY ABSORBED WILL BE
GIVEN BY
-------------------(5)
HENCE IN THE PRESENT CASE J’ = J + 1.
-------------------(6)
TO EXPRESS IN TERMS OF WAVE NUMBERS…
------------------(7)
THE WAVE NUMBER OF THE LINES OBTAINED WILL BE
2B, 4B, 6B, 8B, …… AND SO ON.
THE VARIOUS LINES IN THE ROTATIONAL SPECTRA WILL BE EQUALLY SPACED.