Spectroscopy Lec1 (UV-Visible) PDF

TERMINOLOGY SPECTROSCOPY = STUDY OF THE INTERACTION BETWEEN LIGHT AND MATTER. SPECTRA = PLOTS OF RADIANT INTENSITY VERSUS WAVELENGTH OR FREQUENCY SPECTROSCOPE = INSTRUMENT FOR VIEWING SPECTRUM SPECTROGRAPH = INSTRUMENT FOR RECORDING SPECTRUM (E.G. ON PHOTOGRAPHIC FILM) SPECTROMETER = INSTRUMENT FOR...

TERMINOLOGY SPECTROSCOPY = STUDY OF THE INTERACTION BETWEEN LIGHT AND MATTER. SPECTRA = PLOTS OF RADIANT INTENSITY VERSUS WAVELENGTH OR FREQUENCY SPECTROSCOPE = INSTRUMENT FOR VIEWING SPECTRUM SPECTROGRAPH = INSTRUMENT FOR RECORDING SPECTRUM (E.G. ON PHOTOGRAPHIC FILM) SPECTROMETER = INSTRUMENT FOR RECORDING SPECTRUM AS A FUNCTION OF RADIANT ENERGY SPECTROPHOTOMETER = SPECTROMETER WITH ASSOCIATED ELECTRONICS WHICH PROVIDES THE RATIO OR A FUNCTION OF RATIO (E.G. LOG) OF THE TWO BEAMS AS A FUNCTION OF SPECTRAL WAVELENGTH. 1  ABSORPTION AND EMISSION SPECTRA:- SPECTROSCOPY MAINLY CONCERNED WITH INTERACTION OF ELECTROMAGNETIC RADIATION WITH MATTER. AFTER INTERACTION THEY MAY VARIATION IN INTENSITY OF EMR WITH FREQUENCY. INSTRUMENT WHICH RECORD THIS VARIATION IN INTENSITY KNOWN AS SPECTROPHOTOMETER TWO WAY IN WHICH INTERACTION MAY OBSERVED- Sample itself emits radiation Called as emission spectra Absorbs radiation from Called as absorption spectra continuous source  TERMINOLOGY WAVELENGTH(Λ):- DISTANCE BETWEEN TWO SUCCESSIVE MAXIMA OF ONE ELECTROMAGNETIC WAVE. EXPRESS IN ANGSTRON UNITS OR (MU) FREQUENCY(Ν):- NUMBER OF WAVELENGTH PASSING THROUGH A GIVEN POINT. PER SEC. UNIT:- HERTZ OR CYCLES PER SECOND WAVE NUMBER:- NUMBER OF WAVES PER CENTIMETER IN VACUUM. RECIPROCAL OF WAVELENGTH, EXPRESS AS PER (CM). RELATION BETWEEN FREQUENCY, VELOCITY & WAVE NUMBER ν=(1/λ)c=(c/v)λ=(v/c)  WAVE PARAMETERS SYMBOL PARAMETER DEFINITION A Amplitude Maximum magnitude of one wave cycle p Period Time for a  to pass a fixed point Unit s: seconds (s) λ Wavelength The distance between any maxima or minima of two (Lambda) successive waves. Units : m, µm, nm, Å Frequency Number of wavelength units that occur in one second (Nu) (unit time). ( = 1/p) Units: s1, Hz. Wavenumber The reciprocal of wavelength. Units : cm1 v Velocity The distance travels per second, i.e. v =  Units: 3x1010 cm/s; 3x108 m/s In vacuum c =  4  Electro Magnetic Spectrum Color Wave 109 107 105 103 101 10-1 10-3 10-5 10-7 10-9 10-11 length (nm) violet 400-435 indigo 435-480 gamma X-rays Violet, indigo, blue blue 480-500 500 Ultra Violet 600 Green, yellow green 500-560 Infra red 700 Orange, red yellow 560-595 microwave orange 595-610 Radio waves red 610-750 SPECTRUM OF ELECTROMAGNETIC RADIATION 6 ORIGIN & THEORY OF ULTRA VIOLET SPECTRA ULTRA VIOLET ABSORPTION SPECTRA ARISES FROM TRANSITION OF ELECTRON OR ELECTRON WITHIN MOLECULE. UV EMISSION SPECTRA ARISES FROM REVERSE TYPE OF TRANSITION. ELECTRON UNDERGOES TRANSITION FROM LOWER TO HIGHER ENERGY LEVEL, THIS ENERGY DIFFERENCE GIVEN BY, E=HΝ ERG BUT ACTUALLY ENERGY DIFFERENCE BETWEEN GROUND & EXCITED STATES OF ELECTRONS E1-E0=HΝ TOTAL ENERGY OF THE MOLECULE IS SUM OF ELECTRONIC, VIBRATIONAL, ROTATIONAL ENERGY. E=Eele +Evib +Erot  MOLECULAR ABSORPTION MOLECULES ABSORB/EMIT LIGHT OVER WIDER RANGE OF  → SPECTRAL BANDS DUE TO 3 TYPES OF QUANTIZED TRANSITIONS : 1. ELECTRONIC TRANSITIONS 2. VIBRATIONAL TRANSITIONS 3. ROTATIONAL TRANSITIONS RELATIVE ENERGIES EEL > EVIB > EROT (10,000:100:1) E = (EEL + EVIB + EROT)2  (EEL + EVIB + EROT)1 8 EACH E FIXES THE  AT WHICH EACH MOLECULE ABSORBED.  MOLECULAR ABSORPTION 9 MOLECULAR ABSORPTION SPECTRA CAN BE CLASSIFIED INTO 3 TYPES: ELECTRONIC SPECTRA = DUE TO CHANGES IN ELECTRONIC (1 TRANSITIONS AND ALSO ASSOCIATED VIBRATIONAL AND ROTATIONAL TRANSITIONS → ULTRAVIOLET-VISIBLE REGION (200-800NM) 2) VIBRATIONAL SPECTRA = DUE TO VIBRATIONAL AND ROTATIONAL TRANSITIONS → INFRARED REGION (2.5-15 UM [400-4000CM-1]) 3) ROTATIONAL SPECTRA = ROTATIONAL TRANSITION ONLY → MICROWAVE REGION (103 – 0.67 NM)  QUANTUM THEORY EVERY ELEMENTARY PARTICLE (ATOM, ION OR MOLECULE) EXISTS IN DISCRETE ENERGY STATES, E0, E1, E2 , E3 ETC. AT ROOM TEMPERATURE MOST PARTICLES ARE IN THEIR LOWEST ENERGY LEVEL E0 (GROUND STATE). WHEN ATOMS ABSORB PHOTON OF RADIATION, THEY CAN BE PROMOTED TO HIGHER ENERGY LEVELS E1, E2 , E3 ETC. ONLY OCCUR IF THE PHOTON MATCHES EXACTLY THE ENERGY DIFFERENCE (E) BETWEEN THE GROUND STATE AND THE HIGHER ENERGY STATES, I.E. E = (EN  E0) = H = HC/ PROMOTED ATOMS ARE SAID TO BE IN THE EXCITED STATES. M + H → M* 10  UV-VISIBLE SPECTROSCOPY IS THE MEASUREMENT OF ABSORBANCE OR TRANSMITTANCE OF RADIATION IN THE ULTRA-VIOLET &VISIBLE REGION OF THE SPECTRUM. IT ARISES FROM TRANSITION OF ELECTRON. STAGE -1 M + HΝ M* Excitation of species by absorption of photon with the STAGE -2 limited life time. M* NEW SPECIES Relaxation by converting M* to the new species by photochemical reaction E Absorption E4 E3 E2 Excited states E1 12 E0 Ground state E Emission E4 E3 E2 Excited states E1 13 E0 Ground state  THREE TYPES OF ELECTRONS ARE INVOLVED IN ORGANIC MOLECULE :- Σ-ELECTRONS- INVOLVED IN THE SATURATED BONDS. FOUND IN THE CARBON, HYDROGEN IN THE PARAFFIN. ENERGY REQUIRED TO EXCITE Σ-PRODUCED IS ELECTRON MORE THAN THE PRODUCED BY THE UV-LIGHT. Π- ELECTRONS- INVOLVED IN UNSATURATED HYDROCARBON. PRESENT IN TRIENS & AROMATIC COMPOUNDS. N-ELECTRONS- IT DOES NOT EVOLVED IN THE BONDING OF THE MOLECULES. Fig. 1 The order of orbital energies and approximate order of electronic transition energies in a hypothetical unsaturated molecule containing a heteroatom with a nonbonded electron pair (n).  THE UV ABSORPTION PROCESS  * and  * transitions: high-energy, accessible in vacuum UV (max π-π*> n-π* Thus, n-π*transition required less energy than a π-π* or σ-σ* transition.  THE VISIBLE SPECTRUM WHEN A BEAM OF LIGHT IS PASSED THROUGH A PRISM, A BAND OF COLOURS IS FORMED (CONTINUOUS SPECTRUM). VISIBLE PART OF EM SPECTRUM (VISIBLE RANGE) EACH COLOUR CORRESPONDS TO WAVES OF A PARTICULAR WAVELENGTH. 18  Ethane C C   hv     H HH H C C max = 135 nm (a high energy transition) H H Absorptions having max < 200 nm are difficult to observe because everything (including quartz glass and air) absorbs in this spectral region.      C C hv = hv =hc/       Example: ethylene absorbs at longer wavelengths: = 10,000 max = 165 nm    C O   n hv n     n  The n to pi* transition is at even lower wavelengths but is not as strong as pi to pi* transitions. It is said to be “forbidden.” Example: Acetone: n max = 188 nm ; = 1860 n max = 279 nm ; = 15 C C 135 nm  165 nm  C C weak 183 nm n H C O 150 nm  188 nm n weak 279 nm n C O 180 nm C O A 279 nm   ABSORPTION OF RADIATION ABSORPTION = CHEMICAL SPECIES IN A TRANSPARENT MEDIUM SELECTIVELY ATTENUATES (DECREASES THE INTENSITY OF) CERTAIN FREQUENCIES OF EM. PLOT OF INTENSITY VS , OR → ABSORPTION SPECTRUM TWO QUALITATIVE PARAMETERS : a) TRANSMITTANCE 23 b) ABSORBANCE  BEER’S LAW cuvette slit source detector A = -LOGT = LOG(P0/P) = EBC T = PSOLUTION/PSOLVENT = P/P0 WORKS FOR MONOCHROMATIC LIGHT COMPOUND X HAS A UNIQUE E AT DIFFERENT WAVELENGTHS  ABSORPTION OF RADIATION P P0 b TRANSMITTANCE ,T = P/P0 % T = P/P0  100 ABSORBANCE,  LOG T = LOG P0/P 25  INSTRUMENTATION COMPONENTS OF SPECTROPHOTOMETER SOURCE MONOCHROMATOR SAMPLE COMPARTMENT DETECTOR RECORDER  Entrance slit Exit slit sample detector amplifier Read out monochromator Light source a)D2 Lamp b)WI Lamp Fig.- block diagrammatic representation of UV-spectrophotometer Chromophore: A covalently unsaturated group responsible for electronic absorption. or Any group of atoms that absorbs light whether or not a color is thereby produced. e.g. C=C, C=O, NO2 etc. A compound containing chromophore is called chromogen.  AUXOCHROME:A SATURATED GROUP WITH NON-BONDING ELECTRON WHEN ATTACHED TO CHROMOPHORE ALTERS BOTH WAVELENGTHS AS WELL AS INTENSITY OF ABSORPTION. E.G. OH, NH2, NHR ETC.  BATHOCHROMIC GROUP: THE GROUP WHICH DEEPENS THE COLOR OF CHROMOPHORE IS CALLED BATHOCHROMIC GROUP. E.G. PRIMARY, SECONDARY AND TERTIARY AMINO GROUPS.  BATHOCHROMIC SHIFT: (RED SHIFT)SHIFT OF LAMBDA MAX (ΛMAX)TO LONGER SIDE OR LESS ENERGY IS CALLED BATHOCHROMIC SHIFT OR READ SHIFT. THIS IS DUE TO SUBSTITUTION OR SOLVENT EFFECT.  HYPSOCHROMIC SHIFT:(BLUE SHIFT)SHIFT OF LAMBDA MAX (ΛMAX)TO SHORTER SIDE AND HIGHER ENERGY IS CALLED HYPSOCHROMIC OR BLUE SHIFT. E.G SOLVENT EFFECT.  HYPERCHROMIC EFFECT: AN INCREASE IN ABSORPTION INTENSITY  HYPOCHROMIC EFFECT: A DECREASE IN ABSORPTION INTENSITY  Hyperchromic Hypsochromic Bathochromic  200nm 700nm Hypochromic  WOODWARD-FEISER RULE IT IS USED FOR CALCULATING THE ABSORPTION MAXIMA WOODWARD (1941) GIVES CERTAIN RULE FOR CORRELATING ΛMAX WITH THE MOLECULAR STRUCTURE THESE RULES ARE MODIFIED BY SCOTT & FEISER. THIS RULE FOR CALCULATING ΛMAX IN CONJUGATED DIENES, TRIENES, POLYENES. HOMOANNULAR DIENES:- CYCLIC DIENES HAVING CONJUGATED DOUBLE BONDS IN THE SAME RING. CH3 E.G. CH3  HATEROANNULER DIENES:- CYCLIC DIENES IN WHICH DOUBLE BONDS IN CONJUGATION ARE PRESENT IN THE DIFFERENT RING. ENDOCYCLIC DOUBLE BONDS:- IT IS THE DOUBLE BOND PRESENT IN RING AS SHOWN. EXOCYCLIC DOUBLE BONDS:- DOUBLE BOND IN WHICH ONE OF THE DOUBLE BONDED ATOM IS THE PART OF RING SYSTEM. e.g. Heteroannuler dienes CH2 CH2 Endocyclic Exocyclic double bond double bond  Woodward’s-Fieser rule for conjugated dienes a)Parent values- 1. acyclic & Heteroannuler conjugated dienes 215 nm 2.Homoannular conjugated dienes 260 nm 3.Acyclic trienes 245 nm b)Increments- 1.Each alkyl substituent or ring residue 5 nm 2.Exocyclic double bond 5 nm 3.Double bond extending conjugation 30 nm 4.auxochromes- -OR 6 nm -SR 30 nm -Cl , Br 5 nm -NR2 60 nm -OCOCH3 0 nm Problems:-1) 1,4- dimethyl cyclohex-1,3,-diene H3C CH3 Parent value for Homoannular diene = 253 nm Two alkyl substituent's 2 ˣ 5 = 10 nm Two ring residues 2ˣ 5 = 10 nm Calculated value = 273 nm Observed value = 265 nm 2) Parent value for Heteroannuler diene = 215 nm Four ring residue 4 ˣ 5 = 20 nm Calculated value = 235 nm Observed value = 236 nm