SPECTROSCOPY UNIT I ELECTROMAGENTIC RADIATION Light is travel in the straight line It can be explained by two theories i Wave theory ii Electromagnetic theory WAVE THEORY The light is travels in the form of waves The radiant energy is emitted by fluctuation of electric charge and magnetic field The various form of electromagnetic radiations such as UV IR X ray etc E C M CHARACTERISTICS Produced electromagnetic waves oscillation of electric charge and magnetic field presenting particles on the atom Electromagnetic waves are characterized by wavelength or frequency or wave number EMV Frequency Each emission or absorption of radiation is quantized and each quantized radiation is called a Photon Visible light is travel to prism it splits into seven colour it is called as Dispersion White light Dispersion angle Prism Green DISPERSION red Orange Yellow Blue Indigo violet b UNITS i Wavelength Distance between the two adjacent crests It is denoted as lambda It is expressed as Angstrom A0 or millimicrons m It is reciprocal of wavelength and it is expressed in per centimeter 1 A0 10 8 cm 1 m 10 7 cm ii Wave number 1 Example 25 2 5 10 4 cm 1 2 5 10 4 cm 1 iii Frequency 4000 cm 1 4000 cm 1 The number of waves can pass through a point in one second It is expressed as 4000 cm 1 Frequency 4000 cm 1 1 Hz 4000 cm 1 Electromagnetic spectrum and absorption of radiations The electromagnetic radiations are arranged in the increasing order of wavelengths or decreasing frequencies is called as electromagnetic spectrum 10 14 10 12 10 10 10 8 10 6 10 4 10 2 1 102 104 UV 190 400 nm Visible 400 800 nm IR 667 4000 cm 1 Radio frequency 60 300 Hz Electron beam impact 70 eV 6000 KJmol 1 QUANTISATION OF DIFFERENT FORM OF ENERGIES IN MOLECULES UV SPECTRA It is measurement of energy absorption when the electrons promoted to higher energy levels Vacuum UV UV VISIBLE 10 200nm 200 380nm 380 780 nm It is difficult to P and d Orbitals Electrons are Measure bonds are conjugated and conjugated move to visible Range E g carotene max 451 nm BEER LAMBERT S LAW Where 10 C l A 0 I0 Intensity of incident light I intensity of emergent light Molar absorption C Concentration of solute m lit l path length cm A absorbance LIMITATIONS Different forms of the absorbing molecules are in equilibrium Solute and solvent form associating complex Thermal equilibrium between a ground electronic state and a higher excited state Compounds are charged by irradiation UV VISIBLE SPECTROSCOPY INSTRUMENTATION UV Valence electronic excitation 10 2 100 102 104 106 108 Wavelength cm Gamma ray Visible X ray Infrared UV microwave Radio frequency INTRODUCTION The alternative title for this technique is electronic spectroscopy It involves the promotion of electrons form the ground State to higher energy state The energy absorbed corresponds to the amount necessary to promote an electron from orbital to another USES OF UV It is very useful to measure the number of conjugate double bonds and also conjugation with in the various molecules It is also distinguished between conjugated and non conjugated system It detects the percentage of transmittance of lights when light of certain intensity and frequency passed through the sample It is compared with the intensity of the transmitted light with the incident light UV spectrometer is simplicity versatility speed accuracy and cost effectiveness REGION OF UV REGION WAVELENGTH Far UV 10 200 nm Near UV 200 300 nm Visible 300 780 nm INSTRUMENTATION FOR MEASURING THE ABSORPTION OF UV RADIATION ARE MADE UP OF FOLLOWING COMPONENTS BLOCK DIAGRAM slit m1 mirror SAMPLE prism m2 Light Optical wedge source mirror pen Spectrum Electronic amplifier Motor Source Monochromator Sample cell Amplifier Detector Recording device LIGHT SOURCE W Filament lamp Hydrogen Deuterium discharge lamp W FILAMENT LAMP It is particularly rich in red radiations Radiations wavelength 375m HYDROGEN DEUTRIUM LAMP The intensity of the light is 360 above MONOCHORMATOR This is a beam device which isolates a narrow band wavelength of the get coming from the source All monochromator containing following parts 1 An entrance slit 2 A collimating lens 3 Dispersing device Both prism and grating are used 4 A focusing lens 5 An exit slit WAVELENGTH SELECTOR Filters and monochromator SAMPLE CELL OR SAMPLE CONTAINER Fused silica quartz and silica DETECTORS The photomultiplier is used in UV spectroscopy It contains phototube PMT photodiode photodiode array CCD array SPECTROMETER Ordinary spectrometer absorbed 220 800 m Better instruments absorbed 285 m Oxygen absorbs 200 m The study of below 200 m is evacutted vacuum ultra region Nitrogen absorbed 150 m so flushing the instrument Mostly used double beam instrument WORKING Primary source of light is divided into two beams equal intensity Before dividing the to beam the incident radiation dispersed with the helping of rotating prism One of the beams is selected monochromatic light is passed through the sample as well as reference These substances are taken in cell and glass cannot be used because it absorbed strongly the UV region and silica Then these must be stored and handled properly Usually samples are scanned in dilute solution One mg of the compound undergoes investigation weighed and dissolved in situation solvent to make 100 ml volume at silica cell P1 and P2 receive an intense beam from reference cell weak from sample cell This results the pulsating or alternating current flow from photovoltaic cell to electronic amplifier Amplifier is coupled to small servo meter and turn coupled pen record These records absorb the absorption bands automatically Amplifier is coupled to small servo meter which derives an optical wedge to resonance beam until the photoelectric cell receive light in equal intensity in a sample as well as resonance beam TYPES OF ELECTRONIC TRANSITION A molecule is excited from a bonding to an antibonding orbital by absorption of UV or visible light Anti bonding n n n non bonding Bonding n n n transitions Bonding electron is promoted to antibonding orbital Its process is high energy very strong transitions Don t absorb at 180 400 m Methane ethane propane etc Example C C C C n transitions Non bonding electrons n is promoted into antibonding sigma electrons Saturated compounds containing one hetero atom with unshared pair of electrons Halides alcohols ethers aldehydes ketones Example transitions Promotion of electrons to an antibonding orbital Compound containing double or triple bonds or