Chemistry Module 3 PDF

text Your paragraph text yt fb in ln wp MOLECULAR SPECTROSCOPY AND ANALYTICAL TECHNIQUES SYLLABUS Molecular Spectroscopy and Analytical Techniques 1.Spectroscopy-Types of spectra- Molecular energy levels - Beer Lambert’s law – Numerical problems 2. Electronic Spectroscopy – Principl...

text Your paragraph text yt fb in ln wp MOLECULAR SPECTROSCOPY AND ANALYTICAL TECHNIQUES SYLLABUS Molecular Spectroscopy and Analytical Techniques 1.Spectroscopy-Types of spectra- Molecular energy levels - Beer Lambert’s law – Numerical problems 2. Electronic Spectroscopy – Principle, Types of electronic transitions –Role of conjugation in absorption maxima- Instrumentation-Applications 3.Vibrational spectroscopy – Principle- Number of vibrational modes - Vibrational modes of CO2 and H2O – Applications 4.Thermal Analysis: Dielectric Thermal Analysis (DETA) of Polymers- Working and Application. 5.Electron Microscopic Techniques: SEM - Principle, instrumentation and Applications.  SPECTROSCOPY It is the branch of science that deals with study of interaction matter with light or It is the branch of science that deals with study of interaction of matter with electromagnetic radiation Electromagnetic radiation of particular wavelength or range of wave length is used for qualitative and quantitative analysis of matter  Matter : anything which can occupy space in universe and having mass electromagnetic radiation : wave produced by motion of electrically charged particle consist of 2 component:electrical and magnetic field Properties of waves Wavelength(λ):distance between two nearest crest or trough Frequency: Number of wavecycle in a given time,measured in hertz(Hz) Amplitude: wave`s height or length  Relationship between frequency and wavelength velocity (speed) of propogation : Relation between frequency of light and energy ELECTROMAGNETIC RADIATION  The particle in matter has different types of energy associated with them They are 1. Rotational energy 2. Vibrational energy 3. Electronic energy INTERACTION OF ELECTROMAGNETIC RADIATION WITH MATTER Absorption of Radiation 1. Electronic energy level molecules are present at lowest energy level E0 molecule absorb energy(UV/Visible)promoted to higher energy level Difference in energy ( ΔE) = En – E0 = 35 to 71 kcl/mol 2. Vibrational energy level Molecule absorb energy (IR)-promoted from one vibrational level to another vibrational level Difference in energy ( ΔE) = 0.01 to 10 kcl/mol Less energy required than electronic energy level Rotational energy level Energy required is smaller than vibrational energy level ΔErota < ΔEVibr < ΔE elec Emission of radiation Electromagnetic radiation is produced when excited particle (ions,atoms,molecule) relax to lower energy level by giving up their excess energy as photons  TYPES OF SPECTRA 1.Emmision spectrum Whenever electromagnetic radiation interacts with atoms and molecules of matter, the electrons in these atoms may absorb energy and jump to a higher energy state, losing their stability. In order to regain their stability, they need to move from the higher energy state to the previous lower energy state. To accomplish this job, these atoms and molecules emit radiation in various regions of the electromagnetic spectrum. This spectrum of radiation emitted by electrons in the excited atoms or molecules is known as an emission spectrum. However, the emission spectrum of atoms in the gas phase do not exhibit a continuous spread of wavelength from one colour to others. Rather, the emitted light consists of a specific wavelength having dark spaces existing between them. Such kind of spectra is known as atomic spectra or line spectra. 2.Absorbtion spectrum This spectrum is constituted by the frequencies of light transmitted with dark bands when the electrons absorb energy in the ground state to reach higher energy states. This type of spectrum is produced when atoms absorb energy The dark line pattern is seen exactly in the same place where coloured lines are seen in the emission spectrum. The spectrum thus obtained is known as the absorption spectrum. Emission spectra can emit all the colours in an electromagnetic spectrum, while the absorption spectrum can have a few colours missing due to the redirection of absorbed photons. The wavelengths of light absorbed help figure out the number of substances in the sample. BEER – LAMBERT LAW When a monochromatic radiation is passed through the solution, thedecrease in the intensity of radiation with thickness of the solution is directly proportional to the intensity of the incident light as well as concenteration of the solution Let I be the intensity of incident radiation,X be the thickness of the solution.C be the concenteration of the solution.Then, NUMERICAL PROBLEM Electronic spectroscopy(UV Visible spectroscopy) EM radiation in the 10-800 nm region is capable of interactin with the molecule give rise toelectronic transition 10-400 : UV Region 400-800: visible region UV radiation having wavelength less than 200 nm is difficult to handle and not commonly used. It involve the study of electronic transition from lower to higher energy levels in a molecular species by absorption of energy in the UV and Visible light FRANCK – CONDON PRINCIPLE The principle useful in explain the nature of electronic transition and vibrational structure of electronic spectra In a molecule ,individual atom vibrate about mean position and therefore ,the internuclear distance changes continuously According to FC principle,an electronic transition is rapid that the vibrating molecule doesnot change its internuclear distance appreciably during transition During electronic transition,the internuclear distance remain same and straight line representing the transition between electronic state will be vertical The transition between electronic state occur vertically in a potential energy diagram The intensity distribution of line in the spectra of molecule is not same.The variation of intensity are explained on the basis of FC principle Types of electronic transitions A molecule undergo various type of electronic transition All molecule have σ orbital and the σ*orbital Molecule with doublebond having π and π* orbitals Molecule containing heteroatoms(O,N,S) have nonbonding orbital(n) Role of conjugation in absorbtion maxima Instrumentation  Application Identification of compound:identification of aromatic hydrocarbon containing fused rings such as naphthalene ,anthracene.The greater the number of fused ring ,the more complex the spectrum is.The spectrum of each compound is uniquely characteristic of its structure Detection of functional group Identification of geometrical isomers:trans isomers have higher absorption maximum compared to cis counterpart Study of conjugation:presence of conjugation generally shift higher absorption maxima to higher value.it is useful in distinguishing conjugated compound from non conjugated compound Impurity detection Study of strain  Study of tautomeric equilibria Study of coordination complex Provide a host of information regarding nature and type of metal ion,ligand present and type of interaction present in them Quantitative estimation in organic and inorganic chemistry  Vibrational spectroscopy (IR spectroscopy) PRINCIPLE: The principle of IR spectroscopy is related to the vibrational and rotational energy of a molecule When a frequency of the IR radiation is equal to the natural frequency of vibration,the molecule absorb IR radiation Absorbtion of IR radiation causes an excitation of molecule from a lower to the higher vibrational energy level Each vibrational level is associated with the number of closely packed rotational level.This is also called vibrational rotational spectroscopy The band correspond to the characteristic functional group and the band present in a chemical substance.IR spectrum of a compound is considered as a fingerprint for its chemical identification Normal mode of vibration Vibrational mode of CO2 carbondioxide is triatomic molecule (n=3).For molecule has 3n degree of freedom.For linear molecule there will be (3n -5) fundamental vibration 3n-5 =3(3)-5 = 9-5 =4 The mode of vibrations  In symmetrical stretching,there is a partial charge separation due to 2 oxygen atom,the magnetic moment get cancelled each other, molecule is nonpolar –IR inactive In asymmetrical stretching: one end is stretched and other end is compression.this produce a fluctuating dipole moment – IR active Bending: there are 2 bending vibration ,differ only in their direction and have same frequency and IR active Vibrational mode of H2O DIELECTRIC THERMAL ANALYSIS (DETA) OF POLYMERS DETA is a combination of dielectric analyisis and thermal analysis to study the thermal and dielectric property of metal Dielectric thermal analysis (DETA) is a materials science technique in which an oscillating electric field is used to analyze changes in the physical properties of a number of polar materials This thermal analysis technique can be used with materials in a range of forms, from thin films and sheet materials to powders or liquids PRINCIPLE The sample is placed in contact with two electrodes (the dielectric sensor). When a sinusoidal voltage is applied, the charge carriers inside the sample are forced to move: positively charged particles migrate to the negative pole and vice versa. This movement results in a sinusoidal current with a phase shift. In the frequency range (up to 1 MHz), the charge carriers are mainly ions (often present as catalysts or impurities) and additionally dipole alignment takes place within the electrical field. Sample is subjecting to a varying electric field while simultaneously measuring its temperature  1.Dielectric Analyser This is the instrument used for dielectric measurement It applies an electricfield to the sample and measure its response in terms of dielectric permittivity & dielectric loss as a function of temperature and frequency 2. Furnace The sample is subjected to controlled temperature changes during the analysis The furnace maintain the desired temperature while dielectric properties are measured 3.sample holder The sample holder consist of 2 parallel plate made of conductive material sandwich the sample being analysed The plate serve as electrodes for applying the electric field to sample  4.Frequency generator It is the important component of dielecric analyser DETA involves measurement at different frequencies It allow the user to study the dielectric response over a range of frequencies 5.Data Acquistition system This system records and processes the data obtained during the analysis It capture the dielectric response of the sample and convert it in to meaningful data,such as dielctric loss and dielectric permitivitty 6.Thermocouples Used to maintain desired temperature inside the furnace Thermocouples are temperature sensors placed near the sample to monitor the sample temperature DATA INTERPETATION APPLICATION Dielectric thermal analysis can be used in investigations into the curing behavior of thermosetting resin systems, adhesives, paints, and composite materials. Dielectric thermal analysis can also be used to characterize polar materials including: PVC – polyvinyl chloride PVDF – polyvinylidene fluoride PMMA – poly(methyl methacrylate) PVA – poly(vinyl acetate)  Polymer properties examined by DETA are permittivity (ε’), the measure of the degree of alignment of the molecular dipoles to the electrical field, and the loss factor (ε”), which represents the energy required for the reorientation of the dipoles and ions. The dissipation factor (tan δ = ε”/ε’) and the conductivity (σ-1) are also examined. With DETA, the dielectric constant and polarizability of polymers are easily detected during phase transitions such as the glass transition (Tg), a unique characteristic of polymers in which a material will transition from a hard state (glassy) to a viscous state as the temperature of the polymer is increased. Other transitions include melting and crystallization and secondary transitions. Other applications of dielectric thermal analysis are monitoring curing kinetics of epoxy and urethane systems. Used to study the dielectric properties of various material ADVANTAGE Data can be gathered from samples in a wide range of forms, from solids and liquid, to powders or pellets. DETA is also more sensitive than DSC, allowing it to obtain better data on samples dielectric thermal analysis is able to offer frequencies beyond 100 Hz. DISADVANTAGE Sensitivity of DETA can be affected by various factors such as sample size,sample preparation Preparation of sample for DETA can be difficult for obtaining accurate results Interpretation of DETA require experience and expertise DETA is not available like other technique DETA performed in a limited and specific temperature range,which is difficult to maintain SCANNING ELECTRON MICROSCOPY (SEM) SEM is a type of electron microscope that is capable of producing high resolution image of a sample surface SEM image have characteristic 3D appearance and is useful in judging the surface structure of a sample Principle: SEM work on the principle of scattering of electron on the surface of sample It is a type of electron microscope which produce images of sample by scanning the surface with a focused beam of electron Electron will interact with the atom present in the sample and produce various signal Inforamation of the sample is obtained Instrumentation Electron beam is thermionically emmited from an electric gun fitted with a tungsten filament cathode The beam is recorded by anode The beam travel through electromagnetic field,lens which focus the beam down towards the sample When the beam touches the surface of sample ,it produce 1.secendory electron 2.back scattered electron 3.Xrays The emitted secondary electron are collected by secondary electron detector and converted in to signal that is sent to the screen which produce final image Additional detector collect other rays produce corresponding image Main components are 1.electron gun : 2 types: 1.thermionic gun :apply thermal energy to the tungsten filament to produce electron Tungsten is used because a.high melting point b. low vapourpressure c. it is cheap 2. field emission gun : creat strong electricfield to pull electron away from atom 2. Condenser lens: Made up of magnet capable of bending the path of electron Condenser lens focus and control electron beam 3.Scanning coil : This consist of 2 solenoid oriented in such a way that to create 2 magnetic field perpendicular to each other 4. Chamber This is the place where specimen is kept 5.Detector The device detect various way that electron beam interact with sample object Secondary electron detected by Secondary electron detector,this detector produce most detailed images of an object surface Other detectors such as backscattered electron detector and xray detector APPLICATION Used in a variety of industrial,commercial and research application Used in material science for research Forensic investigation Analysis of gunshot residue Measuring effect of climate Identifying new bacteria Work with in the field of genetics Identifying diseases and viruses Testing new medicines  ADVANTAGE Provide 3D topography image of sample Important research tool to study surface morphology in biological,medical,forensic science Cheaper than TEM It needed minimal sample preparation Scanning speed is high & produce image in lessthan in 5 minute  DISADVANTAGE Size of SEM is large Require vaccum for its operator It can image only solid and inorganic sample Require special training to operate Require stable power supply,cooling system text