S.M.Shazzad Rahman Lecturer, Textile Engineering Department Northern University Bangladesh Course Title: Chemistry Cour

1. S.M.Shazzad Rahman • Lecturer, Textile Engineering Department • Northern University Bangladesh • Course Title: Chemistry • Course Code: CH1201,CH1101

2. What is Spectroscopy • The study of structure and properties of atoms and molecule by means of the spectral information obtained from the interaction of electromagnetic radiant energy with matter • It is the base on which a main class of instrumental analysis and methods is developed & widely used in many areas of modern science • What to be discussed • Theoretical background of spectroscopy • Types of spectroscopy and their working principles in brief • Major components of common spectroscopic instruments • Applications in Chemistry related areas and some examples

3. Electromagnetic Radiation • Electromagnetic radiation (e.m.r.) • Electromagnetic radiation is a form of energy • Wave-particle duality of electromagnetic radiation • Wave nature - expressed in term of frequency, wave-length and velocity • Particle nature - expressed in terms of individual photon, discrete packet of energy when expressing energy carried by a photon, we need to know the its frequency • Characteristics of wave • Frequency, v - number of oscillations per unit time, unit: hertz (Hz) - cycle per second • velocity, c - the speed of propagation, for e.m.rc=2.9979 x 108 m×s-1 (in vacuum) • wave-length, l - the distance between adjacent crests of the wave wave number, v’, - the number of waves per unit distance v’ =l-1 • The energy carried by an e.m.r. or a photon is directly proportional to the frequency, i.e. where h is Planck’s constant h=6.626x10-34J×s

4. Electromagnetic Radiation • Electromagnetic radiation X-ray, light, infra-red, microwave and radio waves are all e.m.r.’s, difference being their frequency thus the amount of energy they possess • Spectral region of e.m.r.

5. Interaction of e.m.r. with Matter • Interaction of electromagnetic radiant with matter • The wave-length, l, and the wave number, v’, of e.m.r. changes with the medium it travels through, because of the refractive index of the medium; the frequency, v, however, remains unchanged • Types of interactions • Absorption • Reflection • Transmission • Scattering • Refraction • Each interaction can disclose certain properties of the matter • When applying e.m.r. of different frequency (thus the energy e.m.r. carried) different type information can be obtained

6. 1.0 intensity 0.5 0.0 350 400 450 wave length cm-1 Spectrum • Spectrum is the display of the energy level of e.m.r. as a function of wave number of electromagnetic radiation energy The energy level of e.m.r. is usually expressed in one of these terms • absorbance (e.m.r. being absorbed) • transmission (e.m.r. passed through) • Intensity The term ‘intensity’ has the meaning of the radiant power that carried by an e.m. r.

7. 1.0 intensity 0.5 0.0 350 400 450 wave length cm-1 Spectrum • What an spectrum tells • A peak (it can also be a valley depending on how the spectrum is constructed) represents the absorption or emission of e.m.r. at that specific wavenumber • The wavenumber at the tip of peak is the most important, especially when a peak is broad • A broad peak may sometimes consist of several peaks partially overlapped each other - mathematic software (usually supplied) must be used to separate them case of a broad peak (or a valley) observed • The height of a peak corresponds the amount absorption/emission thus can be used as a quantitative information (e.g. concentration), a careful calibration is usually required • The ratio in intensity of different peaks does not necessarily means the ratio of the quantity (e.g. concentration, population of a state etc.)

8. Energy Wave number v’ cm-1 Type of quantum transition Wavelength l cm Type of radiation Frequency v Hz Type of spectroscopy Electron vole eV kcal/mol Gamma ray 9.4x107 4.1x106 3.3x1010 3.0x10-11 1021 9.4x105 4.1x104 3.3x108 3.0x10-9 1019 9.4x103 4.1x102 3.3x106 3.0x10-7 1017 9.4x101 4.1x100 3.3x104 3.0x10-5 1015 9.4x10-1 4.1x10-2 3.3x102 3.0x10-3 1013 9.4x10-3 4.1x10-4 3.3x100 3.0x10-1 1011 9.4x10-5 4.1x10-6 3.3x10-2 3.0x101 109 9.4x10-7 4.1x10-8 3.3x10-4 3.0x103 107 Gamma ray emission Nuclear X-ray absorption emission Electronic (inner shell) X-ray UV absorption Vac Ultra Violet absorption emission fluorescence Electronic (outer shell) UV Vis Visible Infrared IR absorption Raman Molecular vibration Molecular rotation Microwave Microwave absorption Electron paramagnet resonance Magnetically induced spin states Nuclear magnetic resonance Radio Spectral properties, applications, and interactions of electromagnetic radiation

9. Problems 1. A laser emits light with a frequency of 4.69x1014 s-1. (h = 6.63 x 10-34Js) A) What is the energy of one photon of the radiation from this laser in kcal? B) If the laser emits 1.3x10-5J during a pulse, how many photons are emitted during the pulse? Ans: A) Ephoton= 3.11 x 10-22 kJ B) No. of photons = 4.2x1013 2. The brilliant red colours seen in fireworks are due to the emission of red light at a wave length of 650nm. What is the energy of one photon of this light? (h = 6.63 x 10-34Js) Ans: Ephoton= 3.06x10-19J 3: Compare the energies of photons emitted by two radio stations, operating at 92 MHz (FM) and 1500 kHz (MW)? Ans: Ephoton = 6.1 x 10-26J, 9.9 x 10-28J

10. Atomic Spectra • Shell structure & energy level of atoms • In an atom there are a number of shells and of subshells where e-’s can be found • The energy level of each shell & subshell are different and quantised • The e-’s in the shell closest to the nuclei has the lowest energy. The higher shell number is, the higher energy it is • The exact energy level of each shell and subshell varies with substance • Ground state and excited state of e-’s • Under normal situation an e- stays at the lowest possible shell - the e- is said to be at its ground state • Upon absorbing energy (excited), an e- can change its orbital to a higher one - we say the e- is at is excited state.

11. Atomic Spectra • Electron excitation • The excitation can occur at different degrees • low E tends to excite the outmost e-’s first • when excited with a high E (photon of high v) an e- can jump more than one levels • even higher E can tear inner e-’s away from nuclei • An e- at its excited state is not stable and tends to return its ground state • If an e- jumped more than one energy levels because of absorption of a high E, the process of the e- returning to its ground state may take several steps, - i.e. to the nearest low energy level first then down to next …

12. Atomic Spectra • Atomic spectra • The level and quantities of energy supplied to excite e-’s can be measured & studied in terms of the frequency and the intensity of an e.m.r. - the absorption spectroscopy • The level and quantities of energy emitted by excited e-’s, as they return to their ground state, can be measured & studied by means of the emission spectroscopy • The level & quantities of energy absorbed or emitted (v & intensity of e.m.r.) are specific for a substance • Atomic spectra are mostly in UV (sometime in visible) regions

13. S2 S1 v4 v4 v4 v4 v3 v3 v3 v3 v2 v2 v2 v2 T1 v1 v1 v1 v1 S0 Molecular Spectra • Motion & energy of molecules • Molecules are vibrating and rotating all the time, two main vibration modes being • stretching - change in bond length (higher v) • bending - change in bond angle (lower v) (other possible complex types of stretching & bending are: scissoring / rocking / twisting • Molecules are normally at their ground state (S0) S (Singlet) - two e-’s spin in pair E T (Triplet) - two e-’s spin parallel J • Upon exciting molecules can change to high E states (S1, S2, T1 etc.), which are associated with specific levels of energy • The change from high E states to low ones can be stimulated by absorbing a photon; the change from low to high E states may result in photon emission

14. S2 S1 T1 absorption A v4 v4 v4 v4 v3 v3 v3 v3 v2 v2 v2 v2 A v1 v1 v1 v1 S0 Molecular Spectra • Excitation of a molecule • The energy levels of a molecule at each state / sub-state are quantised • To excite a molecule from its ground state (S0) to a higher E state (S1, S2, T1 etc.), the exact amount of energy equal to the difference between the two states has to be absorbed. (Process A) i.e. to excite a molecule from S0,v1 to S2,v2, e.m.r with wavenumber v’ must be used • The values of energy levels vary with the (molecule of) substance. • Molecular absorption spectra are the measure of the amount of e.m.r., at a specific wavenumber, absorbed by a substance.

15. Inter- system crossing Internal transition B E1 S2 B E2 S1 A D C T1 Fluorescence v4 v4 v4 v4 v3 v3 v3 v3 Phosphorescence v2 v2 v2 v2 F v1 v1 v1 v1 B S0 Jablonsky diagram Molecular Spectra • Energy change of excited molecules An excited molecules can lose its excess energy via several processes • Process B - Releasing E as heat when changing from a sub-state to the parental state occurs within the same state • The remaining energy can be release by one of following Processes (C, D & E) • Process C - Transfer its remaining E to other chemical species by collision • Process D - Emitting photons when falling back to the ground state - Fluorescence • Process E1 - Undergoing internal transition within the same mode of the excited state • Process E2 - Undergoing intersystem crossing to a triplet sublevel of the excited state • Process F - Radiating E from triplet to ground state (triplet quenching) - Phosphorescence

16. B S2 T1 phosphor-enscence Fluore-scence A v4 v4 v4 v3 v3 v3 F v2 v2 v2 D v1 v1 v1 S0 Molecular Spectra • Two types of molecular emission spectra • Fluorescence • In the case fluorescence the energy emitted can be the same or smaller (if heat is released before radiation) than the corresponding molecular absorption spectra. e.g. adsorption in UV region - emission in UV or visible region (the wavelength of visible region is longer than that of UV thus less energy) • Fluorescence can also occur in atomic adsorption spectra • Fluorescence emission is generally short-lived (e.g. ms) • Phosphorescence • Phosphorescence generally takes much longer to complete (called metastable) than fluorescence because of the transition from triplet state to ground state involves altering the e-’s spin. If the emission is in visible light region, the light of excited material fades away gradually

17. Atomic Spectra & Molecular Spectra • Comparison of atomic and molecular spectra • Quantum mechanics is the basis of atomic & molecular spectra • The transitional, rotational and vibrational modes of motion of objects of atomic / molecular level are well-explained.

18. Incident light, I0 (UV or visible) Emergent light, I ultraviolet visible infra-red 200 - 400 400 - 800 800 - 15 nm nm nm nm nm mm C b UV & Visible Spectrophotometry • Observations When a light of intensity I0 goes through a liquid of concentration C & layer thickness b • The emergent light, I, has less intensity than the incident light I0 • scattering, reflection • absorption by liquid • There are different levels of reduction in light intensity at different wavelength • detect by eye - colour change • detect by instrument • The method used to measure UV & visible light absorption is called spectrophotometry (colourimetryrefers to the measurement of absorption of light in visible region only)

19. b x s I0 I s dx UV & Visible Spectrophotometry • Theory of light absorption Quantitative observation • The thicker the cuvette - more diminishing of light in intensity • Higher concentration the liquid - the less the emergent light intensity These observations are summarised by Beer’s Law: Successive increments in the number of identical absorbing molecules in the path of a beam of monochromatic radiation absorb equal fraction of the radiation power travel through them Thus fraction of light light absorbed number of molecules N-Avogadro number Absorbance

20. UV & Visible Spectrophotometry • Terms, units and symbols for use with Beer’s Law Name alternative name symbol definition unit Path length - b (or l) - cm Liquid concentration - c - mol / L Transmittance Transmission TI / I0 - Percent transmittance - T% 100x I / I0% Absorbance Optical density, A log(I / I0) - extinction Absorptivity Extinction coeff., a (or e, k) A/(bc) [bc]-1 absorbance index Molar absorptivity Molar extinction coeff., aA/(bc) molar absorbancy index [or aMAM/(bc’) ] M-molar weight c’ -gram/L

21. UV & Visible Spectrophotometry • Use of Beer’s Law • Beer’s law can be applied to the absorption of UV, visible, infra-red & microwave • The limitations of the Beer’s Law • Effect of solvent - Solvents may absorb light to a various extent, e.g. the following solvents absorb more than 50% of the UV light going through them 180-195nm sulphuric acid (96%), water, acetonitrile 200-210nm cyclopentane, n-hexane, glycerol, methanol, ethanol 210-220nm n-butyl alcohol, isopropyl alcohol, cyclohexane, ethyl ether 245-260nm chloroform, ethyl acetate, methyl formate 265-275nm carbon tetrachloride, dimethyl sulphoxide/formamide, acetic acid 280-290nm benzene, toluene, m-xylene 300-400nm pyridine, acetone, carbon disulphide • Effect of temperature • Varying temperature may cause change of concentration of a solute because of • thermal expansion of solution • changing of equilibrium composition if solution is in equilibrium

22.  * Antibonding Antibonding non-bonding Bonding * n * n * * * Energy n   UV & Visible Spectrophotometry • What occur to a molecule when absorbing UV-visible photon? • A UV-visible photon (ca. 200-700nm) promotes a bonding or non-bonding electron into antibonding orbital - the so called electronic transition • Bonding e-’s appear in s & p molecular orbitals; non-bonding in n • Antibonding orbitals correspond to the bonding ones • e-’s transition can occur between various states; in general, the energy of e-’s transition increases in the following order: (n®p*) < (n®s*) < (p®p*) < (s®s*) • Molecules which can be analysed by UV-visible absorption • Chromophores functional groups each of which absorbs a characteristic UV or visible radiation.

23. UV & Visible Spectrophotometry • The functional groups & the wavelength of UV-visible absorption Group Example lmax, nm Group Example lmax, nm C=C 1-octane 180 arene benzene 260 naphthalene 280 C=O methanol 290 phenenthrene 350 propanone 280 anthracene 375 ethanoic acid 210 pentacene 575 ethyl ethanoate 210 ethanamide 220 conjugated 1,3-butadiene 220 1,3,5-hexatriene 250 C-X methanol 180 2-propenal 320 trimethylamine 200 b-carotene (11 C=C) 480 chloromethane 170 bromomethane 210 each additional C=C +30 iodomethane 260

24. UV & Visible Spectrophotometry • Instrumentation UV visible Light source Hydrogen discharge lamp Tungsten-halogen lamp Cuvette QUARTZ glass Detectors photomultiplier photomultiplier

25. UV & Visible Spectrophotometry • Applications • Analysis of unknowns using Beer’s Law calibration curve • Absorbance vs. time graphs for kinetics • Single-point calibration for an equilibrium constant determination • Spectrophotometric titrations – a way to follow a reaction if at least one substance is colored – sudden or sharp change in absorbance at equivalence point

26. IR-Spectroscopy • Atoms in a molecule are constantly in motion • There are two main vibrational modes: • Stretching - (symmetrical/asymmetrical) change in bond length - high frequency • Bending - (scissoring/stretch/rocking/twisting) change in bond angle - low freq. • The rotation and vibration of bonds occur in specific frequencies • Every type of bond has a natural frequency of vibration, depending on • the mass of bonded atoms (lighter atoms vibrate at higher frequencies) • the stiffness of bond (stiffer bonds vibrate at higher frequencies) • the force constant of bond (electronegativity) • the geometry of atoms in molecule • The same bond in different compounds has a slightly different vibration frequencies. • Functional groups have characteristic stretching frequencies.

27. IR-Spectroscopy • IR region • The part of electromagnetic radiation between the visible and microwave regions 0.8 m to 50 m (12,500 cm-1-200 cm-1). • Most interested region in Infrared Spectroscopy is between 2.5m-25 m (4,000cm-1-400cm-1), which corresponds to vibrational frequency of molecules • Interaction of IR with molecules • Only molecules containing covalent bonds with dipole moments are infrared sensitive • Only the infrared radiation with the frequencies matching the natural vibrational frequencies of a bond (the energy states of a molecule are quantitised) is absorbed • Absorption of infrared radiation by a molecule rises the energy state of the molecule • increasing the amplitude of the molecular rotation & vibration of the covalent bonds • Rotation - Less than 100 cm-1 (not included in normal Infrared Spectroscopy) • Vibration - 10,000 cm-1 to 100 cm-1 • The energy changes through infrared radiation absorption is in the range of 8-40 KJ/mol

28. IR-Spectroscopy • Use of Infra-Red spectroscopy • IR spectroscopy can be used to distinguish one compound from another. • No two molecules of different structure will have exactly the same natural frequency of vibration, each will have a unique infrared absorption spectrum. • A fingerprinting type of IR spectral library can be established to distinguish a compounds or to detect the presence of certain functional groups in a molecule. • Obtaining structural information about a molecule • Absorption of IR energy by organic compounds will occur in a manner characteristic of the types of bonds and atoms in the functional groups present in the compound • Practically, examining each region (wave number) of the IR spectrum allows one identifying the functional groups that are present and assignment of structure when combined with molecular formula information. • The known structure information is summarized in the Correlation Chart

29. Source: R. Thomas, “Choosing the Right Trace Element Technique,” Today’s Chemist at Work, Oct. 1999, 42. Atomic Absorption/Emission Spectroscopy • Atomic absorption/emission spectroscopes involve e-’s changing energy states • Most useful in quantitative analysis of elements, especially metals • These spectroscopes are usually carried out in optical means, involving • conversion of compounds/elements to gaseous atoms by atomisation. Atomization is the most critical step in flame spectroscopy. Often limits the precision of these methods. • excitation of electrons of atoms through heating or X-ray bombardment • UV/vis absorption, emission or fluorescence of atomic species in vapor is measured • Instrument easy to tune and operate • Sample preparation is simple (often involving only dissolution in an acid)

30. Source P0 P Wavelength Selector Signal Processor Readout Detector Chopper Sample Atomic Absorption Spectrometer (AA) TypeMethod of AtomizationRadiation Source atomic (flame) sample solution aspirated Hollow cathode into a flame lamp (HCL) atomic (nonflame) sample solution HCL evaporated & ignited x-ray absorption none required x-ray tube

31. P Wavelength Selector Signal Processor Readout Detector Source Sample Atomic Emission Spectrometer (AES) TypeMethod of AtomizationRadiation Source arc sample heated in an electric arc sample spark sample excited in a high voltage spark sample argon plasma sample heated in an argon plasma sample flame sample solution aspirated into a flame sample x-ray emission none required; sample bombarded w/ e- sample

32. Wavelength Selector Signal Processor Readout Detector P0 P Chopper 90o Source Sample Atomic Fluorescence Spectrometer (AFS) TypeMethod of AtomizationRadiation Source atomic (flame) sample solution aspirated into a flame sample atomic (nonflame) sample solution sample evaporated & ignited x-ray fluorescence none required sample