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Introduction to IR spectroscopy

Introduction to IR spectroscopy. Yongsik Lee 2004. 6. IR spectrum. 16A theory of IR ABS. Energy of IR photon insufficient to cause electronic excitation But can cause vibrational or rotational excitation Fundamentals

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Introduction to IR spectroscopy

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  1. Introduction toIR spectroscopy Yongsik Lee 2004. 6

  2. IR spectrum

  3. 16A theory of IR ABS • Energy of IR photon • insufficient to cause electronic excitation • But can cause vibrational or rotational excitation • Fundamentals • Molecular electric field (dipole moment) interacts with IR photon electric field (both dynamic) • Magnitude of dipole moment determined by • charge • separation of charge • Vibration or rotation causes varying separation

  4. Dipole moment change • Molecule must have change in dipole moment due to vibration or rotation to absorb IR radiation • Absorption causes increase in vibration amplitude/rotation frequency • Molecules with permanent dipole moments (µ) are IR active • HCl, CO, • H2, N2, • CO2

  5. IR active/inactive

  6. Electric Dipole moment

  7. Types of Molecular Vibrations • Stretching • change in bond length • Symmetric / asymmetric • bending • change in bond angle • symmetric scissoring • asymmetric wagging • rocking • twisting/torsion

  8. Molecular vibration

  9. Classical vibrational motion • Mechanical model • Two masses • A spring • Simple harmonic motion

  10. Energy of the Hooke’s law • Negative sign force • Restoring force • Direction of the force is opposite to the displacement

  11. Harmonic Ocsillator Potential • Potential high • When the spring is compressed or stretched • Parabola function • E=(1/2)kx2 • Minimum at equilibrium position • Maximum at max amplitude A

  12. Classical vibrational frequency • F = ma = m(d2y/dt2) • F=-ky • Solution of differential equation • Y = A cos (2pnt) • D2y/dt2 = -4p2n2A cos (2pnt) • Reduced mass for two masses

  13. Quantum treatment of vibrations

  14. Anharmonic Oscillator • Must modify harmonic oscillator potential for • electron repulsion • steeper at small distances • dissociation • bond breaks at large distances

  15. anharmonicity • Harmonic at low n • DE becomes smaller at high n • broadens band • Selection rule fails Dn = ±1 and Dn = ±2... • fundamentals • overtones

  16. Vibrational Normal modes • Number of possible vibrations in a polyatomic molecule • 2 atoms (H2) - 1 vibration (stretch n) • 3 atoms (H2O) - 3 vibrations (n s, n as, s) • 3 atoms (CO2) - 4 vibrations (n s, n as, s, w) • 4 atoms (H2CO) - 6 vibrations (n s, n as, s, w, r(CH2) n(C=O)) • 5 atoms ... • 3N - 6 Non - linear molecule • 3N - 5 Linear molecule • 3N degrees of freedom for N atoms • 3 translation • 3(or 2) rotation – rotation about the bond axis is not possible • Orhters are "Normal modes"

  17. Fewer experimental peaks • Fewer peaks • Symmetry of the molecule • degenracy • Energies of two or more vibrations are identical • Or nearly identical • Undetectable low absorption intensity • Out of the instrumental detection range • More peaks • Overtone • Combination bands

  18. Applications of FT-IR • Chemical Analysis: • Match spectra to known databases • Identifying an unknown compound, Forensics, etc. • Monitor chemical reactions in-situ • Structural ideas: • Can determine what chemical groups are in a specific compound • Electronic Information: • Measure optical conductivity • Determine if Metal, Insulator, Superconductor, Semiconductor • Band Gaps, Drude model

  19. Vibrational coupling • Coupling of different vibrations shifts frequencies • Energy of a vibration is influenced by coupling • Coupling likely when • common atom in stretching modes • common bond in bending modes • common bond in bending+stretching modes • similar vibrational frequencies • Coupling not likely when • atoms separated by two or more bonds • symmetry inappropriate

  20. Carbon dioxide • C=O bond • In methanol 1034 cm-1 • In ethanol 1053 cm-1 • In butanol 1105 cm-1 • In CO2 • Asym 2330 cm-1 • sym cm-1 • Degenerate bending 667 cm-1

  21. Water vibrations • Non-linear water • 3 x 3 – 6 = 3 vib • Stretching 3650 and 3760 • Bending 1595 • Positive identification of a specific compound

  22. Global climate change • Recent years have seen a huge rise in the number of abnormal weather events. • Meteorologists agree that these exceptional conditions are signs that Global Climate Change is happening already. • Scientists agree that the most likely cause of the changes are man-made emissions of the so-called "Greenhouse Gases" that can trap heat in the earth's atmosphere in the same way that glass traps heat in a greenhouse. • Although there are six major groups of gases that contribute to Global Climate Change, the most common is Carbon Dioxide (CO2).

  23. Greenhouse gas level hits record high • The level of the major greenhouse gas, carbon dioxide, in the Earth's atmosphere has hit a record high. • The new data from the US National Oceanic and Atmospheric Administration also suggest that the rate of increase of the gas may have accelerated in the last two years. • Carbon dioxide emissions, mainly from burning fossil fuels, are thought to be a principle cause of global warming. • Recordings from a volcano-top observatory, NOAA's Mauna Loa Observatory on Hawaii, showed carbon dioxide levels had risen to an average of about 376 parts per million (ppm) for 2003. • This is 2.5 ppm up from the average for 2002. It is not the highest leap in year-on-year atmospheric carbon dioxide levels recorded by NOAA. But it is the first to be sustained, with 2002 levels up 2.5 ppm from 2001. • http://www.newscientist.com/news/news.jsp?id=ns99994802

  24. Greenhouse effect is real

  25. Day after tomorrow? • 2002年8月ミニアルバム『day after tomorrow』でデビュー。misono（18歳京都府出身）・北野正人（28歳大阪府出身）・鈴木大輔（24歳神奈川県出身）の3人組。楽曲全て、作詞はmisono、サウンドプロデューサーは元Every Little Thingの五十嵐充が担当。10月には韓国で開催された「Japan Festival in Korea」に出演、初海外LIVEを行った。

  26. The day after tomorrow

  27. 16B IR sources and detectors • Heated inert solid • 1500 – 2200 K • Blackbody radiation • Max at 5000 – 5900 cm-1

  28. Globar source • Globar • Silicon carbide rod • Diameter 5 mm x length 50 mm • Heated 1300-1500 K • Water cooling to prevent arcing • Compare to Nernst glower • Spectral energies are comparable • At < 5 mm, Globar provides a greater output

  29. Carbon dioxide laser

  30. Carbon dioxide (CO2 ) gas laser • One of the most versatile for materials processing applications • emits IR radiation with a wavelength between 9 and 11µm, although emission at 10.6µm is the most widely used. • Types • Sealed tube • Waveguide • TEA

  31. TEA CO2 Lasers • Discharge instabilities prevent operation of CW CO2 lasers at pressures above about 100mbar. • Pulses in the nanosecond to microsecond duration range can be produced by passing a pulsed current transversely through the lasing gas. • TEA (transversely excited atmospheric) lasers operate at gas pressures of one atmosphere and above in order to obtain high energy output per unit volume of gas.

  32. Home built CO2 gas laser • J&K Laser Productions

  33. IR Transducers • General types • Thermal – thermocouple, bolometer • Pyroelectric • Photoconducting (PC)

  34. 16C IR instruments • IR instruments • Dispersive grating IR instruments • Multiplex instruments using FT • Most widely used since 1980’s • Nondisperive photometers • Quantitative determination

  35. FT instruments • Multiplex instrument Types • Interferogram (FT) type • Tabletop size • Reliable, Reproducible • Maintenace • Cheaper ($15000 – 20000) • Computer cost down • Most widely used • High S/N ratios - high throughput • Rapid (<10 s) • High resolution (<0.1 cm-1) • Handamard transform types • Dispersive instrument • Moving mask at the focal plane of a monochromator

  36. Bio-Rad FTS-40 FT-IR

  37. FT advantage • Jaquinot or throughput advantage. • Few optics, no slits mean high intensity • Usually to improve resolution, decrease slit width but less light makes spectrum "noisier" - signal to noise ratio (S/N) • Fellget or multiplex advantage. • Simultaneously measure all spectrum at once saves time

  38. Dispersive IR • Similar to UV-Vis spectrophotometer • BUT sample after source and before monochromator in IR • (sample after monochromator in UV-Vis - less incident light) • Grating 10-500 lines per mm • Single beam and double beam (in time and space) • Double beam is much more useful • eliminates atmospheric gas interference

  39. Homework • Questions and problems • 16-2, 16-7, 16-8, 16-10

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