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Chapter 16

Chapter 16. An IR spectrum contains information about the functional groups in a molecule, and this is used to uniquely identify the compound. Infrared Absorption Spectroscopy. UV. Vis. Near IR. Mid IR. Far IR. 400 nm. 780 nm. 2500 nm or 4000 cm -1. 50,000 nm or 200 cm -1.

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Chapter 16

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  1. Chapter 16 An IR spectrum contains information about the functional groups in a molecule, and this is used to uniquely identify the compound. Infrared Absorption Spectroscopy

  2. UV Vis Near IR Mid IR Far IR 400 nm 780 nm 2500 nm or 4000 cm-1 50,000 nm or 200 cm-1 Wavenumbers (cm-1) are used since they are directly proportional to energy – e.g. convert 2.5 µm to wavenumbers (cm-1)

  3. Typical Infrared Spectrum “overtones” C-H C-C C=O Higher energy vibrations

  4. Mechanical Model of a Stretching Vibration in a Diatomic Molecule Treats the vibrating bond like a spring with a given “stiffness” -- • = frequency of vibration • k = force constant (spring “stiffness”) • µ = “reduced mass”

  5. + - Quantum Mechanical Treatment of Normal Modes Only certain vibrations are “allowed” The vibrational quantum number v = 0, 1, 2, 3…… Only transitions between adjacent energy levels are possible, i.e. v =  1 The frequency of the photon has to equal the frequency of the vibration(see next slide) The molecule must have a change in “dipole moment” as a result of the vibration Dipole moment = charge difference X separation distance Homonucleardiatomicsnever have an Infrared spectrum. Why?

  6. + + - - IR Absorption if the vibration results in a change in dipole moment Frequency of absorbed light  frequency of vibration S1 + + + v =  1 - - So

  7. Common Types of Vibrations (“Normal Modes”) Higher energy modes Lower energy modes

  8. The number of normal modes = 3N-6 For linear molecules it’s 3N-5 Example– predict the normal modes of CO2

  9. Which normal modes are Infrared active?

  10. Example 16-1, p.436 Calculate the approximate wavenumber and wavelength of the fundamental absorption peak due to the stretching vibration of a C=O group (k = 1.0 x 103 N/m)

  11. Force Constants increase with Bond Strength (a). (b) (a).as  then frequency (b) When masses approx. equal, then see peak at same wavenumbers * all bonds are stretches @ consider these as being approx. equal

  12. Instrumentation IR Sources Nichrome – Ni/Cr alloy, resistively heats up and emits IR, 1100 K Globar – SiC rod, 1500 K Nernst Glower – electrically heated rare earth-oxide ceramic, 2000 K Output from a Nernst Glower cm-1

  13. Optics – IR spectra are measured in the mid-IR, so have to use halides such as NaCl and KBr UV Vis Near IR Mid IR Far IR 400 nm 780 nm 2500 nm or 4000 cm-1 50,000 nm or 200 cm-1 Never use aqueous samples, or water to clean salt plates

  14. Sample Handling – a few drops of “neat” sample between “salt plates”

  15. Solutions

  16. Detectors for the Mid-IR – 200-4000cm-1 Pyroelectric – crystal of DTGS (DeuteratedTriglyceine Sulfate) Nice link: http://www.doitpoms.ac.uk/tlplib/pyroelectricity/printall.php?question=2&type=1 DTGS maintains polarization when heated to just below the Curie Point Above the Curie Point, the permanent polarization of the DTGS crystal disappears. The closer to the Curie Point, the more responsive the detector Much faster response so can take a spectrum much more quickly (FTIR)

  17. Detectors for the Mid-IR – 200-4000cm-1 Photoconduction - Mercury-Cadmium-Tellurium (MCT) Semiconductor-based Cooled to 77 K using liquid N2 Much faster response so can take a spectrum much more quickly (FTIR) Liquid N2 @ 77K http://www.newport.com http://www.boselec.com/pdf/Laser_Focus_World.PDF

  18. Fourier Transform Instrumentation Definition of the Fourier Transform () - The Inverse Fourier Transform - The functions f(t) and () are called "transform pairs" f(t) ()

  19. Examples of Fourier Transforms time (t)  frequency (s-1, Hz)

  20. Examples of Fourier Transforms e.g. FTIR distance (cm)  wavenumber (cm-1) backgroundinterferogram background spectrum = Po sampleinterferogram sample spectrum = P P/Po =IR spectrum

  21. Examples of Fourier Transforms e.g. FT-NMR time (s)  ppm (Hz) NMR Spectrum (ppm, Hz) Free Induction Decay (FID) Signal (time)

  22. Fourier Transforms "invert dimensionality" –signal domainFT domainInstrument • time (t) frequency (s-1, Hz) oscilloscope • mirror wavenumber (cm-1) FTIRdistance (cm) • free induction ppm (from Hertz) FT-NMRdecay (t) • Fast Fourier Transform (FFT) on computers - • Cooley-Tukey algorithm • very efficient on a PC

  23. fixed mirror -  ¼   IR source + + + - - movable mirror → 50/50 beamsplitter D interferogram /4 2 /4 3/4 4/4 mirror distance, x (cm) →

  24. If polychromatic radiation enters the interferometer, then each wavelength produces a separate interferogram. The output from the interferometer will therefore be a superposition of all wavelengths - 1  1 2  2 3  3 ←mirror distance (-x, cm) mirror distance (+x, cm) →

  25. signal atdetector S(x) this term contains the IR spectrum Taking the Fourier Transform of S(x) results in the IR spectrum, A() -

  26. Resolution of an FTIR Depends on the distance the mirror moves in the Michelson Interferometer – the further it moves, the greater the resolution. "retardation“ = 2x minimum detectable difference in wavenumber x = distance mirror moves e.g. What length of mirror drive in a Michelson Interferometer is required to separate 20.34 and 20.35 m?

  27. Fourier Transform Infrared (FTIR) Spectrometers The Multiplex (Fellgett's) Advantage - entire spectrum obtained virtually instantaneously; results in a higher S/N because during the time a scanning instrument is slowly obtaining the spectrum, a multiplex instrument such as an FTIR can acquire 100’s pf spectra. Averaging these extra spectra causes the increased S/N (see next slide) and allows quantitative IR. S/N increases as where N = number of "resolution elements" e.g. scan range 400 - 4000 cm-1 at a resolution of 2 cm-1 2 cm-1 4000 cm-1 400 cm-1 "resolution elements" so the S/N increases by IF the FTIR can obtain the entire spectrum for the same amount of time a slower, scanning instrument requires to acquire only one resolution element.

  28. Signal-to-Noise Ratio (S/N)

  29. Typical FTIR Instrumental Configuration Single-Beam – run background first (see next slide) and store in memory (Po) Higher energy throughput (Jacquinot’s Advantage) and no stray light problems

  30. Typical IR background spectrum (Po)

  31. FTIR Performance • inexpensive (~ $25K) • range = 350 – 7800 cm-1 (29 to 1.3 m) • max resolution = 4 cm-1 • scan time = as fast as 1 sec • detector = DTGS • expensive (~ $100K) • range = 10 – 50,000 cm-1 (1000 m to 200 nm) • max resolution < 0.01 cm-1 • scan time = can be minutes at high-res • detector(s) = DTGS, MCT

  32. Advantages over Scanning Instruments • fast scan times • higher S/N because of signal averaging • higher resolution • because there are no slits, there’s a higher energy throughput (Jacquinot’s Advantage); means larger signals for the same concentration (therefore lower LODs) • no stray light problems • Disadvantages • higher cost than scanning instruments • difficult to align the interferometer (automation helps) • IR optics water soluble (beamsplitter made of KBr)

  33. Chapter 17 Applications of IR Spectrometry

  34. 1. Qualitative Analysis

  35. "group frequency region" "fingerprint region"

  36. Library Searching

  37. 2. Quantitative Analysis Much less sensitive (i.e. higher LODs) in the IR compared to the UV-Visbecause of - • lower source powers than in UV-Vis (in FTIR Jacquinot's Advantage partially offsets) • detectors suffer from thermal noise (i.e. larger backgrounds or blank) • salt plates have a very short path length (A = bc) • Signal averaging with FTIR's has improved the S/N enough to allow more sensitive quantitative work in the IR.

  38. 3. Diffuse Reflectance Accessory

  39. Typically used for powdered samples, i.e. forensic drug analyses

  40. Kubelka-Munk Units = converts the reflectance spectrum to the equivalent of an absorbance spectrum. • R’ = sample refectivity/KBr reflectivity • f(R’) = (1 - R’)2/2 R’ = k/S • k = 2.303  C •  = the molar absorptivity • C is the sample concentration • S = "scattering coefficient" • For a sample to follow a linear relationship between f(R’) and concentration, the following criteria must be met: • the sample must be diluted in a non-absorbing matrix such as KBr or KCl. • the scattering coefficient S must remain constant over the entire spectrum. • there must be no specular, or regular reflectance off the surface of the sample.

  41. 4. Attenuated Total Internal Reflectance (ATR) Accessory Samples - conventional (solutions, liquids, etc) as well as powders, pastes, suspensions, colloids Total Internal Reflection and the "evanescent wave" -

  42. 5. Infrared Microscopy

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