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Chapter 16 – Infrared Spectroscopy

Chapter 16 – Infrared Spectroscopy. Introduction. Useful range from about 2.5 m m to 50 m m. Infrared used to determine the major functional groups present. Quantitative measurements possible but subject to large amount of error.

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Chapter 16 – Infrared Spectroscopy

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  1. Chapter 16 – Infrared Spectroscopy

  2. Introduction • Useful range from about 2.5 mm to 50 mm. • Infrared used to determine the major functional groups present. • Quantitative measurements possible but subject to large amount of error. • Atoms or groups of atoms in molecules are in continuous motion with different modes of vibration relative to each other. • Absorption of radiation changes amplitude of vibration but not frequency. Chapter 16 - 2

  3. Vibrational Modes • Increased amplitude produces a change in dipole moment. •  = q×r. where  = dipole moment, q = change displacement, and r = displacement from equilibrium. • Only vibrations that cause this change in electric dipole moment will be associated with an absorption of infrared radiation. • E.g. Symmetric and antisymmetric modes of vibration are possible with CO2; • symmetric mode of vibration has no net change in its dipole moment, while antisymmetic mode has net change in dipole moment. The antisymmetric mode would be infrared active and the symmetric mode would not. Chapter 16 - 3

  4. VIBRATIONAL MODELS • Mechanical model: spring connects one or two moving bodies. Restoring force, F, pulling on atoms to return to initial positions. • Force related to force constant [stiffness of spring (bond)]. • F = ky. Negative sign means restoring force. (Hook’s law) • dE = Fdy or upon integrating between equilibrium position and y gives E = ½ky2. • Potential energycurve parabolic; maximum when spring is stretched or compressed and minimum at equilibrium position. Chapter 16 - 4

  5. Vibrational frequency • F = ma where a = acceleration and m = mass of the substance moving. • Acceleration is written as: • Solution: y = A cos 2pumt where • .um = vibrational frequency and • A = maximum amplitude of the motion. • Leads to • Two moving particles: frequency uses reduced mass . Chapter 16 - 5

  6. Quantum Mechanical Vibrations • Harmonic oscillator is used to obtain wave equation for potential energy of oscillator. • where v = vibrational quantum # (+ integer). • Quantum Mechanical Model of Atomic Movement only certain energies are allowed. • near equilibrium position, molecular vibrations similar to mechanical model.. • Frequency of the vibration from the mechanical model as a reasonable estimate of true vibrational frequency: . • But quantum mechanical energy is: . • Quantum mechanical selection rule which states that Dv = ± 1 (due to conservation of momentum of the combined photon and molecular system). and where k = force constant (N/m) and m = reduced mass (kg). • In wavenumbers: Chapter 16 - 6

  7. GROUP FREQUENCIES Estimation of frequencies of vibration for various groups possible when force constant known. E.g.1 force constant of C=O bond is 1.23x103 N/m, determine vibrational frequency of this C=O group. • mO = • reduced mass of • Substituting: • Units: Chapter 16 - 7

  8. Sample Problem • E.g.2: C-H stretch of alkane occurs at  2900 cm1; determine frequency of deuterated analog using mechanical equation: • ratio of two equations for two forms of compound or reduced mass for each of two bonds will be: mC = 1.99x1026 kg, • mD = 3.32x1027 kg, mH = 1.66x1027 kg and • .mC-H = 1.53x1027 kg and mC-D = 2.84x1027 kg • Substituting: • Deuterating convenient way to confirm presence of particular type of bond, since frequency shift is relatively large and predictable. Chapter 16 - 8

  9. INSTRUMENTATION • Instrumentation is same as for other absorption instruments except that the sources, detector and the optical material are designed specifically for this spectral region. • Sources: We have mentioned these detectors in our discussion of general features of the absorption spectrophotometer. • Nernst Glower = cylinder composed of rare-earth oxides which is heated to some temperature before current can be passed directly through it. • Since it has a negative temperature coefficient, it is necessary to regulate the current (T kept at about 1800K).. • Globar = SiC rod (T kept at about 1600K) kept at desired temperature by passing current through it; positive temperature coefficient.. • Both of these sources suffer from having low intensities ( » 107 - 109 W) and has led authors to claim that source is energy limited. Chapter 16 - 9

  10. Detectors • Low flux of photons and low energy of infrared radiation make it more difficult to detect. • photon detector- based upon photoconductive effect that occurs in certain semiconductor materials. Absorption of light causes resistance to decrease. Voltage drop across load resistor measured: HgCdTe must be cooled to 77K and PbS2 can be operated at room temperature. • Photon detectors (also called quantum detectors) have rapid response and thus are used with FTIR . • Material deposited on surface of a nonconducting material and is sealed in an evacuated tube to protect the semiconductor from reaction with the atmosphere. Chapter 16 - 10

  11. Thermocouple or therompile. Instrumental Methods of Analysis, Willard, Merritt, Dean & Settle, p. 193. Null Point Wheatsone Bridge, Undergraduate Instrumental Analysis ,Robinson, p. 141. Thermal Detectors • Absorption of IR radiation produces a heating effect which alters a physical property of the detector. Thermal detectors are usable over a wide wavelength range. • Thermocouple or therompile.Instrumental Methods of Analysis, Willard, Merritt, Dean & Settle, p. 193.thermocouples-piece of blackened foil placed over 2 wires made of dissimilar metals. • detector is placed inside vacuum to improve its response (minimize heat loss due to conduction). • Several thermocouples in series = thermopile; has higher sensitivity since voltages of individual thermocouples are additive. • Temperature differences of about 106 K can be determined this way. • Thermocouple is low impedance circuit so that high impedance preamplifier is necessary to avoid signal modification by the amplifier circuitry. The response time of these detectors is on order of 100 ms. 2s • Null Point Wheatsone Bridg • thermistor or bolometer- change in resistance is measured and related to amount of photons hitting detector. • Resistance measured with Wheatstone bridge • Radiation falls on R1;R2 adjusted until circuit balanced (no voltage drop between C and D. Then R1/R2 = R3/R4 • Knowledge of all other resistances makes it possible to determine R1 Chapter 16 - 11

  12. INSTRUMENTS • Dispersive instruments: have been the traditional instrument design for IRs; high resolution is possible, FT-IR is essentially the same in modern instruments. The FT-IR has a much higher sensitivity which is very important due to the low signal levels in the IR region. This extra sensitivity makes it possible to use the FT-IR for quantitative work. • Sample Handling: Solids, liquids and gases can be analyzed with IR. • Gases are generally are often constructed with a cylindrical glass body and are usually about 10 cm in length. • The pressure can be from a few mm Hg up to several atm. depending upon the absorption characteristics of sample. • Liquids and solutions: • Only a short pathlength (»1mm) is required for liquids. • Some solvents absorb in IR region interfering with signal from sample. • Pathlength of thin IR sample holder can be determined by observation of the interference patterns associated with constructive interference from reflection of light from the two internal surfaces of cell: . E.g. Determine the pathlength from the fringes below: Chapter 16 - 12

  13. FT-IR • Michaelson Interferometer. • Laser-fringe reference system provides sampling-interval. Signal averaging can only be accomplished if the positioins of the mirror are precisely know. This is achieved by using a helium-neon laser as a reference. Radiation at exactly 632.8 nm traverses the same optical path as the IR beam. • A separate detector measures the interferogram produced, giving a sinusoidal signal with maxima separated by the laser frequency at 15,803 cm-1. • This signal is used to trigger the sampling of the IR signal very reproducibly • IR Source • Detector Chapter 16 - 13

  14. Instrumental Analysis, Christian & O’Reilly, p. 241. Quantitative Analysis • IR has traditionally been used for qualitative analysis. • Difficult to use quantitatively due to chemical or instrumental effects. • Large sloping background often interferes with normal spectrum. • The base line method corrects involves selection of absorption band of the substance under analysis which is sufficiently separated from other matrix peaks and corrected as shown below. • Mixtures can be determined by same methods described earlier. Need to set up the correct number of simultaneous equations. • Convenient for measuring concentrations of gases Chapter 16 - 14

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