CHM3122 F2010 IR.ppt

CHM3122 F2010 IR.ppt - Chemistry 3122 Applied Spectroscopy...

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Chemistry 3122 Applied Spectroscopy Infrared Spectroscopy Module Prof. Fogg Fall 2010
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2 Infrared Spectrophotometer Requirements: source of IR radiation, sample, detector
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3 IR Spectrophotometer
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4 Absorption Spectrum measure change in intensity of light at range of λ I ! I ! o sample I ! I ! " ! o = Transmittance (T) # optical density (D) # extinction (E) A = log I ! I ! o $ A = - log T Note transmission & absorption are opposing qualities 0% T = opaque
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5 Infrared Radiation • ~ any compound containing covalent bonds will absorb in this region of the electromagnetic spectrum • between visible light (400-800 nm) and microwaves (>1mm) • vibrational portion is 4000-400 cm -1 (wavenumbers, ) or 2.2-25 μ m (wavelength) (NB: cm -1 directly proportional to Energy : cm -1 = 10 4 ) • energy is absorbed molecular vibration • frequency of vibration depends on: - bond strength - mass of atoms μ m !
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6 IR Absorption IR absorptions are characteristic of entire molecule; essentially a molecular Fngerprint • vibration spectrum appears as bands not peaks (molecular vibration is not a single energy level ; also depends on molecular rotation) • band intensities expressed as % transmittance (T) or absorbance (A) (often both, i.e. %T on left, A on right; latter in arbitrary units from 0-2. Recall 100%T = 0 A)
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7 Molecular Vibrations Stretching : rhythmical movement along a bond Vibrations that result in change of dipole moment give rise to IR absorptions Bending : change in angle (twisting, rocking, torsional) • alternating electric Feld produced by changing dipole in bond couples molecular vibration to the oscillating electric Feld of the incident radiation • IR frequencies that match the natural vibrational frequencies of the molecule are absorbed • band intensity re±ects probability of absorption (thus symmetrical vibrations like N ! N not observed)
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8 • reduced mass ( μ ) is measure of mass of groups at either end of bond • strength of bond is reFected in a force constant, f • frequency of stretching vibration given by m 1 m 2 μ = m 1 m 2 m 1 + m 2 Hooke’s Law 1 2 ! c f μ " = Bond Stretching: Harmonic Oscillator
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9 Assignments of Bands (1): Effect of Δ f Force constant for single bonds is ca. 5 x10 5 dyne/cm (double bonds 2x more, triple bonds 3x more) C C C C C C 2150 cm -1 > Thus 1650 cm -1 1200 cm -1 > increasing bond strength (increasing f) C-H stretch: calc 3040 cm -1 actual CH 3 : 2960-2850 cm -1
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And if ν C- H is 2900 cm -1 , ν C- D is thus ~ (2900)( ), or ~ 2050 cm -1 The Harmonic Oscillator: effect of increasing mass Simplest way of looking at effect of mass without perturbing (changing) any other factors such as the force constant : just change 1 H for 2 H 1 2 ! c f μ C-H " C-H = 1 2 ! c f μ C-D C-D 1 2 (this assumes force constant ~ identical for C-D vs. C-H) Ratio of μ Η / μ D is simply Thus Compare aliphatic C-D and C-H bonds: ! 2 =
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This note was uploaded on 01/16/2011 for the course CHM 3122 taught by Professor Bryce during the Fall '09 term at University of Ottawa.

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CHM3122 F2010 IR.ppt - Chemistry 3122 Applied Spectroscopy...

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