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CH_318N_Lecture7-08 notes_Spring_08

CH_318N_Lecture7-08 notes_Spring_08 - Lecture 7 Infrared...

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1 Chemistry 318N Lecture 7 Infrared spectroscopy February 7, 2008 Chemistry 318N The IR Chart 4000 400 2.5 20 0 100 Micrometers Wavenumber (cm -1 ) Transmittance (%)
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2 Chemistry 318N Infrared Spectrum Chemistry 318N Functional group absorptions => =>
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3 Chemistry 318N Molecular Vibrations circle6 For a molecule to absorb IR radiation, the bond undergoing vibration must be polar ( change dipole moment ) circle6 Covalent bonds that do not meet this criterion are said to be IR inactive the C-C double and triple bonds of symmetrically substituted alkenes and alkynes, for example, do not absorb IR radiation because they are not polar bonds H 2 N 2 CO 2 ?????? Chemistry 318N THE ENERGY OF ELECTROMAGNETIC WAVES (nu-bar) represents wavenumber, the number of wavelengths in 1 cm λ υ λ υ λυ c υ λ ν hc c h h E = = = = = 1 10 microns (micrometers) = 1000 cm -1
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4 Chemistry 318N Harmonic Oscillator Model μ K 2 π 1 m 1 m 2 ν = m m ) m )( m ( μ 2 1 2 1 + = E ν = h ν o ( ν + 1/2) quantized vibrational energy h = Planck’s constant ν o = characteristic frequency ν = 0, 1, 2, 3,…. In real molecules, oscillation is anharmonic . Chemistry 318N Energy quantization of the oscillator classical and quantum behavior are very different: energy classical: all possible values of total energy are allowed quantum: only certain ‘energy levels’ allowed note that although the energy levels are equally spaced for the harmonic oscillator this is not true for all QM systems note also that it is not possible for the quantum harmonic oscillator to have zero energy – it must have some energy even at 0 K 0 0 1 2 3 4 n . . . . ‘quantum number’
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5 Chemistry 318N IR Spectra of chloroform and deuterochloroform 652 671 C-Cl bending 737 773 C-Cl stretching 912 1219 C-H bending 2256 3020 C-H stretching CDCl 3 CHCl 3 Mode of vibration * Spartan ’02 AM1 minimization Shift of peak due to replacement of H with D (2x mass)
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