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Unformatted text preview: electric field Polar Molecular Bonds Vibrate produce an oscillating electric field HCl example dipole moment All Molecules Vibrate H-Cl + +H---Cl
equilibrium H-----Cl +
extended compressed Polar Molecular Bonds Vibrate produce an oscillating electric field Dipole moment = qr 30 IR Theory Discussed on Board in Class Hooks Law F = -kx (harmonic oscillator) k = strength of spring Equation for frequency of bond vibration Frequency of Vibration of Bond = (1/2 c)[k(M1+ M2)/M1M2]0.5 k = strength of bond M1 = mass of atom 1, M2 = mass of atom 2 c = speed of light Potential Energy versus atomic distance curve. Excitation of vibration from ground state to excited state by a photon of IR radiation. 31 IR Theory Frequency of Vibration of Bond = (1/2 c)[k(M1+ M2)/M1M2]0.5 C-H bond vibration absorbs infrared radiation at this wavenumber. [or frequency] Energy of photon = = = (c/ ) Vibrating Bond Generates an Oscillating Electric Field with a specific frequency. When frequency of IR radiation equals the frequency of the bond vibration, a photon of energy is absorbed and the molecule is excited to a higher energy vibrational state.
32 IR Theory Bond must have dipole moment IR Active HCl, H2O, CO2, CH3Cl, etc IR Inactive He, Ar, N2, O2, H2 Bonds with large dipole moments have large peaks C-O, C=O, C-F, C-N, Molecular symmetry important HCCH has small CC peak CH3CCH has larger CC peak
33 Infrared Spectrometer Thermo-Nicolet Cost ~$40,000 Samples Gases Liquids (1 drop) Solids (100 mg) 34 Infrared Energy Modes IR energy absorption corresponds to specific modes, corresponding to combinations of atomic movements, such as bending and stretching of bonds between groups of atoms called "normal modes" bonding Energy is characteristic of the atoms in the group and their Corresponds to vibrations and rotations 35 Regions of the Infrared Spectrum 4000-...
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This note was uploaded on 04/07/2008 for the course CHGN 222 taught by Professor Cowley during the Spring '08 term at Mines.
- Spring '08