When a change in vibration from one state to another

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Unformatted text preview: in vibration from one state to another involves a change in charge distribution (i.e., a change in dipole moment). – Very well suited for distinguishing/identifying functional groups 15 Collecting an IR Spectrum CH4 IR Source Spectrum CH4 16 8 Functional groups are distinguished by their spectra Wavenumber = Reciprocal in cm-1 Proportional to and to the vibrational energy E=h c= c: speed of light in vacuum h: Planck’s constant Wavelength: 1 micron = 1 micrometer = 1 m 17 Functional groups are distinguished by their spectra Pairs with higher bond with higher bond energies will generally vibrate at a higher frequency and have shorter wavelength IR absorption, absorption, c = * Wavelength: 1 micron = 1 micrometer = 1 m 18 9 Molecular vibrations: IR absorption • Atoms bonded together are not stationary, they vibrate. Bonds stretch and angles bend. • Simple model: two atoms connected by a spring • Adding energy (via IR absorption) makes the atoms vibrate with more energy; the vibrational energies are quantized 19 (just like n levels in atoms) Motions of CO2 • Go to CO2 to CO2 20 10 Interpreting IR stretching frequencies • IR absorption frequencies depend on absorption frequencies depend on – Mass of atoms bonded together – Strength of the interaction between the atoms m1 m2 21 Interpreting IR stretching frequencies • For the same elements, larger mass atoms will vibrate at a lower frequency (more slowly) • The C-D stretching frequency (2100 cm-1) is...
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This note was uploaded on 01/24/2014 for the course CHEM 115 taught by Professor L during the Fall '02 term at Purdue University-West Lafayette.

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