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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 cm1
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 CD stretching frequency (2100 cm1) 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 UniversityWest Lafayette.
 Fall '02
 l

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