CHEM 156 Study Guide - Final Guide: Infrared Spectroscopy, Broad Peak, Evanescent Field
IR band is lower frequency and higher wavelength; can be absorbed by
organic molecules
-
IR region of electromagnetic spectrum has the right energy to be absorbed
by molecules, and excite them to higher vibrational energy levels
-
IR radiation corresponds to the range encompassing the stretching and
bending vibrational frequencies of the bonds in most covalent molecules
-
IR spectroscopy:
Transmittance and absorbance are inversely proportional
○
Y axis: Transmittance of the incident light
-
Wavenumber is directly proportional to energy of vibration
○
More energy required to excite stronger bonds to higher vibrational
modes and the absorptions appear at higher wavenumbers
○
X axis: wavenumber
-
Contain broad peaks due to large number of possible vibration/rotation
modes available to molecules
-
Only motions that change the molecular dipolar moment are active in
absorption of IR frequency
-
IR spectroscopy used to prove existence of certain functional groups
-
Anatomy of IR spectrum:
ATR mode: to streamline and simplify measurements of IR spectra
-
Eliminates preparation steps because no matrix or plate necessary to hold
sample in the light path
-
Light not reflected directly by boundary surface, but by virtual layer
within optically less dense sample
○
Evanescent wave: fraction of light reaching into sample
○
IR beam enters ATR crystal and is totally reflected at crystal to sample
interface
-
IR beam exits ATR crystal after several internal reflections, and is directed to
IR detector
-
Refractive index of crystal must be significantly higher than that of the
sample
-
Instrumentation: ATR mode
Carbonyl group (C=O): strong peak at 1820 - 1600 cm^-1
-
If C=O is present:
-
Interpreting IR spectrum
Spectroscopic Characterization of Macromolecules
Wednesday, May 9, 2018
7:44 AM
Chem 156 Page 1
Document Summary
Ir band is lower frequency and higher wavelength; can be absorbed by organic molecules. Ir region of electromagnetic spectrum has the right energy to be absorbed by molecules, and excite them to higher vibrational energy levels. Ir radiation corresponds to the range encompassing the stretching and bending vibrational frequencies of the bonds in most covalent molecules. Wavenumber is directly proportional to energy of vibration. More energy required to excite stronger bonds to higher vibrational modes and the absorptions appear at higher wavenumbers. Contain broad peaks due to large number of possible vibration/rotation modes available to molecules. Only motions that change the molecular dipolar moment are active in absorption of ir frequency. Ir spectroscopy used to prove existence of certain functional groups. Atr mode: to streamline and simplify measurements of ir spectra. Eliminates preparation steps because no matrix or plate necessary to hold sample in the light path.
