Infrared Spectroscopy Transmission Vs ATR红外光谱透射比ATR.ppt

Infrared Spectroscopy: Comparison of Transmission and ATR Techniques
Matt Herring
Lewis University
Overview
IR spectroscopy theory
Transmission vs. Attenuated Total Reflectance (ATR) techniques
Based on study by:
Koulis, Cynthia, et. al. Comparison of Transmission and Internal Reflection Infrared Spectra of Cocaine. Journal of Forensic Sciences, 2001.
What is Spectroscopy?
Spectroscopy - study of the interaction of atoms and molecules with ic radiation
Probe the world at an atomic and molecular level
qualitative analysis
Bond lengths
Bond Force Constants
Molecular structures
quantitative analysis
Quantum States
Absorption of ic radiation  excitation to higher energy states
hv = |E2 – E1|
Electronic > Vibrational > Rotational
Infrared Radiation
Frequencies - 12800 to 10 cm-1
(cm-1 = wavenumbers)
Divided into three Regions
Near : 12800 to 4000 cm-1
Middle: 4000 to 200 cm-1
Far: 200 to 10 cm-1
Radiation of most use for spectroscopy falls in the mid-infrared region between 4000 to 670 cm-1
50
Infrared Absorption
Absorption of IR radiation  excitation of vibrational and rotational energy levels
not sufficient energy for electronic excitation
Vibrational energy states are of greatest importance for IR spectroscopy
Dynamic Dipole Moment
Absorption of IR radiation is only observed in molecules with a dynamic dipole moment
An induced dynamic dipole moment is observed in polar molecules
Magnitude of dipole moment depends on overlap of electron densities in molecule
Vibration results in change in internuclear distance and therefore fluctuations in magnitude of dipole
Homonuclear diatomic molecules (. H2, N2, O2) don’t absorb IR radiation, b/c of no dynamic dipole moment
IR absorption
IF vibrational frequency of molecule = frequency of radiation THEN energy is absorbed
Results promotion to higher energy level and increase in amplitude of molecular vibration
Molecular Vibrations
Molecular Vibrational and Bending modes
Single absorption peak appears f