Infrared Spectroscopy Flashcards
Explain Infrared Process
When a wave of infrared light (from spectrophotometer) encounters this oscillating EM field generated by the oscillating dipole of the same frequency, the two waves couple, and IR light is absorbed
As covalent bond oscillates due to oscillation of dipole of the molecule, ____ is produced
varying electromagnetic field is produced
The greater the dipole moment change through vibration, the ___ the EMF that is generated
more intense
when a wave of IR light encounters this oscillating EMF, the waves ___ and IR light is ___
coupled; absorbed
Coupled wave vibrates with ___ the amplitude
twice
Types of bond stretching (2)
Symmetric and asymmetric
Types of bond bending (4)
in-plane rocking
in-plane scissoring
out-of-plane wagging
out-of-plane twisting
a molecule can vibrate in many ways, and each way is called ___
vibrational mode
Vibrational modes for
linear?
non linear?
linear: 3x-5
nonlinear: 3x-6
x = number of atoms
Basic Instrumentation (IR)
Radiation source
Sample Holder
Wavelength Selector (Monochromator)
Detector
inert solid that is electrically heated to a temperature between 1500 to 2200 K
blackbody sources
Common blackbody sources
Globar
Nernst Glower
Incandescent Wire Source
*** (others)
CO2 laser
Mercury Arc Lamp
Tubgsten Halogen Lamp
:rare earth oxides (Ze, Ce, Th) heated electrically formed into a cylinder having a diameter of 1 to 2 mm
: with platinum leads
Nernst Glower
Balckbody source that uses silicon carbide rod; water cooling is needed; more expensive
Globar
Blackbody source that has lower intensity but longer life than globar and nernst glower
a tightly wound spiral of nichrome wore or rhodium-wire heated to about 1100K by an electrical current
incandescent wire source
Blackbody source for far-infrared region
quartz-jacketed tube containing mercury vapor at a pressure greater than one atmosphere
Mercury Arc Lamp
CO2 laser consists of
70% He, 15% CO2, 15% N2
Liquid sample cells can be sandwiched using liquid sample cells of highly purified ____, such as ___
alkali halides; NaCl, KBr, CaF2
_____ solvents cannot be used because they cannot dissolve alkali halides
Aqueous solvents
For most liquids, the sample cell thickness is ___
0.01-0.05 mm
Sample cell for gas samples is made up of and with path length of?
alkali halides such as NaCl, KBr
5-10 cm because of weak absorbance
Sample holder techniques for solid samples (4)
Mull technique
Solid run in solution
case film technique
pressed pellet technique
finely crushed sample is mixed with Nujol in a marble or agate mortar, with pestle to make a thick paste. a thin film is applied onto the salt plates and mounted in a path of IR beam
Mull technique
A drop of solution is placed on the surface of alkali metal disc and solvent is evaporated to dryness leaving a thin film of solute
solid run in solution
amorphous solid is deposited on surface of KBr or NaCl cell by evaporation of solution of solid; film is not too thick
case film technique
small amount of finely ground solid sample is mixed with 100 times its weight of KBr and compressed into a thin transparent pellet
Pressed pellet technique
Common wavelength selector for IR
reflective grating
Why glass prism is not suitable for IR Spectroscopy
glass prism absorbs IR
Detector for IR Spectroscopy (4)
Thermal detectors such as
Thermocouple
Bolometer
Photoconducting Detectors
Pyroelectric Detectors
Types of Instruments (IR) (3)
Non-dispersive
Dispersive
Fourier Transform
used to detect gas and measure concentration of carbon oxides
non-dispersive
scanning instrument uses a grating monochromator to resolve the IR radiation into individual frequencies
Dispersive
FTIR is based on
Michelson Interferometer
Advantages of FTIR from Normal IR (3)
Jacquinot advantage
Connes advantage
Felgett advantage
Since the instrument have few optical elements and no slits to attenuate the radiation, the ___ reaching the detector is ____
What kind of advantage?
power of radiation; greater
Jacquinot advantage.
Type of advantage associated with extremely high wavelength accuracy and precision
Connes advantage
Type of FTIR advantage: Since all elements of the source reach the detector simultaneously, an entire spectrum can be obtained in a brief period
Felgett advantage
IR Techniques (3)
Specular Reflectance
Diffuse Reflectance
Attenuated Total Reflectance
IRRAS
Infrared reflection absorption spectroscopy
used for analysis of both organic and inorganic samples having large, flat, thin reflective surfaces
IRRAS
measures the energy that is reflected off the surface of a sample (refractive index)
IRRAS
IRRAS advantages (2)
sensitivity to monolayer samples
nondestructive analysis
DRIFTS
Diffuse Reflectance Infrared Fourier Transform Spectroscopy
useful for analysis of both organic and inorganic samples that can be ground into fine powder and mixed in a powder matrix such as KBr
DRIFTS
useful for samples that are difficult to deal with such as solid of limited solubility, films, threads, etc
Attenuated Total Reflectance
operates by measuring the changes that occur in an internally reflected IR beam when the beam comes into contact with sample
Attenuated Total Reflectance
used for shiny or rough surface
attenuated total
used for shiny materials
specular reflectance
used for rough surfaces
diffuse reflectance
Qualitative Analysis (IR) (4)
General Scheme
Group Frequency Region
Fingerprint Region
Computer Searches
The area of an IR band (peak) is ____ to the concentration of functional groups producing the peak
not directly
IR Spectroscopy has good selectivity since____ but not as good as UV/Vis in terms of ___
many compounds have different IR spectra
accuracy and precision
