Infrared Spectroscoy Flashcards
What are the 3 parts of the IR region?
Near IR (Nearest to visble); 12800-4000 cm^-1
Mid IR (4000 to 200 cm^-1)
Far IR (200 to 10 cm^-1)
The most common range is Mid (4000 to 200 cm^-1)
What type of process is IR Absorption and what energy states are involed for the changes?
IR is a quanitzed process
IR involes changes in rotational and vibrational enegry states
What is the selection rule to absorb IR radiation
There must a dipole moment of the bond. (Difference In electronegativity).
Mechnaical model for Vibrations
Consider two masses, m1 and m2 both connected by a spring. This allows for harmonic oscillations between the two masses. We can calculate the the force, and it will obey hookes law.
Different types of Vibrations?
Simple vibrations;
Stretching involes changes in the iteratomic distances
Bending involes change i the angle between bonds.
Complex vibrations:
Scissoring
Rocking
Waggign Twistings
what is natural frequcney and step to calculated it?
found by a given equation:
1) convert molar mass to kg
2) divde mass by avergos # to find mass per atom.
3) find the reduced mass with (m1m2)/(m1+m2).
4) plug into another eqaution that used hookes law.
5) depending on the bond (single double triple) there will be a different force constant (K).
Quantum theory of IR
indicates that the only transistion that can take place are those in which the quantum number changes by unity.
This is all called the Infrared selection rule
Vibrational frequencies trend
1) the heavier the mass the lower the frenqencey
2) the strong the bond the the higher the frenquency
Vibrational Modes of a molecule?
Linear molecule the number of vibrational modes are 3N-5
Non-linear molecule the number of vibrational modes are 3N-6
Dispersive instruments
Uses a monpchromator to disperse the IR radiation, can be used in both single and double beam spectrophometer.
Fourier Transform Spectrophotometers
all modren IR spectrophotometers are based on a mathematical operation known as a Fourier transform (FT).
FT spectrophotometers are based n a michelosn interferometer (MT)
MT consists of a beam splitter and move able and fixed mirror. An MT uses difference in optical pathlength to create an interference pattern
What is a interferogram?
is a plot of intensity versues retardation. Using a fourier transfom to convert an interferogram into a spectrum
What is Apodization?
It is a process of removing the side lobs of a interfromgram.
There are 3 types:
Traiangular Apodization
Box Car
Advanatges of Foueir transform
1) the S/N ratio can be improved by collecting many spectra
2)Jacquinot or high through put advantage. This means there is a high throught put because there are fewer slits compared toa dispersive system, leading to a higher signal to noise raito.
3) Fellget advantage (multiplex advantage); all wavelengths from the source reach the detector simultaneously.
4) entire spectrum is collected in a few seconds and stored in a computer
What is beer’s law?
It is an equation to measure the IR absorbance of substance and it depends on 3 factors, the concration of the substance, the molarabsortivity of the substance, and optical pathlength of the substance.
Note: in IRL situtaions will be diffrerence in beers law.
Sample Preparation for liquid
1) Ionic substances; Salt plates, NaCl or KBR. a sample is placed between two salt plates which are ploished from there are squeezed together. this is also called a neat spectrum since there is no solvent being used
Sample preparation for solids
1) KBr pellet; the solid sample is grounding into a paste, then you add powdered KBr. with force you create a pellet with your sample.
2) Nujol mull; the solild is mixed with mineral oil, then is placed between two salt plates.
3) Solid sample in a solvent; the sample is dissolved in a solvent usually carbon tetrachloride
IR sources
Most common is a globar; silicon carbide rod heated to about 1500 degrees.
A Nernst Glower is a cycinder of zirconum and yttrium
A coil of nichrome (nickel-chromium
a tunebale carbon dioxide laster
What is a thermal couple and whats its purpose in IR?
A thermo couple consisits of one or more dissimiliar metal junctios that deveople a potential difference when thier temperature differ. The magniditue of the potenitla depends on the temperaturee difference.
Pyroelectric detectors
are made from ferroelectric materials.
Ferroelectric material the dipole moments of molecues reman aliegned in the absence of an external eletric field. Gives material a permanent electric polarization.
When they absorb IR Radiation the pyroelectric effects leads to a cahnge in voltage, which is the signal in a pyroelectric detector
Photovolatic detectors
Contain Pn junctins, across which an electric field exists.
The absorbs of IR raidaition creates holes and electrons which are attracted to oppsite sides of the junction and cahnge the voltage across the junction.
Most common photovoltaic detector uses mercury, Cadmium, and tellurium (MCT).
Must be cooled to reduce thermal noise.
High speeds, and low IR signals.
Diffuse Reflecentance
is a Inrfrared Fourier Transform Spectroscpy techinuique that collects and analyzes scattered IR energy
Attenuated Total Reflectance Spectra
The sample is placed in contact with the surface of an optical device of high refreactive index called a waveguide.
IR radition is passed through the waveguide.
THe IR radation reflects through the waveguide, penetrationg into the sample a finte amount with each reflection along the top surface via the so called “evensacent wave”.
The beam is attenuated before being totally internally reflected
Aka ATR-IR
What is ATR used for?
ATR-IR is used for the analysis of the surface of materials. Its also suitable for characterization of materials which are either too thick or too strong to absorbed by transmission spectroscopy
