Infrared Spectroscoy

Question 1

What are the 3 parts of the IR region?

Answer 1

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)

Question 2

What type of process is IR Absorption and what energy states are involed for the changes?

Answer 2

IR is a quanitzed process
IR involes changes in rotational and vibrational enegry states

Question 3

What is the selection rule to absorb IR radiation

Answer 3

There must a dipole moment of the bond. (Difference In electronegativity).

Question 4

Mechnaical model for Vibrations

Answer 4

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.

Question 5

Different types of Vibrations?

Answer 5

Simple vibrations;

Stretching involes changes in the iteratomic distances

Bending involes change i the angle between bonds.

Complex vibrations:
Scissoring
Rocking
Waggign Twistings

Question 6

what is natural frequcney and step to calculated it?

Answer 6

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).

Question 7

Quantum theory of IR

Answer 7

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

Question 8

Vibrational frequencies trend

Answer 8

1) the heavier the mass the lower the frenqencey

2) the strong the bond the the higher the frenquency

Question 9

Vibrational Modes of a molecule?

Answer 9

Linear molecule the number of vibrational modes are 3N-5

Non-linear molecule the number of vibrational modes are 3N-6

Question 10

Dispersive instruments

Answer 10

Uses a monpchromator to disperse the IR radiation, can be used in both single and double beam spectrophometer.

Question 11

Fourier Transform Spectrophotometers

Answer 11

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

Question 12

What is a interferogram?

Answer 12

is a plot of intensity versues retardation. Using a fourier transfom to convert an interferogram into a spectrum

Question 13

What is Apodization?

Answer 13

It is a process of removing the side lobs of a interfromgram.

There are 3 types:
Traiangular Apodization
Box Car

Question 14

Advanatges of Foueir transform

Answer 14

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

Question 15

What is beer’s law?

Answer 15

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.

Question 16

Sample Preparation for liquid

Answer 16

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

Question 17

Sample preparation for solids

Answer 17

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

Question 18

IR sources

Answer 18

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

Question 19

What is a thermal couple and whats its purpose in IR?

Answer 19

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.

Question 20

Pyroelectric detectors

Answer 20

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

Question 21

Photovolatic detectors

Answer 21

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.

Question 22

Diffuse Reflecentance

Answer 22

is a Inrfrared Fourier Transform Spectroscpy techinuique that collects and analyzes scattered IR energy

Question 23

Attenuated Total Reflectance Spectra

Answer 23

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

Question 24

What is ATR used for?

Answer 24

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