Optical Properties Test-sua Lai

8/8/2019 Optical Properties Test-sua Lai

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Optical properties test

IR and Raman spectroscopy

Ultraviolet spectroscopy NMR spectroscopy

X-ray spectroscopy


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Symmetric Stretch Antisymmetric Stretch Bend

Infrared spectroscopy

Infrared (IR) spectroscopy measures the bond vibrationfrequencies in a molecule and is used to determine thefunctional group

The bonds between atoms in the molecule stretch and

bend, absorbing infrared energy and creating the infraredspectrum.


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selection rules

Covalent bonds vibrate at only certain allowablefrequencies.


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An electric dipole moment of the moleculemust change during the vibration


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Fig 3. spectrum of novolak resin


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Raman spectroscopy

Is a light scattering technique, and can bethought of in its simplest form as a processwhere a photon of light interacts with a sampleto produce scattered radiation of differentwavelengths

Raman spectroscopy is SCATTERINGspectroscopy Scattering involves amomentary distortion of the electrons distributedaround a molecular bond. Thus, the molecule is

temporarilyPolarized that disappears upon relaxation andreemission. For example, H2 and N2 can be detected


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Arrow up = laser photon in; Arrow down = Ramanscattering out


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Elastic scattering:

Rayleigh scattering

Photon in Photon out

No vibration No vibrationIf incident photon energy E; vibration energy v,then

In terms of energy, photon out has energy:

E-vStokes scattering

E+vanti-Stokes scattering

E Rayleigh scattering


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Selection rule for Raman spectrum

Vibration is active if it has a change in

polarizability, .Polarizability is the ease of distortion of a bond.For Raman-active vibrations, the incident radiationdoes not cause a change in the dipole moment of the

molecule, but instead a change in polarizability.

In starting the vibration going, the electric field ofthe radiation at time t, E, induces a separation ofcharge (i.e. between the nuclear protons and the

bonding electrons). This is called the induced dipolemoment, P. (Dont confuse it with the moleculesdipole moment, or change in dipole moment, becausethis is often zero).

P = E


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NMR spectroscopy

NMR is the most powerful tool availablefor organic structure determination.

It is used to study a wide variety ofnuclei:

1H

13C

15N

19F

31 P


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An electron has a spin quantum number of 1/2 with allowedvalues of +1/2 and -1/2 this spinning charge creates an associated magnetic

field in effect, an electron behaves as if it is a tiny bar

magnet and has what is called a magnetic moment The same effect holds for certain atomic nuclei

any atomic nucleus that has an odd mass number, an oddatomic number, or both also has a spin and a resultingnuclear magnetic moment

the allowed nuclear spin states are determined by thespin quantum number, I, of the nucleus

a nucleus with spin quantum number II has 22II + 1+ 1 spinstates; if I = 1/2, there are two allowed spin states


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Fig. spectrum ofPET

8.16 ppm

4.82 ppm: the ethylenedioxyl


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Fig. spectrum of

PEA

1.66 ppm (CH2)2-

2.35 ppm: CH2COO

4.26 ppm:the ethylenedioxyl group


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UV spectroscopyObserved electronic transitions

The lowest energy transition (and most often obs. by UV)

is typically that of an electron in the Highest OccupiedMolecular Orbital (HOMO) to the Lowest UnoccupiedMolecular Orbital (LUMO)

For any bond (pair of electrons) in a molecule, themolecular orbitals are a mixture of the two contributing

atomic orbitals; for every bonding orbital created fromthis mixing (s, p), there is a corresponding anti-bondingorbital of symmetrically higher energy (s*, p*)

The lowest energy occupied orbitals are typically the s;likewise, the corresponding anti-bonding s* orbital is of

the highest energy p-orbitals are of somewhat higher energy, and theircomplementary anti-bonding orbital somewhat lower inenergy than s*.

Unshared pairs lie at the energy of the original atomicorbital, most often this energy is higher than p or s (sinceno bond is formed, there is no benefit in energy)


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Energy

s*

p

s

p*

n

Atomic orbitalAtomic orbital

Molecular orbitals

Occupied levels

Unoccupied levels


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Energy

s*

p

s

p*

n

s

s

p

n

n

s*

p*

p*

s*

p*

alkanes

carbonyls

unsaturated cmpds.

O, N, S, halogens

carbonyls


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In the manufacture of PLED display using a polymer

it can be stimulated luminescence when a suitable

energy level, they will emit color in the energy

received, based on this ability of the polymer

luminescence Scientists have built PLED screen

PLED display (www.oled.at/Oled-tv-the-next-flat-tv-technology.htm)


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*

*

n

poly(phenylene vinylene)

*

*

n

polyfluorene

**

n

polyfluorene


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X-ray spectroscopyn= 2 dsin

Constructive interference only occurs forcertain s correlating to a (hkl) plane,specifically when the path difference is equalto n wavelengths.

Optical Properties Test-sua Lai

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