Chap1 UV-VIS LectureNote

8/12/2019 Chap1 UV-VIS LectureNote

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UV-VIS SPECTROSCOPY

General info:

Spectrometryis based on the absorption ofphotons by the analyte.Spectrometric method: the sample solution absorbselectromagnetic radiation from an appropriate source

and the amount absorbed is related to theconcentration of the analyte in the solution.

Eg: copper solution is blue because it absorbs the

yellow color from white light and transmits theremaining blue light. The higher[cu], the more yellowlight is absorbed and the deeper the resulting bluecolor of the solution.


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The electromagnetic spectrum- Review

A form of radiant energy that is propagated as a

transverse wave Vibrate perpendicular to the direction of

propagation - a wave motion to the radiation


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Continue The relationship between wavelength and

frequency:;

= wavelength in cm

= frequency in s-1or Hzc = velocity of light (3x1010cm/s) The wavelength varies from a few to several

meters, for UV & Visnanometer for IRmicrometer, commonly used as wave

number, in cm-1

c

v


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Continue

Electromagnetic radiation possesses a

certain amount of energy called photon,related to the or by:

energy of the photon in ergs= Planks constant,

Note: Shorter wavelength, greater energyUV from the sun burns you!!!!

v

hchvE

E

h sec1062.6 34

Joule


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Working ranges of the UV-Vis and IR spectra

UV 200-380 nmVis 380-780 nmNear IR 0.78-2.5 mMid IR 2.5-15 m

The most

analytically region


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How does matter absorb radiationThree basic processes by which a molecule can absorb radiation;all involve raising the molecule to a higher internal energy level,the increase in energy being equal to the energy of theabsorbed radiation, . Three type of internal energy arequantized;

Rotational transition: the molecule rotates about various axes,the energy of rotation being a definite energy levels, so the

molecule may absorb radiation and be raised to a higherrotational energy level.

Vibrational transition: the atoms or groups of atoms within amolecule vibrate relative to each other, and the energy of this

vibration occurs at definite quantized levels. The molecules maythen absorb a discrete amount of energy and be raised to ahigher vibrational energy level.

Electron transition: the electron of a molecule may be raised tohigher electron energy. UV-VIS spectroscopy principal/theory

hv


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Energy level diagram illustrating energy changes associated with absorption ofelectromagnetic radiation

A = pure rotational changer (far IR)B = rotational + vibrational changes (near IR)C = rotational + vibrational + electronic transition(Vis + UV)Eo= electronic ground stateE1= first electronic exited state


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Electronic Spectra and Molecule Structure

The electronic transition in the UV-VIS regions are due tothe absorption of radiation by specific types of: Groups,

bonds and functional groups within the molecule Kinds of transitions: Electrons in a molecule can be

classified into 4 different types:i. Closed shell electronnot involved in bondingii. Covalent single bond electron,iii. Paired non bonding outershell electron (n electrons)

such as those on N, O, S, and halogens. These are lesstightly held then electrons and can be excited by visibleor UV radiation

iv. Electron in orbitals, in = or bonds. These are themost readily excited and are responsible for a majorityof electronic spectra in the UV-VIS regions.

* and n electrons responsible for UV-VIS spectrum


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Other electron remain/occupate in orbitals

A molecule also possesses unoccupied

orbitals called antibonding orbitals; thesecorrespond to excited- state energy levels(either * or * ).Therefore, absorption ofradiation results in an electronic transition

to antibonding orbitals. The most common transition:

* orn *n *

above 200 nm

occur at very short


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HOMO: Highest Occupied Molecular OrbitalLUMO: Lowest Unoccupied Molecular Orbital

Example, molecule H2


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Example of transition * and n *in ketone (acetone)

* n *The probability of * is higher thann *

Molar absorptivities,* (1000-100,000)

n * (less than 1000)


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ABSORPTION BY ISOLATED CHROMOPHORES

The absorbing groups (the electrons responsible forthe absorption) in a molecules are calledchromophores

A molecule containing a chromophorechromogen

The present of auxochrome enhance theabsorption by a chromophore or shift the ofabsorption.

Example: hydroxyl (OH), amine (-NH2), halogens(X)

posses n electronsinteract with electronsin the chromophores (n ) conjugation.


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Spectra changes can be classed as:

Bathochromic shift (red shift) shifted to shorter

wavelength. It may caused by removal of conjugation. Hypsochromic shift (Blue shift) - shifted to shorter

wavelength. It may caused by removal of conjugation.Eg. The conjugation of the lone pair electrons on the

N atom on aniline with the bond system of thebenzene ring is removed on protonationNeutral aniline absorbs at = 230 nm

= 8600

But in acid solution (protonated) = 203 nm= 7500

max

max

max

max


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Spectra changes can be classed as: continue.

Hyperchromic effectincreased absorption

intensity ( ) Hypochromic effectdecreased absorption

intensity ( )

Note: = the wavelength of the absorptionmaximum

= Absorption [ ] of a 1%solution in a cell with 1 cmpathlength- used when MW of a

compound is not known

max

%11cm )(log 010 I

I


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BEERS LAW-ABSORPTION LAW

Fraction of radiant energy transmitted decays exponentially withpathlength.

Transmittance in exponential form:

is a constant (eq 1)

Putting in logarithmic form,

0I Ic

b

Transmittedradiation

Incidentradiation

Pathlength concentration

kb

I

IT

100

k

kb

I

IT

0

loglog (Eq 2


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BEERS LAW-ABSORPTION LAWcontinue

Similar law holds for the dependence of T on the concentration,

Combining (Eq 1) and (Eq 3), describes the dependence of T on thepathlength and concentration.

Omit theve sign, and rearrange

Hence,

c

ck

IIT '0

10

'k is a new constant (Eq 3)

ckI

IT 'loglog

0

(Eq 4)

abc

I

IT 10

0

a is combined constant of kand k (Eq 5)

abcIIT 0

loglog

abcI

I

TTA

0log1

loglog

abcA

Absorbance Absorptivity Pathlength in cm

Conc in g/L


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BEERS LAW-ABSORPTION LAWcontinue

Note: in absorption spectrum, the absorbance varies withwavelength in direct proportion to a (b & c are held

constant). The product of the absorptivity and themolecular weight of the absorbing species is called themolar absorptivity,

Thus bcA )( 11 Lmolcm


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Example

Amines, RNH2react with picric acid to form amine picrates, which

absorb strongly at 359 nm (=1.25 x 104). An unknown amine

(0.1155 g) is dissolved in water and dilute to 100 mL. A 1 ml aliquotof this is diluted to 250 ml for measurement. If this final solution

exhibit an absorbance of 0.454 at 359 nm using a 1.00 cm cell,

what is the formula weight of the amine? What is a probable

formula?

Solution:

in original flask

bcA ccmLmolcm 00.11025.1454.0 114

Lmolc /1063.3 5

mL

mLLLmol 00.1

100)250.0)(/1065.3(

5

mol41008.9

1

4 127

1008.9

1155.0

gmol

mol

g


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Measurement of the UV Spectrum:

Using a dilute solution: 1 mg of compound with MW

(100-400 g/mol) is dissolved in 100 mL volumetric

flask. Hexane is used as a solvent. Portion of the

solution is transferred to a cuvett quartz of 1 cm

(Styrene, MW = 104 g/mol

max=250 nm


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Choice of solvent

Solvent consideration: cheap, good solvent transparent downto about 210 nm.

Most commonly used95% ethanolSolvent Effect

Electronic Transition, *: atom do not move, howeverelectrons of the atom and solvent molecules may

reorganise Transition state is more polar than ground state due to

dipole-dipole interaction with solvents, thus lower the

energy of the excited state.

Eg: Styrene in ethanol (max

= 270 nm)

Styrene in hexane (max= 250 nm)

about 20 nm shift (red shift, longer wavelength)


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Electronic transition, n * (the weaktransition of the O lone pair in ketones

Chap1 UV-VIS LectureNote

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