Kuliah UV-Molecular Spectrometry

8/21/2019 Kuliah UV-Molecular Spectrometry

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UV-VIS Molecular

Spectroscopy

Chapter 13-14

From 190 to 900 nm!


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Reflection and Scattering Losses


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LAMBERT-BEER LAW

ionconcentrat

pathlength

tyabsorptivi

loglog0

0

c

b

a

kcabcA

P

PTA

P

P

P

PT

solvent

solution

Power of radiation

after passing

through the solvent

Power of radiation after

passing through the

sample solution


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Absorption Variables


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Beers law and mixtures

Each analyte present in the solution absorbs light!

The magnitude of the absorption depends on its e

A total = A1+A2++An A total = e1bc1+e2bc2++enbcn

If e1 = e2 =en then simultaneous determination is

impossible Need nls where es are different to solve the

mixture


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Assumptions

Ingle and Crouch, Spectrochemical Analysis


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Deviations from Beers Law

Successful at low analyte concentrations (0.01M)!High concentrations of other species may also affect

2

12

2

12

0 )(

)(

I

Ir


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Chemical EquilibriaConsider the equilibrium:

A + C AC

If eis different for A and AC then the absorbancedepends on the equilibrium.

[A] and [AC] depend on [A]total.

A plot of absorbance vs. [A]totalwill not be linear.


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Instrumental deviation with

polychromatic radiation


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Effects of Stray Light

kcabcA

PP

PPA

TA

PP

PPT

P

S

S

S

S

S

0

0

log

log

lightstray

1000

PPS


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Instrument Noise


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1 2 3 4 5 6 7 8 9 10 11

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

% RELATIVE CONCENTRATION UNCERTAINTIES

TRANSMISSION

Effects of Signal-to-Noise

Bad at Low T

Bad at High T


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Components of instrumentation:

Sources

Sample Containers

Monochromators

Detectors


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Components of instrumentation:

Sources:Agron,Xenon, Deuteriun, or Tungsten lamps

Sample Containers: Quartz, Borosilicate, Plastic

Monochromators: Quarts prisms and all gratings

Detectors: Pohotomultipliers


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SourcesDeuterium and hydrogen lamps (160375 nm)

D2+ Ee D2* D + D + h

Excited deuterium

molecule with fixed

quantized energy

Dissociated into two

deuterium atoms with

different kinetic energies

Ee= ED2*= ED+ ED+ hv

Ee is the electrical energy absorbed by the molecule. ED2*is the fixed quantizedenergy of D2*, EDand EDare kinetic energy of the two deuterium atoms.


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SourcesTungsten lamps (350-2500 nm)

Blackbody type , temperature dependent

Why add I2in the lamps?

W + I2 WI2

Low limit: 350 nm

1)Low density

2)Glass envelope


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General Instrument Designs

Single beam

Requires a stabilized voltage supply


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Double Beam: Space resolved

Need two detectors


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Double Beam: Time resolved


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Double Beam Instruments

1.Compensate for all but the most short term fluctuation in

radiant output of the source

2.Compensate drift in transducer and amplifier

3.Compensate for wide variations in source intensity withwavelength


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Multi-channel Design


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Molar absorptivities

e= 8.7 x 10 19P A

A: cross section of molecule in cm2(~10-15)

P: Probability of the electronic transition (0-1)

P>0.1-1 allowable transitions

P


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Visible Absorption Spectra


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The absorption of UV-visible radiation

generally results from excitation of bonding

electrons. can be used for quantitative and qualitative

analysis


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Molecular orbitalis the nonlocalized fieldsbetween atoms that are occupied by bonding

electrons. (when two atom orbitals combine, eithera low-energy bonding molecular orbital or a highenergy antibonding molecular orbital results.)

Sigma () orbitalThe molecular orbital associated with single bondsin organic compounds

Pi () orbitalThe molecular orbital associated with paralleloverlap of atomic P orbital.

n electrons

No bonding electrons


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Molecular Transitions

for UV-Visible Absorptions

What electrons can we use for these

transitions?


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MO Diagramfor

Formaldehyde

(CH2O)

HC

H

O

= =

n =


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Singlet vs. triplet

In these diagrams, one electron has been excited (promoted)

from the n to * energy levels (non-bonding to anti-bonding). One is a Singlet excited state, the other is a Triplet.


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Type of Transitions

*High energy required, vacuum UV range

CH4: = 125 nm n *

Saturated compounds, CH3OH etc ( = 150 - 250 nm)

n * and *Mostly used! = 200 - 700 nm


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Examples of

UV-Visible Absorptions

LOW!


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UV-Visible Absorption Chromophores


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Effects of solvents

Blue shift (n- *) (Hypsocromic shift) Increasing polarity of solvent better solvation of

electron pairs (n level has lower E)

peak shifts to the blue (more energetic) 30 nm (hydrogen bond energy)

Red shift (n- * and *) (Bathochromic shift) Increasing polarity of solvent, then increase the

attractive polarization forces between solvent andabsorber, thus decreases the energy of the unexcitedand excited stateswith the later greater

peaks shift to the red

5 nm

i i A i C


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UV-Visible Absorption Chromophores

T i l UV Ab i S


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Typical UV Absorption Spectra

Chromophores?


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Effects of Multiple Chromophores


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The effects of substitution

Auxochrome

function group

Auxochrome is a functional group that does not absorb in UV region but

has the effect of shifting chromophore peaks to longer wavelength as well

As increasin their intensit .


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Now solvents are your container

They need to be transparent and do not erase thefine structure arising from the vibrational effects

Polar solvents generallytend to cause this

problem

Same solvent must be

Used when comparing

absorption spectra for

identification purpose.


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Summary of transitions for organic

molecules

* transition in vacuum UV (single bonds)

n* saturated compounds with non-bondingelectrons

l~ 150-250 nm e~ 100-3000 ( not strong)

n*, * requires unsaturated functionalgroups (eq. double bonds) most commonly used,

energy good range for UV/Vis l~ 200 - 700 nm

n* : e~ 10-100

*: e~ 1000

10,000


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List of common chromophores and their

transitions


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Organic Compounds

Most organic spectra are complex

Electronic and vibration transitions superimposed Absorption bands usually broad

Detailed theoretical analysis not possible, but semi-quantitative orqualitative analysis of types of bonds is possible.

Effects of solvent & molecular details complicate comparison


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If greater then one single bond apart

- eare relatively additive (hyperchromic shift)

- lconstant

CH3CH2CH2CH=CH2 lmax= 184 emax= ~10,000

CH2=CHCH2CH2CH=CH2 lmax=185 emax= ~20,000

If conjugated- shifts to higher ls (red shift)

H2C=CHCH=CH2 lmax=217 emax= ~21,000

Rule of thumb for conjugation


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Spectral nomenclature of shifts


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What about inorganics? Common anions n * nitrate (313 nm), carbonate (217 nm)

Most transition-metal ions absorb in the UV/Vis region.

In the lanthanide and actinide series the absorption processresults from electronic transitions of 4f and 5f electrons.

For the first and second transition metal series the absorption

process results from transitions of 3d and 4d electrons. The bands are often broad.

The position of the maxima are strongly influenced by the chemical

environment.

The metal forms a complex with other stuff, called ligands. The presence

of the ligands splits the d-orbital energies.


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Transition metal ions


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Charge-Transfer-Absorption

A charge-transfer complex consists of anelectron-donor group bonded to anelectron acceptor. When this productabsorbs radiation, an electron from thedonor is transferred to an orbital that islargely associated with the acceptor.

1) Large molar absorptivity (max>10,000)

2) Many organic and inorganic complexes


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Kuliah UV-Molecular Spectrometry

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