#1 uv visibel

Ultravolet-Visible Spectroscopy

for Inorganic Compound, Coordination Compound and Inorganic Material

Electromagnetic Spectrum

• The radiation which is absorbed has an energy which exactly matches the energy difference between the ground state and the excited state.

• These absorptions correspond to electronic transitions.

UV-Visible Spectroscopy

Electron Spectra

The UV/Vis spectra are used to determine the value of ∆o for the complex.

The spectra arise from electronic transitions between the t2g and eg sets of molecular orbitals.

Electron-electron interactions can greatly complicate the spectra.

Only in the case of a single electron is interpretation of the spectrum straightforward.

Obtaining ∆o

For a d1 configuration, only a single peak is seen. It results from the electron promotion from the t2g orbitals

to the eg orbitals. The “toothed” appearance of the peak is due to a Jahn-

Teller distortion of the excited state. The energy of the peak = ∆o.

General Observations

d1, d4, d6 and d9 usually have 1 absorption.


d2, d3, d7 and d8

usually have 3 absorptions, one is often obscured by a very intense charge transfer band.


d5 complexes consist of very weak, relatively sharp transitions which are spin-forbidden, and have a very low intensity.

Electronic Transitions

• Absorption of light occurs when electrons are promoted from lower to higher energy states.

• Interactions between electrons causes more than one peak in the UV/Vis spectra of these complexes.

• The electrons are not independent of each other, and the spin angular momentum and orbital angular momentum interact.

– Absorption spectroscopy from 160 nm to 780 nm– Measurement of transmittance• Conversion to absorbance

A = -log T = log(IO/I) = b c

A = Absorbance (optical density)T = TransmittanceIO = Intensity of light on the sample cell

I = Intensity of light leaving the sample cellc = molar concentration of soluteb = length of sample cell (cm) = molar absorptivity (molar extinction coefficient)


• The Beer-Lambert Law is rigorously obeyed when a single species is present at relatively low concentrations.


• The Beer-Lambert Law is not obeyed:

– High concentrations– Solute and solvent form complexes– Thermal equilibria exist between the ground state

and the excited state– Fluorescent compounds are present in solution


• The size of the absorbing system and the probability that the transition will take place control the absorptivity ().

• Values above 104 are termed high intensity absorptions.

• Values below 1000 indicate low intensity absorptions which are forbidden transitions.


• Highest occupied molecular orbital (HOMO)• Lowest unoccupied molecular orbital (LUMO)


Terminology for Absorption Shifts

Nature of Shift Descriptive Term

To Longer Wavelength Bathochromic

To Shorter Wavelength Hypsochromic

To Greater Absorbance Hyperchromic

To Lower Absorbance Hypochromic

1. Bathochromic shift (red shift)– lower energy, longer wavelength

– CONJUGATION.

2. Hypsochromic shift (blue shift)– higher energy, shorter wavelength.

3. Hyperchromic effect– increase in intensity

4. Hypochromic effect– decrease in intensity


Terminology of shifts in the position of an absorption band

Bathochromic

Hyperchromic

Hypochromic

HypsochromicAb

sorb

ance

λλmax

Quantitative Analysis for Inorganic species (Colorimetric Analysis)


• The ions and complexes of elements of the first two transition series absorb broad bands of visible radiation in at least one of their oxidation states and are, as a consequence, coloured.


• Absorption involves transitions between filled and unfilled d-orbitals.

• The energy differences between the d-orbitals (and thus the position of the corresponding absorption peak) depend upon the the position of the element in the periodic table, its oxidation state, and the nature of the ligand bonded to it.


• Charge transfer absorption is of particular interest to analytical chemists because molar absorptivities are usually large (max > 10,000).

• Methods based upon this type of absorption are highly sensitive.


• Many organic and inorganic complexes exhibit charge transfer absorption and are known as charge transfer complexes.

• Thiocyanate and phenolic complexes of iron(III).


• Charge transfer complexes generally contain both an electron donor group and an electron acceptor group.

• Absorption of radiation involves the transfer of an electron from the donor group to an orbital associated with the acceptor group.


• Six criteria for a successful analysis– Specificity of the colour reaction– Proportionality between colour and

concentration– Stability of the colour– Reproducibility– Clarity of the solution– High sensitivity.


• Specificity of the colour reaction

– Very few reactions are specific for a particular substance.

– However, many only give colours for a small group of related substances.

– Therefore they can be considered to be selective.

– Alteration of oxidation states and pH enhances selectivity.


• Proportionality between colour and concentration

– It is desirable that the system under investigation follows the Beer-Lambert law.

• Stability of the colour

- Not all complexes are stable with respect to time.

- The stability of the complex should be sufficient to allow precise measurements to be made.


• Reproducibility

– The colorimetric procedure must give reproducible results under the experimental conditions.

– The reaction need not represent a stoichiometrically quantitiative change.


• Clarity of the solution

– The solution must be free of precipitates. – Turbidity scatters and absorbs light.

• High sensitivity

– It is desirable that the colour reaction be highly sensitive.

– i.e. is very large.


• Steps in carrying out a colorimetric analysis.

– Choose the wavelength of maximum absorbance.– Prepare a calibration curve using known quantities of

the complex measured at this wavelength.– Measure the absorbance of your unknown sample.– Calculate the concentration from the equation of the

best fit line.


• Calibration Curves

y = 1.2469x - 0.0219

0

1

2

3

4

0 1 2 3

Nickel Concentration (mg/l)

Abs

R2 = 0.9992


• Advantages of colorimetric analysis.

– Better at low concentrations than titrimetric or gravimetric analysis.

– Can be applied under conditions where there are no satisfactory titrimetric or gravimetric procedures.

– Very rapid once a calibration curve as been obtained.


#1 uv visibel

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