UV-VIS Molecular SpectroscopyChapter 13-14

From 190 to 900 nm!

Reflection and Scattering Losses

LAMBERT-BEER LAWPower of radiation after passing through the solventPower of radiation after passing through the sample solution

Absorption Variables

Beers law and mixturesEach analyte present in the solution absorbs light! The magnitude of the absorption depends on its eA total = A1+A2++An A total = e1bc1+e2bc2++enbcnIf e1 = e2 = en then simultaneous determination is impossibleNeed nls where es are different to solve the mixture

AssumptionsIngle and Crouch, Spectrochemical Analysis

Deviations from Beers LawSuccessful at low analyte concentrations (0.01M)!High concentrations of other species may also affect

Chemical EquilibriaConsider the equilibrium:If e is different for A and AC then the absorbance depends on the equilibrium.

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

A plot of absorbance vs. [A]total will not be linear.

Instrumental deviation with polychromatic radiation

Effects of Stray Light

Instrument Noise

Effects of Signal-to-NoiseBad at Low TBad at High T

Components of instrumentation:SourcesSample ContainersMonochromatorsDetectors

Components of instrumentation:Sources: Agron, Xenon, Deuteriun, or Tungsten lampsSample Containers: Quartz, Borosilicate, PlasticMonochromators: Quarts prisms and all gratingsDetectors: Pohotomultipliers

SourcesDeuterium and hydrogen lamps (160 375 nm)

D2 + Ee D2* D + D + h Excited deuterium molecule with fixed quantized energyDissociated into two deuterium atoms with different kinetic energiesEe = ED2* = ED + ED + hvEe is the electrical energy absorbed by the molecule. ED2* is the fixed quantized energy of D2*, ED and ED are kinetic energy of the two deuterium atoms.

SourcesTungsten lamps (350-2500 nm)

Blackbody type , temperature dependentWhy add I2 in the lamps?W + I2 WI2Low limit: 350 nm

Low densityGlass envelope

General Instrument DesignsSingle beamRequires a stabilized voltage supply

General Instrument DesignsDouble Beam: Space resolvedNeed two detectors

General Instrument DesignsDouble Beam: Time resolved

Double Beam Instruments

Compensate for all but the most short term fluctuation inradiant output of the source

Compensate drift in transducer and amplifier

Compensate for wide variations in source intensity withwavelength

Multi-channel Design

Molar absorptivitiese = 8.7 x 10 19 P AA: cross section of molecule in cm2 (~10-15)P: Probability of the electronic transition (0-1)P>0.1-1 allowable transitionsP

Visible Absorption Spectra

The absorption of UV-visible radiation generally results from excitation of bonding electrons. can be used for quantitative and qualitative analysis

Molecular orbital is the nonlocalized fields between atoms that are occupied by bonding electrons. (when two atom orbitals combine, either a low-energy bonding molecular orbital or a high energy antibonding molecular orbital results.)Sigma () orbitalThe molecular orbital associated with single bonds in organic compoundsPi () orbitalThe molecular orbital associated with parallel overlap of atomic P orbital.n electronsNo bonding electrons

Molecular Transitions for UV-Visible AbsorptionsWhat electrons can we use for these transitions?

MO Diagram for Formaldehyde (CH2O)HCHOs =p =n =

Singlet vs. tripletIn 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.

Type of Transitions *High energy required, vacuum UV rangeCH4: = 125 nmn *Saturated compounds, CH3OH etc ( = 150 - 250 nm)n * and *Mostly used! = 200 - 700 nm

Examples of UV-Visible AbsorptionsLOW!

UV-Visible Absorption Chromophores

Effects of solvents Blue shift (n- p*) (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- p* and p p*) (Bathochromic shift)Increasing polarity of solvent, then increase the attractive polarization forces between solvent and absorber, thus decreases the energy of the unexcited and excited states with the later greater peaks shift to the red5 nm

UV-Visible Absorption Chromophores

Typical UV Absorption SpectraChromophores?

Effects of Multiple Chromophores

The effects of substitutionAuxochrome function groupAuxochrome is a functional group that does not absorb in UV region but has the effect of shifting chromophore peaks to longer wavelength as wellAs increasing their intensity.

Now solvents are your containerThey need to be transparent and do not erase the fine structure arising from the vibrational effectsPolar solvents generally tend to cause this problem Same solvent must beUsed when comparingabsorption spectra foridentification purpose.

Summary of transitions for organic moleculess s* transition in vacuum UV (single bonds)n s* saturated compounds with non-bonding electrons l ~ 150-250 nme ~ 100-3000 ( not strong)n p*, p p* requires unsaturated functional groups (eq. double bonds) most commonly used, energy good range for UV/Visl ~ 200 - 700 nmn p* : e ~ 10-100p p*: e ~ 1000 10,000

List of common chromophores and their transitions

Organic CompoundsMost organic spectra are complexElectronic and vibration transitions superimposedAbsorption bands usually broadDetailed theoretical analysis not possible, but semi-quantitative or qualitative analysis of types of bonds is possible.Effects of solvent & molecular details complicate comparison

If greater then one single bond apart- e are relatively additive (hyperchromic shift)- l constant

CH3CH2CH2CH=CH2lmax= 184 emax = ~10,000

CH2=CHCH2CH2CH=CH2lmax=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

Spectral nomenclature of shifts

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

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

In the lanthanide and actinide series the absorption process results 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.

Transition metal ions

Charge-Transfer-Absorption

A charge-transfer complex consists of an electron-donor group bonded to an electron acceptor. When this product absorbs radiation, an electron from the donor is transferred to an orbital that is largely associated with the acceptor.

Large molar absorptivity (max >10,000)Many organic and inorganic complexes

*