Applications of Applications of UV/VISUV/VIS

Yongsik LeeYongsik Lee2004. 62004. 6

14B absorbing species14B absorbing species

► ExcitationExcitation Formation of M*Formation of M* Lifetime 1-10 nsecLifetime 1-10 nsec Experience relaxationExperience relaxation

► RelaxationRelaxation Photochemical rxnPhotochemical rxn LuminescenceLuminescence heatheat

Three types of electronic Three types of electronic transitiontransition

►Three typesThree types Involving Involving , and n electrons, and n electrons Involving d and f orbital electronsInvolving d and f orbital electrons Charge transfer electronsCharge transfer electrons

Molecular orbitals (MO)Molecular orbitals (MO)

► Sigma orbitalSigma orbital Rotaionally symmetric charge density around the axis of the bondRotaionally symmetric charge density around the axis of the bond

► Pi orbitalPi orbital Parallel overlap of atomic p orbitalsParallel overlap of atomic p orbitals Nodal plane along the axis of the bondNodal plane along the axis of the bond Maximum density in regions above and below the planeMaximum density in regions above and below the plane

MO in formaldehydeMO in formaldehyde

►Nonbonding electronsNonbonding electrons Unshared electrons between atomsUnshared electrons between atoms Not participate in chemical bondingNot participate in chemical bonding

Molecular energy levelsMolecular energy levels

Sigma-sigma* transitionsSigma-sigma* transitions

►Requires large energyRequires large energy Usually in VUVUsually in VUV

►Not used much in UV/VISNot used much in UV/VIS

For C-H bondFor C-H bond►Methane = abs max at 125 nmMethane = abs max at 125 nm►Ethane = abs max at 135 nmEthane = abs max at 135 nm

For C-C bondFor C-C bond►Less bonding energy, longer abs wavelengthLess bonding energy, longer abs wavelength

N-sigma* transitionsN-sigma* transitions

►RegionRegion 150-250 nm150-250 nm Table 14-1 some examples of absorptionTable 14-1 some examples of absorption Bond itself dependent not chemical strucuture of Bond itself dependent not chemical strucuture of

the moleculethe molecule Solvent effectSolvent effect

►Shift to shorter wavelength in the presence of polar sShift to shorter wavelength in the presence of polar solventsolvents

►Water or ethanolWater or ethanol

n – pi*, pi-pi* transitionsn – pi*, pi-pi* transitions

►200-700 nm►Unsaturated absorbing

center required► Ideal for UV-Vis spectro

metry of organic chromophore

Spectrum Spectrum max shiftmax shift

List of simple chromophoresList of simple chromophores

► chromophoreschromophores only molecular moieties likely to absorb light in the 200 to 800 nm regiononly molecular moieties likely to absorb light in the 200 to 800 nm region pi-electron functionspi-electron functions hetero atoms having non-bonding valence-shell electron pairs. hetero atoms having non-bonding valence-shell electron pairs. The oxygen non-bonding electrons in alcohols and ethers do not give rise to absorption above 16The oxygen non-bonding electrons in alcohols and ethers do not give rise to absorption above 16

0 nm. Consequently, pure alcohol and ether solvents may be used for spectroscopic studies.0 nm. Consequently, pure alcohol and ether solvents may be used for spectroscopic studies.► The presence of chromophores in a molecule is best documented by UV-Visible spectroscThe presence of chromophores in a molecule is best documented by UV-Visible spectrosc

opyopy► but the failure of most instruments to provide absorption data for wavelengths below 200 but the failure of most instruments to provide absorption data for wavelengths below 200

nm makes the detection of isolated chromophores problematic. nm makes the detection of isolated chromophores problematic.

Natural organic pigmentsNatural organic pigments

Terminology for Absorption Terminology for Absorption Shifts Shifts

                                                          

 

► each additional double boneach additional double bond in the conjugated pi-electrd in the conjugated pi-electron system on system shifts the absorption maximushifts the absorption maximu

m about 30 nm in the same dm about 30 nm in the same direction. irection.

► Also, the molar absorptivity Also, the molar absorptivity (ε) roughly doubles with eac(ε) roughly doubles with each new conjugated double boh new conjugated double bond.nd.

► extending conjugation generextending conjugation generally results in bathochromic ally results in bathochromic and hyperchromic shifts in and hyperchromic shifts in absorption absorption

                         

                                 

 

Conjugated dienesConjugated dienes

Unsaturated ketoneUnsaturated ketone

► The spectrum of the unsatThe spectrum of the unsaturated ketone illustrates thurated ketone illustrates the advantage of a logarithmie advantage of a logarithmic display of molar absorptivc display of molar absorptivity. ity.

► The π The π __>__> π* absorption lo π* absorption located at 242 nm is very strcated at 242 nm is very strong, with an ε = 18,000. ong, with an ε = 18,000.

► The weak n The weak n __>__> π* absorpt π* absorption near 300 nm has an ε ion near 300 nm has an ε = 100. = 100.

UV/VIS of Aromatoc compoundUV/VIS of Aromatoc compound

► E2 bandE2 band Exhibits very strong light absorption near 180 nm Exhibits very strong light absorption near 180 nm

(ε > 65,000)(ε > 65,000) weaker absorption at 200 nm (ε = 8,000) weaker absorption at 200 nm (ε = 8,000)

► B bandB band a group of much weaker bands at 254 nm (ε = a group of much weaker bands at 254 nm (ε =

240)240) Only this group of absorptions are completely Only this group of absorptions are completely

displayed displayed because of the 200 nm cut-off characteristic of because of the 200 nm cut-off characteristic of

most spectrophotometers. most spectrophotometers.

Added conjugation of Added conjugation of benzenebenzene

► The added conjugation in naphthThe added conjugation in naphthalene, anthracene and tetracene alene, anthracene and tetracene -> bathochromic shifts of absorp-> bathochromic shifts of absorption bands.tion bands.

► All the absorptions do not shift bAll the absorptions do not shift by the same amounty the same amount for anthracene and tetracene thfor anthracene and tetracene th

e weak absorption is obscured be weak absorption is obscured by stronger bands that have expey stronger bands that have experienced a greater red shift. rienced a greater red shift.

► As might be expected from their As might be expected from their spectra, naphthalene and anthraspectra, naphthalene and anthracene are colorless, but tetracencene are colorless, but tetracene is orange. e is orange.

Woodward-Fieser Rules for CalculatinWoodward-Fieser Rules for Calculating the λmax of Conjugated Dienes and g the λmax of Conjugated Dienes and

PolyenesPolyenes

UV data sheetUV data sheet

Calculating the π Calculating the π ->-> π* λmax of Conj π* λmax of Conjugated Carbonyl Compounds ugated Carbonyl Compounds

Woodward-Fieser RulesWoodward-Fieser Rules

UV data sheetUV data sheet

Physical states & spectraPhysical states & spectra

Inorganic ionsInorganic ions

►Most transition metal ions are colored (absorb in UV-vis) due to d -> d electronic transitions

Color of the sampleColor of the sample

►Remember: Solution absorbs red appears blue-green Solution absorbs blue-green appears red

Five d orbitalsFive d orbitals

► Electron density distribution of d orbitals Xy, xz, yz are similar in space (between 3 axes)Xy, xz, yz are similar in space (between 3 axes) X2-y2, z2 are along the axesX2-y2, z2 are along the axes

Effect of ligand field on d-orbital enerEffect of ligand field on d-orbital energiesgies

► Ligands cause different interactions with d electrons ligand field “splitting” theory

Ligand field strengthLigand field strength

► Ligand field increase -> Ligand field increase -> increase increase The The max decreasemax decrease

HomeworkHomework

►14-1, 14-6, 14-714-1, 14-6, 14-7