Applications of UV/VIS Yongsik Lee 2004. 6

14B absorbing species Excitation Relaxation Formation of M* Lifetime 1-10 nsec Experience relaxation Relaxation Photochemical rxn Luminescence heat

Three types of electronic transition Involving p, s, and n electrons Involving d and f orbital electrons Charge transfer electrons

Molecular orbitals (MO) Sigma orbital Rotaionally symmetric charge density around the axis of the bond Pi orbital Parallel overlap of atomic p orbitals Nodal plane along the axis of the bond Maximum density in regions above and below the plane

MO in formaldehyde Nonbonding electrons Unshared electrons between atoms Not participate in chemical bonding

Molecular energy levels

Sigma-sigma* transitions Requires large energy Usually in VUV Not used much in UV/VIS For C-H bond Methane = abs max at 125 nm Ethane = abs max at 135 nm For C-C bond Less bonding energy, longer abs wavelength

N-sigma* transitions Region 150-250 nm Table 14-1 some examples of absorption Bond itself dependent not chemical strucuture of the molecule Solvent effect Shift to shorter wavelength in the presence of polar solvents Water or ethanol

n – pi*, pi-pi* transitions 200-700 nm Unsaturated absorbing center required Ideal for UV-Vis spectrometry of organic chromophore

Spectrum lmax shift

List of simple chromophores only molecular moieties likely to absorb light in the 200 to 800 nm region pi-electron functions 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 160 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 spectroscopy but the failure of most instruments to provide absorption data for wavelengths below 200 nm makes the detection of isolated chromophores problematic.

Natural organic pigments

Terminology for Absorption Shifts                                                              Terminology for Absorption Shifts each additional double bond in the conjugated pi-electron system shifts the absorption maximum about 30 nm in the same direction. Also, the molar absorptivity (ε) roughly doubles with each new conjugated double bond. extending conjugation generally results in bathochromic and hyperchromic shifts in absorption                                                             

Conjugated dienes

Unsaturated ketone The spectrum of the unsaturated ketone illustrates the advantage of a logarithmic display of molar absorptivity. The π __> π* absorption located at 242 nm is very strong, with an ε = 18,000. The weak n __> π* absorption near 300 nm has an ε = 100.

UV/VIS of Aromatoc compound E2 band Exhibits very strong light absorption near 180 nm (ε > 65,000) weaker absorption at 200 nm (ε = 8,000) B band a group of much weaker bands at 254 nm (ε = 240) Only this group of absorptions are completely displayed because of the 200 nm cut-off characteristic of most spectrophotometers.

Added conjugation of benzene The added conjugation in naphthalene, anthracene and tetracene -> bathochromic shifts of absorption bands. All the absorptions do not shift by the same amount for anthracene and tetracene the weak absorption is obscured by stronger bands that have experienced a greater red shift. As might be expected from their spectra, naphthalene and anthracene are colorless, but tetracene is orange.

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

UV data sheet

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

Woodward-Fieser Rules

UV data sheet

Physical states & spectra

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

Color of the sample Remember: Solution absorbs red appears blue-green Solution absorbs blue-green appears red

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

Effect of ligand field on d-orbital energies Ligands cause different interactions with d electrons ligand field “splitting” theory

Ligand field strength Ligand field increase -> D increase The lmax decrease

Homework 14-1, 14-6, 14-7