UV-Vis Spectroscopy Chem 744 Spring 2013
UV-Vis Spectroscopy Every organic molecule absorbs UV-visible light Energy of electronic transitions saturated functionality not in region that is easily accessible (obscured by solvent and atmosphere) Conjugation ultraviolet 200 300 400 2
Basic Instrument Design 3
Electronic Transitions possible electronic transitions σ ΔE = [E excited - E ground ] = hν π σ σ alkanes E n σ π π π carbonyls alkenes, carbonyls, alkynes, etc. π n σ heteroatoms -, N, S, X, etc. σ n π carbonyls 4
Beer-Lambert Law A = log ( I0 / I1 ) = ε l c A is absorbance (no units) ε is the molar absorptivity or extinction coefficient (L mol -1 cm -1 ) (how strongly it absorbs - intrinsic) l is the path length of the sample (cm) c is the concentration of the comppound (mol L -1 ) I0 is the intensity of the incident light I1 is the intensity of the transmitted light 5
rganic Molecules UV-Vis Characteristics Most organic molecules absorb in UV region unless highly conjugated Most common detector for HPLC best to have conjugated chromophore Spectra are broad (why?) making it useful for qualitative identification Can quantitate using Beer s law analysis 6
Presentation of Spectra many vibrational bands - many slightly different absorbances Absorption of light occurs in 10-15 s, faster than vibrational changes Franck-Codon principle - absorption occurs via a vertical transition - all bond lengths, angles, conformations and solvationare conserved in the transition. hyperchromic A hypsochromic (blue) λ max bathochromic (red) hypochromic λ max 200 nm 800 nm 7
Presentation of Spectra 8
Solvents Measuring UV-Vis spectra <200 nm is impractical Quartz glass cutoff 210 nm fused synthetic silica can get down to 190 nm Solvent Cutoffs solvent cut-off (nm) water 205 acetonitrile 210 cyclohexane 210 diethyl ether 210 ethanol 210 methanol 210 dioxane 220 solvent cut-off (nm) THF 220 dichloromethane 235 chloroform 245 carbon tet 265 benzene 280 acetone 300 9
Solvent effects on spectra H 10
Selection Rules Not all transitions are observed Depends on symmetry and multiplicity Forbidden Transitions (e.g. n-π*) can be seen but are weak Molecular vibrations can disrupt the symmetry σ π possible electronic transitions E n π σ 11
Transitions 12
Nature of absorption Ethane - λmax = 135 nm H H H C C H hν H H 13
Nature of absorption Acetone - λmax = ~166 nm (n-σ * ; ε = >10000) λmax = 188 nm (π-σ * ; ε = 1860) λmax = 279 nm (n-π * ; ε = 15) 14
Tetraphenyldicyclopentadienone 15
Ethylene Ethylene π-π * λmax = 165 nm (n-σ * ; ε = 16,000) Substitution with an atom containing non-bonding electrons ( H, R, NH2, NHR, SH, SR, Hal) results in a bathochromic shift > the non-bonding electrons interact with the π-orbitals of the double bond the energy difference between the HM and LUM decreases 16
Conjugation 17
Conjugation Conjugation of two or more double bonds results in decreasing energy difference between the HM and LUM λ max 217 253 220 227 227 256 263 18
Conjugation 19
Conjugation Woodward-Fieser Rules work well up to 4 double bonds base 217 2 alkyls 10 exo bond 5 total 232 (actual 237) base 214 3 alkyls 15 exo bond 5 total 234 (actual 235) for more than 4 conjugated double bonds: Fieser-Kuhn Rules λ max = 114 + 5(# alkyl substituents) + n(48-1.7n) - 16.5(# endo) - 10(# exo) 20
Conjugation Lycopene and beta-carotene λ max = 114 + 5(# alkyl substituents) + n(48-1.7n) - 16.5(# endo) - 10(# exo) λ max = 114 + 5(8) + 11(48-1.7 11) - 0-0 = 476 nm λ max (actual) = 474 λ max = 114 + 5(10) + 11(48-1.7 11) - 16.5(2) - 0 = 453.3 nm λ max (actual) = 452 21
Benzene 22
Aromatic Substituent Effects 23
Aromatic Substituent Effects 24
Polyaromatic Systems 25
Carbonyl Compounds 26
Carbonyl Compounds C aldehyde or ketone n -> σ * 166 nm ε = 16,000 π -> π * 189 nm ε = 900 n -> π * 166 nm ε = 10-20 279 279 288 H 285 290 H Et 204 207 220 NH 2 295 H 292 Cl 235 299 H 290 225 27
Carbonyl Compounds 28