Electronic Excitation by UVVis Spectroscopy Xray core electron

Electronic Excitation by UV/Vis Spectroscopy : X-ray: core electron excitation UV: valance electronic excitation IR: molecular vibrations Radio waves: Nuclear spin states (in a magnetic field)

Spectroscopic Techniques UV-vis region bonding electrons Atomic Absorption UV-vis region atomic transitions (val. e-) FT-IR IR/Microwave vibrations, rotations Raman IR/UV vibrations FT-NMR Radio waves nuclear spin states X-Ray Spectroscopy X-rays inner electrons, elemental X-ray Crystallography X-rays 3 -D structure

The wavelength and amount of light that a compound absorbs depends on its molecular structure and the concentration of the compound used. The concentration dependence follows Beer’s Law. A=ebc Where A is absorbance (no units, since A = log(P 0 / P ) e is the molar absorbtivity with units of L mol-1 cm-1 b is the path length of the sample - that is, the path length of the cuvette in which the sample is contained (typically in cm). c is the concentration of the compound in solution, expressed in mol L-1

Characteristics of UV-Vis spectra of Organic Molecules • Absorb mostly in UV unless highly conjugated • Spectra are broad, usually to broad for qualitative identification purposes • Excellent for quantitative Beer’s Lawtype analyses • The most common detector for an HPLC

Molecules have quantized energy levels: ex. electronic energy levels. energy hv } = hv Q: Where do these quantized energy levels come from? A: The electronic configurations associated with bonding. Each electronic energy level (configuration) has associated with it the many vibrational energy levels we examined with IR.

Broad spectra • Overlapping vibrational and rotational peaks • Solvent effects

Ethane max = 135 nm (a high energy transition) Absorptions having max < 200 nm are difficult to observe because everything (including quartz glass and air) absorbs in this spectral region.

= hv =hc/ Example: ethylene absorbs at longer wavelengths: max = 165 nm = 10, 000

The n to pi* transition is at even lower wavelengths but is not as strong as pi to pi* transitions. It is said to be “forbidden. ” Example: Acetone: n max = 188 nm ; = 1860 n max = 279 nm ; = 15

135 nm 165 nm n 183 nm weak n n 150 nm 188 nm 279 nm weak 180 nm A 279 nm

Conjugated systems: Preferred transition is between Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO). Note: Additional conjugation (double bonds) lowers the HOMOLUMO energy gap: Example: 1, 3 butadiene: max = 217 nm ; = 21, 000 1, 3, 5 -hexatriene max = 258 nm ; = 35, 000

Similar structures have similar UV spectra: max = 238, 305 nm max = 240, 311 nm max = 173, 192 nm

Woodward-Fieser Rules for Dienes Parent Homoannular =253 nm Heteroannular =214 nm =217 (acyclic) Increments for: Double bond extending conjugation Alkyl substituent or ring residue Exocyclic double bond Polar groupings: -OC(O)CH 3 -OR -Cl, -Br -NR 2 -SR Homoannular heteroannular +30 +5 +5 +0 +6 +5 +60 +30 acyclic exocyclic For more than 4 conjugated double bonds: max = 114 + 5(# of alkyl groups) + n(48. 0 -1. 7 n)

max = 114 + 5(8) + 11*(48. 0 -1. 7*11) = 476 nm max(Actual) = 474.